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Mineralogy of Permeable Reactive Barriers for the Attenuation of Subsurface Contaminants
Abstract
Permeable reactive barriers (PRBs) are a relatively recent development of a passive system to remediate subsurface waters containing organic or inorganic contaminants. Groundwater fl ow under a natural gradient passes through a permeable curtain of treatment medium that either precipitates the contaminants as relatively insoluble compounds or transforms the contaminants into environmentally acceptable or benign species. The most widely adopted treatment medium is submillimetric zero-valent iron, a substance that is highly reactive, environmentally acceptable, and is readily available as a manufactured product derived from the recycling of scrap iron and steel. Organic compost wastes have also been used to ameliorate inorganic contaminants, and two case studies of the utilization of composts to reduce sulfate and precipitate metals are presented, primarily from a mineralogical perspective. In cores of the reacted treatment media, the most abundant secondary product formed in situ is Fe oxyhydroxide, but a variety of precipitates has been identifi ed. For example, secondary pyrite, greigite, and native nickel are present at a site at which replacement of organic material by sulfi des is common. At an industrial site, secondary pyrite, covellite, chalcopyrite, and bornite have formed in the treatment medium, and whereas replacement of organic material by Fe oxyhydroxides is widespread, replacement by sulfi des is rare. The secondary sulfi des and metals are volumetrically small and are unlikely to impede the perme-ability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of zero-valent iron requires further monitoring to determine whether the secondary precipitates and the consumption of Fe 0 will appreciably lessen the effectiveness of such PRBs over the long term. Current indications are that PRBs are both an environmentally effective and a cost-effective technique of remediation.
... Permeable reactive barriers are expensive in its purchase, too, but after installement in the aquifer they work for long periods of time. The barriers are inserted downgradient of the contaminant source and work in-situ, decontaminating the groundwater by physical, chemical or biological processes (Jambor et al., 2005;Henderson et al., 2007). There are the so called funnel-and-gate systems as well as reactive curtains. ...
... Elemental iron (Fe 0 ) is the most widely used material for contaminant removal in permeable reactive barriers. Efficient decontamination by Fe 0 has been reported for organic and inorganic contaminants, including heavy metals and radionuclides (Jambor et al., 2005;Henderson et al., 2007). The successful removal of viruses has also been reported (You et al. 2005). ...
... The use of elemental iron (Fe 0 ) for the remediation of contaminated groundwater has been an area of intensive research during the past fifteen years (Gillham et al., 1994;Matheson et al., 1994;Jambor et al. 2005, Henderson et al., 2007. Reductive degradation/precipitation has been considered to be the main removal process in dealing with reducible contaminants. ...
The reactivity of elemental iron, Fe0 is widely investigated for use in permeable
reactive barriers. Fe0 is known for its efficient removal of a wide range of contaminants, like organic and inorganic substances, heavy metals and viruses.
Beside its efficacy, Fe0 is unexpensive and available in quantities huge enough for
the use in permeable reactive barriers. Aqueous decontamination by Fe0 (e.g., in Fe0-H2O systems) may proceed by (i) contaminant adsorption onto Fe0, aged and nascentFe0 corrosion products, (ii) contaminant co-precipitation with nascent Fe0
corrosion products, (iii) contaminant reduction by Fe0 (direct reduction), and (iv) contaminant reduction by FeII and atomic or molecular hydrogen (indirect reduction). Investigations for the efficacy of Fe0 are abundantly performed under shaken conditions.
In this work the influence of shaking intensities on the decontamination in “Fe0-H2O” systems was investigated. Methylene blue (MB) was used as a model contaminant for the characterization of the removal efficiency of Fe0. MB is both
easy in handling and inexpensive and its adsorption behavior is comparable with
several organic contaminants. Besides, it is a contaminant of the textile industry
itself. MnO2 and GAC were added as comparable systems to the “Fe0-H2O” system
for a better understanding of proceeding processes in this system. Investigations were performed under non-shaken, mild and violent shaken conditions. Shaking duration was either one day, three or five days.
It could be shown, that shaking intensity as well as shaking duration has a significant influence on the discoloration of MB and thus decontamination. The generation of corrosion products was accelerated and the contaminant removal, which is mainly due to adsorption and co-precipitation on in-situ generated corrosion products, increased significantly. Furthermore, the pattern of discoloration, i.e. decontamination, under shaken conditions differed strongly from the pattern seen in the undisturbed experiments.
Data obtained by this work indicate, that experiments performed under shaken conditions might be difficult in transfer to reality and when compared with each other. Therefore, investigation of decontamination efficiency or removal mechanisms should be performed under undisturbed (non-shaken) conditions
... A.15, A.16). Geochemical modeling also indicates the potential precipitation of secondary minerals previously observed in reactive barrier studies, including magnetite (Fe 3 O 4 ), maghemite (Fe 2 O 3 ), sphalerite ((Zn, Fe)S), greigite (Fe 3 S 4 ), and amorphous FeS (Gu et al., 1999;Jambor et al., 2005;Lindsay et al., 2008;Rao et al., 2009;Figs. A.17, A.18). ...
... Other secondary products such as magnetite and maghemite likely contributed to Fe removal (Gu et al., 1999;Jambor et al., 2005). ...
... Corrosion of ZVI grains, or replacement by secondary precipitates, was observed on grain boundaries and in the form of alteration rims, or on the darkened sections of the grains. Jambor et al. (2005) observed the replacement of ZVI by Fe (oxy)hydroxides as rims and veins around the grain or on the exterior of the grain at sites containing graphite. The Fe (oxy)hydroxides observed in these column experiments are similar in appearance to those observed by Jambor et al. (2005). ...
Acid mine drainage and the associated contaminants, including As and metals, are ongoing environmental issues. Passive remediation technologies have the potential to remove As from mine waste effluents. A series of laboratory column experiments was conducted to evaluate the effectiveness of varying mixtures of organic carbon (OC), zero-valent iron (ZVI), and limestone for the treatment of As, metals, SO42-, and acidity in groundwater from an abandoned gold mine. The onset of bacterially-mediated SO42- reduction was indicated by a decrease in Eh, a decline in aqueous SO42- concentrations coupled with enrichment of δ34S, and the presence of sulfate-reducing bacteria and H2S. Removal of As was observed within the first 3 cm of reactive material, to values below 10 µg L-1, representing > 99.9% removal. An increase in pH from 3.5 to circumneutral values and removal of metals including Al, Cu, and Zn was also observed. Synchrotron results suggest As was removed through precipitation of As-crystalline phases such as realgar and orpiment, or through adsorption as As(V) on ferrihydrite. The results indicate the potential for a mixture of OC and ZVI to remove As from acidic, mine-impacted water.
... Iron permeable reactive barriers (iron walls) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1][2][3]. Since its development in 1990, by Canadian 1 hydrogeologist, numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [1][2][3][4]. Laboratory tests have demonstrated that other metals (e.g., Zn, Mg, Sn) are also effective in removing contaminates from water. ...
... Iron permeable reactive barriers (iron walls) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1][2][3]. Since its development in 1990, by Canadian 1 hydrogeologist, numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [1][2][3][4]. Laboratory tests have demonstrated that other metals (e.g., Zn, Mg, Sn) are also effective in removing contaminates from water. The advantage of iron over other metals is that they are non-toxic (environmentally friendly), inexpensive and readily available in bulk quantities (cost-effective). ...
... The advantage of iron over other metals is that they are non-toxic (environmentally friendly), inexpensive and readily available in bulk quantities (cost-effective). Fe "zero valent iron", "elemental iron", "iron fillings" or "granular iron", are commercially available and produced from scrap iron and steel [2,3]. ...
A review of the approach used by environmental remediation researchers to evaluate the reactivity of Fe0-based alloys reveals the lack of consideration of the results available from other branches of science. This paper discusses the limitations of the current approach. The discussion provided here suggests that the current assumption that redox-sensitive species serve as corrosive agents for Fe0 maybe incorrect because water as the solvent is also corrosive. A new approach is proposed in which water is considered as the primary Fe0 oxidizing agent and the impact of individual relevant solutes (including contaminants) should be assessed in long-term laboratory experiments. It is expected that the application of the proposed approach will help to reliably characterize the reactivity of Fe0 materials within a few years.
... Recently, the introduction of in situ permeable reactive barriers for groundwater remediation [1,2] which contains a removing agent. The groundwater passing through a reactive barrier is ideally completely freed from contaminants [3][4][5]. ...
... In the last two decades metallic iron (Fe 0 ) has been extensively used in remediation schemes to effectively remove a wide variety of inorganic and organic contaminants in reactive barriers [3][4][5][6]. Ideally, Fe 0 is oxidized only from the oxidized form of the contaminant (Ox) which reduction yields a corresponding reduced form (Red) (Eq. 1 -Tab. 1). ...
... The most important feature characterizing a Fe 0 /H 2 O system is that the weight fraction of iron corrosion products increases from zero at the beginning of the experiment to more or less higher proportions depending on the reaction progress (Fe 0 consumption [4][5][6]11,12,17]. It has been recently suggested that the chemical reactivity of Fe 0 (iron corrosion) has not been properly considered while using Fe 0 as remediation medium [15,16]. ...
Aqueous contaminant removal in the presence of metallic iron (e.g. in Fe0/H2O systems) is characterized by the large diversity of removing agents. This paper analyses the synergistic effect of adsorption, co-precipitation and reduction on the process contaminant removal in Fe0/H2O systems on the basis of simple theoretical calculations. The system evolution is characterized by the percent Fe0 consumption. The results showed that contaminant reduction by Fe0 is likely to significantly contribute to the removal process only in the earliest stage of Fe0 immersion. With increasing reaction time, contaminant removal is a complex interplay of adsorption onto iron corrosion products, co-precipitation or sequestration in the matrix of iron corrosion products and reduction by Fe0, FeII or H2/H. The results also suggested that in real world Fe0/H2O systems, any inflowing contaminant can be regarded as foreign species in a domain of precipitating iron hydroxides. Therefore, current experimental protocols with high contaminant to Fe0 ratios should be revisited. Possible optimising of experimental conditions is suggested.
... Iron permeable reactive barriers (iron walls) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters123. Since its development in 1990, by Canadian hydrogeologist, numerous papers have been written on the topic and approximately 120 iron walls installed worldwide1234. Laboratory tests have demonstrated that other metals (e.g., Zn, Mg, Sn) are also effective in removing contaminates from water. ...
... Iron permeable reactive barriers (iron walls) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters123. Since its development in 1990, by Canadian hydrogeologist, numerous papers have been written on the topic and approximately 120 iron walls installed worldwide1234. Laboratory tests have demonstrated that other metals (e.g., Zn, Mg, Sn) are also effective in removing contaminates from water. ...
... The advantage of iron over other metals is that they are non-toxic (environmentally friendly), inexpensive and readily available in bulk quantities (cost-effective). Fe 0 -based alloys commonly referred to as " zero valent iron " , " elemental iron " , " iron fillings " or " granular iron " , are commercially available and produced from scrap iron and steel [2,3]. Since the introduction of the iron wall technology, reductive transformations of compounds (degradation of organics and precipitation of inorganics) by Fe 0 mostly under anoxic conditions has been extensively investigated [1,2,3]. ...
A review of the approach used by environmental remediation researchers to evaluate the reactivity of Fe0-based alloys reveals the lack of consideration of the results available from other branches of science. This paper discusses the limitations of the current approach. The discussion provided here suggests that the current assumption that redox-sensitive species serve as corrosive agents for Fe0 maybe incorrect because water as the solvent is also corrosive. A new approach is proposed in which water is considered as the primary Fe0 oxidizing agent and the impact of individual relevant solutes (including contaminants) should be assessed in long-term laboratory experiments. It is expected that the application of the proposed approach will help to reliably characterize the reactivity of Fe0 materials within a few years.
... Permeable reactive barriers containing metallic iron as a reactive filler material (Fe 0 PRBs) is an established technology for groundwater remediation, e.g., www.itrcweb.org (accessed: 09.03.2012) [1][2][3][4][5][6][7][8][9][10]. At present, more than 120 Fe 0 PRBs have been installed worldwide, and effective performance has typically been reported [10][11][12][13]. ...
... At present it is suggested that the mechanism of permeability loss in Fe 0 PRBs is due to the accumulation of insoluble minerals within the PRB pore network [10,13]. Relevant minerals include siderite (FeCO 3 ), aragonite (CaCO 3 ), and iron (hydr)oxides (e.g., Fe(OH) 2 , Fe(OH) 3 , FeOOH, Fe 2 O 3 , and Fe 3 O 4 ) [10,13,[20][21][22][23][24][25][26]. Another mechanism reported attributes the permeability loss to the build-up of H 2 gas, formed due to the hydrolysis of water during Fe 0 corrosion [11,27,28]. ...
... For pH > 4.5 the solubility of iron is very low and the flux of Fe (m Fe,flux ) can be largely neglected assuming that water flow rate is slow enough that the dissolution/precipitation reactions are at pseudo-equilibrium. Equation (2) can be re-written as: ...
Over the past 30 years the literature has burgeoned with in-situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe0) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe0 barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe0 corrosion in porous media is presented. Subsequent implications for the design of Fe0 filtration systems are modelled. The results suggest that: (i) for the pH range of natural waters (> 4.5), the high volumetric expansion of Fe0 during oxidation and precipitation dictates that Fe0 should be mixed with a non-expansive material; (ii) naturally-occurring solute precipitates have a negligible impact on permeability loss compared to Fe0 expansive corrosion; and (iii) the proliferation of H2 metabolising bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe0 PRB system as a physical (size-exclusion) water filter device.
... In cores of the reacted treatment media, the most abundant secondary product formed in situ is Fe oxyhydroxide (iron corrosion products – iron hydroxides and oxides), but a variety of precipitates has been identified. For example, secondary pyrite, greigite, cove llite , chalcopyrite, and bornite have formed in the treatment medium (Jambor et al. 2005, Mackenzie et al. 1999). The secondary sulfides are volumetrically small and are unlikely to impede the permeability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of Fe 0 requires further monitoring to determine whether the secondary precipitates and the consumption of Fe 0 will appreciably lessen the effectiveness of such PRBs over the long term. ...
... The secondary sulfides are volumetrically small and are unlikely to impede the permeability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of Fe 0 requires further monitoring to determine whether the secondary precipitates and the consumption of Fe 0 will appreciably lessen the effectiveness of such PRBs over the long term. Current indications are that PRBs are both an environmentally effective and a cost-effective technique of remediation (Henderson and Demond 2007, Jambor et al. 2005, Laine and Cheng 2007). A trend persists in the scientific literature terming iron PRBs as a reduction technology (Kim et al. 2008, Laine and Cheng 2007 ) although contaminant reduction has not been traceably demonstrated. ...
... The long term feasibility of Fe 0 reactive barriers in the cleanup of contaminated groundwaters has been demonstrated in laboratory column studies and confirmed by field installations. Column studies and fields installations indicate that Fe 0 maintains its reactivity over long periods of time (Jambor et al. 2005). Many forms of iron have been proposed for water treatment. ...
Many of the reasons behind the anthropogenic contamination problems in rural environments of developing countries lie in changes
in the traditional way of life and the ignorance on the toxic potential of introduced manufactured products. A generalization
trend exists within the international community suggesting that water in developing countries is of poor quality. However,
the water quality is rarely analytically determined. Existing potabilization solutions may be prohibitively expensive for
the rural populations. Therefore, efficient and affordable technologies are still needed to ameliorate the water quality.
In the recent two decades, elemental iron has shown the capacity to remove all possible contaminants (including viruses) from
the groundwater. This paper presents a concept to scale down the conventional iron barrier technology to meet the requirements
of small communities and households in rural environments worldwide.
... [18] All of the reactive substrates in the five columns were very effective for removing dissolved Cu (Figure 2(d)) from the influent concentrations of 0.87 and 2.57 mg/L for oxide and sulfide waters, respectively, except for Col 3 during Phase 5 which showed a slightly higher concentration (0.12 mg/L). Cu concentrations may be controlled by the precipitation of chalcocite [Cu 2 S] and covellite [CuS] (see Table 5), which is consistent with Jambor et al. [19] and Jeen et al. [20]. ...
... The precipitations of Ni-bearing sulfide minerals (e.g. millerite; Table 5) and elemental Ni [19] are probably the primary mechanisms of Ni removal in these columns. Nickel removal may also be occurring by adsorption onto the organic materials in the columns. ...
Laboratory column tests for passive treatment systems for mine drainage from a waste rock storage area were conducted to evaluate suitable reactive mixture, system configuration, effects of influent water chemistry, and required residence time. Five columns containing straw, chicken manure, mushroom compost, and limestone, either in layered or mixed configurations, were set up to simulate the treatment system. The columns were operated for a total of 74 days, except one column, which was continued for a total of 214 days to evaluate long-term performance of the system. The results showed that all of the five columns removed metals of concern (i.e., Al, Cd, Co, Cu, Fe, Ni, Zn) with residence time of 15 hours and greater. The organic substrates used in the test provided sufficient redox control for sulfate reduction. The sulfate removal rates ranged between 200 and 600 mg/L/day. Reaction mechanisms responsible for the removal of metals may include sulfate reduction and subsequent sulfide precipitation, precipitation of secondary carbonates and hydroxides, co-precipitation, and sorption on organic substrates and secondary precipitates. The results suggest that the mixed systems containing organic materials and limestone perform better than the layered systems, sequentially treated by organic and limestone layers, due to the enhanced pH adjustment, which is beneficial to bacterial activity and precipitation of secondary minerals. The columns tests provide a basis for design of a field-scale passive treatment system, such as reducing and alkalinity producing system (RAPS) or permeable reactive barrier (PRB).
... The processes occurring at the interface Fe 0 /H 2 O are of great interest for the use of metallic iron in environmental remediation (e.g. in Fe 0 /H 2 O systems). A great deal of work has been reported in this area during the past 20 years [1][2][3][4][5][6][7][8][9][10]. Since the seminal work of Matheson and Tratnyek [1], a substantial amount of literature concerning the removal mechanism of various contaminants in Fe 0 /H 2 O systems has been published. ...
... Since the seminal work of Matheson and Tratnyek [1], a substantial amount of literature concerning the removal mechanism of various contaminants in Fe 0 /H 2 O systems has been published. This is not surprising given that: (i) the concept of permeable reactive barrier (PRB) is regarded as a significant advance in remediation technology [11,12], and (ii) iron PRBs have been demonstrated very efficient to mitigate contaminants in surface and ground waters [3][4][5]. Moreover, Fe 0 /H 2 O systems have been shown to effectively removed aqueous species of various nature. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... Permeable reactive barriers of elemental iron (Fe 0 walls or remediation Fe 0 /H 2 O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1][2][3][4][5][6][7]. Since its development in 1990 by Canadian hydrogeologists numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [4]. ...
... Permeable reactive barriers of elemental iron (Fe 0 walls or remediation Fe 0 /H 2 O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1][2][3][4][5][6][7]. Since its development in 1990 by Canadian hydrogeologists numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [4]. Even though the importance of the mechanism of contaminant removal was recognised in the early stage of technology development [1,8], there is still discussion about the mechanism of contaminant removal in Fe 0 /H 2 O systems [9][10][11][12]. ...
Metallic iron (Fe0) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe0 used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe0 as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe0 in the process of electron transfer at the Fe0/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe0/H2O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO2) by selectively removing metallic iron (Fe0) from reduced ilmenite (RI). This process involves an aqueous oxygen leaching step at near neutral pH. Oxygen leaching suffers from serious limitations imposed by limited mass transport rates of dissolved oxygen across the matrix of iron oxides from initial Fe0 oxidation. In a Fe0/H2O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O2) to the Fe0/H2O interface. Instead of this universal role of oxide layers on Fe0, improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe0/oxide/water interface.
... Several reactive materials have been used including activated carbon, compost, clays, Fe II -bearing minerals, metallic iron, wood chip or zeolites. Two of the most common designs are 'funnel and gate' and 'continuous walls' [3] and metallic iron (Fe 0 ) represents the most commonly used reactive material [5] [11]. ...
... The PRB technology using metallic iron (Fe 0 ) has gained acceptance as an effective passive remediation strategy for the treatment of a variety of organic and inorganic contaminants in groundwater [5] [8] [9] [10] [11] [12] [13] [14]. Even pathogens are efficiently removed in Fe 0 /H 2 O systems [15] [16]. ...
The interpretation of processes yielding aqueous contaminant removal in the presence of elemental iron (e.g., in Fe0/H2O systems) is subject to numerous complications. Reductive transformations by Fe0 and its primary corrosion products (FeII and H/H2) as well as adsorption onto and co-precipitation with secondary and tertiary iron corrosion products (iron hydroxides, oxyhydroxides, and mixed valence FeII/FeIII green rusts) are considered the main removal mechanisms on a case-to-case basis. Recent progress involving adsorption and co-precipitation as fundamental contaminant removal mechanisms have faced a certain scepticism. This work shows that results from electrocoagulation (EC), using iron as sacrificial electrode, support the adsorption/co-precipitation concept. It is reiterated that despite a century of commercial use of EC, the scientific understanding of the complex chemical and physical processes involved is still incomplete.
... Metallic iron is an emergent reactive material increasingly used for water treatment [25,29,33,36]. Fe 0 is the most used reactive material in subsurface permeable reactive barriers [29,34,58]. It was originally used to remove redox-sensitive contaminants from groundwater [26,27,[58][59][60]. ...
... Fe 0 is the most used reactive material in subsurface permeable reactive barriers [29,34,58]. It was originally used to remove redox-sensitive contaminants from groundwater [26,27,[58][59][60]. It is commonplace to consider that the bare Fe 0 surface reacts with the contaminants and converts them into non-toxic/less toxic species (Assumption 1). ...
A new concept for household and large-scale safe drinking water production is presented. Raw water is successively filtered through a series of sand and iron filters. Sand filters mostly remove suspended particles (media filtration) and iron filters remove anions, cations, micro-pollutants, natural organic matter, and micro-organisms including pathogens (reactive filtration). Accordingly, treatment steps conventionally achieved with flocculation, sedimentation, rapid sand filtration, activated carbon filtration, and disinfection are achieved in the new concept in only two steps. To prevent bed clogging, Fe0 is mixed with inert materials, yielding Fe0/sand filters. Efficient water treatment in Fe0/sand filters has been extensively investigated during the past two decades. Two different contexts are particularly important in this regard: (i) underground permeable reactive barriers, and (ii) household water filters. In these studies, the process of aqueous iron corrosion in a packed bed was proven very efficient for unspecific aqueous contaminant removal. Been based on a chemical process (iron corrosion), efficient water treatment in Fe0 beds is necessarily coupled with a slow flow rate. Therefore, for large communities several filters should work in parallel to produce enough water for storage and distribution. It appears that water filtration through Fe0/sand filters is an efficient, affordable, an flexible technology for the whole world.
... Since then intensive efforts have been devoted to remediation with Fe 0 materials. As result Fe 0 is now regarded as a very competent reactive agent for remediation of systems that are contaminated with reducible substances (including chlorinated hydrocarbons, nitrate, nitro aromatics, chromium, uranium) (4)(5)(6)(7)(8). ...
... Fe-based alloys (Fe 0 materials, mostly cast iron and steel) are certainly suitable for environmental remediation because of their low tendency to passivity due to the porosity and the instability of generated oxide layers. Instead of this trivial reason, Fe 0 has been considered as a strong reducing agent for the reductive transformation of several species in natural waters (3)(4)(5)(6)(7)(8)(9)(10). This consideration is not acceptable even from a pure thermodynamic perspective as the electrode potential of iron is almost the same (about -0.44 V) in low alloyed and stainless steels (Fe-based alloys). ...
Aqueous contaminant removal in the presence of metallic iron is often regarded as a reductive transformation mediated by the Fe0 surface. However, successful removal of theoretically non-reducible contaminants has been largely reported. This paper presents a rebuttal of the concept of contaminant reductive transformation. It is argued through a careful examination of the evolution of the volume and adsorptive properties of iron and its corrosion products that contaminants are primarily adsorbed and co-precipitated with iron corrosion products. One may wonder how the Fe0 technology will develop with the new concept.
... Since then intensive efforts have been devoted to remediation with Fe 0 materials. As result Fe 0 is now regarded as a very competent reactive agent for remediation of systems that are contaminated with reducible substances (including chlorinated hydrocarbons, nitrate, nitro aromatics, chromium, uranium)45678. The rediscovery of iron corrosion was followed by a seminal work on the mechanism of aqueous contaminant removal in the presence of Fe 0 (e.g. in Fe 0 / H 2 O systems) [9] . ...
... Fe-based alloys (Fe 0 materials, mostly cast iron and steel) are certainly suitable for environmental remediation because of their low tendency to passivity due to the porosity and the instability of generated oxide layers. Instead of this trivial reason, Fe 0 has been considered as a strong reducing agent for the reductive transformation of several species in natural waters345678910. This consideration is not acceptable even from a pure thermodynamic perspective as the electrode potential of iron is almost the same (about 20.44 V) in low alloyed and stainless steels (Fe-based alloys). ...
Aqueous contaminant removal in the presence of metallic iron is often regarded as a reductive transformation mediated by the Fe0 surface. However, successful removal of theoretically nonreducible contaminants has been largely reported. This article presents a rebuttal of the concept of contaminant reductive transformation. It is argued through a careful examination of the evolution of the volume and adsorptive properties of iron and its corrosion products that contaminants are primarily adsorbed and coprecipitated with iron corrosion products. One may wonder how the Fe0 technology will develop with the new concept. © 2009 American Institute of Chemical Engineers Environ Prog, 2010
... The absence of a pre-edge peak suggests that the dominant Fe-S phase is not in tetrahedral coordination and indicates the presence of a Fe-S phase other than Fe 1+x S. Hydrogen sulfide production within RM1 was observed to continue following removal of >97% of aqueous Fe (Figs. 1 and 2). This excess of dissolved H 2 S, as compared to Fe, could lead to the sulfurization of mackinawite, with Fe 3 S 4 and pyrrhotite [Fe 1Àx S] as possible reaction products (Posfai et al., 1998;Neretin et al., 2004;Jambor et al., 2005). Greigite occurs as a mixed tetrahedral-octahedral structure whereas Fe 1Àx S exhibits octahedral coordination of Fe with S. The occurrence of Fe 3 S 4 and Fe 1Àx S has previously been observed in PRBs for AMD remediation, and was thought to be preceded by Fe 1+x S precipitation (Herbert et al., 2000;Jambor et al., 2005). ...
... This excess of dissolved H 2 S, as compared to Fe, could lead to the sulfurization of mackinawite, with Fe 3 S 4 and pyrrhotite [Fe 1Àx S] as possible reaction products (Posfai et al., 1998;Neretin et al., 2004;Jambor et al., 2005). Greigite occurs as a mixed tetrahedral-octahedral structure whereas Fe 1Àx S exhibits octahedral coordination of Fe with S. The occurrence of Fe 3 S 4 and Fe 1Àx S has previously been observed in PRBs for AMD remediation, and was thought to be preceded by Fe 1+x S precipitation (Herbert et al., 2000;Jambor et al., 2005). In addition to Fe and S removal, precipitation of Fe-S phases, such as mackinawite, has been shown to contribute to the removal of divalent metals and trace elements (Arakaki and Morse, 1993;Gallegos et al., 2007;Jeong et al., 2007). ...
A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe0). Modest increases in SO4 reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe0. However, OC was essential for supporting SO4 reducing bacteria (SRB) and therefore SO4 reduction. This observation suggests a general absence of autotrophic SRB which can utilize H2 as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L−1 d−1 g−1 OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe0 only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO4 reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe0, and S-rich in the absence of Fe0 amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe0 enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe0 and OC mixtures.
... Metallic iron is an emergent reactive material increasingly used for water treatment [25,29,33,36]. Fe 0 is the most used reactive material in subsurface permeable reactive barriers [29,34,58]. It was originally used to remove redox-sensitive contaminants from groundwater [26,27,585960. ...
... Fe 0 is the most used reactive material in subsurface permeable reactive barriers [29,34,58]. It was originally used to remove redox-sensitive contaminants from groundwater [26,27,585960. It is commonplace to consider that the bare Fe 0 surface reacts with the contaminants and converts them into non-toxic/less toxic species (Assumption 1). ...
A new concept for household and large-scale safe drinking water production is presented. Raw water is successively filtered through a series of sand and iron filters. Sand filters mostly remove suspended particles (media filtration) and iron filters remove anions, cations, micro-pollutants, natural organic matter, and micro-organisms including pathogens (reactive filtration). Accordingly, treatment steps conventionally achieved with flocculation, sedimentation, rapid sand filtration, activated carbon filtration, and disinfection are achieved in the new concept in only two steps. To prevent bed clogging, Fe0 is mixed with inert materials, yielding Fe0/sand filters. Efficient water treatment in Fe0/sand filters has been extensively investigated during the past two decades. Two different contexts are particularly important in this regard: (i) underground permeable reactive barriers and (ii) household water filters. In these studies, the process of aqueous iron corrosion in a packed bed was proven very efficient for unspecific aqueous contaminant removal. Been based on a chemical process (iron corrosion), efficient water treatment in Fe0 beds is necessarily coupled with a slow flow rate. Therefore, for large communities several filters should work in parallel to produce enough water for storage and distribution. It appears that water filtration through Fe0/sand filters is an efficient, affordable, a flexible technology for the whole world.
... Among the remaining PRBs, the highest sulfate removal percentage (25-78%) with documented t R is reported for the PRBs at Sudbury, where the average t R is as high as 90 days [7,34]. SEM and XRD analysis of solid cores from this PRB showed that precipitation of metal sulfides was a major sink for Fe (the most abundant metal) and SO 4 2− [35,36]. This finding was corroborated by high increases of SRB populations within the PRB compared to the upgradient aquifer [7,36]. ...
... SEM and XRD analysis of solid cores from this PRB showed that precipitation of metal sulfides was a major sink for Fe (the most abundant metal) and SO 4 2− [35,36]. This finding was corroborated by high increases of SRB populations within the PRB compared to the upgradient aquifer [7,36]. However, even for this case, which is characterized by an extremely high t R , a decline of 30% in the sulfate removal was observed after 3 years of operation [34]. ...
Following on the accident occurred in Aznalcóllar in 1998, whereby a huge amount of acid mine drainage and heavy metal-bearing pyritic sludge was released to the Agrio river valley with the subsequent contamination of groundwater, a subsurface permeable reactive barrier (PRB) was installed to mitigate the long-term impacts by the spillage. The PRB material consisted of a mixture of limestone and vegetal compost. A particular characteristic of the Agrio aquifer is its high water flow velocity (0.5-1 m/d), which may pose difficulties in its remediation using PRB technology. The present study reports the 36-month performance of the PRB. Vertical differences in water velocity were observed within the PRB, with the deeper part being slower and more effective in neutralizing pH and removing heavy metals (Zn, Al, Cu). On the other hand, partial sulfate removal appeard to be restricted to the bottom of the PRB, but with no apparent influence on downgradient water quality. The results are finally compared with the other four reported existing PRBs for AMD worldwide.
... However, mineralogical investigations in field iron PRBs showed that microbial activity is an important promoter of sulfate reduction and subsequent precipitation of am FeS. Amorphous iron sulfide can be disordered/transformed to mackinawite [Fe (1+x) S] [31], a mineral phase that can present high metal removal capacity even at low pH [32]. ...
... Sulfate is also known to adsorb on iron (oxy)hydroxide surfaces such as goethite and maghemite exhibiting net positive charge at low pH [33]. Nevertheless, this behaviour may inhibit metal removal as the presence of SO 4 2− slows down the transformation of primary iron hydroxides into more crystallized and reactive oxides [31]. ...
A continuous column experiment was carried out under dynamic flow conditions in order to study the efficiency of low-cost permeable reactive barriers (PRBs) to remove several inorganic contaminants from acidic solutions. A 50:50 w/w waste iron/sand mixture was used as candidate reactive media in order to activate precipitation and promote sorption and reduction-oxidation mechanisms. Solid phase studies of the exhausted reactive products after column shutdown, using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), confirmed that the principal Fe corrosion products identified in the reactive zone are amorphous iron (hydr)oxides (maghemite/magnetite and goethite), intermediate products (sulfate green rust), and amorphous metal sulfides such as amFeS and/or mackinawite. Geochemical modelling of the metal removal processes, including interactions between reactive media, heavy metal ions and sulfates, and interpretation of the ionic profiles was also carried out by using the speciation/mass transfer computer code PHREEQC-2 and the WATEQ4F database. Mineralogical characterization studies as well as geochemical modelling calculations also indicate that the effect of sulfate and silica sand on the efficiency of the reactive zone should be considered carefully during design and operation of low-cost field PRBs.
... Several reactive materials have been used including activated carbon, compost, clays, Fe II -bearing minerals, metallic iron, wood chip or zeolites. Two of the most common designs are 'funnel and gate' and 'continuous walls' [3] and metallic iron (Fe 0 ) represents the most commonly used reactive material [5,11] . The PRB technology using metallic iron (Fe 0 ) has gained acceptance as an effective passive remediation strategy for the treatment of a variety of organic and inorganic contaminants in groundwater [5,891011121314 . ...
... Two of the most common designs are 'funnel and gate' and 'continuous walls' [3] and metallic iron (Fe 0 ) represents the most commonly used reactive material [5,11] . The PRB technology using metallic iron (Fe 0 ) has gained acceptance as an effective passive remediation strategy for the treatment of a variety of organic and inorganic contaminants in groundwater [5,891011121314 . Even pathogens are efficiently removed in Fe 0 /H 2 O sys- tems [15,16]. ...
The interpretation of processes yielding aqueous contaminant removal in the presence of elemental iron (e.g. in Fe(0)/H(2)O systems) is subject to numerous complications. Reductive transformations by Fe(0) and its primary corrosion products (Fe(II) and H/H(2)) as well as adsorption onto and co-precipitation with secondary and tertiary iron corrosion products (iron hydroxides, oxyhydroxides, and mixed valence Fe(II)/Fe(III) green rusts) are considered the main removal mechanisms on a case-to-case basis. Recent progress involving adsorption and co-precipitation as fundamental contaminant removal mechanisms have faced a certain scepticism. This work shows that results from electrocoagulation (EC), using iron as sacrificial electrode, support the adsorption/co-precipitation concept. It is reiterated that despite a century of commercial use of EC, the scientific understanding of the complex chemical and physical processes involved is still incomplete.
... Among these materials, the most frequently utilized are: Zero valent iron (ZVI), organic carbon, apatite, zeolites, clays, and calcite/limestone (Blowes et al., 2000;Obiri-Nyarko et al., 2014). In addition, discrete Ni-bearing minerals, such as Ni hydroxide, Ni sulfides, and elemental Ni, have been shown to control Ni fate in mineimpacted (Jambor et al. 2005, Yeheyis et al., 2009Shaw et al., 2011;Essilfie-Dughan et al., 2012;Poaty et al., 2021) and euxinic environments (Ferris et al., 1987. ...
The determination of fractionation factors associated with important biogeochemical processes controlling Ni availability in the environment is necessary to confidently use Ni isotope signatures as tracers in environmental studies. In this paper we present experimental results on Ni isotope fractionation during the precipitation of Ni secondary minerals (Ni hydroxide, Ni hydroxycarbonate and Ni sulfide minerals) that can control Ni fate and mobility in settings where high dissolved Ni(II) concentrations may pose a serious threat to the environment. Results show a preferential partition of lighter isotopes into the solid phase with associated fractionation factors ε of -0.40‰ ± 0.04‰, -0.50‰ ± 0.02‰, and -0.73‰ ± 0.08‰ relative to the hydroxide, carbonate and sulfide systems, respectively. Early kinetically induced isotope fractionation, followed by possible re-equilibration with the reacting solutions was suggested for the three investigated systems. Distortions of Ni-O bonds in the octahedra constituting the structures of Ni hydroxide and hydroxycarbonate minerals could have also contributed to the isotopic fractionation measured in these systems. In contrast, for Ni sulfide system, the magnitude of Ni isotope fraction-ation seemed to be determined by water and sulfide ligands exchange in solution prior to mineral nucleation. Although there is insufficient evidence to determine whether complete equilibrium occurred in the three studied systems, the fractionation factors reported in this study can provide useful indicators of Ni isotope fractionation associated with fast precipitation of secondary minerals involved in the sequestration of Ni from contaminated environments. These findings also contribute to the characterization of Ni isotope systematics which is still in the early stages of development.
... « l'élimination » des contaminants dans le système Fe 0 /H2O(Noubactep 2015b). Reconnaître donc le Fe 0 comme générateur d'agents réducteurs permettrait de rediriger les efforts et surmonter les problèmes majeurs qui confinent encore aujourd'hui la technologie du Fe 0 en dépit de plusieurs décennies de recherches intensives.5 Les problèmes majeurs de la technologie du Fe 0 et qui entravent sa vulgarisationLa filtration sur lit de Fe 0 est considérée comme le moyen ingénié de filtration réactive le plus prometteur(Anderson 1885, Osgton 1885, Tucker 1892, Burton 1898, Don et Chisholm 1911, Tratneyk et al. 2003, Jambor et al. 2005, Henderson et Demond 2007, Thiruvenkatachari et al. 2008, Noubactep 2010a, et qui jouit en plus aujourd'hui d'une acceptation universelle grandissante(Henderson et Demond 2007, Johnson et al. 2008, Comba et al. 2011. Néanmoins, en dépit d'une efficience bien documentée, la technologie fait toujours face à une série de problèmes qui entravent sa standardisation et donc sa vulgarisation, Mielczarski et al. 2005, Henderson et Demond 2007, Johnson et al. 2008, Jiao et al. 2009, Comba et al. 2011, Noubactep 2011a, Ghauch 2015. ...
This thesis deals with metallic iron (Fe(0))for water treatment.
Steel wool was tested as Fe(0) source, and characterized for both its intrinsic reactivity (material screening) and efficiency (for water treatment) for the first time. Other achievements encompassed (I) testing the suitability of pozzolan as an alternative material to sand for the construction of metallic iron filters, and (II) testing the suitability of steel Fe(0)-based filters for water defluoridation. The work concludes that steel wool holds good promise as Fe(0)-bearing material for the construction of efficient, low-cost and reliable decentralized water treatment systems.
... Abiotic oxidation of Mn(II) occurs preferentially at pH .8.0 [19,20]. Moreover, anaerobic conditions are favorable for the formation of some Mn compounds such as rhodochrosite (MnCO 3 ) [21] or MnS [22,23]. In the current study, the enrichment of Mn(II) and carbon-rich complex medium resulted 10 mM labeled in lanes; from different depths; 0, 1, and 10 mM are the Mn(II) concentration). ...
Manganese-oxidizing bacteria in the aquatic environment have been comprehensively investigated. However, little information is available about the distribution and biogeochemical significance of these bacteria in terrestrial soil environments. In this study, stratified soils were initially examined to investigate the community structure and diversity of manganese-oxidizing bacteria. Total 344 culturable bacterial isolates from all substrata exhibited Mn(II)-oxidizing activities at the range of 1 mM to 240 mM of the equivalent MnO 2. The high Mn(II)-oxidizing isolates (.50 mM MnO 2) were identified as the species of phyla Actinobacteria, Firmicutes and Proteobacteria. Seven novel Mn(II)-oxidizing bacterial genera (species), namely, Escherichia, Agromyces, Cellulomonas, Cupriavidus, Microbacterium, Ralstonia, and Variovorax, were revealed via comparative phylogenetic analysis. Moreover, an increase in the diversity of soil bacterial community was observed after the combined enrichment of Mn(II) and carbon-rich complex. The phylogenetic classification of the enriched bacteria represented by predominant denaturing gradient gel electrophoresis bands, was apparently similar to culturable Mn(II)-oxidizing bacteria. The experiments were further undertaken to investigate the properties of the Mn oxide aggregates formed by the bacterial isolates with high Mn(II)-oxidizing activity. Results showed that these bacteria were closely encrusted with their Mn oxides and formed regular microspherical aggregates under prolonged Mn(II) and carbon-rich medium enrichment for three weeks. The biotic oxidation of Mn(II) to Mn(III/IV) by these isolates was confirmed by kinetic examinations. X-ray diffraction assays showed the characteristic peaks of several Mn oxides and rhodochrosite from these aggregates. Leucoberbelin blue tests also verified the Mn(II)-oxidizing activity of these aggregates. These results demonstrated that Mn oxides were formed at certain amounts under the enrichment conditions, along with the formation of rhodochrosite in such aggregates. Therefore, this study provides insights into the structure and diversity of soil-borne bacterial communities in Mn(II)-oxidizing habitats and supports the contribution of soil-borne Mn(II)-oxidizing bacteria to Mn oxide mineralization in soils.
... Abiotic oxidation of Mn(II) occurs preferentially at pH .8.0 [19,20]. Moreover, anaerobic conditions are favorable for the formation of some Mn compounds such as rhodochrosite (MnCO 3 ) [21] or MnS [22,23]. In the current study, the enrichment of Mn(II) and carbon-rich complex medium resulted 10 mM labeled in lanes; from different depths; 0, 1, and 10 mM are the Mn(II) concentration). ...
Manganese-oxidizing bacteria in the aquatic environment have been comprehensively investigated. However, little information is available about the distribution and biogeochemical significance of these bacteria in terrestrial soil environments. In this study, stratified soils were initially examined to investigate the community structure and diversity of manganese-oxidizing bacteria. Total 344 culturable bacterial isolates from all substrata exhibited Mn(II)-oxidizing activities at the range of 1 mM to 240 mM of the equivalent MnO 2. The high Mn(II)-oxidizing isolates (.50 mM MnO 2) were identified as the species of phyla Actinobacteria, Firmicutes and Proteobacteria. Seven novel Mn(II)-oxidizing bacterial genera (species), namely, Escherichia, Agromyces, Cellulomonas, Cupriavidus, Microbacterium, Ralstonia, and Variovorax, were revealed via comparative phylogenetic analysis. Moreover, an increase in the diversity of soil bacterial community was observed after the combined enrichment of Mn(II) and carbon-rich complex. The phylogenetic classification of the enriched bacteria represented by predominant denaturing gradient gel electrophoresis bands, was apparently similar to culturable Mn(II)-oxidizing bacteria. The experiments were further undertaken to investigate the properties of the Mn oxide aggregates formed by the bacterial isolates with high Mn(II)-oxidizing activity. Results showed that these bacteria were closely encrusted with their Mn oxides and formed regular microspherical aggregates under prolonged Mn(II) and carbon-rich medium enrichment for three weeks. The biotic oxidation of Mn(II) to Mn(III/IV) by these isolates was confirmed by kinetic examinations. X-ray diffraction assays showed the characteristic peaks of several Mn oxides and rhodochrosite from these aggregates. Leucoberbelin blue tests also verified the Mn(II)-oxidizing activity of these aggregates. These results demonstrated that Mn oxides were formed at certain amounts under the enrichment conditions, along with the formation of rhodochrosite in such aggregates. Therefore, this study provides insights into the structure and diversity of soil-borne bacterial communities in Mn(II)-oxidizing habitats and supports the contribution of soil-borne Mn(II)-oxidizing bacteria to Mn oxide mineralization in soils.
... In retrospect, Fe 0 remediation research was born with a mistake as the reductive transformation paradigm has never been convincingly established [13][14][15][16], but the flaw has been 'difficult' to recognize in the midst of success stories [17][18][19][20][21][22][23][24]. During the past two decades, Fe 0 remediation researchers have (re) demonstrated the potential of Fe 0 and Fe 0 -based filtration to remediate many cases of pollution including wastewater [10][11][12] and safe drinking water [25][26][27][28][29][30]. ...
The well-accepted assumption of metallic iron (Fe0) acting as electron donor for environmental remediation has created an unstable domain of knowledge for the past 23 years. This assumption is discouraging some outstanding and prospective scientists from correctly interpreting their experimental results. Such a situation is a recipe for long-term decline. The critical situation cannot be solved with simplistic approaches. It is now imperative to develop an understanding to defend the difficulties of this assumption and re-orient Fe0 mediated remediation research as a whole.
... .5: Solid phases used in PHREEQC model, arranged in order of ascending molar volume normalized to moles Fe. These solid phases have been commonly found in PRBs (Jambor et al. 2005;Liang et al. 2003; To better understand the factors associated with solids production and the potential for failure, a statistical analysis of data from field PRBs composed of ZVI was conducted. ...
Permeable reactive barriers (PRBs) are an in situ technology for remediation of contaminated groundwater. Most employ ZVI as the reactive medium, and although many achieve remediation goals, the performance of others is compromised by the precipitation of naturally-occurring solutes. Therefore, this research aimed to understand the precipitation of solids in these systems. Recent work has suggested the suitability of reduced iron sulfide (FeS) as a reactive material for PRB applications, so this research compared the solids production and hydraulic performance of pure ZVI and FeS-coated sands.
To better understand the factors associated with solids production and the potential for failure, a statistical analysis of data from field PRBs composed of ZVI was conducted. Based on this statistical analysis, a series of column experiments was conducted utilizing a simulated groundwater with high calcium (280 mg/L), total carbonate (420 mg/L), and chloride (405 mg/L) as a base solution, to which oxidants (0, 2, or 8 mg/L dissolved oxygen or 100 mg/L nitrate) were added.
Characterization of the aqueous phase in the ZVI column effluents indicated that both both calcium and carbonate were removed from solution. Production of gas bubbles was also observed. In the ZVI columns, increasing oxidant levels corresponded to higher hydraulic conductivity losses. Yet the spectroscopic analysis of solids produced and mass balances on the aqueous phase could not account for all of the hydraulic conductivity loss. Geochemical modeling of the systems was also used to estimate the potential for solids formation and gas production. Results of this modeling also suggested that hydraulic conductivity losses due to the gas phase may be a crucial component of permeability loss.
In the FeS columns, no calcium or carbonate was removed in the columns, no hydraulic conductivity loss was measured, and no solids were detected on the surface of the solids. Modeling of the FeS and ZVI systems on an equal mass basis indicated the potential for solids formation with FeS is much less than that with ZVI. Based on these hydraulic considerations, FeS may have significant advantages over ZVI for PRB applications.
... ZVI was very popular during the early 1990's when research on PRB technology began; its popularity is still increasing. About three-quarters of all full-scale PRBs worldwide utilise ZVI as the reactive material Jambor et al., 2005) because of its capacity to remove a range of contaminants such as chlorinated organic solvents, reducible metals and inorganic contaminants such as nickel (Ni), chromium (Cr), uranium (U), radionuclides, SO 4 2-, As, and NO 3 2- (Gillham and O'Hannesin, 1994;Powell et al., 1995;Blowes et al., 1997;Blowes et al., 2000). The widespread use of ZVI is also due to its ability to act as a strong reducing agent in groundwater causing the abiotic reductive degradation of organic compounds such as chlorinated hydrocarbon. ...
This is a Thesis. Some Chapters have unpublished data. This thesis can be used as reference for other research purpose. But the contents in this thesis (any data or figures used from the thesis) are not allowed to use for any articles and if found to be used for any article either journal or conference without my consent, it is considered as plagiarism.
... ZVI was very popular during the early 1990's when research on PRB technology began; its popularity is still increasing. About three-quarters of all full-scale PRBs worldwide utilise ZVI as the reactive material Jambor et al., 2005) because of its capacity to remove a range of contaminants such as chlorinated organic solvents, reducible metals and inorganic contaminants such as nickel (Ni), chromium (Cr), uranium (U), radionuclides, SO 4 2-, As, and NO 3 2- (Gillham and O'Hannesin, 1994;Powell et al., 1995;Blowes et al., 1997;Blowes et al., 2000). The widespread use of ZVI is also due to its ability to act as a strong reducing agent in groundwater causing the abiotic reductive degradation of organic compounds such as chlorinated hydrocarbon. ...
The effectiveness of a permeable reactive barrier (PRB) to remediate contaminated groundwater from acid sulphate soil on the Shoalhaven Floodplain, southeast New South Wales (NSW), Australia was investigated. High concentrations of dissolved aluminium (Al3+), total iron (Fe), and sulphate (SO42-) in the groundwater along with low pH were evidence of acidic conditions due to pyrite oxidation at the study site. Groundwater manipulation using engineering solutions such as weirs and modified floodgates drains are not effective in low-lying ASS terrain, as they cannot remediate the acidity already present in the soil nor significantly prevent pyrite oxidation in areas far from nearby drains. This study combined laboratory, field and numerical analyses in order to determine the feasibility and performance of a PRB utilising zero-cost recycled concrete for the remediation of acidic groundwater in ASS terrain.
Long-term laboratory column experiments were carried out using synthetic and real groundwater from the study site. The column experiments investigated the acid neutralisation reactions occurring within the PRB and the precipitation of Al and Fe from the acidic groundwater. Three distinct pH-buffering reactions were ascertained: (i) the dissolution of carbonate/bicarbonate alkalinity from concrete at nearly neutral pH, (ii) the re-dissolution of aluminium hydroxide precipitates at pH ~4, and (iii) the re-dissolution of ferric oxyhydroxides minerals at pH <3. However, carbonate/bicarbonate buffering was the most significant because of the maintenance of near neutral pH and complete removal of Al3+ and total Fe from the influent.
Chemical armouring and physical clogging, which are considered the major factors in reducing the efficiency of any reactive material, were also studied by evaluating the duration of buffering periods for maintaining neutral pH and also the changes in physical parameters (e.g. hydraulic conductivity and flow rate) due to mineral precipitation. Chemical armouring by secondary Al- and Fe- precipitates decreased the ANC of the recycled concrete by ~50% compared to its theoretical ANC. Furthermore, high concentrations of Al3+ and total Fe caused a rapid decrease in ANC efficiency due to accelerated armouring. Application of larger size concrete aggregates reduced the threat of physical clogging in the pilot-scale PRB. Furthermore, mineralogical and morphological analysis was carried out to characterise the recycled concrete used in the column experiments and the precipitates formed. Correlation between CaO reduction in the armoured concrete and the reduction in ANC validated the decline in ANC by chemical armouring. 3D image analysis was demonstrated to be a useful tool for the examination of the porous architecture, and the performance of PRB reactive materials in a novel yet quantifiable manner.
A comprehensive field study involved the monitoring of groundwater via piezometers and observation wells, installed up-gradient, within and down-gradient of the PRB, to observe changes in the level of the phreatic surface along with water quality parameters (e.g. pH, electrical conductivity (EC), oxidation reduction potential (ORP), temperature and concentration of anion and cations). Groundwater pH inside the PRB was maintained near neutral throughout the monitoring period. The concentration of Al3+ and total Fe were maintained below the Australian and New Zealand Environment and Conservation Council (ANZECC) (2000) criteria, in a similar manner to what was observed in the column experiments. Steady piezometric head observed within the PRB throughout the monitoring period confirmed that chemical and physical clogging did not occur within the PRB to an extent that would affect the permeability of the reactive material.
One-dimensional, simple reactive transport modelling was carried out based on data from a laboratory column experiment, mineralogical analysis of the recycled concrete and the PRB. Numerical modelling using MIN3P provides insights into the neutralisation mechanisms and geochemical evolution of groundwater along a flow path inside the PRB. The ability to make comparisons between the geochemically complex transport scenarios within the column experiments and pilot-scale PRB confirm that it can be used as an analysis tool for investigating the performance of PRBs in ASS terrain.
Overall, this study contributes a better understanding of the acid neutralisation processes occurring inside the PRB for the remediation of contaminated groundwater from ASS terrain and offers novel field, laboratory and modelling techniques to investigate and quantify these processes. The findings from the first pilot-scale PRB using recycled concrete as the reactive material confirms that it is a suitable environmentally friendly and cost-effective alternative to other conventionally utilised techniques (e.g. watertable manipulation, lime neutralisation) for the spot treatment of acidic groundwater in ASS terrain.
(Note: This thesis can be used as reference for other research purpose, however, if the contents in this thesis (any data or figures used from the thesis) are found to be used for any article either journal or conference without my consent is considered as plagiarism).
... Permeable reactive barriers are barriers to remove specific chemicals of concern that are placed in the path of groundwater flow (69,98). Permeable reactive barriers have a subsurface reactive section for the groundwater to flow through and to be treated. ...
... Cuprite precipitation was unlikely given the presence of dissolved sulfide in the columns. Jambor et al. (2005) identified the copper sulfides chalcocite and covellite in samples collected from a reactive barrier installed at a site in Vancouver. The precipitation of these sulfides, as well as coprecipitation of Cu with Fe sulfides and Fe oxides is probably responsible for the low effluent Cu concentrations. ...
Reactive mixtures to be used in a permeable reactive barrier (PRB) for the treatment of low quality groundwater derived from a mine waste rock storage site were evaluated. Low pH drainage water from the site contained high concentrations of sulfate and dissolved metals, including Al, Co, Ni, and Zn. Column experiments were conducted to evaluate whether mixtures containing either peat moss (as an organic carbon source) or a mixture of peat moss and granular zero-valent iron (ZVI) filings, in addition to small amounts of lime and/or limestone, were suitable treatment materials for removing these metals from the water. The experimental results showed that the mixtures promote bacterially-mediated sulfate reduction and metal removal by precipitation of metal sulfides, metal carbonate/hydroxide precipitation, and adsorption under relatively high pH conditions (pH of 7-8). Both reactive mixtures removed influent dissolved metals to near or below the limit of detection in the effluent throughout the experiment; however, influent-level concentrations of the metals of interest gradually moved through the column containing peat alone, as the pH neutralizing ability in the mixture was consumed. In contrast, the column containing both peat and ZVI showed very little breakthrough of the influent metals, suggesting that the longevity of the mixture including ZVI will be much longer than the mixture containing peat alone. The results show that both reactive mixtures should be effective in a PRB installation as long as neutral pH conditions and microbial activity are maintained. The cost to performance ratio of the two reactive mixtures will be a key factor in determining which mixture is best suited for a particular site. (c) 2014 Elsevier Ltd. All rights reserved.
... Elemental iron (Fe 0 ) is a well known material for the abiotic removal of organic and inorganic contaminants from groundwater, soils, sediments, and waste streams [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Fe 0 is widely termed in the literature on permeable reactive barriers as zerovalent iron (ZVI) and is available as Fe 0 -based alloys (Fe 0 materials), mostly cast iron and low alloy steel. ...
In an attempt to characterize material intrinsic reactivity, iron dissolution from elemental iron materials (Fe0) was investigated under various experimental conditions in batch tests. Dissolution experiments were performed in a dilute solution of ethylenediaminetetraacetate (Na2-EDTA - 2 mM). The dissolution kinetics of eighteen Fe0 materials were investigated. The effects of individual operational parameters were assessed using selected materials. The effects of available reactive sites [Fe0 particle size (≤2.0 mm) and metal loading (2-64 g L–1)], mixing type (air bubbling, shaking), shaking intensity (0-250 min–1), and Fe0 pre-treatment (ascorbate, HCl and EDTA washing) were investigated. The data were analysed using the initial dissolution rate (kEDTA). The results show increased iron dissolution with increasing reactive sites (decreasing particle size or increasing metal loading), and increasing mixing speed. Air bubbling and material pre-treatment also lead to increased iron dissolution. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A unified experimental procedure for the investigation of processes in Fe0/H2O systems is suitable. Alternatively, a parameter (τEDTA) is introduced which could routinely used to characterize Fe0 reactivity under given experimental conditions.
... Iron-based alloys (metallic iron, elemental iron or Fe 0 materials) have been used as an abiotic contaminant reducing reagent for organic and inorganic groundwater contaminants for over 15 years [1][2][3][4][5][6][7][8][9][10][11][12][13]. In this context, Fe 0 materials are widely termed as zerovalent iron (ZVI) materials, contaminants have been denoted as reductates [14], and the bare surface of Fe 0 as reductant. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe0/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe0-materials (material A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe0/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min-1 and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (kEDTA). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on kEDTA from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe0 surface.
... The use of elemental iron (Fe 0 ) for water treatment has attracted much attention thanks to its great potential for removing several classes of substances from the aqueous phase123456789. Fe 0 has been proven the most efficient material for subsurface reactive permeable barriers (reactive walls) [6, 10, 11]. Actually, there are about 180 Fe 0 reactive walls installed worldwide [6]. ...
A knowledge system (KS) is a knowledge that is unique to a given group of persons. This form of knowledge may have a local or natural origin and is linked to the community that has produced it. On the contrary, the core of mainstream science (MS) is the desire to profoundly understand processes, through sequential studies such as hypothesis formulation, experiment and prediction. Thus, KS is communitarian and MS is universal. KS can be understood and rendered universal through MS. In general, a process discovery (know-how) may be intuitive, accidental, conjectural or inspirational but outcomes should be predictable and repeatable as soon as the know-why is achieved by MS. This paper argues that the technology of using metallic iron for water treatment has all the characteristics of a KS and that promoters of this technology have deliberately rejected scientific arguments leading to the know-why of the fortuitous discovery. Consequently, the technology has developed into an impasse where controversial discoveries are reported on all relevant aspects. It is concluded that the integrity of science in endangered by this communitarian behaviour.
... The primary mechanisms of contaminant removal by elemental metals (M 0 ) are considered to 29 be adsorption, chemical reduction, complexation, co-precipitation, incorporation and size- 30 exclusion123456. Chemical reduction at the M 0 surface has often been cited as a key 31 mechanism of contaminant removal. ...
In the mining industry, the separation of economically valuable metals from gangue materials is a well established process. As part of this field, hydrometallurgy uses chemical fluids (leachates) of acidic or basic pH to dissolve the target metal(s) for subsequent concentration, purification and recovery. The type and concentration of the leach solution is typically controlled to allow selective dissolution of the target metal(s), and other parameters such as oxidation potential, temperature and the presence of complexing/chelating agents. In the remediation industry the use of elemental metals (M0) for the removal of aqueous contaminant species is also a well established process. Removal is achieved by the oxidative corrosion of the M0 and associated pH and/or redox potential change. Whilst the two processes are directly opposed and mutually exclusive they both stem from the same theoretical background: metal dissolution/precipitation reactions. In the mining industry, with each prospective ore deposit physically and chemically unique, a robust series of tests are performed at each mine site to determine optimal hydrometallurgical fluid composition and treatment conditions (e.g. fluid temperature, flow rate) for target metal dissolution/yield. In comparison, within the remediation industry not all such variables are typically considered. In the present communication a comparison of the processes adopted in both industries are presented. The consequent need for a more robust empirical framework within the remediation industry is outlined.
... Permeable reactive barriers containing metallic iron as a reactive filler material (Fe 0 PRBs) is an established technology for groundwater remediation [1][2][3][4][5][6][7][8][9][10]. At present, more than 120 Fe 0 ...
Over the past 30 years the literature has burgeoned with in-situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe0) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe0 barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe0 corrosion in porous media is presented. Subsequent implications for the design of Fe0 filtration systems are modelled. The results suggest that: (i) for the pH range of natural waters (> 4.5), the high volumetric expansion of Fe0 during oxidation and precipitation dictates that Fe0 should be mixed with a non-expansive material; (ii) naturally-occurring solute precipitates have a negligible impact on permeability loss compared to Fe0 expansive corrosion; and (iii) the proliferation of H2 metabolising bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe0 PRB system as a physical (size-exclusion) water filter device.
... Abiotic oxidation of Mn(II) occurs preferentially at pH .8.0 [19,20]. Moreover, anaerobic conditions are favorable for the formation of some Mn compounds such as rhodochrosite (MnCO 3 ) [21] or MnS [22,23]. In the current study, the enrichment of Mn(II) and carbon-rich complex medium resulted 10 mM labeled in lanes; from different depths; 0, 1, and 10 mM are the Mn(II) concentration). ...
Manganese-oxidizing bacteria in the aquatic environment have been comprehensively investigated. However, little information is available about the distribution and biogeochemical significance of these bacteria in terrestrial soil environments. In this study, stratified soils were initially examined to investigate the community structure and diversity of manganese-oxidizing bacteria. Total 344 culturable bacterial isolates from all substrata exhibited Mn(II)-oxidizing activities at the range of 1 µM to 240 µM of the equivalent MnO2. The high Mn(II)-oxidizing isolates (>50 mM MnO2) were identified as the species of phyla Actinobacteria, Firmicutes and Proteobacteria. Seven novel Mn(II)-oxidizing bacterial genera (species), namely, Escherichia, Agromyces, Cellulomonas, Cupriavidus, Microbacterium, Ralstonia, and Variovorax, were revealed via comparative phylogenetic analysis. Moreover, an increase in the diversity of soil bacterial community was observed after the combined enrichment of Mn(II) and carbon-rich complex. The phylogenetic classification of the enriched bacteria represented by predominant denaturing gradient gel electrophoresis bands, was apparently similar to culturable Mn(II)-oxidizing bacteria. The experiments were further undertaken to investigate the properties of the Mn oxide aggregates formed by the bacterial isolates with high Mn(II)-oxidizing activity. Results showed that these bacteria were closely encrusted with their Mn oxides and formed regular microspherical aggregates under prolonged Mn(II) and carbon-rich medium enrichment for three weeks. The biotic oxidation of Mn(II) to Mn(III/IV) by these isolates was confirmed by kinetic examinations. X-ray diffraction assays showed the characteristic peaks of several Mn oxides and rhodochrosite from these aggregates. Leucoberbelin blue tests also verified the Mn(II)-oxidizing activity of these aggregates. These results demonstrated that Mn oxides were formed at certain amounts under the enrichment conditions, along with the formation of rhodochrosite in such aggregates. Therefore, this study provides insights into the structure and diversity of soil-borne bacterial communities in Mn(II)-oxidizing habitats and supports the contribution of soil-borne Mn(II)-oxidizing bacteria to Mn oxide mineralization in soils.
... There has been growth in development of remediation technologies that involve manipulation of in situ chemical conditions in soils or groundwater aquifers to transform and/or immobilize metal contaminants (Jambor and Raudsepp, 2005; Palmisano and Hazen, 2003). In general, these technologies are designed around three different approaches: (1) introduction of soluble chemical amendments that cause precipitation of the contaminant in a specific form (e.g., injection of phosphate compounds TABLE 6.2 Literature examples where solid-phase speciation employing synchrotron radiation was employed to characterize contaminant speciation in source zones prior to remedy selection ...
Synchrotron techniques are powerful tools in environmental research. In relation to regulatory development, the quality and impact of synchrotron data is beginning to show its merit. The momentum of synchrotron research in the United States Environmental Protection Agency (USEPA) is leading to a robust approach to site and risk assessment to understand bioavailability of metals as well as remedial design at contaminated sites. The continuous success of synchrotron studies to address complex environmental issues will lead to more acceptance by those in the regulatory community, resulting in well-informed policy decisions. We encourage our national and international peers to produce high-quality outputs utilizing synchrotron techniques and push these capabilities to the limit to address important future issues. USEPA researchers understand the vital impact of synchrotron research in reducing the uncertainty and dynamic nature of scientific knowledge. We will continue to educate risk assessors and decision makers on the value of synchrotron data and its importance in protecting human health and the natural environment.
... Under anaerobic conditions, however, Mn, a common metal contaminant in MIW (Johnson and Hallberg, 2005), is extremely difficult to remove (Benner et al., 1999;Doshi, 2006;Johnson and Younger, 2005;Sibrell et al., 2007). Partial Mn removal under reducing conditions has been reported and attributed to its precipitation as rhodochrosite (MnCO 3 ) (Benner et al., 1999) or MnS (Jambor et al., 2005). However, the most common approach for the removal of Mn in field applications is an additional aerobic/oxidative step in which Mn is oxidized and subsequently precipitated as MnO 2 (Doshi, 2006;Johnson and Hallberg, 2005;Johnson and Younger, 2005). ...
The chemical and physical treatment mechanisms by which crab shell removes metals from mine impacted water (MIW) were evaluated under anaerobic and biologically limited conditions in closed systems and kinetic tests. Raw (R-SC20) and deproteinized (DP-SC20) crab shell were tested and compared to limestone to quantify the contribution of chitin-associated minerals and proteins to alkalinity generation and metal precipitation. Single-metal closed systems (initial Mn and Fe=0.18mM and Al=0.34mM) containing 5g/L of either R- or DP-SC20, yielded an increase in pH from 3 to 9.2–10.2, generation of 0.83–1.87mM of alkalinity, and resulted in ⩾95% removal of metals within 72h. In contrast, 5–125glimestone/L only raised the pH to 7.8–8.3, produced lower alkalinity (0.56–0.63mM), and resulted in less metal removal (⩽85%). In kinetic tests with 5g-DP-SC20/L, removal of ⩾95% of the initial metal load was achieved after 0.5, 6, and 48h for Al, Fe, and Mn, respectively. Geochemical calculations (PHREEQC) indicate that limestone-treated systems were close to equilibrium with calcite (CaCO3), whereas octacalcium phosphate (Ca4H(PO4)3) appears to be a controlling phase in systems treated with R- and DP-SC20. The probable mechanisms for Mn removal are the precipitation of rhodochrosite (MnCO3) and/or sorption. In the case of Al and Fe, geochemical calculations point to the precipitation of hydroxides; however, visual observations in Fe systems suggest the formation of green rust, a precursor of other, more stable phases like goethite or lepidocrocite. Several factors may account for the faster changes observed with R- and DP-SC20 compared to limestone: increased dissolution and degree of supersaturation, the presence of phosphates, the release of organic compounds, and a significantly larger surface area. These results are the first to verify and quantify the capacity of crab shell-associated minerals to treat MIW under biologically limited conditions.
Metallic iron (Fe0) based filtration systems have the potential to significantly contribute to the achievement of the United Nations (UN) Sustainable Development Goals (SDGs) of substantially improving the human condition by 2030 through provision of clean water. Recent knowledge on Fe0-based safe drinking water filters is addressed herein. They are categorized into two types: Household and community filters. Design criteria are recalled and operational details are given. Scientists are invited to co-develop knowledge enabling the exploitation of the great potential of Fe0 filters for sustainable safe drinking water provision (and sanitation).
The research community on using metallic iron (Fe0) for environmental remediation is virtually divided in two schools, characterized each by a different research tradition. The two are arbitrarily termed the ‘conventional’ and ‘critical’ schools. The conventional school has discovered Fe0 for environmental remediation and the critical school has conciliated the discovery with the mainstream corrosion science. It is very difficult to understand how both schools are suffering from a ‘dialogue of the deaf’. This communication clarifies the view of the critical school and demonstrates that there is no need for a third approach to conciliate both schools. All is needed is an holistic approach of the Fe0/H2O system, obeying to the laws of chemical thermodynamics.
Cite as:
Ebelle T.C., Makota S., Tepong-Tsindé R., Nassi A., Noubactep C. (2019): Metallic iron and the dialogue of the deaf. Fresenius Environmental Bulletin 28, 8331–8340.
Mining and mineral processing generates large volumes of waste, including waste rock, mill tailings, and mineral refinery wastes. The oxidation of sulfide minerals in the materials can result in the release of acidic water containing high concentrations of dissolved metals. Recent studies have determined the mechanisms of abiotic sulfide-mineral oxidation. Within mine wastes, the oxidation of sulfide minerals is catalyzed by microorganisms. Molecular tools have been developed and applied to determine the activity and role of these organisms in sulfide-mineral-bearing systems. Novel tools have been developed for assessing the toxicity of mine-waste effluent. Dissolved constituents released by sulfide oxidation may be attenuated through the precipitation of secondary minerals, including metal sulfate, oxyhydroxide, and basic sulfate minerals. Geochemical models have been developed to provide improved predictions of the magnitude and duration of environmental concerns. Novel techniques have been developed to prevent and remediate environmental problems associated with these materials.
In this manuscript, we report that a bacterial multicopper oxidase (MCO266) catalyzes Mn(II) oxidation on the cell surface, resulting in the surface deposition of Mn(III) and Mn(IV) oxides and the gradual formation of bulky oxide aggregates. These aggregates serve as nucleation centers for the formation of Mn oxide micronodules and Mn-rich sediments. A soil-borne Escherichia coli with high Mn(II)-oxidizing activity formed Mn(III)/Mn(IV) oxide deposit layers and aggregates under laboratory culture conditions. We engineered MCO266 onto the cell surfaces of both an activity-negative recipient and wild-type strains. The results confirmed that MCO266 governs Mn(II) oxidation and initiates the formation of deposits and aggregates. By contrast, a cell-free substrate, heat-killed strains, and intracellularly expressed or purified MCO266 failed to catalyze Mn(II) oxidation. However, purified MCO266 exhibited Mn(II)-oxidizing activity when combined with cell outer membrane component (COMC) fractions in vitro. We demonstrated that Mn(II) oxidation and aggregate formation occurred through an oxygen-dependent biotic transformation process that requires a certain minimum Mn(II) concentration. We propose an approximate electron transfer pathway in which MCO266 transfers only one electron to convert Mn(II) to Mn(III) and then cooperates with other COMC electron transporters to transfer the other electron required to oxidize Mn(III) to Mn(IV).
The vast majority of PRB currently in use utilize zero valent iron (ZVI) as the reactive medium. In this paper, three laboratory columns were set up and operated under conditions simulating those anticipated in the groundwater to investigate the feasibility and efficiency of the enhanced Fe-0 PRB for the remediation of the PCBs contaminated groundwater. Operating under 10 degrees C and an effective porosity of 61% to 67% and infiltration velocity of groundwater of 0.7 to 0.8m.d(-1), the average iron concentration of effluent was 0.241mg.L-1, 0.129mg.L-1 and 0.201mg.L-1, respectively, and the average dechlorination efficiency reached 49.6%, 72.6% and 58.6%, respectively, the Fe-0/Zn-0 based columns can accomplish 94% of PCBs removal and pH value raised from 6.87 to 10.2. Comprehensive consideration suggested that Fe-0/Zn-0 based PRB technology is feasible for the remediation of PCBs contaminated groundwater.
Scientific progress is in nature a permanent accumulation of experimental observations and data. However, pure accumulation is of limited value. A profound look behind the data is necessary to recognize relations between apparently remote observations and express these relations in universally valid concepts and models. Usually, such concepts are cornerstones for further scientific progress.
Based on the above premise and the experimental evidence that metallic iron (Fe0) do remove more substances or substance classes from aqueous solutions than could be predicted for a reducing agent (Fe0), the objective of the present work was to critically review the literature on “water treatment with Fe0” and discuss the consequences for the further development of the technology of “using Fe0 for water treatment”.
The first observation was that the approach to investigate processes in Fe0/H2O systems has been more pragmatic than systematic. In fact, iron walls have first been reported to effectively degrade solvents in groundwater. Subsequently, the ability of Fe0 to treat other contaminants has been evaluated on a case-by-case basis. Quantitative removal of non-reducible species, oxidable species and species without redox properties has been reported as well. Therefore, the concept considering Fe0 as reducing agent has been questioned and proven inconsistent. A new concept has been introduced and validated which considers adsorption (and adsorptive size exclusion in column studies), and co-precipitation as fundamental contaminant removal mechanisms. Because removed contaminants are enmeshed in the matrix of transforming iron corrosion products, they are necessarily long-term stable under experimental conditions. Thus, Fe0 is a universal material for water treatment and in particular for safe drinking water production.
Next to the profound understanding of the mechanism of contaminant removal in packed Fe0 beds, the volumetric expansive nature of iron oxidative dissolution at pH > 4.5 was properly considered. The result was the suggestion of Fe0 volumetric proportions between 30 and 60 % for safe drinking water production at household level. Ideally, Fe0 is mixed with porous inert materials which sustain the reactivity of Fe0 by storing in-situ generated iron hydroxides.
The efficiency of a Fe0 bed mostly depends on: (i) the intrinsic reactivity of used Fe0, (ii) the thickness of the bed, and the water flow rate (or the residence time within the bed). Future experimental works should be focused on characterizing the intrinsic reactivity of potential affordable materials. It can be emphasized that Fe0 beds will allow for the provision of household and remote small communities with safe drinking water.
Despite two decades of intensive laboratory investigations, the removal mechanism of several contaminants from aqueous solutions by elemental iron (e.g. in Fe0/H2O systems) are not really elucidated. Two of the major reasons for this are: (i) the failure to consider Fe0/H2O systems as consisted of the elemental iron material (Fe0) covered by a layer of corrosion products (oxide-film), and (ii) the failure to treat properly the combined problem of mass transport and chemical reaction in these complex systems. Well-mixed batch experiments that have been undertaken in order to circumvent the mass-transport problem associated with bulk solutions have not always adequately addressed these key issues. Mixing intensity may not only affect the hydrodynamic but also the chemical dynamics, in particular the formation of the oxide-film. The present work presents a critical review on the process of oxide-film formation and its impact on the process of mass-transport to the Fe0 surface. It is shown that well-mixed batch systems are not necessarily an effective tool for investigating the mechanism of contaminant removal by Fe0 since mixing may increase corrosion rate, avoid/delay the formation of oxide-films and/or provoke their abrasion. This discussion suggests that quantitative abiotic contaminant reduction in Fe0/H2O systems may mostly occur within the oxide-film as result of: (i) electron transfer from Fe0 surface, (ii) catalytic activity of secondary reductants (FeII, H2/H). Non-shaken batch experiments are proposed as a simple tool to investigate mass-transport limitation through oxide-films at laboratory scale. Working with stationary Fe0 samples and controlled stirring speeds may allow the investigation of oxide-film effect under more realistic conditions.
The concept of hazard in the current interpretation of the regulations is directed to local and short term effects and its assessment is based on the distribution in space of the concentration of contaminants in abiotic and biotic compartments. We criticize this concept and introduce a potential hazard with full space-time dimension, i.e. from short term to long term, and from local to regional. This way of describing the hazard is biogeochemical and based on the scale specific processes of metals mobility. The short term hazard of a contaminated area (and its future hazards in different environmental scenarios) depends on the stocks of metals, on the fluxes of out-going elements, and on the retention time of the elements (ratio between stock and sum of fluxes). Different hazard situations can result from the relative importance of the intensity of the carrier flux and the mobilization of metals by the carrier flux. The analyses of long term hazard can relocate a contaminated site from one hazard situation to another because of changes in the intensity of the carrier flux or/and of the mobility of metals. The mineralogical aspects controlling the stocks of metals in contaminated areas and the outgoing fluxes of metals are discussed discussed analytically by type of source and type of flux and research directions are identified. Finally the situation of hazard evaluation for Romanian tailing dams based on the approach introduced in this chapter is presented.
In this research, we have evaluated iron sulfide for treating heavy metal contaminated groundwater plumes for PRB systems. Our approach was to test the effectiveness of reduced iron sulfide (FeS) as both a sorbent and reducing agent in PRB applications for long-term sequestration of heavy metal ions. Cadmium (Cd) and Arsenic (As) were the targeted contaminants. Mechanistic information on the metal removal mechanisms was obtained by molecular-scale surface techniques including synchrotron-based XAS and XRD, and microscopic tools such as HRTEM and SEM-EDS. FeS performance under various geochemical conditions was investigated using batch and column reactor systems. Two different forms of reactive sorbent media were prepared, nanoscale FeS and FeS-coated sand for two emplacement methods, colloidal injection and physical packing of porous media, respectively. Rejuvenation of FeS using sulfate reducing micro-organisms for biogenic formation of FeS from iron oxidation products was examined and shown to be feasible. Finally, a reactive transport model was developed using batch isotherm and column arsenic breakthrough data. The overall results provide tools needed to design and apply the FeS PRB media for effective long-term treatment of mixed-metal ion plumes at contaminated groundwater DOD sites.
Mine wastes are the largest volume of materials handled in the world
(ICOLD, 1996). The generation of acidic drainage and the release of
water containing high concentrations of dissolved metals from these
wastes is an environmental problem of international scale. Acidic
drainage is caused by the oxidation of sulfide minerals exposed to
atmospheric oxygen. Although acid drainage is commonly associated with
the extraction and processing of sulfide-bearing metalliferous ore
deposits and sulfide-rich coal, acidic drainage can occur wherever
sulfide minerals are excavated and exposed to atmospheric oxygen.
Engineering projects, including road construction, airport development,
and foundation excavation are examples of civil projects that have
resulted in the generation of acidic drainage. On United States Forest
Service Lands there are (2-5)×104 mines releasing acidic drainage
(USDA, 1993). Kleinmann et al. (1991) estimated that more than 6,400 km
of rivers and streams in the eastern United States have been adversely
affected by mine-drainage water. About (0.8-1.6)×104 km of streams
have been affected by metal mining in the western United States. The
annual worldwide production of mine wastes exceeded 4.5 Gt in 1982
(ICOLD, 1996). Estimated costs for remediating mine wastes
internationally total in the tens of billions of dollars ( Feasby et
al.,1991).
Contaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe 0 /H 2 O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe 0) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe 0 is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe 0 /H 2 O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) com-pounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO 2 corrosion, electrocoagulation using Fe 0 electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe 0 /H 2 O systems. Therefore, the 'iron technology' as a whole is to be revisited as the 'know-why' of contaminant removal is yet to be properly addressed.
Permeable reactive barriers (PRBs) have shown great promise as an alternative to pump and treat for the remediation of groundwater containing a wide array of contaminants including organics, metals, and radionuclides. Analyses to date have focused on individual case studies, rather than considering broad performance issues. In response to this need, this study analyzed data from field installations of in situ zero-valent iron (ZVI) PRBs to determine what parameters contribute to PRB failure. Although emphasis has been placed on losses of reactivity and permeability, imperfect hydraulic characterization was the most common cause of the few PRB failures reported in the literature. Graphical and statistical analyses suggested that internal EH, influent pH, and influent concentrations of alkalinity, NO3 − and Cl− are likely to be the strongest predictors of PRBs that could be at risk for diminished performance. Parameters often cited in the literature such as saturation indices, dissolved oxygen, and total dissolved solids did not seem to have much predictive capability. Because of the relationship between the predictive parameters and corrosion inhibition, it appears that reactivity of the ZVI, rather than the reduction in permeability, is more likely the factor that limits PRB longevity in the field. Due to the sparseness of field monitoring of parameters such as EH, the data available for these analyses were limited. Consequently, these results need to be corroborated as additional measurements become available. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/63236/1/ees.2006.0071.pdf
This paper describes reactive transport simulations conducted to assess the impact of mineral fouling on the long-term performance of permeable reactive barriers employing granular zero valent iron (ZVI). Three minerals were assumed to form in the pore space (CaCO3, FeCO3, and Fe(OH) 2) and the inflowing groundwater was assumed to have the following composition: DO = 10-8 M, Fe2+ = 10-10 M, Ca2+ = 10-3 M, OH- = 10-7M, HCO 3- = 10-3 M, and CO32- = 10-7 M. Results of the simulations show that the porosity and hydraulic conductivity of the ZVI decrease over time and that flows are redistributed throughout the PRB in response to fouling of the pore space. Seepage velocities in the PRB increase, and residence times decrease, due to porosity reductions caused by accumulation of minerals in the pore space. Under the assumed conditions, only subtle changes occur within the first 10 years (i.e. the duration of the current field experience record with PRBs) and the most significant changes do not occur until the PRB has operated for at least 30 years. However, after 30-50 years, reductions in residence time of the order of 50% occurred. More rapid and extensive changes are likely to occur for conditions that result in greater precipitation rates (e.g. groundwater with higher ionic strength, higher velocity).
The Hayle Estuary, Cornwall, acted as an effective sediment trap for mine waste tailings and smelt waste released into the river catchments draining into the estuary. The stratigraphy of two 3 m cores recovered from Copperhouse Pool, Hayle comprises interbedded muds (interpreted as mine waste slimes) and sands in the upper 50 cm, passing down into sands composed predominantly of carbonate shell debris. Vacuum resin-impregnated core plugs sampled from more organic-rich intervals in the upper 150 cm of both cores were examined using scanning electron microscopy. Detrital heavy and opaque minerals include abundant grains of cassiterite, chalcopyrite, Fe oxides, arsenopyrite, sphalerite, polymetallic slags, detrital angular pyrite, ilmenite, monazite, zircon, wolframite, ?loellingite (As-Fe), galena, chalcocite/bornite and pyromorphite. Abundant diagenetic sulphide minerals also occur in these samples, and include Cu-Fe-(As) sulphides (probably chalcopyrite), As sulphides and pyrite. The precipitation of chalcopyrite occurred under reducing conditions with the reaction buffered by the Fe system. Possible copper concentrations in equilibrium with the authigenic chalcopyrite are so low that it is likely that there was an influx of more oxidising pore waters carrying higher levels of copper, which was then precipitated on reaching more reducing conditions. The precipitation of the As sulphides occurred from pore fluids with a high arsenic concentration under less reducing conditions than the chalcopyrite.
Permeable reactive barriers, comprised of zero-valent iron (ZVI), have been installed and operated at U.S. DOE Sites located at the Y-12 Plant in Oak Ridge, Tennessee, and at the uranium mill tails repository near Durango, Colorado. The ZVI medium is intended to remove select toxic solutes, especially uranium (U), from contaminated groundwater. Core samples from these barrier installations were sampled after prolonged exposure to contaminated groundwater (~ 1.2 year of operation at the Y-12 Site, and up to 3 years of operation at the Durango Site). The core samples were protected from exposure to the ambient atmosphere by packaging them in argon-purged containers for shipment to an off-site laboratory. The concern was to protect the media from exposure to oxygen, which could alter the valence state of the treatment medium itself and of the metal deposits therein. The samples were subjected to a battery of analytical techniques, including X-ray Photoelectron Spectroscopy (XPS), a surface-sensitive technique that can be used to determine the average valence state of elements. Major findings in this study were (1) that the ZVI media had extensive surface deposition of various mineral phases (e.g., calcite and amorphous iron sulfides), (2) that U was present within the media at somewhat modest levels (e.g., < 0.2 wt%), and (3) that U on the ZVI surface was at least partially oxidized.
A major concern with contaminated sites is their potential for off-site environmental effects by the leaching
of contaminants to adjacent lands and groundwater aquifiers. As groundwater underlies 60% of Australia
(some 5,226,440 square kilometres) it is therefore a vital source of water that is constantly under threat of
contamination. The use of in situ engineering treatment methods such as permeable reactive barriers (PRB’s)
is a cost effective and environmentally sound method of removing contamination from point sources. PRB’s
are passive walls containing a chemically active material that reacts with groundwater contaminants as they
pass through the barrier. However, reactive barriers still have many technical uncertainties associated with
them, especially in regard to their long term performance.
Spent potliner (SPL), a by-product of the aluminium smelting process, is listed by various
environmental bodies including the US EPA as a designated hazardous waste primarily due to the high
levels of leachable fluoride, cyanide and pH (>9.5). World-wide it is estimated that the total amount of SPL
produced is approximately one million tonnes per year with the bulk of this being stored in purpose built
storage facilities. At the Hydro Aluminium smelter near Kurri Kurri, NSW, Australia, SPL was dumped by
previous owners in an unlined waste repository from 1969 to 1992 resulting in localized contamination of
the groundwater aquifer with high levels of fluoride.
This project utilized the groundwater remediation test facility at the University of Newcastle,
Australia to assess the use of calcite as a reactive barrier substrate for the remediation of fluoride from
groundwater contaminated with SPL leachate. The facility houses a 3.0m x 2.0m x 1.3m high pilot cell that
can be configured with up to 40 individual cells to represent a reactive barrier installation. Pilot tests were
setup to examine optimal methods of injecting CO2 gas for pH control and thereby fluoride removal.
Methods included injection upstream of, and directly into, the calcite barrier using slow release gas
permeable tubing or high flow injection using ~50mm diameter spear point installations.
Over a period of 28 days, approximately 26,000 litres of actual SPL contaminated groundwater (pH
9.5; [F-] ~500 mg/L) was allowed to flow through a simple aquifer of clean sand and a single barrier of
calcite (1.8m high x 1.3m wide x 0.3m thick). Contaminant data (fluoride, pH, electrical conductivity, and
major cations and anions) were collected at defined intervals (~6 hourly) in a 1-dimensional array from
upstream of the barrier, 3 points within the barrier, and 2 points downstream. Data was also periodically
collected from within the barrier to characterize chemical changes vertically.
Results show that, following breakthrough (based on the non-reactive chloride tracer), little or no
fluoride was removed due to inhibition by an as yet undefined reaction between the SPL leachate and the
calcite substrate as previously reported by the authors. Injection of 100% CO2 upstream of the barrier has
very little effect on fluoride removal, whilst injection directly into the calcite barrier showed that >90%
fluoride removal can be attained with the added benefit of removing the coffee coloured taint from the SPL
contaminated groundwater. Changes in the porosity of the calcite barrier were also examined using cores
obtained by injection of resin into a 100mm diameter tube inserted into the calcite barrier before and after
testing. Results indicate that after 28 days and 26,000L of leachate there was no significant change in the
porosity of the calcite barrier.
Shallow groundwaters in the western U.S. often contain elevated levels of trace oxyanions such as selenate, chromate, and uranyl. A potential remedia-tion method is to react the water with zero-valent iron. Both in-situ permeable reactive barrier walls and pump-n-treat technologies are being used for to treat these contaminated waters (Wilson, 1995). In this reaction, the iron serves as both an electron source and as a catalyst. The objectives of this research were to determine the factors that affect the rate of reaction and the types of secondary products formed, including the iron hydroxide mineralogy and the fate of the trace elements. Variables that we studied included pH, ionic strength, 02, competitive ions and forms of iron. Materials and methods Reactions between zero-valent iron metal and aqueous solutions were carried out in 1.3 L stirred batch reactors with controlled pH and gas composi-tion (mixtures of N2, CO2, and 02). Variable concentrations of selenate, chromate, and uranyl were reacted with coarse iron filings and iron foil. Reacting solutions were sampled every few hours and analysed for trace elements and soluble iron. Selenate and selenite were determined using hydride generation atomic absorption spectroscopy (AAS), chromate and chromite by the diphenylcarbazide colorimetric method and AAS, and total soluble uranium by ICP-MS. The solid phases were collected after 1 to 10 days and the iron hydroxide products separated from the metal by agitation and filtration. X-ray diffraction of powder mounts was used for mineralogical identification of the secondary products. X-ray absorption near edge spectroscopy (XANES) was conducted using the Beamline 4-2 at the Stanford Synchrotron Radiation Laboratory. Both fluorescence and X-ray transmission intensity were monitored at the Se and Fe absorption edges. Standard reference materials including iron oxides and selenium compounds were used to identify absorption edge shifts and fine-structure features for the different oxidation states and coordination environments (Pickering et al., 1995). X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and scanning tunneling micro-scopy (STM) were used to characterized the surface chemistry of iron foils reacted in aqueous solutions. The STM images were collected in aqueous solution periodically for several hours. The reacted foils were then rinsed with deionized water and dried under N2 prior to being placed in the ultra-high vacuum chamber for XPS analysis.
Acid-mine drainage (AMD) can introduce elevated concentrations of sulfate, ferrous iron and other dissolved metals to groundwater and receiving surface water. Permeable reactive barriers (PRBs) offer an approach for the passive interception and in situ treatment of AMD-impacted groundwater. Three field-scale applications and supporting laboratory columns in the past six years have shown that several thousands of mg/L sulfate, more than 1,000 mg/L iron, and several tens of milligrams per liter of other metals can be removed from plume or tailings groundwater. The reactive materials, which incorporate various forms of organic carbon, promote microbially mediated sulfate reduction, the generation of hydrogen sulfide, and the subsequent precipitation of sparingly soluble iron and other metal, such as Cd, Ni, Co, Cu, Zn, As or Zn, sulfide minerals. The applications include PRBs for the treatment of a plume at full scale from a mine-tailings impoundment and at demonstration-and full-scale at a former metal processing facility. These PRBs have removed sulfate and metals from groundwater. Similar materials were also used to create reactive layers within test cells directly in a tailings impoundment to evaluate the potential removal iron and sulfate from pore water before it migrates from the tailings impoundment. PRBs have been considered as an alternative for plume control and remediation at many contaminated sites (Powell et al. 1998). PRBs have two essential functions. The PRB must facilitate the interception or capture of a contaminant plume at some distance down-gradient of the source, and provide treatment or removal of contaminants to acceptable levels. Treatment is achieved within or down-gradient of the barrier by physical, chemical or biological processes. AMD is caused by the oxidation of residual sulfide minerals in the vadose zone of mine tailings and waste rock. AMD effluent is acidic and contains elevated concentrations of sulfate, ferrous iron [Fe(II)] and dissolved trace metals. Although buffering of the pH to near-neutral conditions may occur in groundwater flow systems, the oxidation of Fe(II) to Fe(III) occurs upon discharge of AMD to receiving surface water. This generates additional acidity and results in the precipitation of ferric oxyhydroxides, and can have adverse impacts on aquatic ecosystems by lowering the pH and enhancing the mobility of trace metals in surface water.
We identified submicrometer-sized framboidal sphalerite (ZnS) below the base of supergene oxidation in a Carlin-type gold deposit of Eocene age in Nevada, United States, where the framboidal sphalerite forms a blanket-like body containing >400,000 metric tons of zinc. Framboidal sphalerite <0.1 μm in diameter, formed in the early Miocene, ranges from <0.1 to 0.35 mol% FeS; the δ34S values range from -25‰ to -70‰, the lowest values measured in a marine or terrestrial environment. These S isotope data demonstrate the involvement of sulfate-reducing bacteria and provide the first documentation that sphalerite can form significant supergene sulfide-enrichment blankets.
This chapter presents a study in which a permeable iron reactive barrier was installed in late November 1997 at the U.S. Department of Energy's Y-12 National Security Complex in Oak Ridge, Tennessee. The overall goal of this study was to determine the effectiveness of the use of zero valent iron (Fe0) to retain or remove uranium and other contaminants such as technetium and nitrate in groundwater. The long-term performance issues were investigated by studying the biogeochemical interactions between Fe0 and groundwater constituents and the mineralogical and biological characteristics over an extended field operation. Results from nearly 3 years of monitoring indicated that the Fe0 barrier was performing effectively in removing contaminant radionuclides such as uranium and technetium. In addition, a number of groundwater constituents such as bicarbonates, nitrate, and sulfate were found to react with the Fe0. Both nitrate and sulfate were reduced within or in the influence zone of the Fe0 with a low redox potential. An increased anaerobic microbial population was also observed within and in the vicinity of the Fe0 barrier, and these microorganisms were at least partially responsible for the reduction of nitrate and sulfate in groundwater. Decreased concentrations of Ca2+ and bicarbonate in groundwater occurred as a result of the formation of minerals such as aragonite (CaCO3) and siderite (FeCO3), which coincided with the Fe0 corrosion and an increased groundwater pH. A suite of mineral precipitates was identified in the Fe0 barrier system, including amorphous iron oxyhydroxides, goethite, ferrous carbonates and sulfides, aragonite, and green rusts, which were found to be responsible for the cementation and possibly clogging of Fe0 filings observed in a number of core samples from the barrier.
This chapter focuses on a permeable reactive barrier (PRB), which was installed at a site near Monticello, Utah, in June 1999 to remediate contaminated groundwater at a former uranium-processing mill. Laboratory and field column tests were used to evaluate reactive materials and zero valent iron (ZVI) was selected for the installation, as it effectively removes various contaminants such as arsenic, molybdenum, nitrate, selenium, uranium, and vanadium present in the groundwater at this site and is also available in large quantities at low cost. The PRB is 31 m long and contains 251 metric tons of ZVI and the installation includes slurry walls that direct contaminated groundwater in an alluvial aquifer through the PRB. The PRBwas keyed at least 0.3 m into impermeable bedrock and after 1 year of operation, effluent from the PRBcontinued to meet regulatory concentration goals for all contaminants. Low iron (Fe) concentrations in effluent from the PRB contrasted with high Fe concentrations in effluent from column tests, which is explained by the longer residence times in the PRB that led to higher pH values and precipitation of Fe(OH)2. Groundwater mounded immediately upgradient of the PRB, which resulted partly due to a reduction in the width of the alluvial aquifer because of the impermeable slurry walls, and possibly due to a zone of decreased permeability caused by steel sheet pile installation during PRB construction. Results of tracer tests and downhole flow measurements suggest that groundwater was flowing though the PRB at nearly the average linear design velocity of 5.7 m per day.
This chapter presents a study in which a zero valent iron (ZVI) permeable reactive barrier (PRB) was installed in a shallow, colluvial aquifer contaminated with uranium in Fry Canyon, Utah, in September 1997. Aerobic and anaerobic iron corrosion reactions in the ZVI PRB have created a highly reducing, oxygen-depleted, and hydrogen gas-enriched geochemical environment in the PRB that is favorable for sulfate-reducing bacteria (SRB). Stable sulfur isotope, microbiologic, and geochemical evidence indicates that SRB are active in the ZVI PRB. The stable sulfur isotope and SO42– data from wells in the ZVI PRB and a downgradient well show that sulfur is removed by DSR through a Rayliegh-type distillation process. The enrichment factor computed from the Rayleigh plot is close to the values measured in other field investigations of DSR in groundwater systems. The thermodynamic speciation calculations and stable sulfur isotope data indicate that sulfide precipitation is the only sink for sulfur in the PRB. The distribution of SO42- concentrations indicates that most of the sulfide precipitation is occurring in the first 0.15 m of the PRB.
This research investigated the mechanisms controlling the kinetics of arsenate and chromate removal from water by zerovalent iron media. Batch experiments were performed to determine the kinetics of As(V) and Cr(VI) removal as a function of their aqueous concentration. The effect of arsenic and chromium on the corrosion rate of iron was investigated via analysis of Tafel polarization diagrams. Column experiments were conducted to investigate the long-term performance of zerovalent iron for arsenate and chromate removal. Arsenate removal kinetics were first-order at low aqueous concentrations, but approached zeroth-order in the limit of high concentrations. This was attributed to increasing adsorption site saturation with increasing aqueous concentration. Arsenate removal at low aqueous concentrations was limited by diffusion of As(V) through iron corrosion products to adsorption sites. At high concentrations, arsenate removal was limited by the generation rate of adsorption sites. The dividing line between the high and low concentration behavior depends on the iron corrosion rate, which is largely determined by the dissolved oxygen concentration. Rates of Cr(VI) removal could not be described by a simple kinetic model due to the effect of precipitated chromium compounds on the reactivity of the iron media. Absolute rates of chromate removal declined with increasing concentration due to passivation of the iron surfaces by adsorbed chromium compounds. This surface passivation was incomplete, and steady-state effluent concentrations from a column reactor were observed over a period spanning more than 1,200 pore volumes. The surface passivation was also reversible upon lowering the influent Cr(VI) concentration, indicating that the performance of iron media for Cr(VI) removal was hysteretic.
Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate- releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.
The chemical and microbial activity of corroding iron metal is examined in the acid rock drainage (ARD) resulting from pyrite oxidation to determine the effectiveness in neutralizing the ARD and reducing the load of dissolved heavy metals. ARD from Berkeley Pit, MT, is treated with iron in batch reactors and columns containing iron granules. Iron, in acidic solution, hydrolyzes water producing hydride and hydroxide ion resulting in a concomitant increase in pH and decrease in redox potential. The dissolved metals in ARD are removed by several mechanisms. Copper and cadmium cement onto the surface of the iron as zerovalent metals. Hydroxide forming metals such as aluminum, zinc, and nickel form complexes with iron and other metals precipitating from solution as the pH rises. Metalloids such as arsenic and antimony coprecipitate with iron. As metals precipitate from solution, various other mechanisms including coprecipitation, sorption, and ion exchange also enhance removal of metals from solution. Corroding iron also creates a reducing environment supportive for sulfate reducing bacteria (SRB) growth. Increases in SRB populations of 5,000-fold are observed in iron metal treated ARD solutions. Although the biological process is slow, sulfidogenesis is an additional pathway to further stabilize heavy metal precipitates.
The use of permeable reactive barriers (PRB) for the treatment of contaminated groundwater is gaining interest as an alternative to conventional (pump and treat) methods. Laboratory experiments investigating the potential for treatment of arsenic contaminated groundwater through the use of potential reactive barrier materials have been in progress since October 2000. For this investigation, arsenic contaminated groundwater collected from a mine located in Ontario, Canada, is continuously passed through two laboratory columns, one containing 100% zero valent iron and the other a mixture containing 20% zero valent iron. During the experiments, the influent groundwater has contained between 9 and 15 mg/L of dissolved arsenic. The AsIII:AsV ratio of the groundwater input varies from dominantly AsIII to dominantly AsV. Flow through the columns (0.2 to 0.4 pore volumes/day; (PV/day) or 25 to 65 m/a) is several times the groundwater velocity measured at the mine site (10-40 m/a). Results indicate that both reactive mixtures are capable of removing the dissolved arsenic to below current Canadian interim guideline concentration of 0.025 mg/L and the new U.S. EPA maximum contaminant level of 0.01 mg/L for more than 200 pore volumes. After treating 200 PV of water containing an average of 12 mg/L As, concentrations exceeding 0.1 mg/L of As migrate less than 5 cm into the reactive material. Concentrations exceeding 0.05 mg/L have not moved more than 12 cm into the reactive material. The position of the reaction front has been immobile for approximately 2 years. Current results suggest almost a 2:1 mass ratio of As removed vs. reactive material consumed. These results indicate rapid removal of the dissolved As and a negligible decrease in the reactivity since the start of the experiment. Under field conditions, (e.g. a 1 m thick in situ PRB installation), this represents the potential for several years or decades of treatment.
Sulfide minerals are known to be important hosts for metals in many freshwater and marine sediments, but little is known about their petrography, particularly in fine-grained recent freshwater sediments. In this paper the results of a Cryogenic SEM investigation of sulfidc textures in two fine-grained anaerobic canal-bed muds are reported. The technique allows direct observation of the undisrupted mineral and organic textures in the canal-bed mud. Iron sulfides occur in two forms: coatings on biofilms and framboids. Iron sulfide-coated biofilms have iron sulfur ratios in the range FeS to Fe 3S 4. Framboid structures display a continuum of textures from greigite proto-framboids with a poorly developed crystallite texture to pyrite framboids with well developed crystallites. Proto-framboids tend to be smaller than framboids. A positive correlation was observed between crystallite diameter and framboid diameter. By dividing the framboid diameter by the crystallite diameter it was deduced that framboids tend not to have single crystallites at their centers. Copper occurs as a discrete sulfide with a composition similar to chalcopyrite. Zinc occurs as a zinc iron sulfide with metal-to-sulfur ratios in the range 0.59 to 0.87. Copper sulfides tend to nucleate on surfaces whereas the zinc sulfides occur both on surfaces and as floccular precipitates in open pore space.
In bioreactor systems for the treatment of metal-contaminated water, pretreatment with zerovalent Fe can be exploited for oxygen consumption and H2 production. In this study, a column experiment is used to investigate the changes in surface chemistry and solid phase products that result from the reaction of a Zn-sulphate-lactate solution with zerovalent Fe filings. The results of this study indicate that zerovalent Fe is very effective in immobilizing dissolved Zn with a porewater residence time of 1.3 -3.1 days. A combination of X-ray diffractometry, X-ray photoelectron spectroscopy, and mineral equilibria calculations indicates that Zn precipitates as Zn(OH)2 and zincite at pH 9 -10. At pH 6, Zn primarily adsorbs to abundant ferric oxyhydroxides, although incorporation in green rust is also considered. During the course of the experiment, the surface mineralogy changes from magnetitelepidocrocite-goethite to green rust-akaganéite-goethite. The results suggest that the zerovalent Fe surface becomes passivated by a surface film of ferric oxyhydroxides, green rust and organic material, so that the rate of electron transfer and proton consuming reactions (i.e. oxygen consumption, H2 generation) declines, resulting in a decrease in solution pH.
The generation and release of acidic drainage containing high concentrations of dissolved metals from decommissioned mine wastes is an environmental problem of international scale. A potential solution to many acid drainage problems is the installation of permeable reactive walls into aquifers affected by drainage water derived from mine waste materials. A permeable reactive wall installed into an aquifer impacted by low-quality mine drainage waters was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. The reactive mixture, containing organic matter, was designed to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation. The reactive wall is installed to an average depth of 12 feet (3.6 m) and is 49 feet (15 m) long perpendicular to ground water flow. The wall thickness (flow path length) is 13 feet (4 m). Initial results, collected nine months after installation, indicate that sulfate reduction and metal sulfide precipitation is occurring. The reactive wall has effectively removed the capacity of the ground water to generate acidity on discharge to the surface. Calculations based on comparison to previously run laboratory column experiments indicate that the reactive wall has potential to remain effective for at least 15 years.
In situ permeable reactive barriers (PRBs) consist of zones of reactive material, such as granular iron or other typically reduced metal, lime, electron donor-releasing compounds, or electron acceptor-releasing compounds, installed in the path of a plume of contaminated groundwater. As the groundwater flows through this zone, contaminants are degraded to innocuous components through chemical and/or biological reactions, adsorbed, or chemically altered so that they form insoluble precipitates. This article represents a summary review of representative literature on permeable reactive barrier technology. It consists of a description of the technology, a list of treatable contaminants, the processes necessary for its implementation, considerations for conducting performance monitoring, a discussion of the positive and negative attributes and costs of the technology, and lessons learned during recent applications. Where conditions are favorable and time factors are appropriate, this technology appears promising. The main characteristic in its favor is the lack of the need to operate pumps or treatment vessels, thereby saving operation and maintenance costs and allowing the economic value of property to be restored during remediation. Its reliance on natural advec-tive processes to move contaminants through the treatment zone, resulting in long treatment time frames, can be a disadvantage under some circumstances. There are also uncertainties about the long-term effectiveness of the reactive media. Regulators need to continue the trend toward being more receptive of this technology, as well as other innovative technologies, so that it can be improved. This receptiveness will benefit all stakeholders involved.
There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225-ft long x 2-ft wide x 31-ft deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (10-150 m thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 m thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.
Bornite was prepared at atmospheric temperature and pressure by the addition of sulfide ion to an acid ferrous sulfate solution;
the bornite was formed on a small piece of Cu metal. Possible environments of formation are suggested and localities mentioned
where chalcopyrite and bornite might have formed in this way.
Recent advances in understanding the chemistry of iron sulfides in sedimentary environments are beginning to shed more light on the processes involved in the global sulfur cycle. Pyrite may be formed via at least three routes including the reaction of precursor sulfides with polysulfides, the progressive solid-state oxidation of precursor iron sulfides and the oxidation of iron sulfides by hydrogen sulfide. The kinetics and mechanism of the polysulfide pathway are established and those of the H2S oxidation pathway are being investigated. Preliminary considerations suggest that the relative rates of the three pathways are H2S oxidation > polysulfide pathway > > solid-state oxidation. The kinetics and mechanisms of iron(II) monosulfide formation suggest the involvement of iron bisulfide complexes in the pathway and iron bisulfide complexes have now been identified by voltammetry and their stabililty constants measured. The framboidal texture commonly displayed by sedimentary pyrite appears to be an extreme example of mosaicity in crystal growth. Framboidal pyrite is produced through the H2S oxidation reaction. Frontier molecular orbital calculations are beginning to provide theoretical underpinning of the reaction mechanisms. Recent progress in understanding iron sulfide chemistry is leading to questions regarding the degree of involvement of precursor iron sulfides in the formation of pyrite in sediments. Spin-offs from the work are addressing problems relating to the involvement of iron sulfides in the origin of life, the nature of metastability, the mechanism of precipitation reactions and the use of iron sulfides in advanced materials.
The initial stages of corrosion of iron by unstirred saturated aqueous HâS solutions at 21°C and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of HâSOâ or NaOH), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal FeS/sub 1-x/) is formed by both solid-state and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid-state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; the quantity of precipitated material peaks near pH = 4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pH = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21°C was monitored by x-ray diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.
Permeable reactive barriers designed to enhance bacterial sulfate reduction and metal sulfide precipitation have the potential to prevent acid mine drainage and the associated release of dissolved metals. Two column experiments were conducted using simulated mine-drainage water to assess the performance of organic carbon-based reactive mixtures under controlled groundwater flow conditions. The simulated mine drainage is typical of mine-drainage water that has undergone acid neutralization within aquifers. This water is near neutral in pH and contains elevated concentrations of Fe(II) and SO4. Minimum rates Of SO4 removal averaged between 500 and 800 mmol d(-1) M-3 over a 14-month period. Iron concentrations decreased from between 300 and 1200 mg/L in the influent to between <0.01 and 220 mg/L in the columns. Concentrations of Zn decreased from 0.6-1.2 mg/L in the input to between 0.01 and 0.15 mg/L in the effluent, and Ni concentrations decreased from between 0.8 and 12.8 mg/L to <0.01 mg/L. The pH increased slightly from typical input values of 5.5-6.0 to effluent values of 6.5-7.0. Alkalinity, generally <50 mg/L (as CaCO3) in the influent, increased to between 300 and 1300 mg/L (as CaCO3) in the effluent from the columns. As a result of decreased Fe(II) concentrations and increased alkalinity, the acid-generating potential of the simulated mine-drain age water was removed,and a net acid-consuming potential was observed in the effluent water.
The formation pathways of pyrite are controversial. Time resolved experiments show that in reduced sulphur solutions at low temperature, the iron monosulphide mackinawite is stable for up to 4 months. Below 100°C, the rate of pyrite formation from a precursor mackinawite is insignificant in solutions equilibrated solely with H2S(aq). Mackinawite serves as a precursor to pyrite formation only in more oxidised solutions. Controlled, intentional oxidation experiments below 100°C and over a wide range of pH (3.3–12) confirm that the mackinawite to pyrite transformation occurs in slightly oxidising environments. The conversion to pyrite is a multi-step reaction process involving changes in aqueous sulphur species causing solid state transformation of mackinawite to pyrite via the intermediate monosulphide greigite. Oxidised surfaces of precursors or of pyrite seeds speed up the transformation reaction.Solution compositions from the ageing experiments were used to derive stability constants for mackinawite from 25°C to 95°C for the reaction:FeS(s)+2H+⇔Fe2++H2SThe values of the equilibrium constant, logKFeS, varied from 3.1 at 25°C to 1.2 at 95°C and fit a linear, temperature-dependent equation: logKFeS=2848.779/T−6.347, with T in Kelvin. From these constants, the thermodynamic functions were derived. These are the first high temperature data for the solubility of mackinawite, where Fe2+ is the dominant aqueous ferrous species in reduced, weakly acidic to acidic solutions.
Permeable Reactive Barriers show promise as an inexpensive and effective remediation technology alternative to Pump and Treat for removal of a wide range of contaminants including radionuclides from groundwater. Reactions within a reactive barrier either degrade contaminants to non -toxic forms or transfer contaminants to an immobile phase. Three permeable reactive barriers (PRBs) were installed near Fry Canyon, Utah, in August 1997. The overall objective of this project is to demonstrate the use of PRBs to control the migration of uraniu m (U) in ground water. A funnel and gate design was used to construct the three PRBs, which consist of (1) bone-char phosphate (PO4), (2) zero-valent iron (ZVI) pellets, and (3) amorphous ferric oxyhydroxide (AFO). During the first 28 months of PRB operation (September 1997 through December 1999), the ZVI PRB was the most effective at lowering U concentrations in the contaminated ground water. The median U removal in the ZVI PRB was higher than 99.5 percent, in the AFO barrier 95 percent, and in the bone-char phosphate barrier, 81 percent. Geochemical modeling techniques were used to define and quantify the amount and type of mineral precipitates that may be forming in the ZVI PRB. Modeling results indicate that most of the mineral precipitation occurs within the first 1.0 foot of barrier material. On the basis of water -chemistry data collected during May 1999, the downgradient two-thirds of the ZVI PRB has not been affected by mineral precipitation. Geochemical modeling results were consistent with solid-phase analyses of reactive material collected from the ZVI PRB.
Because of the limitations of conventional pump-and-treat systems in treating groundwater contaminants, permeable barriers are potentially more cost-effective than pump-and-treat systems for treating dissolved chlorinated solvent plumes, which may persist in the saturated zone for several decades. Other contaminants, such as chromium or other soluble heavy metals, can also be treated with this technology. The authors discuss the types of permeable barriers, their design and construction, and how they can be monitored to evaluate compliance. It provides practical guidance on reactive media selection, treatability testing, hydrogeologic and geochemical modeling, and innovative installation techniques for the evaluation and application of this promising new technology. The types of permeable barriers discussed include: trench-type and caisson-based reactive cells; innovative emplacements, such as horizontal trenching and jetting; and continuous reactive barriers versus funnel-and-gate systems. The material is organized in an easy-to-read format, with topics and illustrations that will be useful to readers already familiar with barriers, and to those just being introduced to the technology.
Lessons Learned/New Directions was prepared by the ITRC Permeable Reactive Barriers Team to update previous guidance written by the team. The goal for this document was to compile the information and data on permeable reactive barriers (PRBs) that have been generated over the last 10 years of technology development and research, as well as to provide information on noniron-based reactive media that can be used in PRBs. This document also provides an update on a developing technology somewhat related to PRBs in which source zone contamination is treated with iron-based reactive media. A PRB is defined as an in situ permeable treatment zone designed to intercept and remediate a contaminant plume. Zero-valent iron is the most common media used in PRBs to treat a variety of chlorinated organics, metals, and radionuclides. Reactive media such as carbon sources (compost), limestone, granular activated carbon, zeolites, and others had also been deployed in recent years to treat metals and some organic compounds. The proper design of a PRB is highly dependent on a complete and accurate site characterization. A conceptual site framework is discussed as a means to perform a detailed characterization for PRB deployment. Collection of hydrogeologic, geochemical, microbial, and geotechnical data along with the complete vertical and horizontal plume delineation are necessary to characterize a PRB site. The Triad concept is also introduced as a means to gather site data. The design of a PRB can be enhanced using probabilistic modeling to incorporate the variability of the input design parameters. Construction advancements include the use of biopolymer for trench stabilization or the use of vertical hydraulic fracturing for reactive media emplacement.
Irrigation drainage and industrial wastewaters often contain elevated levels of toxic oxyanions and oxycations such as selenate, chromate, and uranyl. A potential remediation method is to react contaminated water with zero-valent iron, which transforms the mobile contaminants into immobile forms. In this work, iron foil was exposed to aqueous solutions containing the relevant ions, and the reacted surfaces were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM images collected in situ show that the protrusions on the foil surface associated with iron oxides are smoothed out by the reaction. XPS indicates that partially reduced Se(IV) and Cr(III) are adsorbed on the surface, while uranium is deposited as U(VI), i.e., without reduction. More Se and Cr are deposited when the atmospheric gases are removed from solution because of the elimination of a competing process in which dissolved O2 increases the thickness of the iron oxide overlayer to the point where the reduction reaction is quenched. The amount of U deposited is greatly increased when the atmospheric gases are removed because of the elimination of dissolved CO2, which can form carbonate complexes with uranium.
The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17−33 mM HCO3-) and sulfate (7−20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3- ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.
A permeable reactive barrier, designed to remove metals and generate alkalinity by promoting sulfate reduction and metal sulfide precipitation, was installed in August 1995 into an aquifer containing effluent from mine tailings. Passage of groundwater through the barrier results in striking improvement in water quality. Dramatic changes in concentrations of SO4 (decrease of 2000−3000 mg/L), Fe (decrease of 270−1300 mg/L), trace metals (e.g., Ni decreases 30 mg/L), and alkalinity (increase of 800−2700 mg/L) are observed. Populations of sulfate reducing bacteria are 10 000 times greater, and bacterial activity, as measured by dehydrogenase activity, is 10 times higher within the barrier compared to the up-gradient aquifer. Dissolved sulfide concentrations increase by 0.2−120 mg/L, and the isotope 34S is enriched relative to 32S in the dissolved phase SO42- within the barrier. Water chemistry, coupled with geochemical speciation modeling, indicates the pore water in the barrier becomes supersaturated with respect to amorphous Fe sulfide. Solid phase analysis of the reactive mixture indicates the accumulation of Fe monosulfide precipitates. Shifts in the saturation states of carbonate, sulfate, and sulfide minerals and most of the observed changes in water chemistry in the barrier and down-gradient aquifer can be attributed, either directly or indirectly, to bacterially mediated sulfate reduction.
This study was undertaken to determine the effectiveness of zero-valent iron (Fe0) and several adsorbent materials in removing uranium (U) from contaminated groundwater and to investigate the rates and mechanisms that are involved in the reactions. Fe0 filings were used as reductants, and the adsorbents included peat materials, iron oxides, and a carbon-based sorbent (Cercona Bone-Char). Results indicate that Fe0 filings are much more effective than the adsorbents in removing uranyl (UO22+) from the aqueous solution. Nearly 100% of U was removed through reactions with Fe0 at an initial concentration up to 76 mM (or 18 000 mg of U/L). Results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe0 is the major reaction pathway. Only a small percentage (<4%) of UO22+ appeared to be adsorbed on the corrosion products of Fe0 and could be desorbed by leaching with a carbonate solution. The study also showed that the reduced U(IV) species on Fe0 surfaces could be reoxidized and potentially remobilized when the reduced system becomes more oxidized. Results of this research support the application of the permeable reactive barrier technology using Fe0 as a reactive media to intercept U and other groundwater contaminants migrating to the tributaries of Bear Creek at the U.S. Department of Energy's Y-12 Plant located in Oak Ridge, TN.
This work was carried out with the purpose of developing effective reagents for decontamination of groundwater contaminated with chlorocarbons. Zinc metal as a reducing agent for carbon tetrachloride (CT), chloroform (Chl), and methylene chloride (MC) in aqueous solution has been studied in some detail, especially regarding activated forms of the metal. Chlorocarbon concentrations were monitored at certain time intervals by gas chromatography/mass spectrometry (GC/MS) analysis of the headspace and water phase. Reaction mixture headspace was additionally studied by a GC/headspace analysis system to detect the formation of hydrocarbons. Chloroform, methylene chloride, methyl chloride, methane, and acetylene were found to be products from CT reduction. For methylene chloride reduction, traces of cis and trans-1,2-dichloroethene (DCE) were also found. Activated by cryo or mechanical treatment, metallic zinc caused an increase in CT dechlorination rate and conversion into methane. After the first 2.5 h, more than 20% of CT was converted into methane by cryochemically activated zinc in comparison to 1.2% by conventional zinc dust. Furthermore, CT reduction by activated zinc caused the formation of DCEs and TCE. Pathways are proposed to account for the observed methane/methylene chloride ratio and DCEs and TCE formation that include sequential reductive dechlorination through organometallic and carbonoid species on the Zn surface. Furthermore, it seems likely that some methane can be formed in “one metal contact”, since significant amounts are formed early in the reaction. In attempts to learn more about morphological changes in the zinc during its consumption, pore volume/pore radii were determined, and atomic force microscope images were obtained. Zinc corrosion takes place rapidly at edges/corners leading to the formation of cavities with wide openings, large volumes, and increased specific surface areas. Pyramidal zinc “pillars” are formed during the process.
Synthetic green rusts, GRs, are prepared by oxidation of Fe(OH)2 incorporating Cl-, SO42-, or CO32- ions. Eh−pH diagrams are drawn, and thermodynamic data are derived. A GR incorporating OH- ions, GR1(OH-), is suspected to exist like similar other M(II)−M(III) compounds. GRs form as corrosion products of steels, implying microbially induced corrosion. Mössbauer and Raman spectroscopies allowed the identification of GR in samples extracted from hydromorphic soils scattered over Brittany, France. This mineral has a varying Fe(III)/Fe(II) ratio. At Fougères, it increases with depth till the oc currence of more oxidized ferric oxyhydroxide. In the same sites, soil solutions are collected and prevented from any oxidation and photoreduction. In large ranges of pH, pe, and Fe(II) concentration variations, soil solutions are in equilibrium with a Fe(II)−Fe(III) compound, a GR1 mineral with pyroaurite-like structure incorporating OH- ions and having the formula [FeII(1-x)FeIIIx(OH)2]+x·[xOH]-x ≡ Fe(OH)(2+x). Computation of ionic activity products (IAP) of the equilibria between minerals and solutions leads to molar ratio x from 1/3 to 2/3, in agreement with the Fe(III)/Fe(II) ratios obtained from Mössbauer spectroscopy. The GR mineral plays a key role for controlling iron in soil solutions, and equilibria between soil and suspension constrain the Fe(III)/Fe(II) ratios of the iron(II)−iron(III) hydroxide.
The contaminant of most concern in groundwater at the Oak Ridge Y-12 Plant's Bear Creek Valley Characterization Area is soluble uranium. The removal mechanism of soluble uranium from groundwater by zero-valent iron (ZVI, Fe0) was investigated. X-ray photoelectron spectroscopy (XPS, ESCA) was used to determine the uranium oxidation state at the Fe0 or iron oxide surface. Product speciation and relative reaction kinetics for the removal of soluble uranium under aerobic and anaerobic conditions with ZVI are presented. Under aerobic conditions, U6+ is rapidly and strongly sorbed to hydrous ferric oxide particulates (“rust”), whereas U6+ is slowly and incompletely reduced to U4+ under anaerobic conditions.
Permeable-reactive redox walls, placed below the ground surface in the path of flowing groundwater, provide an alternative remediation approach for removing electroactive chemicals from contaminated groundwater. Four types of Fe-bearing solids, siderite [FeCO3], pyrite [FeS2], coarse-grained elemental iron [Fe0], and fine-grained Fe0, were assessed for their ability to remove dissolved Cr(VI) from solution at flow rates typical of those encountered at sites of remediation. Batch studies show that the rate of Cr(VI) removal by fine-grained Fe0 is greater than that for pyrite and coarse-grained Fe0. Results from column studies suggest that partial removal of Cr(VI) by pyrite and coarse-grained Fe0 and quantitative removal of Cr(VI) by fine-grained Fe0 occur at rapid groundwater flow velocities. The removal mechanism for Cr(VI) by fine-grained Fe0 and coarse-grained Fe0 is through the reduction of Cr(VI) to Cr(III), coupled with the oxidation of Fe0 to Fe(II) and Fe(III), and the subsequent precipitation of a sparingly soluble Fe(III)−Cr(III) (oxy)hydroxide phase. Mineralogical analysis of the reactive material used in the batch tests indicates that Cr is associated with goethite (α-FeOOH). These results suggest that Cr(III) is removed either through the formation of a solid solution or by adsorption of Cr(III) onto the goethite surface. The effective removal of Cr(VI) by Fe0 under dynamic flow conditions suggests porous-reactive walls containing Fe0 may be a viable alternative for treating groundwater contaminated by Cr(VI).
Recent studies have shown promising results for subsurface remediation of dissolved chromate using permeable-reactive redox walls. Chromate reduction in the presence of iron filings and quartz grains was studied to determine the fate of reduced chromium in proposed wall material. Using a flow-through column apparatus, iron filings mixed with quartz grains were reacted with solutions that contained about 20 mg/L dissolved Cr(VI). Reacted iron filings developed coatings comprised of goethite with chromium concentrated in the outermost edges. Surface analysis showed all detectable chromium occurred as Cr(III) species. In addition, in regions of increased chromium concentration, goethite acquired chemical and structural characteristics similar to Fe2O3 and Cr2O3. Results of the study show that complete reduction of Cr(VI) to Cr(III) occurred and that Cr(III) was incorporated into sparingly soluble solid species.
Chlorinated ethenes can be reduced in metallic (zero-valent) iron/water systems to produce a suite of non-chlorinated hydrocarbons. When 13C-labeled trichloroethylene is reduced, 13C-labeled hydrocarbons are produced. In the absence of chlorinated ethenes, however, lower concentrations of many of the same hydrocarbons (methane and C2−C6 alkanes and alkenes) are also produced. Hardy and Gillham (1996) proposed that these background hydrocarbons were due to the reduction of aqueous CO2 by metallic iron. In the present study, we examined the production of these hydrocarbons by various batch experi ments. Several of the systems examined produced hydrocarbons in excess of the carbon available from aqueous CO2. In addition, carbon from aqueous 13C-labeled CO2 was not incorporated into the hydrocarbons produced. The reduction of aqueous CO2 was not a major source of carbon for the background hydrocarbons. Acid dissolution of gray cast irons containing both carbide and graphite carbon yielded hydrocarbons and a substantial amount of graphite residual. The dissolution of metallic irons contain ing only carbide carbon yielded total carbon conversion to hydrocarbons. Carbide carbon in the iron appears to be the most likely carbon source for the production of the background hydrocarbons. Mechanisms, analogous to the Fischer−Tropsch synthesis of hydrocarbons, are proposed for hydrocarbon production from carbide carbon. Similar mechanisms may also contribute to the formation of some of the hydrocarbons produced during the reduction of chlorinated ethenes by metallic iron.
A combination of new and previously reported data on the kinetics of dehalogenation by zero-valent iron (Fe0) has been subjected to an analysis of factors effecting contaminant degradation rates. First-order rate constants (kobs) from both batch and column studies vary widely and without meaningful correlation. However, normalization of these data to iron surface area concentration yields a specific rate constant (kSA) that varies by only 1 order of magnitude for individual halocarbons. Correlation analysis using kSA reveals that dechlorination is generally more rapid at saturated carbon centers than unsaturated carbons and that high degrees of halogenation favor rapid reduction. However, new data and additional analysis will be necessary to obtain reliable quantitative structure−activity relationships. Further generalization of our kinetic model has been obtained by accounting for the concentration and saturation of reactive surface sites, but kSA is still the most appropriate starting point for design calculations. Representative values of kSA have been provided for the common chlorinated solvents.
The reduction of aqueous CO2 by zero-valent iron was studied in batch and column experiments. Ten hydrocarbons up to C5 were identified as products of the reduction process and were shown to have Anderson−Schulz−Flory (ASF) product distributions. A direct consequence of the ASF product distribution is that a significant mass of hydrophobic hydrocarbons may remain sorbed to the iron surface. Based on a reaction mechanism proposed for the electroreduction of aqueous CO2 with nickel electrodes, iron acts as both a reactant by corroding to supply electrons and as a catalyst by promoting the formation and growth of hydrocarbon chains. Water is also a reactant in the system. When iron is used to enhance the dechlorination of chlorinated organic compounds, the slow desorption of the hydrocarbon products may become the rate-limiting step in the reaction.
The purpose of this investigation was (i) to test the effectiveness of a barrier engineered to remove Cr(VI) from leachates of higher pH and salinity typical of coal burning ashes and (ii) to determine which geochemical processes control Cr immobilization. Laboratory column and batch desorption experiments show that a barrier composed of sand, Fe(0), and bentonite irreversibly immobilizes Cr. Concentrations fall from 25 mg Cr L-1 in the leachate to below detection limits (0.0025 mg Cr L-1) and solution pH increases by about two units. Solid-phase analytical techniques such as SEM, EDS, XPS, and TOF-SIMS were used to characterize the barrier material prior to and after exposure to the Cr leachate. In the barrier material, Cr(III) was found associated with Fe(III)-oxides, as separate Cr oxides and as a Ca,Cr phase, probably Ca-chromite, CaCr2O4. The attenuating barrier can be an alternative to traditional liners and leachate collection systems at coal ash storage and disposal sites.
THE reduction of gaseous oxides such as CO2 and H2O is an important concern in industrial processes and pollution control. Here we report the reduction of carbon dioxide to carbon with an efficiency of nearly 100% at 290 °C using cation-excess magnetite (Fe3+deltaO4, delta =0.127). In this reaction, the oxygen in the CO2 is transferred, in the form of O2-, to the cation-excess magnetite, and no gas is evolved. The carbon in the CO2 is reduced to carbon (zero valence) by the addition of an electron donated from the cation-excess magnetite to maintain electrical neutrality during the transfer of the O2- to the magnetite. When we used H2O in place of CO2, hydrogen gas was evolved, indicating that the same mechanism can also reduce H2O.
Adherent films of the gold-colored metal sulfides pyrite (FeS2), pyrrhotite (Fe1−xS), and chalcopyrite (CuFeS2) are known to exist on metallic and nonmetallic artifacts of Classic times. Consideration of the action of bacterial consortia containing sulfate reducing bacteria (SRB) during microbiologically influenced corrosion (MIC) and analyses of appropriate trajectories on Eh-pH stability diagrams indicate that these films can be understood either as a result of natural processes over archaeological times or of deliberate pseudogilding. Methods of discriminating natural sulfide deposition from pseudogilding are suggested. © 1993 John Wiley & Sons, Inc.
The hydraulic and geochemical performance of a 366 m long permeable reactive barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluated. The funnel and gate system, which was installed in 1996 to intercept and remediate ground water contaminated with chlorinated aliphatic hydrocarbons (CAHs), contained four 12.2 m wide gates filled with zero-valent iron. Ground water mounding on the upgradient side of the PRB resulted in a tenfold increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRB. Water balance calculations for April 1997 indicate that about 75 % of the ground water moving toward the PRB from upgradient areas moved through the gates. The rest of the water either accumulated on the upgradient side of the PRB or bypassed the PRB. Chemical data from monitoring wells screened down-gradient, beneath, and at the ends of the PRB indicate that contaminants had not bypassed the PRB, except in a few isolated areas. Greater than 99 % of the CAH mass entering the gates was retained by the iron. Fifty-one percent of the CAH carbon entering one gate was accounted for in dissolved C1 and C2 hydrocarbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates displaced contaminated ground water at a distance of at least 3 m downgradient from the PRB by the end of 1997. Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate could reduce gate porosity by about 0.35 % per year. Results from this study indicate that funnel and gate systems containing zero-valent iron can effectively treat ground water contaminated with CAHs. However, the hydrologic impacts of the PRB on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.