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Evaluation of the Effects of Shaking Intensity on the Process of Methylene Blue Discoloration by Metallic Iron
Abstract
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 (Fe(0)). 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 (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). 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) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 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 >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).
... The extent and kinetics of Fe 0 oxidative dissolution are the result of the following: (i) the nature of metal (intrinsic reactivity), and (ii) the interactions between Fe 0 and the environment in which it is placed [76,[122][123][124][125]. Therefore, (i) a change of material, and/or (ii) a change in environment results in changes in the rate and the extent of corrosion. ...
... However, shortterm laboratory experiments are always a simplification and this should be borne in mind when interpreting achieved results [76,129,131,132]. Given the diversity of operational parameters that have been proven to influence iron corrosion from individual studies, one can be overwhelmed by their number and the fact that each material is unique in its corrosion behaviour [11,124,125,133]. Therefore, a first attempt toward a systematic investigation of relevant influencing factors goes through the consideration of the electrochemical nature of aqueous iron corrosion. ...
... Then and now, most research groups consider the shaking intensity as a relevant operational parameter to be investigated almost in all instances without a quiescent system as a reference. However, there is no given convincing reason for the chosen mixing intensities tested in these experiments [124,125]. ...
A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe 0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe 0 specimens for water treatment are typically small in size. Before the advent of this technology and it application for environmental remediation, such small Fe 0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe 0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe 0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe 0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe 0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe 0 technology are demonstrated. The misuse of the terms "reactivity", and "efficiency", and adsorption kinetics and isotherm models for Fe 0 systems is also elucidated. The immense importance of Fe 0 /H 2 O systems in
... Under groundwater flow condition, advection is dominant near the oxide film layer but large flow scale is restricted near the Fe 0 surface due to presence of tubercle. Tubercle are mounds of rust formation due to corrosion (Sarin et al. 2004a) and thereby turbulence is absent in the vicinity of Fe 0 surface (Noubactep 2009b, Noubactep et al 2009c, Noubactep 2010b, Noubactep 2012b. The migration of the contaminants across the oxide film is mainly due to molecular diffusion and electro-migration depending on the pore structure and tortuosity (film permeability) (van der Kamp et al. 1996, Nordsveen et al. 2003. ...
... Shaking certainly accelerates processes leading to dye discoloration by adsorption and co-precipitation. In earlier experimental works the critical shaking intensity enabling an undisturbed formation of an oxide-scale on Fe 0 was found to be 50 rpm (Kurth 2008, Noubactep 2008a, 2008b, Noubactep et al. 2009c. Herein a shaking intensity of 75 rpm is used as it was demonstrated to yield reproducible results (Miyajima 2012). ...
... The main reason for this is the duration of the Master thesis and the number of opportunities to investigate. It should be delineated that the large majority of batch experiments reported in the literature are performed under more vigorous shaking speeds and for experimental durations rarely exceeding two days (Noubactep 2009a, Noubactep et al. 2009c, Tepong-Tsindé et al. 2015a, Noubactep 2015c. For the used shaking intensity, 75 rpm, the process of oxide scale formation on Fe 0 is not significantly disturbed (conditions in nature are nearly reproduced) (Miyajima 2012). ...
Elemental iron (Fe0) has been successfully tested and used for water treatment for decades due to its worldwide availability and inexpensiveness. Fe0-based filtration technology has been used for (i) environmental remediation (e.g. subsurface permeable reactive barriers) (ii) wastewater treatment and (iii) safe drinking provision. The evidence that Fe0 oxidative dissolution and subsequent precipitation at pH > 4.0 is a volumetric expansive process (Voxide > Viron) implies that Fe0 should be amended with non-expansive aggregates such as activated carbon, manganese oxides, pumice or sand. Only such hybrid systems are likely to be sustainable.
The present work focuses on the characterization of the ion selective nature of Fe0-based filters using three azo dyes: methylene blue (cationic), Orange II and Reactive Red 120 (anionic). The dyes are used as indicators for the reactivity of Fe0/H2O system in both batch and column experiments. The idea is to demonstrate that downwards from a Fe0 / sand system; available aggregates are in-situ coated with iron oxide, such that in the medium to long term, the whole system is ion-selective. The selectivity being fixed by positively charged iron oxides.
The characterization of the Fe0/H2O system is realized herein by amending (i) Fe0 with sand and MnO2 in batch experiments and (ii) Fe0 with sand in column experiments. Sand is a pure adsorbent with a negatively charged surface while MnO2 is reactive in nature. MnO2 addition enables the control of the availability of in-situ generated iron corrosion products and thus the role of corrosion product in the process of contaminant removal. The investigated systems in batch mode are (i) pure sand, (ii) pure MnO2, (iii) pure Fe0, (iv) Fe0/sand mixture, (v) Fe0/MnO2 mixt ure and (vi) Fe0/sand/MnO2 mixtures with various amounts of sand and MnO2 loadings. Column experiments were performed with the following systems: (i) pure sand (0 % Fe0), (ii) pure Fe0 (100 % Fe0), and (iii) Fe0/sand (50 % Fe0- vol/vol).
Results of batch experiment showed that sand is a good adsorbent for MB and has negligible effect on anionic dyes. MnO2 also favors MB discoloration. Pure Fe0 favors discoloration of both cationic and anionic dyes but shows best discoloration efficiency for Orange II. Among the Fe0 amended systems, the Fe0/sand system is most efficient for dye discoloration. The discoloration efficiency in Fe0-based systems is 75 % for MB and > 95 % for Orange II and RR120. Results confirmed quantitative adsorptive MB discoloration and negligible adsorption of anionic dyes on negatively charged sand. Quantitative discoloration of the anionic dyes (Orange II and RR120) in Fe0-based systems was attributed to high affinities of both species to positively charged iron corrosion products. The ion selective nature of the Fe0/H2O system is elegantly demonstrated.
... In particular, mixing of the solution should neither delay nor prevent the formation of an oxide-film in the vicinity of the Fe 0 surface [16,17]. This aspect of mixing has been mostly overseen since mixing is essentially used as a tool to accelerate contaminant transport to Fe 0 surface [18,19]. This example illustrates the necessity of exploring and/or revisiting the influence of operational parameters on the processes of iron dissolution which is coupled to contaminant removal. ...
... Such mixing may allow contaminant transport to the Fe 0 /H 2 O interface, an interface which can not exist in nature [24,63,65]. On this basis it can be argued that sample mixing and agitation may yield unrealistic results and should therefore be avoided when testing the reactivity of Fe 0 materials for commercial use in reactive barriers [18,19]. Note that all types of mixing devices can be used for above ground water treatment systems using Fe 0 . ...
... However, mixing always increases iron dissolution and the Fe 0 surface is permanently covered with corrosion products. Therefore, it may be advantageous to conduct initial work under stagnant conditions and progressively increase the mixing intensity to discover which mixing speeds can be used without major iron precipitation interference [19]. Clearly, works investigating the same process can only be comparable if conducted under similar τ EDTA conditions. ...
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.
... The material is available as fillings with a particle size between 0. 3 for MB. In particular, discoloration agents are progressively generated in-situ [41,42]. ...
... Usually these experiments are performed under mixing conditions (mixing type, mixing intensity) which are no relevant for experimental situations [45]. Recent works have shown that while using the experimental design of the present study, the shaking intensity should not exceed 50 rpm [23,41,42] to be relevant for field situations. ...
... MB discoloration as used in this work is not interchangeable with MB removal as the DOC values were not characterized. However, from a pure thermodynamic perspective MB is not redox sensitive in Fe 0 /H 2 O systems[41,42] (see section 2). ...
The influence of granular sand on the efficiency of metallic iron (Fe0) for the discoloration of a methylene blue (MB) solution was investigated in the current work. The initial MB concentration was 10 mg L−1 and mass loadings within the range of 0–90 g L−1 for sand and 0–45 g L−1 for Fe0 were applied. The batch reaction vessel used was a graduated essay tube containing 22.0 mL of the MB solution. Shaking intensities of 0 and 75.0 rpm were applied for experimental durations of 7, 21 and 45 days. Results provide clear evidence that both Fe0 and sand were independently effective for the discoloration of MB. However, the latter material was significantly less effective, recording 54.0% compared to 82.0% recorded for the Fe0 after 45 days in experiment with 45.0 g L−1 of each material. Similarly, mixing 90 g L−1 sand with 45.0 g L−1 of Fe0 depicted a MB discoloration efficacy of 72.0% demonstrating that the discoloration capability of the Fe0 was significantly ‘masked’ by the presence of sand. This observation provides clear evidence to question the common approach of using adsorbents for contaminant accumulation in the vicinity of Fe0 materials in order to facilitate chemical reduction by Fe0. Further research is required to determine the relative affinity of different materials that can be used in Fe0 mixtures for maximum contaminant removal efficacies.
... In particular, the observed lag time between Fe 0 supply and quantitative contaminant removal is a strong argument against direct reductive transformations (electrons from Fe 0 ). Evidently, the cited papers (Noubactep, 2007; Noubactep, 2008; Noubactep, 2010;) and related papers (Noubactep, 2009a-g; Noubactep and Schöner, 2009; Noubactep et al., 2009a; 2009b; Noubactep, 2010b; Noubactep and Caré, 2010a; Noubactep and Schöner, 2010a) are not clear enough in their explanation to convince many authors of current articles dealing with contaminant removal in Fe 0 /H 2 O systems (Elsner et al., 2007; Jeen et al., 2008; Katsoyiannis et al., 2008; Wang and Cheng, 2008; Kang and Choi, 2009; Tratnyek and Salter, 2010). An understanding of how aqueous contaminants are effectively removed in the presence of Fe 0 is essential for proper designing of the reactive ...
... Readers interested in more detail on the process of co-precipitation are encouraged to read 2 excellent works by Crawford et al. (1993a Crawford et al. ( , 1993b). It should be noted that: @BULLET Co-precipitation is a primarily unspecific removal mechanism that occurs whenever an abundant species (iron) precipitates (here as oxide) in the presence of trace amounts of foreign species (here contaminants) @BULLET Co-precipitated contaminants are not likely to be released into the environment unless iron oxides are dissolved (Stipp et al., 2002; Noubactep et al., 2003; Noubactep et al., 2006a; Noubactep et al., 2006b; Noubactep, 2009c; Noubactep et al., 2009a; Ghauch et al., 2010a). Therefore, although contaminants are not necessarily reduced in Fe 0 /H 2 O systems, they are successfully removed and strongly sequestrated in the matrix of iron oxides. ...
... This conclusion is supported by the observation that even bacteria (Hussam and Munir, 2007; Diao and Yao, 2009) and viruses (You et al., 2005) are successfully removed in Fe 0 /H 2 O systems. The latter observation has led to the suggestion of metallic iron as a universal filter material in small above-ground walls and household filters for treatment of waters of unknown quality (Noubactep and Woafo, 2008; Noubactep et al., 2009b; Noubactep and Schöner, 2010b). If successfully developed, this simple idea could enable universal access to safe drinking water in remote areas around the world. ...
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.
... A structural modification of the adsorbate at the solid/H 2 O interface or within the pores of the adsorbent for better packing with lateral interactions of adsorbate molecules is possible 23 . In natural ecosystems, adsorption competition between molecules of different natures and molecular weights should be additionally considered 24 . Finally, the surface interaction (whether physically or chemically) of the adsorbate at the adsorbent surface and the possible desorption should, moreover, be addressed. ...
... To expedite the mass transfer of MB to the sorbent surface, the influence of shaking rate, a hydrodynamic parameter, was then examined. Noubactep et al. investigated previously the impact of shaking rate between 0 and 300 min −1 on the adsorption of MB onto scrap iron (Fe 0 ), granular activated carbon (GAC), and deep-sea manganese nodules (MnO 2 ) 24 . While non-shaken experiments lasted for up to 50 days, shaken experiments were executed in less than a day. ...
Phytoremediation is a promising, cost-effective, and eco-friendly process for wastewater treatment. Herein, the dry biomasses of Vossia cuspidata (Roxb.) Griff. leaves (PL) and rhizomes including aerial stems (PR) were used to effectively remediate methylene blue (MB) dyes. Interestingly, the adsorption uptake and removal efficiency of MB by PR were higher than those of PL; exceeding 97 and 91% in 35 and 25 min for 0.1 and 0.4 g/L MB, respectively. The MB diffusion within the PL and PR was insignificant and the adsorption kinetics was principally controlled by the surface MB–adsorbent interaction, as consistently approved by the pseudo-second order kinetic model. In addition, the adsorption increased rapidly with the plant dosage with high dependence on the initial MB concentration. Moreover, the impact of shaking speed on the adsorption was minor but temperature played a critical role where the highest efficiencies were recorded at 30 and 40 °C on PL (91.9%) and PR (93.3%), respectively. The best removal efficiencies were attained with PR at pH 6, but with PL at pH 8. The Temkin isotherm could perfectly simulate the experimental data (R² > 0.97); suggesting a linear decrease of the adsorption heat of MB with the plant coverage.
... (3) "Redox condition": During the weathering process, the occurrences of olivine (Fe 2+ ) and birnessite (Mn 4+ ) indicate reduction and oxidation environments, respectively. The two oxidation states of Mn (Mn 2+ and Mn 4+ ) and Fe (Fe 2+ and Fe 3+ ), which are common states in natural systems, can serve as redox indicator in a various of geochemical systems (e.g., Noubactep et al., 2009;Schaller and Wehrli, 1997). ...
... That is probably attributed to facilitating the formation of Fe 2+ to Fe 3+ by Mn 4+ oxide (birnessite) as a catalyst. That is also supported by the findings: (1) the oxidation of Fe 2+ by birnessite was thermodynamically feasible (Weast, 1981); and (2) birnessite, which is common in natural environments, played a catalytic role in promoting the transformation of Fe 2+ to Fe 3+ in a common pH range of soils and sediments (Krishnamurti and Huang, 1987;Noubactep et al., 2009). ...
Because ferromanganese oxides can host significant amounts of Ni during the weathering, the Fe- and Mn-oxide are resultantly considered as the important Ni-bearing minerals. However, little is known about the role of Mn species to influence the Ni behavior during this process. In this study, we employed the techniques of ICP-MS, XRF, XRD, XAFS (XANES and EXAFS), and μ-XRF to interpret the Ni behavior along a laterite profile in Myanmar. Nickel concentrations in the saprolite developed on peridotites can reach up to 11 wt% Ni mainly because of Ni precipitation as fracture-fillings and thin coatings on joints, which produce a promisingly and importantly economic mineral resource of this territory. In the peridotites, Fe and Mn species predominantly consisted of olivine (96%) and biotite (74%), respectively. Nickel XANES spectra indicated that Ni was primarily hosted by olivine (86%) in the protolith under reduction condition, lizardite (90%) and Ni-bearing goethite (10%) in the lower garnierite vein under strong oxidation condition, and lizardite (37%) and Ni-bearing goethite (63%) in the upper saprolite under medium oxidation condition. Accordingly, minerals of Fe and Mn were progressively oxidized toward Fe³⁺ and Mn⁴⁺, which occurred primarily as Fe³⁺-bearing goethite and Mn⁴⁺-bearing birnessite. The occurrence of Fe- and Mn-oxide plays an important role in Ni species and suggests that birnessite maybe facilitate the formation of Fe oxide precipitations by means of the oxidation of Fe²⁺ in the olivine to Fe³⁺ in the goethite. The results are of relevant importance, as it fills a gap in the knowledge of ore-formation processes occurring in Myanmar
... The initial pH value and the ionic strength were properly adjusted. Based on literature data [23,24,26] and preliminary studies, a stirring speed of 250 rpm was selected to maintain the GWS particles in suspension. At pre-determined times; 1 mL of the supernatant was extracted for Cr analysis. ...
... Given the huge number of parameters which have been reported to significantly influence adsorption processes [22,28,29], one could wonder how to rationally agree on such a relevant protocol. Experience from studies with metallic iron (a reactive material) for contaminant removal has suggested that non-disturbed batch experiments are the most reproducible designs [24]. Additionally for more comparable results, vessels of similar geometries and sizes should be used and should contain the same volume of the solution to be tested. ...
Researchers have been developing low-cost bio-adsorbents as alternative materials to conventional activated carbons for water treatment over the years. New materials are used either ‘as is’ or slightly chemically or physically modified. Their efficiency is mostly characterized in batch experiments with parameters like (i) adsorption capacity (mg/g), (ii) fitting of adsorption (e.g. Langmuir) and/or kinetics (e.g. first order) models, (iii) optimal experimental conditions (e.g. pH value, duration), (iv) reaction mechanism (e.g. adsorption vs. bioreduction), and (v) removal efficiency (%). The suitability of this approach is questioned in this communication using green walnut shell as adsorbent and CrVI as a model contaminant. It is shown that results from such experiments are highly qualitative and not each other comparable. The two main reasons are (i) the lack of a reference material and (ii) the lack of a standard experimental procedure. Based on the Bernoulli's principle, tools for more comparable results are discussed. The Bernoulli's principle suggests that batch experiments should be designed at constant solution pressure on adsorbing particles.
... However, considering the fact that Fe oxyhydroxides precipitate in the system as well, it is possible that contaminant removal at higher mixing intensities is associated with oxide precipitation and not with the iron surface. This conclusion is supported by recent data on methylene blue discoloration in Fe 0 /H 2 O systems [40,41]. Several studies have concluded that Fe 0 transformation reactions are either transport limited [42][43][44] or reaction limited [45,46]. ...
... This is the reason why more iron dissolved in system V (30 h) than in system VI (48 h). [41] suggested that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe 0 surface using material A should not exceed 50 min -1 . Based on this result, τ EDTA ≤ 90 h (3.75 d) can be adopted as a guide value for the investigation of mass transfer limited processes. ...
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.
... For example, Lackovic et al. [25] reported that the removal mechanism for arsenic contrasted with that of chlorinated hydrocarbons (reductive dechlorination) and hexavalent chromium (reductive precipitation), and involved either adsorption or co-precipitation on the iron surface. However, two important facts challenge the universal validity of the reductive transformation concept: (i) a quantitative removal of redox-insensitive compounds as triazoles [26], methylene blue [27] [28], or zinc [24] were reported, and (ii) Fe 0 /H 2 O systems have been reported to function as a Fenton-like system for the oxidation of several contaminants [29]. ...
... well established and generally accepted among the research community. " However, the validity of the reductive transformation concept was challenged, both theoretically [19] [20] [30] and experimentally [27] [28]. Furthermore, O'Hannesin and Gillham [13] reported that abiotic contaminant reduction coupled with metallic iron oxidation was a " broad consensus " . ...
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.
... A major problem of available data from batch studies is the poor comparability of results from different laboratories using different conditions. Most experimental conditions are not relevant for field situations [25,26]. For example, only shaking intensity lower than 50 m -1 could enable the formation of a universal oxide-scale in the vicinity of the Fe 0 surface as observed in column studies and in full-scale barriers [25]. ...
... Most experimental conditions are not relevant for field situations [25,26]. For example, only shaking intensity lower than 50 m -1 could enable the formation of a universal oxide-scale in the vicinity of the Fe 0 surface as observed in column studies and in full-scale barriers [25]. A careful look behind published data on laboratory column experiments ( [27][28][29][30][31][32]; see Tab. 1) also demonstrates large variability in the experimental design. ...
Despite the amount of data available on investigating the process of aqueous contaminant removal by metallic iron (Fe0), there is still a significant amount of uncertainty surrounding the design of Fe0 beds for laboratory testing to determine the suitability of Fe0 materials for field applications. Available data were obtained under various operating conditions (e.g. column characteristics, Fe0 characteristics, contaminant characteristics, oxygen availability, solution pH) and are hardly comparable to each other. The volumetric expansive nature of iron corrosion has been univocally reported as major drawback for Fe0 beds. Mixing Fe0 with inert materials has been discussed as an efficient tool to improve sustainability of Fe0 beds. This paper discusses some problems associated with the design of Fe0 beds and proposes a general approach for the characterization of Fe0 beds. Each Fe0 column should be characterized by its initial porosity, the composition of the steady phase and the volumetric proportion of individual materials. Used materials should be characterized by their density, porosity, and particle size. This work has introduced simple and reliable mathematical equations for column design, which include the normalisation of raw experimental data prior to any data treatment.
... The voluminous literature on "remediation with corroding iron" is characterized by the overwhelming number of parameters which have been shown to affect the process of aqueous contaminant removal in the presence Fe 0202122232425. These parameters include the nature of the contaminant, the pH of the solution, the nature of Fe 0 (e.g. ...
... The importance of all these factors was traceably demonstrated from isolated sets of experiments. However, due to lack of a standard experimental protocol, available results could be collectively regarded as qualitative as they are not comparable to each other22232425. Taken together, results from Fe 0 -based filters (field walls and household filters) demonstrate the suitability of Fe 0 beds for the removal of all possible contaminants from water. ...
The goal of water treatment in water works is safe drinking water production. Various technological options are available to this end. However, many conventional water treatment technologies are too expensive for extensive deployment in rural communities worldwide. Research over the past two decades has demonstrated the efficiency of metallic iron (Fe0) for the aqueous removal of a wide range of chemical and microbial contaminants (e.g. bacteria, chlorinated organics, dyes, emerging contaminants, heavy metals, radionuclides, viruses). The prevailing concept considers that the mechanism of Fe0 remediation varies depending on the contaminant of interest. This concept was recently revisited and Fe0 was proven an universal material for water treatment. As a consequence Fe0 filtration beds are proposed in this communication to replace ultra-filtration, nano-filtration, and disinfection units in water works. It is anticipated that the success of Fe0 filtration beds in producing safe drinking water in large scale will depend on the ability of researchers to produce adequate reactive materials. Target experimental work is needed to confirm and extend the applicability of this affordable method.
... For example, Lackovic et al. [25] reported that the removal mechanism for arsenic contrasted with that of chlorinated hydrocarbons (reductive dechlorination) and hexavalent chromium (reductive precipitation), and involved either adsorption or co-precipitation on the iron surface. However, two important facts challenge the universal validity of the reductive transformation concept: (i) a quantitative removal of redoxinsensitive compounds as triazoles [26], methylene blue [27,28], or zinc [24] were reported, and (ii) Fe 0 /H 2 O systems have been reported to function as a Fenton-like system for the oxidation of several contaminants [29]. A survey of the spectrum of efficiently removed species (oxidable , reducible and redox-insensitive) suggests that some removal mechanisms may be universal while others are specific. ...
... For example, Kang and Choi [33] stated that questioning the premise of reductive transformation is " hardly acceptable since the role of the direct electron transfer in Fe 0 -mediated reactions is well established and generally accepted among the research community. " However, the validity of the reductive transformation concept was challenged, both theoretically [19,20,30] and experimentally [27,28]. Furthermore, O'Hannesin and Gillham [13] reported that abiotic contaminant reduction coupled with metallic iron oxidation was a " broad consensus " . ...
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.
... However, considering the fact that Fe oxyhydroxides precipitate in the system as well, it is possible that contaminant removal at higher mixing intensities is associated with oxide precipitation and not with the iron surface. This conclusion is supported by recent data on methylene blue discoloration in Fe 0 /H 2 O systems [40,41]. Several studies have concluded that Fe 0 transformation reactions are either transport limited424344 or reaction limited [45,46]. ...
... The results of such concerted investigations could be critical EDTA values (guide values) at which specific experiments have to be performed. For example, results of Noubactep et al. [41] suggested that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe 0 surface using material A should not exceed 50 min −1 . Based on this result, EDTA ≤ 90 h (3.75 days) can be adopted as a guide value for the investigation of mass-transfer limited processes. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe(0)/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 Fe(0)-materials (materials 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 Fe(0)/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 (k(EDTA)). 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 k(EDTA) 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 Fe(0) surface.
... Depending on the film permeability structure, the contamination migrates because of molecular diffusion and electromigration (van der Kamp et al. 1996, Nordsveen et al. 2003. The process of iron corrosion (1) produces Fe 2+ ions (Stratmann and Müller 1994), (2) exhaust O2, H + ions, in addition to the contamination near the surface of Fe 0 , and (3) causes a change in concentration between the contaminants and particles crossways the oxide film and create the molecular diffusion (Noubactep 2008b, 2009a, Noubactep 2009b, Noubactep et al 2009b, Noubactep et al 2009c, Noubactep 2010b, Gunawardana et al. 2011, Noubactep 2012b. The oxide film is considered as an electron conductor, through which electrons are transferred from Fe 0 surface to the bulk solution and contaminants. ...
The concept that metallic iron (Fe0) is a reducing agent under environmental conditions has urged the large-scale application of Fe0 filters for environmental remediation and water treatment. During the past two decades, some 3,000 scientific articles have widely discussed the importance of processes yielding water treatment in Fe0/H2O systems. The current state-of-the-art knowledge is that Fe0 is the generator of (i) contaminant scavengers (iron hydroxides/oxides), and (ii) reducing agents (e.g. H/H2, FeII, green rusts, Fe3O4). In other words, contaminant reductive transformation in the presence of Fe0 is not mediated by electrons from the metal body (direct reduction).
The realization that Fe0 is not the reducing agent in Fe0/H2O systems has redirected fundamental researches on the operating mode of Fe0 filters. In this effort, a cationic azo dye (methylene blue, MB) has been presented as reactivity indicator to characterize changes in Fe0/H2O systems. The present study investigates the impact of contact time on the efficiency of Fe0/H2O systems. The research questions are "is there any direct relationship between experimental duration and system's efficiency?" If yes, how is the efficiency modified in the presence of natural additives such as manganese oxides (MnO2) and sand? Both research questions are justified by the evidence that the Fe0 surface is constantly shielded by an oxide scale which has been reported to mediate a 'reactivity loss' of Fe0 materials.
The methodology consists of (i) varying the experimental duration, and (ii) modifying the Fe0/H2O system by amending it with various amounts of MnO2 and sand. The efficiency of Fe0/sand/MnO2 systems for water treatment is characterized using methylene blue (MB) as reactivity indicator. Batch experiments using various weight ratios of Fe0 and the two additives were performed for up to six weeks (47 days). The impact of the intrinsic reactivity of MnO2 was characterized by using different types of MnO2. The MB discoloration process was investigated both under shaking and non-disturbed conditions.
The results clearly demonstrate the impact of increased contact time on the extent of MB discoloration in all tested systems (Fe0, Fe0/sand, Fe0/MnO2 and Fe0/sand/MnO2). As a rule, MB discoloration was improved by increased experimental duration. It was noted that the extent of MB discoloration is influenced by the diffusive (or advective) transport of MB from the solution to the reactive materials at the bottom of the test tubes. Without shaking, more time is needed for the transport of MB to the particles of tested materials. For experiments lasting for longer times, sand addition prevented Fe0 particles from compaction (cementation) at the bottom of the test-tubes. This enabled the long-term generation of iron hydroxides (new iron corrosion products) for the discoloration of MB by adsorption and co-precipitation. The same observation was made for Fe0/MnO2 systems. In other words, the addition of non-expansive materials (e.g. MnO2, sand) is necessary to sustain the efficiency of Fe0 filters. Shaking the test tubes increased the extent of MB discoloration by two different mechanisms: (i) speeding up the mass transport of MB solution towards the adsorptive materials, and (ii) speeding up the kinetics of Fe0 corrosion, creating new corrosion products. Discoloration processes occur due to MB diffusion and advection which are accelerated during the shaking operation. The results clearly delineate the complexity of the Fe0/MnO2/sand system and suggest that varying the experimental conditions will give more opportunities to discuss the efficiency of Fe0/H2O systems.
... Moreover, future research should extend the MB method to the optimization of Fe 0 design parameters such as mixing methods/intensities, temperature, flow rate, and chemical water composition. To date, Noubactep et al. (2009b) and Tepong-Tsindé et al. (2015) have already tested the influence of shaking intensity in batch studies and Clions respectively on the efficiency of ...
An innovative approach to characterize the reactivity of metallic iron (Fe0) for aqueous contaminant removal has been in use for a decade: The methylene blue method (MB method). The approach considers the differential adsorptive affinity of methylene blue (MB) for sand and iron oxides. The MB method characterizes MB discoloration by sand as it is progressively coated by in-situ generated iron corrosion products (FeCPs) to deduce the extent of iron corrosion. The MB method is a semi-quantitative tool that has successfully clarified some contradicting reports on the Fe 0 /H2O system. Moreover, it has the potential to serve as a powerful tool for routine tests in the Fe 0 remediation industry, including quality assurance and quality control (QA/QC). However, MB is widely used as a 'molecular probe' to characterize the Fe 0 /H2O system, for instance for wastewater treatment. Thus, there is scope to avoid confusion created by the multiple uses of MB in Fe 0 /H2O systems. The present communication aims at filling this gap by presenting the science of the MB method, and its application and limitations. It is concluded that the MB method is very suitable for Fe 0 material screening and optimization of operational designs. However, the MB method only provides semi-quantitative information, but gives no data on the solid-phase characterization of solid Fe 0 and its reaction products. In other words, further comprehensive investigations with microscopic and spectroscopic surface and solid-state analyses are needed to complement results from the MB method.
... In this decade, the use of single-walled carbon nanotubes (SWCNTs) and nanoscale of zero valent iron (NZVI) as a reactive medium for the treatment of toxic chemicals is one of the most significant techniques and has attracted a lot of attention because the iron metal is of low cost, low toxicity, is easy to get, and has good effectiveness and ability to degrading contaminants. Laboratory studies have demonstrated that SWCNTs and NZVI surfaces as adsorbents can effectively transform chlorinated solvents, organochlorine pesticides, organic dyes and heavy metals [4][5][6][7][8][9][10][11] into nontoxic forms. Nanoscale of zero valent iron has drawn great attention as an inexpensive and environmentally friendly strong reducing agent [12,13]. ...
The adsorption of ethidium bromide (EtBr) by single-walled carbon nanotubes (SWCNTs) and nanoscale of zero valent iron (NZVI) were investigated to assess its possible use as adsorbents. The effect of various factors, namely initial adsorbate concentration, adsorbent dosage, and contact time, were studied to identify adsorption capacity of SWCNTs and NZVI surfaces. The experiment demonstrated the maximum EtBr which was obtained at 5 min to attain equilibrium for SWCNTs and NZVI surfaces. Adsorption data were modeled with the Langmuir, Freundlichand, Temkin isotherms. Langmuir adsorption model was used for the mathematical description of the adsorption equilibrium, and the equilibrium data fitted very well with this model for both surfaces as adsorbents. The study showed that SWCNTs and NZVI surfaces could be used as new and efficient adsorbent materials for the removal of EtBr from aqueous solution. Also, the result showed that the SWCNTs were more effective than NZVI in the removal of EtBr from aqueous solution.
... The aim of the study was to sustain Fe 0 reactivity upon nitrate reductive degradation. Their results showed that ultrasound, used alone (US/H 2 suggest in agreement with well documented results from the corrosion science, that Fe 0 is oxidized by acidic dissolution [5]. Moreover this large molar ratio indicate that enough iron oxides was generated in the system to co-precipitate possibly reduced NO 3 -. ...
... The concept that contaminants are fundamentally removed by adsorption and co-precipitation is consistent with many experimental observations which remained non-elucidated by the reductive transformation concept [11,12]. Although researchers are continuing to maintain the validity of the latter concept [24][25][26], the new concept was validated [27,28] and has been independently verified [29,30]. As a matter of course the concept of adsorption/co- ...
Remediation of contaminated groundwater is an expensive and lengthy process. Permeable reactive barrier
of metallic iron (Fe0 PRB) is one of the leading technologies for groundwater remediation. One of
the primary challenges for the Fe0 PRB technology is to appropriately size the reactive barrier (length,
width, Fe0 proportion and nature of additive materials) to enable sufficient residence time for effective
remediation. The size of a given Fe0 PRB depends mostly on accurate characterization of: (i) reaction
mechanisms and (ii) site-specific hydrogeologic parameters. Accordingly, the recent revision of the fundamental
mechanisms of contaminant removal in Fe0/H2O systems requires the revision of the Fe0 PRB
dimensioning strategy. Contaminants are basically removed by adsorption, co-precipitation and size
exclusion in the entire Fe0 bed and not by chemical reduction at a moving reaction front. Principle calculations
and analysis of data from all fields using water filtration on Fe0 bed demonstrated that: (i)
mixing Fe0 and inert additives is a prerequisite for sustainability, (ii) used Fe0 amounts must represent
30–60 vol.% of the mixture, and (iii) Fe0 beds are deep-bed filtration systems. The major output of this
study is that thicker barriers are needed for long service life. Fe0 filters for save drinking water production
should use several filters in series to achieve the treatment goal. In all cases proper material selection is
an essential issue.
... agitation, stirring, vibration) [36,37]. However, one should acknowledge that such mixing operations are not applicable to packed beds and field reactive walls [25,36,38]. As discussed in details elsewhere [25], the use of various mixing systems with the resulting mixing intensities and their impact on the process of contaminant removal in the presence of Fe 0 is the main reason why the inconsistent concept of reductive transformation has survived for more than a decade. ...
The further development of Fe0-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe0/H2O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both process is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe0 as remediation agent compared to thermodynamically more favourable Al0 and Zn0. The validity of the adsorption/co-precipitation concept is corroborated.
... (iii) The adsorption/co-precipitation concept has been validated by Noubactep using methylene blue (C 16 H 18 N 3 SCl) as model contaminant [29,30]. The model was further independently confirmed using clofibric acid (C 10 H 11 ClO 3 ) and diclofenac (C 14 H 11 Cl 2 NO 2 ) [31,32]. ...
The author used a recent article on lindane (γ-hexachloro-cyclohexane) reductive dechlorination by Fe/Pd bimetallics to point out that many other of published works in several journals do not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe0/H2O systems). It is the author’s view that the contribution of adsorbed FeII to the process of contaminant reduction has been neglected while discussing the entire process of contaminant reduction in the presence of bimetallics.
... In the last decade, zero-valent iron (ZVI) has been increasingly used in ground water remediation and hazardous waste treatment. Laboratory studies have demonstrated that ZVI can effectively transform chlorinated solvents, organochlorine pesticides, PCBs, organic dyes and heavy metals [2][3][4][5][6][7][8][9]into nontoxic forms. These zero-valent iron (ZVI) materials were proposed as a reactive material in permeable reactive barriers (PRBs) due to its great ability in reducing and stabilizing different types of pollutants [10][11][12]. ...
This study reports the synthesis, characterisation and application of nano-zero-valent iron (nZVI). The nZVI was produced by a reduction method and compared with commercial available ZVI powder for Pb2+ removal from aqueous phase. Comparing with commercial ZVI, the laboratory made nZVI powder has a much higher specific surface area. XRD patterns have revealed zero-valent iron phases in two ZVI materials. Different morphologies have been observed using SEM and TEM techniques. EDX spectrums revealed even distribution of Pb on surface after reaction. The XPS analysis has confirmed that immobilized lead was present in its zero-valent and bivalent forms. ‘Core–shell’ structure of prepared ZVI was revealed based on combination of XRD and XPS characterisations. In addition, comparing with Fluka ZVI, this lab made nZVI has much higher reactivity towards Pb2+ and within just 15min 99.9% removal can be reached. This synthesized nano-ZVI material has shown great potential for heavy metal immobilization from wastewater.
... Moreover, it is difficult to relate available information to design criteria because 2 experiments are often designed with own past experience or rules of thumb [15,17,57] There are repeated claims that the adsorption/co-precipitation concept [27,41,42] for contaminant removal in Fe 0 /H 2 O systems has been introduced by prolifically refuting extensive work validating the still prevailing reductive degradation/precipitation concept [58,59] without any original experimental work [60][61][62][63][64]. Thus, the adsorption/co-precipitation concept has been mostly ignored by the scientific community. Unfortunately, this argumentation ignored seven important facts: (i) the reductive degradation/precipitation concept has never been univocally accepted [39,[65][66][67], (ii) the author of the adsorption/coprecipitation concept has initially published on the reductive precipitation by Fe 0 using uranium as model contaminant [68], (iii) intensive work with methylene blue as model contaminant has disproved the reductive degradation/precipitation concept [69][70][71], (iv) apart from Ghauch and his colleagues [61] no other researcher or research group who has initially criticized the adsorption/co-precipitation concept has poised to test it, (v) Ghauch et al. [43][44][45] have validated the adsorption/co-precipitation concept using various organic pollutants including clofibric acid and diclofenac, (vi) chemical reduction is definitively not a stand alone removal mechanism for any contaminant [17,28,30,32,47], and (vii) the incriminated prolific literature is a peer-reviewed one. To sum up, the adsorption/co-precipitation concept is currently dismissed because the reductive degradation/precipitation concept is widely accepted [62] or because no laboratory or field work with chlorinated organic compounds has been published to support it [63,64]. ...
Metallic iron (Fe0) is often reported as a reducing agent for environmental remediation. There is still controversy as to whether Fe0 plays any significant direct role in the process of contaminant reductive transformation. The view that Fe0 is mostly a generator of reducing agents (e.g. H, H2 and FeII) and Fe oxyhydroxides has been either severely refuted or just tolerated. The tolerance is based on the simplification that, without Fe0, no secondary reducing agents could be available. Accordingly, Fe0 serves as the original source of electron donors (including H, H2 and FeII). The objective of this communication is to refute the named simplification and establish that quantitative reduction results from secondary reducing agents. For this purpose, reports on aqueous contaminant removal by Al0, Fe0 and Zn0 are comparatively discussed. Results indicated that reduction may be quantitative in aqueous systems containing Fe0 and Zn0 while no significant reduction is observed in Al0/H2O systems. Given that Al0 is a stronger reducing agent than Fe0 and Zn0, it is concluded that contaminant reduction in Fe0/H2O systems results from synergic interactions between H/H2 and FeII within porous Fe oxyhydroxides. This conclusion corroborates the operating mode of Fe0 bimetallics as H/H2 producing systems for indirect contaminant reduction.
... Noubactep et al. compared the shaking intensity under various conditions for the purpose of discoloring methylene blue with metallic iron, where it was determined that the materials considered were scrap iron, granular activated carbon, and deep sea manganese nodules. From fifty days of experimentation, discoloration of methylene blue is related to shaking intensity, where the peak shaking intensity is 50 min −1 [92]. Liu et al. was capable of degrading 2,2,',4,4'-tetrabromodiphenye ether (BDE, 47) consisting of soil-washing and photodestruction processes within non-ionic surfactants under UV-irradation at 253.7 nm. ...
A review of the literature published from 2008 to 2010 on topics related to chemicals and allied products is presented. The review considered several sections such as waste management, physicochemical treatment, aerobic treatment, anaerobic treatment, air emissions, soils and groundwater, and reuse.
... The concept that contaminants are fundamentally removed by adsorption and co-precipitation is consistent with many experimental observations which remained non-elucidated by the reductive transformation concept [11,12]. Although researchers are continuing to maintain the validity of the latter concept242526, the new concept was validated [27,28] and has been independently verified [29,30] . As a matter of course the concept of adsorption/coprecipitation (and size exclusion for packed bed) should have been challenged by researchers working on remediation in Fe 0 /H 2 O systems . ...
Remediation of contaminated groundwater is an expensive and lengthy process. Permeable reactive barrier of metallic iron (Fe0 PRB) is one of the leading technologies for groundwater remediation. One of the primary challenges for the Fe0 PRB technology is to appropriately size the reactive barrier (length, width, Fe0 proportion and nature of additive materials) to enable sufficient residence time for effective remediation. The size of a given Fe0 PRB depends mostly on accurate characterization of: (i) reaction mechanisms and (ii) site-specific hydrogeologic parameters. Accordingly, the recent revision of the fundamental mechanisms of contaminant removal in Fe0/H2O systems requires the revision of the Fe0 PRB dimensioning strategy. Contaminants are basically removed by adsorption, co-precipitation and size exclusion in the entire Fe0 bed and not by chemical reduction at a moving reaction front. Principle calculations and analysis of data from all fields using water filtration on Fe0 bed demonstrated that: (i) mixing Fe0 and inert additives is a prerequisite for sustainability, (ii) used Fe0 amounts must represent 30–60 vol.% of the mixture, and (iii) Fe0 beds are deep-bed filtration systems. The major output of this study is that thicker barriers are needed for long service life. Fe0 filters for save drinking water production should use several filters in series to achieve the treatment goal. In all cases proper material selection is an essential issue.
... To determine effects of temperature on the chlorophenol removal rate, temperature-controlled batch experiments were carried out at 5 • C, 20 • C, 40 • C, and 60 • C. Metalfree control flask containing only the solution was prepared for each batch experiment. All flasks were placed in water bath shakers and shaken at 240 rpm (to dismiss the limitation by external diffusion) [22]. ...
The [Ni|Cu] microcell was prepared by mixing the Ni(0) and Cu(0) particles. The composition and crystal form were characterized by X-ray diffraction (XRD) and scanning electron microscope. The results evidenced the zero-valence metals Ni and Cu were exposed on the surface of particles mixture. The [Ni|Cu] microcell was employed to decompose chlorophenols in aqueous solution by reductive dechlorination. The dechlorination rates of chlorophenols by [Ni|Cu] were >10 times faster than those by [Fe|Cu], [Zn|Cu], [Sn|Cu], and [Fe|Ni] mixtures under the same conditions. [Ni|Cu] is different from other zero valent metals (ZVMs) in that it performed the best at neutral pH. The main products of chlorophenol dechlorination were cyclohexanol and cyclohexanone. The reduction kinetics was between pseudo zero-order and first-order, depending on the pH, concentration, and temperature. These results, combined with electrochemical analysis, suggested that Ni(0) acted as a reductant and catalyst in dechlorination reaction. The H* corridor mechanism from Ni(0) to Cu(0) was also proposed based on hydrogen spillover. The inhibition on the release of Ni(2+) by adding natural organic matters and adjusting pH was investigated.
... (iii) The adsorption/co-precipitation concept has been validated by Noubactep using methylene blue (C 16 H 18 N 3 SCl) as model contaminant [29,30]. The model was further independently confirmed using clofibric acid (C 10 H 11 ClO 3 ) and diclofenac (C 14 H 11 Cl 2 NO 2 ) [31,32]. ...
The author used a recent article on lindane (γ-hexachloro-cyclohexane) reductive dechlorination by Fe/Pd bimetallics to point out that many other of published works in several journals do not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe(0)/H(2)O systems). It is the author's view that the contribution of adsorbed Fe(II) to the process of contaminant reduction has been neglected while discussing the entire process of contaminant reduction in the presence of bimetallics.
... Thus, acceleration the corrosion rate of Fe 0 is one of the challenges for its utilization for high concentrations of As removal. To accelerate the corrosion of Fe 0 , a range of methods have been reported, such as using bimetallics, ultrasound, manganese oxides and activated carbon to combine with Fe 0 [9,22]. Among which activated carbon was cheap and commonly used as adsorbent. ...
Galvanic couples composed of zero-valent iron and activated carbon (Fe(0)/AC) were investigated for As(V) removal from water. The effects of Fe(0) to AC mass ratio (FCR), solution pH, ionic strength and co-existing anions (phosphate, carbonate, silicate, nitrate, chloride and sulfate) and humic acid (HA) on As(V) removal were evaluated. The results showed that the optimum mass ratio was 1:1, and Fe(0)/AC with this ratio was more effective for As(V) removal than Fe(0) and AC alone at pH of 7 and ion strength of 0.03 M NaCl. The enhanced performance for As(V) removal was fulfilled through an accelerated corrosion process of Fe(0), which meant more corrosion products for efficient As(V) removal. The As(V) removal followed a pseudo-first order reaction. The rate constants (k) for 1:1 Fe(0)/AC and Fe(0) alone were 0.802 and 0.330 h(-1), respectively. Potentiodynamic polarization scans further confirmed that Fe(0) corrosion was promoted when Fe(0) was coupled with AC. Except silicates, other co-existing anions promoted As(V) removal. No reduction form of As (As(III) or As(0)) could be detected on iron corrosion products (ICPs) and in solutions. Identified ICPs included poorly crystallized lepidocrocite (gamma-FeOOH) and magnetite/maghemite (Fe(3)O(4)/gamma-Fe(2)O(3)) for both of Fe(0)/AC and Fe(0) systems. In conclusion, the Fe(0)/AC couple exhibited higher As removal performance than that of Fe(0) alone from water.
... agitation, stirring, vibration) [36,37] . However, one should acknowledge that such mixing operations are not applicable to packed beds and field reactive walls [25,36,38]. As discussed in details elsewhere [25] , the use of various mixing systems with the resulting mixing intensities and their impact on the process of contaminant removal in the presence of Fe 0 is the main reason why the inconsistent concept of reductive transformation has survived for more than a decade. ...
The further development of Fe(0)-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe(0)/H(2)O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both processes is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe(0) as remediation agent compared to thermodynamically more favourable Al(0) and Zn(0). The validity of the adsorption/co-precipitation concept is corroborated.
... Obviously, the used power and the frequency of US radiation were too low to induce noticeable NO 3 − transport within 4 h. The combined effect of the mixing intensity and the reaction time were comprehensively discussed by Noubactep et al. [2]. Second, Tsai et al. [1] intended to " investigate the effects of ultrasound and pH on the destruction of passive oxide film " . ...
This thesis deals with the use of metallic iron (Fe0) for water treatment in general and the use of Fe0 for safe drinking water production in particular. The provision with safe drinking water is a real problem for 800 millions of people all over the world.Chapter 1 presents the concept of water treatment with Fe0 in a broader scientific context and
reveals research needs. Chapter 2 presents the 21 peer-reviewed journal articles on which the thesis is based in relation to their contribution to solve the problems from Chapter 1. Chapter 3 presents the same articles in the perspective of using Fe0 for safe drinking water production.Chapter 4 summarizes the major findings or the present work. An outlook is given in form of specific recommendations for future works. Chapter 5 gives an epilogue which is a sort of responses to the comments made by the referees on the submitted thesis. Chapter 6 lists cited references. The 21 papers on which this thesis is formulated are not appended to this version.
The experimental research was carried out at the Department of Applied Geology of the
University of Göttingen (Prof. Martin Sauter) between July 2005 and March 2009 and partly was financed by the German Research Foundation (DFG) under the Grant number DFG NO 626/2-1 and DFG NO 626/2-2.
I would like to thank Angelika Schöner, Paul Waofo and Sabine Caré for the scientific
collaboration during the study.
My acknowledgements also go to my colleagues of the Department of Applied Geology at the University of Göttingen, to my friends and collaborators for religious, cultural and sportive issues in Göttingen (and Krebeck), in Freiberg (Sachsen) and elsewhere. They provided the excellent atmosphere for this work.
Special thanks to: (i) my family for his endless support and (ii) Léonard Kwuida, Sabine Caré, and Ewa Lipczynska-Kochany for reading and re-reading the draft of this thesis.
High air pollution concentrations lead to serious health problems in urbanized and industrialized areas. In Istanbul, Golden Horn is a creek valley that is identified by its
special terrain that makes air pollutants difficult to disperse. The goal of this study is to determine the ozone levels in this region, considering that reducing ozone precursor
emissions accomplishes surface ozone control. Ozone in surface boundary layer is formed by photochemical reactions involving nitrogen oxides and is affected by urbanization, traffic, and industry. In order to investigate the air quality levels in Golden Horn, the surface ozone concentrations and its precursors (NO and NO2) in Alibeykoy and Kagithane regions are temporally analyzed herein. Moreover, the relationship of ozone with precipitation is also determined.
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.
Iron/copper (Fe/Cu) bimetallic particles have been proposed as a viable technology for reduction of nitrobenzene from wastewater, however, little data is currently available on its applicability in the presence of the dissolved oxygen (DO). In this study, the prepared Fe/Cu bimetallic particles with different theoretical copper mass loadings were characterized by SEM, EDS, BET and XRD. Also, the effect of theoretical Cu mass loading and other operating parameters (such as initial pH, initial nitrobenzene concentration, the Fe/Cu bimetallic dosage and the reaction temperature) under oxic conditions (DO > 6.5 mg/L) were investigated, respectively. The results suggest that Cu planted on the iron surface could enhance the reduction, while the dense Cu layer could seriously decrease its reactivity. Under oxic conditions, neutral surroundings could enhance the reduction more obviously. This phenomenon indicated that DO could enhance the direct reduction on the catalytic activity site of iron surface. The result of the reduction efficiency of nitrobenzene under oxidizing atmosphere indicated that besides the reduction, the sorption onto the iron hydroxides generated from Fe0 during treatment and the existence of Fenton-like degradation was also responsible for the removal of nitrobenzene. The data of the chemical oxygen demand (COD) and total organic carbon (TOC) indicated that DO could facilitate the removal of organics. Thus, Fe/Cu bimetallic particles can reach high efficiency for treatment of toxic nitrobenzene-contained wastewater under oxic conditions.
This article critically evaluates recent review articles on using metallic iron (Fe(0)) for environmental remediation in order to provide insight for more efficient Fe(0)-based systems. The presentation is limited to peer-reviewed articles published during 2014 and 2015, excluding own contributions, dealing mostly with granular Fe(0). A literature search was conducted up to June 15th 2015 using Science Direct, SCOPUS, Springer and Web of Science databases. The search yielded eight articles that met the final inclusion criteria. The evaluation clearly shows that seven articles provide a narrative description of processes occurring in the Fe(0)/H20 system according to the concept that Fe(0) is a reducing agent. Only one article clearly follows a different path, presenting Fe(0) as a generator of adsorbing (hydroxides, oxides) and reducing (Fe(II), H/H2) agents. The apparent discrepancies between the two schools are identified and extensively discussed based on the chemistry of the Fe(0)/H20 system. The results of this evaluation indicate clearly that research on 'Fe(0) for environmental remediation' is in its infancy. Despite the current paucity of reliable data for the design of efficient Fe(0)-based systems, this review demonstrates that sensible progress could be achieved within a short period of time, specific recommendations to help guide future research are suggested.
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Effects of pH and dissolved oxygen on mechanisms for decolorization and total organic carbon (TOC) removal of cationic dye methylene blue (MB) by zero-valent iron (ZVI) were systematically examined. Decolorization and TOC removal of MB by ZVI are attributed to the four potential mechanisms, i.e. reduction, degradation, precipitation and adsorption. The contributions of four mechanisms were quantified at pH 3.0, 6.0 and 10.0 in the oxic and anoxic systems. The maximum efficiencies of decolorization and TOC removal of MB were found at pH 6.0. The TOC removal efficiencies at pH 3.0 and 10.0 were 11.0 and 17.0%, respectively which were considerably lower as compared with 68.1% at pH 6.0. The adsorption, which was favorable at higher pH but was depressed by the passive layer formed on the ZVI surface at alkaline conditions, characterized the effects of pH on decolorization and TOC removal of MB. The efficiencies of decolorization and TOC removal at pH 6.0 under the anoxic condition were 73.0 and 59.0%, respectively, which were comparable to 79.9 and 55.5% obtained under the oxic condition. In the oxic and anoxic conditions, however, the contributions of removal mechanisms were quite different. Although the adsorption dominated the decolorization and TOC removal under the oxic condition, the contribution of precipitation was largely superior to that of adsorption under the anoxic condition.
Advanced oxidation process based on thermally activated sodium persulfate (SPS) is used in this work to degrade methylene blue (MB). The impact of temperature (30–70 °C), MB concentration (15.6–311.8 μM), SPS load (1–100 mM) as well as phosphate buffer (PB) capacity were characterized in short term (1–2 h) and long term (1–2 weeks) experiments. Results showed complete MB discoloration under tested conditions. The observed degradation rates (kobs) from the pseudo-first order kinetics model changed upon concentration of MB and were found closely dependent on the [SPS]0/[MB]0 ratio. An acceptable Arrhenius behaviour was noticed for solutions of [SPS]0/[MB]0 ⩾ 32 in which SPS is in excess. Upon addition of SPS to MB at room temperature, the solution turned purple due to the formation of a non-stable flocky precipitate. However, spectral analysis showed complete disappearance of MB and its derivative precipitate after heat as well. The HPLC/MS analysis indicated the formation of non-stable transformation products identified as sulfonic acid and hydroxylated MB derivatives via SO4-andHO oxidation. Those disappeared totally by the end of the treatment making from thermally activated SPS an excellent choice for the treatment of industrial effluents of the dyes industry even at room temperature.
Controlling the reactivity of nano-scale zero-valent iron (nZVI) remains a challenge for its practical application. In the present study, smectite-templated nZVI is hydrophobized by adding N,N,N-trimethyl-1-dodecanaminium salt (DTA+) to yield organo-smectite-ZVI. The obtained material was characterized by XRD, TEM and FTIR. Its reactivity was evaluated for the aqueous removal of 2,4-DCP. Results show that (i) nanosized ZVI clusters of <5 nm are intercalated into the clay interlayers; (ii) hydrophobization of smectite surfaces occurs after binding of DTA+ to the clay minerals; (iii) aqueous 2,4-DCP could be rapidly accumulated in the vicinity of the solid phase; (iv) accumulated 2,4-DCP is then gradually dechlorinated. This demonstrates that hydrophobic conditions in clay interlayer facilitate the 2,4-DCP adsorption. In a 2,4-DCP successive addition systems, dechlorination can be maintained even after five cycles for organo-smectite-ZVI, but just two cycles for smectite-ZVI. This indicates that the hydrophobization of smectite-ZVI could significantly sustain its reactivity and inhibit the rapid consumption of ZVI in the Fe0/H2O system. This statement is supported by XPS analysis. Furthermore, organo-smectite-ZVI provides strong adsorptive affinity to 2,4-DCP and its reaction products. This is beneficial for the long-term stability of removed contaminants.
This study conceptually discusses the feasibility of enhancing the sustainability of conventional iron/sand filter (Fe0/sand filter) for safe drinking water by partially or totally substituting sand (quartz) by porous materials. Relevant materials included activated carbon, dolomites, limestone, pumice, sandstone, and zeolites. The rational was to use the internal volume of porous additives as storage room for in situ generated iron oxyhydroxides (iron corrosion products) and thus delay time to filter clogging. Based on previous works a filter with a volumetric Fe0:quartz ratio of 51:49 was used as reference system. The reference system is clogged upon Fe0 depletion. Results showed that totally substituting quartz by pumice particles having a porosity of 80% yields to a residual porosity of 41%. This encouraging result suggested that the possibility of using Fe0/MnO2/pumice systems for a synergic promotion of Fe0 reactivity (by MnO2) and filter permeability (by pumice) should be investigated in more details.
The past two decades have witnessed a boom of sci-entific articles on relevant processes governing aqueous contaminant removal in the presence of metallic iron (Fe0). Nonetheless, transforming accumulated data into useful knowledge is difficult. The major limitation is that a the-ory of the system is yet to be established or accepted. Generally, the theory of the system is established during the period between a discovery and its market introduction (‘valley of death’). This communication argues that the too short ‘valley of death’ has harmed progresses in Fe0 technology. The introduction of this technology was cou-pled with the consensus that Fe0 is a reducing agent. However, the question as to whether the reductive trans-formation theory was worthy of pursuit remained inade-quately addressed. The aim of this paper is to offer an answer to this question. A critical evaluation of the reduc-tive transformation theory is presented. It is established that pursuing the reductive transformation theory was irrational. It is shown that it is worthy to base future work on the concept that contaminants are removed during the dynamic process of Fe0 oxidative dissolution and subse-quent hydroxide/oxide precipitation.
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.
Nano zerovalent iron (Fe0) was reported as an effective material for azo dye removal, however, similar to other nano-materials, ultra-fine powder has a strong tendency to agglomerate into larger particles, resulting in an adverse effect on both effective surface area and catalyst performance. Here we report the preparation of nano sized Fe0 particles dispersed onto the surface of natural bentonites and their ability to decolourize orange II (OII). X-ray diffraction was used to study the sample phases while scanning electron microscopy and transmission electron microscopy were used to study the morphological changes. Spherical individual Fe0 particles were observed after dispersion onto bentonites, and these samples were used for OII decolourization over a wide pH range. Higher reactivity was attributed to the good dispersion of Fe0 particles on clay mineral surface. This study has great significance in providing novel modified clay based catalysts for the decolourization of azo dye contaminants from wastewater.Highlights► Fe0 particles on clay bring higher efficiency toward azo dye decolourization. ► Fe0 mainly exists as individual spherical shaped particles on clay surface. ► High efficiency can be attributed to the good dispersion of Fe0 particles. ► These Fe0 modified clay samples have wide working pH range.
Effects of dissolved oxygen concentrations on dye removal by zero-valent iron (Fe(0)) were investigated. The Vibrio fischeri light inhibition test was employed to evaluate toxicity of decolorized solution. Three dyes, Acid Orange 7 (AO7, monoazo), Reactive Red 120 (RR120, diazo), and Acid Blue 9 (AB9, triphenylmethane), were selected as model dyes. The dye concentration and Fe(0) dose used were 100 mg L(-1) and 30 g L(-1), respectively. Under anoxic condition, the order for dye decolorization was AO7>RR120>AB9. An increase in the dissolved oxygen concentrations enhanced decolorization and chemical oxygen demand (COD) removal of the three dyes. An increase in gas flow rates also improved dye and COD removals by Fe(0). At dissolved oxygen of 6 mg L(-1), more than 99% of each dye was decolorized within 12 min and high COD removals were obtained (97% for AO7, 87% for RR120, and 93% for AB9). The toxicity of decolorized dye solutions was low (I(5)<40%). An increase in DO concentrations obviously reduced the toxicity. When DO above 2 mg L(-1) was applied, low iron ion concentration (13.6 mg L(-1)) was obtained in the decolorized AO7 solution.
The aqueous removal of diclofenac (DF) by micrometric iron particles (Fe(0)) and amended Fe(0) (Me(0)(Fe(0))) under oxic and anoxic conditions was investigated. Bimetallic systems were obtained by plating the surface of Fe with Co, Cu, Ir, Ni, Pd and Sn. Experimental results confirmed the superiority of (Me(0)(Fe(0))) for DF removal except for IrFe (oxic) and SnFe (anoxic). Under anoxic conditions, Pd was by far the most efficient plating element followed by Ir, Ni, Cu, Co and Sn. However, under oxic conditions, Pd and Cu showed almost the same efficiency in removing DF followed by Ni, Co, Sn and Ir. Oxidative and reductive DF transformation products were identified under oxic and anoxic conditions respectively. In some systems (e.g. CoFe and SnFe oxic/anoxic; PdFe oxic; NiFe anoxic), no transformation products could be detected. This was ascribed to the nature of the plating element and its impact on the process of the formation of metal corrosion products (MCPs). MCPs are known for their high potential to strongly adsorb, bond, sequestrate and enmesh both the original contaminant and its reaction products. Obtained results corroborate the universal validity of the view, that aqueous contaminants are basically removed by adsorption and co-precipitation.
Since the introduction of iron wall technology, the inherent relationship between contaminant removal and iron corrosion has been mostly attributed to electron transfer from the metal body (direct reduction). This thermodynamically founded premise has failed to explain several experimental facts. Recently, a new concept considering adsorption and co-precipitation as fundamental contaminant removal mechanisms was introduced. This consistent concept has faced very skeptic views and necessarily needs experimental validation. The present work was the first independent attempt to validate the new concept using clofibric acid (CLO) as model compound. For this purpose, a powdered Fe(0) material (Fe(0)) was used in CLO removal experiments under various experimental conditions. Additional experiments were performed with plated Fe(0) (mFe(0): Fe(0)/Pd(0), Fe(0)/Ni(0)) to support the discussion of removal mechanism. Main investigated experimental variables included: abundance of O(2), abundance of iron corrosion products (ICPs) and shaking operations. Results corroborated the concept that quantitative contaminant removal in Fe(0)/H(2)O systems occurs within the oxide-film in the vicinity of Fe(0). Additionally, mixing type and shaking intensity significantly influenced the extent of CLO removal. More importantly, HPLC/MS revealed that the identity of reaction products depends on the extent of iron corrosion or the abundance of ICPs. The investigation of the CLO/Fe(0)/H(2)O system disproved the popular view that direct reduction mediates contaminant removal in the presence of Fe(0).
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
Zerovalent iron (ZVI) has been proposed as a reactive material in permeable in-situ walls for groundwater contaminated by metal pollutants. For such pollutants which interact with corrosion products, the determination of the actual mechanism of their removal is very important to predict the long-term stability of reactive walls. From a study of the effects of pyrite (FeS2) and manganese nodules (MnO2) on the uranium removal potential of a selected ZVI material, a test methodology (FeS2-MnO2-method) is suggested to follow the pathway of contaminant removal by ZVI materials. An interpretation of the removal potential of ZVI for uranium in presence of both additives corroborates coprecipitation with iron corrosion products as a major removal mechanism for uranium.
The definition of a "primary method of measurement" [1] has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harried cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Systeme International d'Unites). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [2]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 degreesC. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.
This study reports on the influence of selected background electrolyte parameters (pH, ionic strength, ligand and cation concentration) on the partition of uranium between the aqueous phase and elemental iron and its corrosion products in the presence or absence of arsenic. To this end, batch isotherm and kinetic experiments up to 24 h of equilibration time were performed in the dark using scrap metallic iron as sorbent. In most experiments, the background electrolyte was mainly 10 mM KCl spiked with either 50 µM uranium alone or uranium and arsenic both at the same 50 µM concentration. The subsequent uranium and arsenic speciation calculations points to UO2 2+ and H2AsO4 - as major species for the pH-range 3 to 5 whereas in the pH-range 6 to 9 UO2(OH)2 and HASO4 2- prevail. The sorption of uranium on elemental iron and corrosion products is pH dependent. The presence of arsenic seems to enhance the removal of uranium for the entire pH-range 3-9. The ionic strength also influence uranium behaviour but its effect is much less apparent than the pH. In terms of uranium removal efficiency, within an average of more or less 10 % experimental error there is no evidence of competition from metal cations dissociated from respectively 10 mM KCl, NaCl, BaCl2, CaCl2 and MgCl2 background electrolytes. Uranium and arsenic speciation remained dominated by UO2 2+ and H2AsO4 - for the background electrolyte pH 4.5. A similar uranium removal rate was observed when comparing different ligand effect as respectively 10 mM KCl, KNO3, K2CO3, K3PO4 and K2SO4 despite a slight change in uranium speciation in particular when uranium carbonato species dominated.
Neste trabalho é apresentado um método eficiente para a degradação de corantes, usando uma fonte de pó de ferro zero ambientalmente amigável (resíduo de um processo industrial). A influência de vários fatores experimentais (tais como: pH, massa de ferro, tamanho de partícula, concentração do substrato, atmosfera inerte ou oxidante) sobre a eficiência do ferro zero em reduzir o grupo cromóforo e o teor de carbono orgânico total de um azocorante (Preto Remazol B) foi avaliada. O processo de degradação do corante apresentou uma cinética de primeira ordem com uma constante de 0,153 min -1 . Nas condições otimizadas (pH=3, (Fe) = 5 g L -1 , tamanho de partícula ≤ 250 μm), este processo promoveu uma descoloração de 95% e redução de 70% na concentração de carbono orgânico total de uma solução 100 mg L -1 do corante. Este processo também foi empregado na degradação de um efluente industrial têxtil apresentando bons resultados e demonstrando capacidade para ser uma alternativa para a remediação de sistemas aquáticos poluídos. In this paper an efficient method for azo dye degradation using an environmentally friendly zero-valent iron powder source is presented (iron particles discarded from a manufacturing process). The influence of several experimental parameters (such as pH, iron mass, particle size, substrate concentration, oxidizing and inert atmospheres) on the ability of zero-valent iron to reduce the chromophoric groups and total organic carbon content of the azo dye Remazol Black B was evaluated. Kinetic studies revealed that the azo degradation by Fe 0 , iron particle size ≤ 250 μm), the iron-based process produced net a reduction in color and total organic carbon of about 95% and 70%, respectively. The process was also evaluated for the degradation of textile effluent. The studied process showed good characteristics, which can make it an effective alternative for polluted aquatic system remediation.
The adsorption of a ca. 5.4 x10-5 M aqueous solution of methylene blue has been examined on five activated carbon samples. The equilibrium concentrations (C,) were determined by spectrophotometry studies. The analysis of Freundlich adsorption isotherms obtained provides the adsorption capacity of each carbon sample.
The mechanism of aqueous contaminant removal by elemental iron (Fe0) materials (e.g. in Fe0-H2O systems) has been largely discussed in the “iron technology” literature. Two major removal mechanisms are usually discussed: (i) contaminant adsorption onto Fe0 oxidation products, and (ii) contaminant reduction by Fe0, FeII or H/H2. However, a closer inspection of the chemistry of the Fe0-H2O system reveals that co-precipitation could be the primary removal mechanism. The plausibility of contaminant co-precipitation with iron corrosion products as independent contaminant removal mechanism is discussed here. It shows that the current concept does not take into account that the corrosion product generation is a dynamic process in the course of which contaminants are entrapped in the matrix of iron hydroxides. It is recalled that contaminant co-precipitation with iron hydroxides/oxides is an unspecific removal mechanism. Contaminant co-precipitation as primary removal mechanism is compatible with subsequent reduction and explains why redox-insensitive species are quantitatively removed. Adsorption and co-precipitation precede reduction and abiotic reduction, when its takes place, occurs independently by a direct (electrons from Fe0) or an indirect (electrons from FeII/H2) mechanism.
Environmental Context. Groundwater is the water that fills the spaces between sand, soil, and rock below the water table. It discharges into ecologically sensitive wetlands and is used as drinking water or in agriculture and industry. Inappropriate waste disposal and poor land management can contaminate groundwater and may minimize its use for decades. The common method for pumping contaminated groundwater to the surface for treatment is costly and labour intensive. Zerovalent iron is a new, more cost-effective method of groundwater remediation.
Abstract. Zerovalent iron (ZVI) has been proposed as a reactive material in permeable in situ walls for groundwater contaminated by metal pollutants. For such pollutants that interact with corrosion products, the determination of the actual mechanism of their removal is very important to predict their stability in the long term. From a study of the effects of pyrite (FeS2) and manganese nodules (MnO2) on the uranium removal potential of a selected ZVI material, a test methodology (FeS2–MnO2 method) is suggested to follow the pathway of contaminant removal by ZVI materials. An interpretation of the removal potential of ZVI for uranium in the presence of both additives corroborates coprecipitation with iron corrosion products as the initial removal mechanism for uranium.
The term mixing is confusing because it is used to describe transport mechanisms for both flash mixing (reagent dispersion and homogenisation with water mixing) and agitation (flocculation mixing) because each of these mechanisms requires dif-ferent flow characteristics in order to take place with maximum efficiency. Flash mixing should take place under conditions of mixing on macro-scale with macro-turbulent eddies being formed and agitation under conditions of mixing under micro-scale with micro-turbulent eddies being formed. Agitation takes place under high-or low-intensity agitation. Only the condi-tions of agitation can be characterised by velocity gradient. Differentiation between flash mixing and agitation is discussed.
The definition of a "primary method of measurement" [1] has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harried cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Systeme International d'Unites). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [2]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 degreesC. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe(0)/H(2)O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe(0) and Fe(0) corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO(2)) is not expected. The kinetics of MB discoloration by Fe(0), Fe(2)O(3), Fe(3)O(4), MnO(2), and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe(0) source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min(-1)), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe(0)/H(2)O systems is reiterated.
A central aspect of the contaminant removal by elemental iron materials (Fe0 or Fe0 materials) is that reduction reactions are mediated by the iron surface (direct reduction). This premise was introduced by the pioneers of the reactive wall technology and is widely accepted by the scientific community. In the meantime enough evidence has been provided to suggest that contaminant reduction through primary corrosion products (secondary reductants) does indeed occur (indirect reduction). It was shown for decades that iron corrosion in the pH range of natural waters (4-9) inevitably yields an obstructive oxide film of corrosion products at the metal surface (oxide film). Therefore, contaminant adsorption on to corrosion products and contaminant co-precipitation with corrosion products inevitably occurs. For adsorbed and coprecipitated contaminants to be directly reduced the oxide film should be electronic conductive. This study argues through a literature review a series of points which ultimately lead to the conclusion that, if any quantitative contaminant reduction occurs in the presence of Fe0 materials, it takes place within the matrix of corrosion products and is not necessarily a direct reduction. It is concluded that Fe0 materials act both as source of corrosion products for contaminant adsorption/coprecipitation and as a generator of FeII and H2 (H) for possible catalytic contaminant reduction.
The kinetics of the reaction between
manganese dioxide and ferrous ion in acid solution have been investigated by
using the potential of the ferrous-ferric couple as a measure of the extent of
reaction. The experimental conditions were such that the reaction rate was
independent of ferrous, ferric, manganous ions, and acid concentrations and the
agitation was sufficient to prevent bulk diffusion in the solution from being a
rate-determining factor.
The reaction rate of sized samples of
pyrolusite and γ-MnO2 in ferrous
sulphate solution was proportional to the surface area of the solid and was
constant (i.e. " zero-order ") until 50 per cent. of the solid was
consumed. γ-MnO2 reacted about twice
as rapidly as the pyrolusite. The reaction occurred most readily at certain
active sites on the particles and appeared to proceed along crystal boundaries
in such a manner that the active surface area was not significantly changed
during the first half of the reaction. In ferrous perchlorate the reaction rate
of 10 μ diameter pyrolusite was about one-hundredth of that in sulphate
and the reaction appeared to occur at a more even rate over the whole surface
of the particle so that the zero-order law was no longer obeyed. Activation
energies of 7.4 and 5 kcal in sulphate and perchlorate respectively, for the
temperature range 18 to 40 °C, suggest that the difference in rate is a result
of a change in the entropy factor of the Arrhenius equation. It is suggested
that this difference in rate may result from the activation, by sulphate ions,
of a less reactive lower oxide of manganese which is formed on the surface.
Mass transfer between solid particles and liquids frequently occurs in the presence of inert micro-particles during iron removal by precipitation. In this investigation, the influence of inert micro-particles on mass transfer between a coarse active particle and liquid is simulated with the aid of a cementation reaction between cast iron particles and copper sulphate solution in a mechanically agitated contactor. Kaolin, silicon carbide, iron oxide and anatase were used as the inert micro-particles. The reaction was carried out under conditions of mass transfer control. The experimental data agreed well with Sano's semiempirical expression for a solid-liquid mass transfer coefficient based on the specific power group. The presence of inert micro-particles was found to reduce the solid-liquid mass transfer coefficient significantly, depending on volume fraction, size and density. The relative mass transfer factor, α, defined as the ratio of the solid-liquid mass transfer coefficient with and without the presence of inert micro-particles (κ′sl/κ′sl), was correlated by the expression (SD=8.66%). where φmp=the volume fraction of inert micro-particles, threshold volume fraction below which the inert micro-particles do not influence the mass transfer coefficient; and β=a mass transfer attenuation factor. β and φ∗ are functions of density and size of inert micro-particles and can be expressed as: β=4.33×10−5dmp−0.32ϱ1.05; . The ranges of variables covered by the correlatiion are: 1.0<dmp<13 μm; 2600<ϱmp<5300 kg/m3; 0<φmp<0.06. The relative mass transfer factor, α, was found to be independent of the coarse active solid phase hold-up up to a volume fraction of 0.04.
The funnel-and-gate system for in situ treatment of contaminant plumes consists of low hydraulic conductivity cutoff walls with gaps that contain in situ reactors, such as reactive porous media-, that remove contaminants by abiotic or biological processes. Funnel-and-gate systems can be installed at the front of plumes to prevent further plume growth, or immediately downgradient of contaminant source zones to prevent contaminants from moving into plumes. Cutoff walls (the funnel) modify flow patterns so that ground water flows primarily through high conductivity gaps (the gates). This approach is largely passive in that after installation, in situ reactors are intended to function with little or no maintenance for long periods. This approach contrasts with the energy and maintenance-intensive character of pump-and-treat systems. This paper describes the funnel-and-gate concept, and uses two-dimensional computer simulations to illustrate the effects of cutoff wall and gate configuration on capture zone size and shape and on the residence time for reaction of contaminants in gates.
The reactions of 8 model contaminants with 9 types of granular Fe(0) were studied in batch experiments using consistent experimental conditions. The model contaminants (herein referred to as reductates because they were reduced by the iron metal) included cations (Cu2+), anions (CrO42-; NO3-; and 5,5,7,7-indigotetrasulfonate), and neutral species (2-chloroacetophenone; 2,4,6-trinitrotoluene; carbon tetrachloride; and trichloroethene). The diversity of this range of reductates offers a uniquely broad perspective on the reactivity of Fe(0). Rate constants for disappearance of the reductates vary over as much as 4 orders of magnitude for particular reductates (due to differences in the 9 types of iron) but differences among the reductates were even larger, ranging over almost 7 orders of magnitude. Various ways of summarizing the data all suggest that relative reactivities with Fe(0) varies in the order: Cu2, I4S > 2CAP, TNT > CT, Cr6 > TCE > NO3. Although the reductate h as the largest effect on disappearance kinetics, more subtle differences in reactivity due to the type of Fe(0) suggests that removal of Cr6 and NO3 (the inorganic anions) involves adsorption to oxides on the Fe(0), whereas the disappearance kinetics of all other types of reductants is favored by reduction on comparatively oxide-free metal. Correlation analysis of the disappearance rate constants using descriptors of the reductates calculated by molecular modeling (energies of the lowest unoccupied molecular orbitals, LUMO, highest occupied molecular orbitals, HOMO, and HOMO-LUMO gaps) showed that reactivities generally increase with decreasing ELUMO and increasing EGAP (and, therefore, increasing chemical hardness h).
The reduction of Mn-oxide by Fe2+ was studied in column experiments, using a column filled with natural Mn-oxide coated sand. Analysis of the Mn-oxide indicated the presence of both Mn(III) and Mn(IV) in the Mn-oxide. The initial exchange capacity of the column was determined by displacement of adsorbed Ca2+ with Mg2+. Subsequently a FeCl2 solution was injected into the column causing the reduction of the Mn-oxide and the precipitation of Fe(OH)3. Finally the exchange capacity of the column containing newly formed Fe(OH)3 was determined by injection of a KBr solution. During injection of the FeCl2 solution into the column, an ion distribution pattern was observed in the effluent that suggests the formation of separate reaction fronts for Mn(III)-oxide and Mn(IV)-oxide travelling at different velocities through the column. At the proximal reaction front, Fe2+ reacts with MnO2 producing Fe(OH)3, Mn2+ and H+. The protons are transported downstream and cause the disproportionation of MnOOH at a separate reaction front. Between the two Mn reaction fronts, the dissolution and precipitation of Fe(OH)3 and Al(OH)3 act as proton buffers. Reactive transport modeling, using the code PHREEQC 2.0, was done to quantify and analyze the reaction controls and the coupling between transport and chemical processes. A model containing only mineral equilibria constraints for birnessite, manganite, gibbsite, and ferrihydrite, was able to explain the overall reaction pattern with the sequential appearance of Mn2+, Al3+, Fe3+, and Fe2+ in the column outlet solution. However, the initial breakthrough of a peak of Ca2+ and the observed pH buffering indicated that exchange processes were of importance as well. The amount of potential exchangers, such as birnessite and ferrihydrite, did vary in the course of the experiment. A model containing surface complexation coupled to varying concentrations of birnessite and ferrihydrite and a constant charge exchanger in addition to mineral equilibria provided a satisfactory description of the distribution of all solutes in time and space. However, the observed concentration profiles are more gradual than indicated by the equilibrium model. Reaction kinetics for the dissolution of MnO2 and MnOOH and dissolution of Al(OH)3 were incorporated in the model, which explained the shape of the breakthrough curves satisfactorily. The results of this study emphasize the importance of understanding the interplay between chemical reactions and transport in addition to interactions between redox, proton buffering, and adsorption processes when dealing with natural sediments. Reactive transport modeling is a powerful tool to analyze and quantify such interactions.
Details of the formation of potentials at a metal-oxide/H2O interphase are described. On the one hand the whole potential drop consists of one fixed (HELMHOLTZ layer) and two diffuse parts (space charge in the oxide phase and GOUY layer in the electrolyte), on the other hand it is given as sum of free charges and dipols. Besides the PZC the charge distribution is determined above all by the pH value of the electrolyte under conditions without an external current. The following cases can be qualitatively distinguished in connection with the important pH22°c =7.
The focus of the ongoing research described in this paper is the in situ treatment of contaminated groundwater. The goal is to establish a technology and methodology capable of removing hazardous constituents from the groundwater without the liabilities of resource depletion, treated water disposal, expensive and energy-intensive withdrawal well fields, or expensive surface treatment facilities. Among the possible subsurface treatment processes are sorption, ion exchange, precipitation, and nutrient and/or oxygen-enhanced microbiological degradation.
Mass transport by advection and diffusion is involved in nearly all branches of science, but various scientific disciplines define the two mechanisms of transport differently. Models widely accepted for combining transport resulting from advection and diffusion are shown to be inconsistent with experimental observations. Experimental observations are cited to show that the barycentric velocity of a solution is not synonymous with velocity as determined by the Navier-Stokes equation of fluid motion. The physics of advection and diffusion is analyzed, and requirements for combining advection and diffusion are presented. Advection is defined here as transport responding to a pressure gradient or body force. Molecular diffusion is defined as transport responding to concentration or thermal gradients. Diffusion represents an average velocity component of molecules of a particular constituent relative to a fixed frame of reference external to the solution, rather than to the mean velocity of all constituents in a solution as defined in current literature. Advection and diffusion contribute independently to the total transport.
This paper investigated the removal of humic substances (HS) from drinking water supplies by adsorption onto granular activated carbon (GAC), iron-coated alumina (AAFS) and ferric oxihydroxide (β-FeOOH). The determination of the rate limiting step of the adsorption process was tested using the pseudo-first and second order kinetic models. Two mixing speeds were considered namely; 200 and 300 rpm. Disintegration (attrition) of the adsorbents occurred above 300 rpm while 200 rpm represented the minimum mixing speed required to keep the solution in suspension. During the first few minutes of adsorption at 300 rpm, strong instantaneous adsorption was observed. Among the three adsorbents, AAFS showed the least affinity for the HS with no instantaneous adsorption at all at 200 rpm. Furthermore, the pseudo-first order model failed to represent the whole range of data while the second order model was the best fit for all the experimental range considered in this study.
This article aims to provide an overview of the upcoming technology of permeable reactive barriers for groundwater remediation. A comprehensive list of references and web-links are also provided for further in-depth understanding. A brief discussion on the Australian perspective on this emerging technology is also included.
This investigation describes the adsorption of methylene blue, a cationic organic dyestuff commonly used for tracer studies in water research, on activated carbon.
The adsorption of proteins onto a solid/liquid interface involves a cascade of complex events occuring almost simultaneously: 1.(i)Transport of the molecules onto the interface by diffusion or diffusion /convection processes.2.(ii) Adsorption and desorption of the molecules at the interface formed by an interfacial chemical reaction.3.(iii) Structural modifications of the fixed molecules together for high surface coverages and interactions of the incoming molecules with previously uptaken protein molecules.4.(iv) In real biological systems: adsorption competition between molecules of different nature and molecular weight.The aim of the present study is to carefully analyse the adsorption and desorption behaviours at an apatite interface of 125I-labelled albumin molecules. Taking advantage of an experimental set-up which allows control of the chemical reaction, the effect of the interfacial protein residence times on the adsorption and desorption properties, in particular, were analysed.
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.
The funnel-and-gate system for in situ treatment of contaminant plumes consists of low hydraulic conductivity cutoff walls with gaps that contain in situ reactors, such as reactive porous media, that remove contaminants by abiotic or biological processes. Funnel-and-gate systems can be installed at the front of plumes to prevent further plume growth, or immediately downgradient of contaminant source zones to prevent contaminants from moving into plumes. Cutoff walls (the funnel) modify flow patterns so that ground water flows primarily through high conductivity gaps (the gates). This approach is largely passive in that after installation, in situ reactors are intended to function with little or no maintenance for long periods. This approach contrasts with the energy and maintenance-intensive character of pump-and-treat systems. This paper describes the funnel-and-gate concept, and uses two-dimensional computer simulations to illustrate the effects of cutoff wall and gate configuration on capture zone size and shape and on the residence time for reaction of contaminants in gates.
The combination of detailed multilevel ground water geochemistry samples, a natural-gradient tracer test, minislug tests, and a numerical flow and transport model was used to examine flow through a zero-valent iron permeable reactive barrier (PRB) installed to remove explosives from ground water. After 20 months of operation, the PRB continued to completely remove explosives from the ground water flowing through it. However, the data indicate that a portion of ground water flow was being diverted beneath the PRB. Ground water geochemistry was significantly altered by the PRB, and concentrations of some ions, including sulfate, carbonate, and calcium, were substantially reduced due to precipitation. Field data and numerical model results indicate that, after 20 months of operation, flow through the PRB was reduced to approximately one-third of its expected value.
Core samples taken from a zero-valent iron permeable reactive barrier (ZVI PRB) at Cornhusker Army Ammunition Plant, Nebraska, were analyzed for physical and chemical characteristics. Precipitates containing iron and sulfide were present at much higher concentrations in native aquifer materials just upgradient of the PRB than in the PRB itself. Sulfur mass balance on core solids coupled with trends in ground water sulfate concentrations indicates that the average ground water flow after 20 months of PRB operation was approximately twenty fold less than the regional ground water velocity. Transport and reaction modeling of the aquifer PRB interface suggests that, at the calculated velocity, both iron and hydrogen could diffuse upgradient against ground water flow and thereby contribute to precipitation in the native aquifer materials. The initial hydraulic conductivity (K) of the native materials is less than that of the PRB and, given the observed precipitation in the upgradient native materials, it is likely that K reduction occurred upgradient to rather than within the PRB. Although not directly implicated, guar gum used during installation of the PRB is believed to have played a role in the precipitation and flow reduction processes by enhancing microbial activity.
The use of granular iron for in situ degradation of dissolved chlorinated organic compounds is rapidly gaining acceptance as a cost-effective technology for ground water remediation. This paper describes the first field demonstration of the technology, and is of particular importance since it provides the longest available record of performance (five years). A mixture of 22% granular iron and 78% sand was installed as a permeable “wall” across the path of a contaminant plume at Canadian Forces Base, Borden, Ontario. The major contaminants were trichloroethene (TCE, 268 mg/L) and tetrachloroethene (PCE, 58 mg/L). Approximately 90% of the TCE and 86% of the PCE were removed by reductive dechlorination within the wall, with no measurable decrease in performance over the five year duration of the test. Though about 1% of the influent TCE and PCE appeared as dichloroethene isomers as a consequence of the dechlorination of TCE and PCE, these also degraded within the iron-sand mixture. Performance of the field installation was reasonably consistent with the results of laboratory column studies conducted to simulate the field behavior. However, if a more reactive iron material, or a higher percentage of iron had been used, complete removal of the chlorinated compounds might have been achieved. Changes in water chemistry indicated that calcium carbonate was precipitating within the reactive material; however, the trace amount of precipitate detected in core samples collected four years after installation of the wall suggest that the observed performance should persist for at least another five years. The study provides strong evidence that in situ use of granular iron could provide a long-term, low-maintenance cost solution for many ground water contamination problems.
In-situ thermogravimetric measurements were used in the hydrogen reduction of poly-granular synthetic ilmenite discs at temperatures
in the range 823 to 1173 K and at pressures in the range 1.2 to 13 atm. A symmetrical beam microbalance was used, coupled
with twin reactors and twin furnaces, to minimize buoyancy and drag effects. Stable operation was achieved at high gas flow
rates where gas film transport effects were negligible. Polishing the ilmenite discs prior to reduction eliminated the formation
of dense surface metallic iron films that can impede gas diffusion into the discs. Macroscopically, the reduction reaction
proceeded topochemically and a shrinking core reaction model was found to be appropriate to predict conversion-time relationships.
It was necessary to allow for water vapor adsorption onto the reacting interface in order to model the effect of pressure
on the reduction kinetics. The observed reduction rate increased sharply with pressure up to approximately 3 atm and then
approached a plateau with further pressure increase. The porosity in the reduced ilmenite samples was very fine, with pore
diameters of typically 0.05 to 0.3 µm. Intragrain gas pressure buildup in the fine pores due to the influence of Knudsen diffusion
was incorporated into the modeling of the kinetic data.
Several activated carbons were obtained by H3PO4 at 500 °C, under increasing acid concentrations of 30–70%. Products were characterized by N2 at 77 K, and proved to be highly microporous with high surface area and pore volume that increased with impregnation ratio. Two modified carbons were prepared by concurrently passing N2 during pyrolysis of impregnated precursor with 50% H3PO4 at 500 °C, and post-heat treatment at 800 °C for one carbon. A low reduction in porosity accompanied this treatment. Equilibrium adsorption of methylene blue (MB) proved good uptake of the bulky dye, which improved considerably with impregnant concentration that was related to enhanced porosity. Dynamic removal of MB was carried out by running solutions of influent concentrations, of 100–200 mg/L, through a mini-column. Many column performance parameters were estimated at different stages on the typical S-shaped breakthrough curves: volume treated, amounts uptaken, bed service time, height of mass transfer zone, and column exhaustion characteristics. Increased impregnation ratio improved column performance, as well as forcing N2 pyrolysis or extra heat-treatment. Activated carbon impregnated with 70% H3PO4 and carbonized at 500 °C exhibited the best properties which prevailed upon raising treated dye concentration to 150 and 200 mg/L, although degraded its capacity due to the limited mass of adsorbent and to the short contact time.
Zero valent iron as a reactive barrier material has garnered considerable attention over the past few years because it is relatively inexpensive, abundant, harmless to the environment and effective in reducing organic contaminants. The redox process requires adsorption of the organic contaminant onto the Fe surface, where protonation and electron transfer occur. In this study, nitrobenzene reduction by Fe was investigated using differential pulse polarography. This technique was employed because it is able to simultaneously monitor the disappearance of nitrobenzene and the appearance of Fe2+. Initial studies focused on the efficiency and reaction kinetics as a function of the Fe surface pretreatment. The presence of a pH dependent induction period in the reduction of nitrobenzene using stored Fe was observed. Detailed secondary experiments were also performed using freshly prepared Fe, which was washed with the reaction medium buffer to eliminate solvent effects. Since oxygen is an important factor in iron corrosion, the effect of oxygen on the efficiency of reduction of nitrobenzene was studied by comparing the reaction kinetics of aerated and de-aerated solutions at pH 6 and 7. Furthermore, the adsorptivity of Fe towards anions, intermediate surface species, and organic contaminants, was investigated to elucidate the multifaceted role of the buffer. These studies proved informative, despite the fact that oxygen, mass transfer, and colloidal iron (II) limited our ability to access information about crucial mechanistic details on the chemical processes controlling the reduction of organic compounds at the Fe surface.
A method of continuous mechanical renewal (scouring) of the whole reaction surface of a solid metal electrode has been used for investigation of the kinetics of various characteristic electrode processes. Cathodic processes of hydrogen evolution on Pd, Ni, Fe, Pb, Sn, oxygen ionization on Pd, active anodic dissolution (Fe, Ni, Pb, Sn), anodic passivity (Ti, Cr, Ni), as well as the effect of adsorption of surface-active anions on the kinetics of electrode and corrosion processes on Fe and Ni, have been studied.
A survey of the literature covering the destruction of organic pollutants accomplished under mild reaction conditions is presented. Technologies presented are segregated according to two main reaction pathways; oxidation and reduction. Sub-topics discussed are representative of the main component of the degradation system, including the following; electrochemical reactors, hydrogen as a reducing agent, zero-valent metals, biological based systems, photolytic processes, Fenton reaction, and a recently discovered process that is a form of room temperature and pressure oxygen activation.
Permeable reactive barriers are an emerging alternative to traditional pump and treat systems for groundwater remediation. This technique has progressed rapidly over the past decade from laboratory bench-scale studies to full-scale implementation. Laboratory studies indicate the potential for treatment of a large number of inorganic contaminants, including As, Cd, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Se, Tc, U, V, NO3, PO4 and SO4. Small-scale field studies have demonstrated treatment of Cd, Cr, Cu, Fe, Ni, Pb, NO3, PO4 and SO4. Permeable reactive barriers composed of zero-valent iron have been used in full-scale installations for the treatment of Cr, U, and Tc. Solid-phase organic carbon in the form of municipal compost has been used to remove dissolved constituents associated with acid-mine drainage, including SO4, Fe, Ni, Co and Zn. Dissolved nutrients, including NO3 and PO4, have been removed from domestic septic-system effluent and agricultural drainage.
Zero-valent iron (Fe0), metallic iron, is being evaluated as a permeable reactive barrier material to mitigate the transport of a wide array of highly mobile contaminants in groundwater. Zero-valent iron has previously been shown to destroy effectively numerous chlorinated hydrocarbon compounds via reductive dehalogenation. No references could be found regarding the ability of zero-valent iron to reduce UO2+2, MoO2−4, or TcO−4.A series of kinetic-batch studies was conducted to determine the capability of particulate Fe0 to remove UO2+2, MoO2−4, TcO−4, and CrO2−4 from groundwater. Particulate Fe0 effectively removed each of these contaminants from solution; removal rates decreased as follows: CrO2−4 > TcO−4 > UO2+2 ⪢ MoO2−2. The removal mechanism appears to be reductive precipitation. Thermodynamic equilibrium calculations indicated that the rate of removal of the metals from solution increased as the difference in pe (Δpe) increased between the redox half reaction for the redox couple of interest and the couple. Furthermore, the pe value for a redox couple provided a qualitative indication of the reduction rate by Fe0. These results indicate that the rate of removal of CrO2−4, TcO−4, and UO2+2 from groundwater is rapid, permitting an inexpensive barrier of practical dimensions to be used for in situ remediation purposes.
Reduction of chlorinated solvents by fine-grained iron metal was studied in well-mixed anaerobic batch systems in order to help assess the utility of this reaction in remediation of contaminated groundwater. Iron sequentially dehalogenates carbon tetrachloride via chloroform to methylene chloride. The initial rate of each reaction step was pseudo-first-order in substrate and became substantially slower with each dehalogenation step. Thus, carbon tetrachloride degradation typically occurred in several hours, but no significant reduction of methylene chloride was observed over 1 month. Trichloroethene (TCE) was also dechlorinated by iron, although more slowly than carbon tetrachloride. Increasing the clean surface area of iron greatly increased the rate of carbon tetrachloride dehalogenation, whereas increasing pH decreased the reduction rate slightly. The reduction of chlorinated methanes in batch model systems appears to be coupled with oxidative dissolution (corrosion) of the iron through a largely diffusion-limited surface reaction.
It is often assumed that the negative gradient of chemical potential indicates the direction of transport by diffusion. Diffusion is the transport of a fluid constituent induced by a concentration gradient. The objective of this review paper is to explain the purpose and correct application of the chemical potential. Fick's law is derived here from Newton's second law of motion. Diffusion is a transport mechanism involving the motion of individual molecules, and does not induce a resistance associated with fluid viscosity. Consequently, unlike advection, the mean hydraulic radius of flow channels does not affect resistance to diffusion. A concentration gradient is not reflected in the pressure gradient acting externally on reference volumes of fluid. Both external driving force and a concentration gradient may result in motion of the center of mass of reference volumes of fluid. If a density gradient exists, the vector sum of external forces does not describe the resultant driving force. A concentration gradient induces constituent velocity relative to the fluid boundaries, not relative to the center of mass. Chemical potential includes pressure as a variable, and cannot predict the direction of diffusion because pressure is a driving force for advection, not diffusion. Chemical potential describes equilibrium conditions and is not useful for evaluating mass transport.
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
Column experiments and numerical simulation were conducted to test the hypothesis that iron material having a high corrosion rate is not beneficial for the long-term performance of iron permeable reactive barriers (PRBs) because of faster passivation of iron and greater porosity loss close to the influent face of the PRBs. Four iron materials (Connelly, Gotthart-Maier, Peerless, and ISPAT) were used for the column experiments, and the changes in reactivity toward cis-dichloroethene (cis-DCE) degradation in the presence of dissolved CaCO3 were evaluated. The experimental results showed that the difference in distribution of the accumulated precipitates, resulting from differences in iron corrosion rate, caused a difference in the migration rate of the cis-DCE profiles and a significant difference in the pattern of passivation, indicating a faster passivation in the region close to the influent end for the material having a higher corrosion rate. For the numerical simulation, the accumulation of secondary minerals and reactivity loss of iron were coupled using an empirically-derived relationship that was incorporated into a multi-component reactive transport model. The simulation results provided a reasonable representation of the evolution of iron reactivity toward cis-DCE treatment and the changes in geochemical conditions for each material, consistent with the observed data. The simulations for long-term performance were also conducted to further test the hypothesis and predict the differences in performance over a period of 40 years under typical groundwater conditions. The predictions showed that the cases of higher iron corrosion rates had earlier cis-DCE breakthrough and more reduction in porosity starting from near the influent face, due to more accumulation of carbonate minerals in that region. Therefore, both the experimental and simulation results appear to support the hypothesis and suggest that reactivity changes of iron materials resulting from evolution of geochemical conditions should be considered in the design of iron PRBs.
Reactions involving iron play a major role in the environmental cycling of a wide range of important organic, inorganic and radioactive contaminants. Consequently, a range of environmental clean-up technologies have been proposed or developed which utilise iron chemistry to remediate contaminated land and surface and subsurface waters, e.g. the use of injected zero zero-valent iron nanoparticles to remediate organic contaminant plumes; the generation of iron oxyhydroxide-based substrates for arsenic removal from contaminated waters; etc. This paper reviews some of the latest iron-based technologies in contaminated land and groundwater remediation, their current state of development, and their potential applications and limitations.
Manganese oxide minerals have been used for thousands of years-by the ancients for pigments and to clarify glass, and today as ores of Mn metal, catalysts, and battery material. More than 30 Mn oxide minerals occur in a wide variety of geological settings. They are major components of Mn nodules that pave huge areas of the ocean floor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil composition. Their typical occurrence as fine-grained mixtures makes it difficult to study their atomic structures and crystal chemistries. In recent years, however, investigations using transmission electron microscopy and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial deposits and ocean nodules, were determined by using powder x-ray diffraction data and the Rietveld refinement method. Because of the large tunnels in todorokite and related structures there is considerable interest in the use of these materials and synthetic analogues as catalysts and cation exchange agents. Birnessite-group minerals have layer structures and readily undergo oxidation reduction and cation-exchange reactions and play a major role in controlling groundwater chemistry.
The reactions of eight model contaminants with nine types of granular Fe(0) were studied in batch experiments using consistent experimental conditions. The model contaminants (herein referred to as "reductates" because they were reduced by the iron metal) included cations (Cu2+), anions (CrO4(2-), NO3(-), and 5,5',7,7'-indigotetrasulfonate), and neutral species (2-chloroacetophenone, 2,4,6-trinitrotoluene, carbon tetrachloride, and trichloroethene). The diversity of this range of reductates offers a uniquely broad perspective on the reactivity of Fe(0). Rate constants for disappearance of the reductates vary over as much as four orders of magnitude for particular reductates (due to differences in the nine types of iron) but differences among the reductates were even larger, ranging over almost seven orders of magnitude. Various ways of summarizing the data all suggest that relative reactivities with Fe(0) vary in the order Cu2+, 5,5',7,7'-indigotetrasulfonate > 2-chloroacetophenone, 2,4,6-trinitrotoluene > carbon tetrachloride, CrO4(2-) > trichloroethene > NO3(-). Although the reductate has the largest effect on disappearance kinetics, more subtle differences in reactivity due to the type of Fe(0) suggests that removal of CrO2(2-) and NO3(-) (the inorganic anions) involves adsorption to oxides on the Fe(0), whereas the disappearance kinetics of all other types of reductants is favored by reduction on comparatively oxide-free metal. Correlation analysis of the disappearance rate constants using descriptors of the reductates calculated by molecular modeling (energies of the lowest unoccupied molecular orbitals, LUMO, highest occupied molecular orbitals, HOMO, and HOMO-LUMO gaps) showed that reactivities generally decrease with increasing E(LUMO) and increasing E(GAP) (and, therefore, increasing chemical hardness eta).
Great attention should be paid now to simultaneously removing common pollutants, especially inorganic pollutants such as nitrate and heavy metals, as individual removal has been investigated extensively. Removing common pollutants simultaneously by iron metal is a very effective alternative method. Near neutral pH, heavy metals, such as copper and nickel, can be removed rapidly by iron metal, while nitrate removal very much slower than that of copper and nickel, and copper can accelerate nitrate removal when both are removed simultaneously. Even a little amount of copper can enhance nitrate removal efficiently. Different mechanisms of these contaminants removal by iron metal were also discussed.
An in situ methodology based on covalently bonded redox indicators has been developed for determining when sulfate-reducing conditions exist in environmental samples. Three immobilized redox indicators [thionine (Thi, formal potential at pH 7 (E(0')7) equals 52 mV), cresyl violet (CV, E(0')7 = -81 mV), and phenosafranine (PSaf, E(0')7 = -267 mV)] were tested for their response to sulfide in synthetic solutions and under sulfate-reducing conditions in wastewater slurries. The byproduct of the sulfate-reducing process, sulfide, was found to couple well to CV in the concentration range of 1-100 microM total sulfide ([S(-II)]) and the pH range of 6-8. Thi, the indicator with the highest formal potential, reacts rapidly with sulfide at levels well below 1 microM while PSaf, the indicator with the lowest formal potential, does not couple to sulfide at levels in excess of 100 microM [S(-II)]. The degree of reduction of the indicators (i.e., the fraction of cresyl violet oxidized) in contact with a given level of sulfide can be modeled qualitatively with an equilibrium expression for [S(-II)]-indicator based on the Nernst equation assuming that rhombic sulfur is the product of sulfide oxidation. In a groundwater sample with dechlorinating microbes, reduction of Thi and partial reduction of CV correlated with dechlorination of TCE to cis-DCE.
A daunting challenge facing the water industry and regulators is how to simultaneously control microbial pathogens, residual disinfectant, and disinfection byproducts in drinking water, and to do so at an acceptable cost. Of the different pathogens, viruses are especially problematic due to their small size, high mobility, and resistance to chlorination and filtration. In the past decade, zerovalent iron has been used to treat a wide variety of organic and inorganic contaminants from groundwater. However, iron has not been tested against biological agents. This study examined the effectiveness of commercial zerovalent iron to remove two viruses, phiX174 and MS-2, from water. Removal of these viruses by iron granules in batch reactors was first-order, and the rate was likely controlled by external mass transfer. Most of the viruses removed from solution were either inactivated or irreversibly adsorbed to iron. In a flow-through column containing zerovalent iron (with 20 min of iron contact time), the removal efficiency for both viruses was 4-log in an initial pulse test, and over 5-log in the second pulse test after passage of 320 pore volumes of artificial groundwater. We assume that the improved efficiency was due to continuous formation of new iron (oxyhydr)oxides which served as virus adsorption sites. To our knowledge, this is the first demonstration of biological agent removal from water by zerovalent iron. Results of this study suggest zerovalent iron may be potentially useful for disinfecting drinking water and wastewater, thereby reducing our dependence on chlorine and reducing the formation of disinfection byproducts.
Triazoles, additives in runway de-icers, are found in soil and groundwater at airport sites. To better understand the fate and transport of benzotriazole (BTA) and methylbenzotriazole (MeBTA) and to assess possible remediation options of contaminated groundwater, sorption to various soils and ferrous sorbents has been studied. In batch experiments, limited non-linear sorption of BTA to mineral subsoil from the Oslo International Airport, Gardermoen was observed. The sorption to soil could be described using a Freundlich isotherm. pH affected sorption of BTA to subsoil, although the effect was not strong. Increased sorption was observed to zerovalent iron (Fe(0)). MeBTA showed similar sorption behaviour as BTA although the sorption coefficient was generally higher. Sorption to Fe(0) seems to be controlled by multi-layer coverage. Our data suggest that sorption of triazoles to Fe(2)O(3) is negligible. However BTA sorption to 2-line and 6-line ferrihydrites showed strong sorption. The results demonstrate that triazoles are highly mobile in the subsurface environment, however zerovalent iron can be an effective medium for groundwater remediation. Without remediation, wide distribution of triazoles in the environment can be expected due to its extensive application and limited degradability.
Drinking groundwater contaminated with naturally occurring arsenic is a worldwide public health issue. This work describes the research, development and distribution of a filter used by thousands of people in Bangladesh to obtain arsenic-free safe water. The filter removes arsenic species primarily by surface complexation reactions: =FeOH + H(2)AsO(4)(-) --> =FeHAsO(4)(-) + H(2)O (K=10(24)) and =FeOH + HAsO(4)(2-) --> =FeAsO(4)(2-) + H(2)O (K=10(29)) on a specially manufactured composite iron matrix (CIM). The filter water meets WHO and Bangladesh standards, has no breakthrough, works without any chemical treatment (pre- or post-), without regeneration, and without producing toxic wastes. It costs about $40/5 years and produce 20-30 L/hour for daily drinking and cooking need of 1-2 families. The spent material is completely non toxic-solid self contained iron-arsenate cement that does not leach in rainwater. Approved by the Bangladesh Government, about 30,000 SONO filters were deployed all over Bangladesh and continue to provide more than a billion liters of safe drinking water. This innovative filter was also recognized by the National Academy of Engineering-Grainger Challenge Prize for sustainability with the highest award for its affordability, reliability, ease of maintenance, social acceptability, and environmental friendliness, which met or exceeded the local government's guidelines for arsenic removal.