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Field Evidence for Flow Reduction through a Zero‐Valent Iron Permeable Reactive Barrier
Abstract
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.
... They became the most promising reactive medium for engineered walls (Tratneyk et al. 2003, Jambor et al. 2005, Henderson & Demond 2007, Thiruvenkatachari et al. 2008. Within 20 years the iron wall technology has developed to a standard technology for groundwater remediation and wastewater treatment with worldwide acceptance (Henderson & Demond 2007, Johnson et al. 2008, Comba et al. 2011). ...
... Despite its well-documented efficiency, the Fe 0 remediation currently has three major limitations (Lee et al. 2004, Mielczarski et al. 2005, Henderson & Demond 2007, Johnson et al. 2008, Jiao et al. 2009, Comba et al. 2011): ...
... Fortunately, reducible (CCl 4 , Cr VI ) and non-reducible (e.g. Zn II ) contaminants are quantitatively removed in Fe 0 beds (Morrison et al. 2002, Lai et al. 2006, Johnson et al. 2008, Noubactep et al. 2010. This strong argument and other considerations have yielded to the revision of the still prevailing view, that Fe 0 is a (strong) reducing agent for contaminant reductive transformation (Noubactep 2007, Noubactep 2008). ...
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.
... Available studies evokes that the hydraulic conductivity of a Fe 0 filter is a coefficient dependent on the porous system properties where the flow takes place (particle size, particle shape, distribution and shape of the pores), the properties of inflowing water (viscosity, density) and the saturation of the porous medium. In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. ...
... In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. The dwelling time of water in the Fe 0 system determines the degree of matrix and liquid exchanges. ...
... Published studies in the past two decades [22][23][24][25][26] revealed that physical characteristics of Fe 0 determine both the hydraulic conductivity and the water treatment performance a Fe 0 filter. Significant factors involve the depth of the Fe 0containing layer, the filter media setting arrangement, the size and the smoothness of the media particles. ...
... Available studies evokes that the hydraulic conductivity of a Fe 0 filter is a coefficient dependent on the porous system properties where the flow takes place (particle size, particle shape, distribution and shape of the pores), the properties of inflowing water (viscosity, density) and the saturation of the porous medium. In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. ...
... In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. The dwelling time of water in the Fe 0 system determines the degree of matrix and liquid exchanges. ...
... Published studies in the past two decades [22][23][24][25][26] revealed that physical characteristics of Fe 0 determine both the hydraulic conductivity and the water treatment performance a Fe 0 filter. Significant factors involve the depth of the Fe 0containing layer, the filter media setting arrangement, the size and the smoothness of the media particles. ...
... Available studies evokes that the hydraulic conductivity of a Fe 0 filter is a coefficient dependent on the porous system properties where the flow takes place (particle size, particle shape, distribution and shape of the pores), the properties of inflowing water (viscosity, density) and the saturation of the porous medium. In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. ...
... In fact, these influencing variables are inter-dependant [23][24][25][26].Among granular in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,6,24,26], the investigation is targeting on the Fe 0 diameter. The dwelling time of water in the Fe 0 system determines the degree of matrix and liquid exchanges. ...
... Published studies in the past two decades [22][23][24][25][26] revealed that physical characteristics of Fe 0 determine both the hydraulic conductivity and the water treatment performance a Fe 0 filter. Significant factors involve the depth of the Fe 0containing layer, the filter media setting arrangement, the size and the smoothness of the media particles. ...
Zero-valent iron (Fe0) has been used as a new efficient and affordable filter material for water treatment in
last two decades. Design guidance for cost-effective Fe0 filters still remain a challenge in the scientific
community. Nevertheless, there is a need to give the scientific foundation for the design and assessment of
Fe0 water filters technology. In this work, we suppose equal corrosion of single Fe0 particles and exploit the
radius loss ΔR to evaluate the degree of porosity reduction in the entire system. It is shown, long-lasting
filters should satisfy less than 53% Fe0 (v/v) for R0=1 mm.Some parameters such as the coefficient of
volumetric expansion with is function of the oxygen accessibility, the initial radius, the primary volume of
Fe0, the initial porosity of the system are use to establish a mathematical equation of Fe0 filters.The
hydraulic conductivity of the Fe0 filters system should be improved by included this equation in
modeling.The discussion of available data on porosity reduction of Fe0 system should be enhanced with this
model
... a key design parameter for water treatment [29,30]. Published studies suggest that the permeability of a Fe 0 filter is specific to (i) the nature of used granular media and initial porosity of the filter, (ii) size of the filter, (iii) the characteristics of inflowing water and (iv) the water flow velocity (flow through rate) [31][32][33][34][35][36][37][38][39][40]. In essence, these are four groups of interdependant influencing parameters. ...
... Herein the discussion is focused on the Fe 0 size which is ideally 'just' one of the granular media in hybrid systems (e.g. Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,11,36,40]. The flow through rate determines the residence time of water in the The effect of physical characteristics of Fe 0 and other media on the filter permeability and its inter-relationship with water treatment performance has been addressed in some details during the past two decades [29][30][31][32][33][34][35][36][37][38][39][40]. ...
... Fe 0 /anthracite, Fe 0 /gravel, Fe 0 /pumice, Fe 0 /sand) [1,11,36,40]. The flow through rate determines the residence time of water in the The effect of physical characteristics of Fe 0 and other media on the filter permeability and its inter-relationship with water treatment performance has been addressed in some details during the past two decades [29][30][31][32][33][34][35][36][37][38][39][40]. Relevant characteristics include (i) porosity, shape, size and smoothness of media grains (including Volumetric expansive is inherent to (aqueous) metal corrosion [19] and to iron corrosion at pH > 4.0 [20]. ...
The use of granular metallic iron (Fe0) as filter material is gaining acceptance in the field of water treatment. Few works have been directed at developing design guidance for efficient Fe0 filters. This note consolidates earlier works and provides the scientific basis for the design and evaluation of Fe0 filters for water treatment at any scale. The approach assumes uniform corrosion of individual Fe0 particles and utilises the radius loss (X = R0 - R) to asses the extent of porosity loss in the whole system. Results corroborate that, for R0 £ 1.0 mm, sustainable filters must content less than 53 % Fe0 (v/v). A universal equation of Fe0 filters is provided given X as a function of the initial radius R0, the initial volume of Fe0, the initial porosity of the filter and the coefficient of volumetric expansion (O2 availability). This equation should be routinely incorporated in simulations for modelling the hydraulic conductivity of Fe0 filters. The model improves the discussion of published data on porosity loss.
... A PRB transforms the contaminations into less harmful substances or immobilizes them while allowing groundwater to pass through. The contaminant is either biologically or chemically transformed and/or physically removed [4] [5] [8] [9] [10]. Several reactive materials have been used including activated carbon, compost, clays, Fe II -bearing minerals, metallic iron, wood chip or zeolites. ...
... The PRB technology using metallic iron (Fe 0 ) has gained acceptance as an effective passive remediation strategy for the treatment of a variety of organic and inorganic contaminants in groundwater [5] [8] [9] [10] [11] [12] [13] [14]. Even pathogens are efficiently removed in Fe 0 /H 2 O systems [15] [16]. ...
... Presently, around 120 Fe 0 -PRBs have been installed worldwide and are mostly achieving their remediation goals. Theoretically, barrier performance failure can be related to three issues: (i) continual build-up of mineral precipitates on the Fe 0 surface (surface passivation or reactivity loss), (ii) loss of pore space (porosity loss and/or loss of hydraulic permeability), and (iii) development of preferential flow paths or complete bypass of the Fe 0 barrier resulting in the loss of hydraulic control [8] [17]. ...
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.
... However, in spite of the abundance of laboratory work on reduction of explosives by ZVI (recent examples include Bandstra et al., 2005;Monteil-Rivera et al., 2005;Oh et al., 2002), there have been few well-documented field trials of this approach to remediating explosives contaminated sites (one exception being Comfort et al., 2003). Recently, we have reported results from a field demonstration of a full-scale ZVI PRB for removal of explosives from groundwater at the Cornhusker Army Ammunition Plant (CAAP) near Grand Island, Nebraska Johnson et al., 2008a;Johnson et al., 2008b). ...
... Details of the core sampling and analysis are given in (Johnson et al., 2008a;Johnson et al., 2008b). Prior to core sample collection the overlying soils were removed down to near the water table. ...
... Overall, the most significant result here is probably that the up-gradient impacted zone consistently gives rates of reduction of TNT and RDX that are comparable to the material from the ZVI-containing zone. This is undoubtedly due to reduced iron/sulfur phases that precipitated on the up-gradient aquifer material (due to aspects of the installation process that are somewhat peculiar to this site (Johnson et al., 2008a;Johnson et al., 2008b)). There is little evidence that reduced sulfur species (e.g., mackinawite) will reduce nitro compounds like TNT and RDX, but there is abundant evidence that various forms of Fe(II) will rapidly reduce nitro-containing organics (e.g., Haderlein et al., 2000). ...
We recently completed a pilot-scale permeable reactive barrier (PRB) with zero-valent iron (ZVI) to treat groundwater contaminated with explosives (TNT and RDX) at the Cornhusker Army Ammunition Plant (CAAP) near Grand Island, Nebraska. While the PRB at CAAP continues to be effective at removing explosives from the groundwater, the hydraulic performance is significantly reduced. This may be due to the accumulation of authigenic precipitates slightly up-gradient from the iron-containing zone. It is likely that the accumulation of new solid phases on the matrix materials in and around the treatment zone would also cause the system to be less effective at reducing contaminants. This, however, does not seem to be the case at CAAP. We report here that iron removed from the PRB is still quite reactive—with TNT and with RDX—when re- suspended and tested in laboratory batch experiments. Surprisingly, the up-gradient impacted samples showed reduction of TNT and RDX even though they did not contain ZVI. Also of note is that the core samples gave slower reduction of TNT than the dry ZVI/sand mixture, but the reverse was true for RDX. In all cases, however, the rates of TNT/RDX reduction by materials containing ZVI were within the range given by the de- sign guidelines.
... They became the most promising reactive medium for engineered walls (Tratneyk et al. 2003, Jambor et al. 2005, Henderson & Demond 2007, Thiruvenkatachari et al. 2008). Within 20 years the iron wall technology has developed to a standard technology for groundwater remediation and wastewater treatment with worldwide acceptance (Henderson & Demond 2007, Johnson et al. 2008, Comba et al. 2011). ...
... Despite its well-documented efficiency, the Fe 0 remediation currently has three major limitations (Lee et al. 2004, Mielczarski et al. 2005, Henderson & Demond 2007, Johnson et al. 2008, Jiao et al. 2009, Comba et al. 2011): ...
... Fortunately, reducible (CCl 4 , Cr VI ) and non-reducible (e.g. Zn II ) contaminants are quantitatively removed in Fe 0 beds (Morrison et al. 2002, Lai et al. 2006, Johnson et al. 2008, Noubactep et al. 2010). This strong argument and other considerations have yielded to the revision of the still prevailing view, that Fe 0 is a (strong) reducing agent for contaminant reductive transformation (Noubactep 2007, Noubactep 2008). ...
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.
... The PRB was installed using guar gum biopolymer (Day et al. 1999; Day and Schindler 2004), and ground water chemistry was monitored for a period of approximately 2 years (Environmental Security Technology Certification Program [ESTCP] 2008). The ground water data indicate that, although explosives removal by the PRB continues to be effective, there is evidence that the ground water flow rate through the PRB is significantly less than the regional ground water velocity 20 months after PRB installation (Nurmi et al. 2008; Johnson et al. 2008). As with other ZVI PRBs, significant changes in ground water chemistry occurred at this site as water flowed through the PRB. ...
... The ZVI PRB installed at CAAP has been previously described (ESTCP 2008; Johnson et al. 2008). Briefly, the PRB was 15 m long 3 4.5 m deep 3 0.9 m thick and was composed of 30% iron (Peerless Metals and Abrasives Inc., Detroit, Michigan) by weight in local sand. ...
... While the upgradient values remain fairly constant, the PRB values drop substantially over the study period. Based on data from Johnson et al. (2008), this is believed to be due to an increased rate of sulfate reduction over time. The sulfate behavior is significantly different than for carbonate alkalinity (Figure 1b ), calcium, and a number of other geochemical parameters (not shown), which did not show significant trends over time.Figure 2 shows dissolved sulfate and sulfide concentrations for a transect in the direction of ground water flow collected 20 months after installation of the PRB. ...
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.
... Ideally a PRB transforms the contaminations into less harmful substances or immobilizes them while allowing groundwater to pass through. The contaminant is either biologically or chemically transformed and/or physically removed [4,5,8910 . Several reactive materials have been used including activated carbon, compost, clays, Fe II -bearing minerals, metallic iron, wood chip or zeolites. ...
... Two of the most common designs are 'funnel and gate' and 'continuous walls' [3] and metallic iron (Fe 0 ) represents the most commonly used reactive material [5,11] . The PRB technology using metallic iron (Fe 0 ) has gained acceptance as an effective passive remediation strategy for the treatment of a variety of organic and inorganic contaminants in groundwater [5,891011121314 . Even pathogens are efficiently removed in Fe 0 /H 2 O sys- tems [15,16]. ...
... (i) continual build-up of mineral precipitates on the Fe 0 surface (surface passivation or reactivity loss), (ii) loss of pore space (porosity loss and/or loss of hydraulic permeability), and (iii) development of preferential flow paths or complete bypass of the Fe 0 barrier resulting in the loss of hydraulic control [8,17]. Despite two decades of extensive research, the mechanisms of contaminant removal in Fe 0 /H 2 O systems are not fully under- stood [10,181920. ...
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.
... « l'élimination » des contaminants dans le système Fe 0 /H2O(Noubactep 2015b). Reconnaître donc le Fe 0 comme générateur d'agents réducteurs permettrait de rediriger les efforts et surmonter les problèmes majeurs qui confinent encore aujourd'hui la technologie du Fe 0 en dépit de plusieurs décennies de recherches intensives.5 Les problèmes majeurs de la technologie du Fe 0 et qui entravent sa vulgarisationLa filtration sur lit de Fe 0 est considérée comme le moyen ingénié de filtration réactive le plus prometteur(Anderson 1885, Osgton 1885, Tucker 1892, Burton 1898, Don et Chisholm 1911, Tratneyk et al. 2003, Jambor et al. 2005, Henderson et Demond 2007, Thiruvenkatachari et al. 2008, Noubactep 2010a, et qui jouit en plus aujourd'hui d'une acceptation universelle grandissante(Henderson et Demond 2007, Johnson et al. 2008, Comba et al. 2011. Néanmoins, en dépit d'une efficience bien documentée, la technologie fait toujours face à une série de problèmes qui entravent sa standardisation et donc sa vulgarisation, Mielczarski et al. 2005, Henderson et Demond 2007, Johnson et al. 2008, Jiao et al. 2009, Comba et al. 2011, Noubactep 2011a, Ghauch 2015. ...
... Reconnaître donc le Fe 0 comme générateur d'agents réducteurs permettrait de rediriger les efforts et surmonter les problèmes majeurs qui confinent encore aujourd'hui la technologie du Fe 0 en dépit de plusieurs décennies de recherches intensives.5 Les problèmes majeurs de la technologie du Fe 0 et qui entravent sa vulgarisationLa filtration sur lit de Fe 0 est considérée comme le moyen ingénié de filtration réactive le plus prometteur(Anderson 1885, Osgton 1885, Tucker 1892, Burton 1898, Don et Chisholm 1911, Tratneyk et al. 2003, Jambor et al. 2005, Henderson et Demond 2007, Thiruvenkatachari et al. 2008, Noubactep 2010a, et qui jouit en plus aujourd'hui d'une acceptation universelle grandissante(Henderson et Demond 2007, Johnson et al. 2008, Comba et al. 2011. Néanmoins, en dépit d'une efficience bien documentée, la technologie fait toujours face à une série de problèmes qui entravent sa standardisation et donc sa vulgarisation, Mielczarski et al. 2005, Henderson et Demond 2007, Johnson et al. 2008, Jiao et al. 2009, Comba et al. 2011, Noubactep 2011a, Ghauch 2015. Les quatre principaux sont: (i) le paradigme révisé n'est pas encore univoquement accepté (problème 1); (ii) il existe une difficulté liée au contrôle externe de la réactivité (problème 2); (iii) la perte de réactivité des matériaux de Fe 0 utilisés (problème 3); et (iv) la perte de perméabilité du système (problème 4).6 La voie vers une innovation frugaleConstruire un filtre à Fe 0 efficient et durable revient surtout, outre la considération des approches de dimensionnement établis, à considérer et/ou solutionner les problèmes susprésentés. ...
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.
... If the porosity is 0.35, this would correspond to an average groundwater velocity over the lifetime of the PRB of ~1 cm/d, which is only 5% of the expected value. This result is consistent with the groundwater tracer data and numerical modeling presented in Johnson et al., 2007a. XPS analysis of the core samples also showed significant increases in the surficial concentrations of iron precipitates in the upgradient impacted zone, as well as in the PRB. ...
... There are a few ways in which a PRB can fail: (1) the PRB becomes plugged with oxides, biofilms, or other groundwater constituents, in which case the groundwater flow goes around the PRB; and (2) the iron particles become passivated and the reactivity of the particles decreases. Johnson et al. (2007a) that flow through the CAAP PRB may be reduced due to plugging. We show here that the reactivity of the particles in the PRB is still reactive and that particles in the upgradient impacted zone are also reactive toward TNT and RDX. ...
... If the porosity is 0.35, this would correspond to an average groundwater velocity over the lifetime of the PRB of ~1 cm/d, which is only 5% of the expected value 3 . This result is consistent with the groundwater tracer data and numerical modeling presented in (Johnson et al. 2007a). XPS analysis of the core samples also showed significant increases in the surficial concentrations of iron precipitates in the up-gradient impacted zone, as well in the PRB. ...
... There are a few ways in which a PRB can fail; (i) the PRB becomes plugged with oxides, biofilms, or other groundwater constituents in which case the groundwater flow goes around the PRB, and (ii) the iron particles become passivated and the reactivity of the particles decreases. Johnson et al., (2007a) showed that flow through the CAAP PRB may be reduced due to plugging. We show here, that the reactivity of the particles in the PRB are still reactive and that particles in the up-gradient impacted zone are also reactive toward TNT and RDX. ...
This final technical report documents the demonstration of a zero-valent iron (ZVI) permeable reactive barrier (PRB) for the removal of explosives from groundwater. The demonstration was conducted at the Cornhusker Army Ammunition Plant (CAAP) near Grand Island, Nebraska. The primary objective of this project was to evaluate the cost and performance of the ZVI PRB Performance of the PRB was evaluated by monitoring groundwater concentrations of explosives downgradient of the PRB. Data obtained during the demonstration were used to assess the costeffectiveness of this approach for long-term removal of explosives from groundwater. The primary advantages of ZVI PRBs for groundwater remediation are: 1. No aboveground remediation equipment is required 2. Rapid conversion of groundwater to reducing conditions 3. Low operation and maintenance costs 4. Long-lasting (>20 years) in situ treatment 5. Cost-effective. The cost-effective use of ZVI PRBs may be limited by the depth to groundwater and the ability to install the PRB in some geologic media. However, at sites without these physical constraints, the approach can be highly effective.
... This is apparent from the rebuttals to Noubactep's commentaries, where the original authors clearly have struggled to find constructive ways to respond. The rebuttals published in this journal (8)(9)(10)(11)(12) are representative: they conscientiously show that most of Noubactep's inductive arguments from specific data are erroneous (e.g., that the available data show removal of U VI involves only coprecipitation and not reduction (9,12)) and his deductive arguments from "accepted" principles are spurious (e.g., that degradation of chlorinated organics is unimportant because some metals are removed mainly by sequestration (8,12)). Since the points he makes about our paper are analogous, we will not repeat the same approach to rebut them. ...
... This is apparent from the rebuttals to Noubactep's commentaries, where the original authors clearly have struggled to find constructive ways to respond. The rebuttals published in this journal (8)(9)(10)(11)(12) are representative: they conscientiously show that most of Noubactep's inductive arguments from specific data are erroneous (e.g., that the available data show removal of U VI involves only coprecipitation and not reduction (9,12)) and his deductive arguments from "accepted" principles are spurious (e.g., that degradation of chlorinated organics is unimportant because some metals are removed mainly by sequestration (8,12)). Since the points he makes about our paper are analogous, we will not repeat the same approach to rebut them. ...
... Thus, intermittent flow operation of ZVI-based filters can delay pore clogging and requires lower hydraulic heads to maintain acceptable filtration rates as compared to constant operation via its impact on Fe phase transformations, but possibly also via its effects on CaCO 3 formation. Our findings on Fe-phase formation and (cyclic) transformation may also be relevant with respect to the previously observed loss in hydraulic conductivity 61,62 in ZVI-based permeable reactive barriers for groundwater remediation 61−64 that could possibly be mitigated by periodic operation of push or pull wells. ...
Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.
... Flow velocity and column boundary conditions influence the rate and long-term PRB performance. The reduction in the hydraulic conductivity of field-scale observed due to the precipitation, the plugging in laboratory column, and the accumulation of irons led to reduced reactivity, permeability of ZVI beds, and treatment efficiency (Johnson et al., 2008). An increase in groundwater velocity enhances source depletion. ...
... However, it is evident that pure Fe 0 (100% Fe 0 ) beds are not sustainable. However, permeability 10 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 loss and failure of Fe 0 barriers have been attributed to all possible arguments, but not really the expansive corrosion of Fe 0 [59,101,102]. An evident merit of considering the volumetric expansive nature of iron corrosion has been to end the discussion as to whether mixing Fe 0 and inert aggregates (e.g., gravel, MnO 2 , sand) is beneficial for packed beds. ...
Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.
... Some cases of failure have been reported (Morrison et al. 2006, Johnson et al. 2008) but reporting performance underachievments has not been the rule, even not in technical literature (Warner 2015). ...
Metallic iron (Fe 0) has been suggested as an affordable, applicable and efficient material for environmental remediation. Mixed to soil or filled in reactive walls, Fe 0 is a feasible pathway to control contamination in seepage waters. Available information in the literature however presents discrepant evidence on the efficiency of this (still innovative) technology. On basis of a profound literarture study over the past 160 years, it is outlined that these discrepancies are explained by the aqueous chemistry of iron (corrosion). Neglected aspects contributing to the apparent complexity of the Fe 0 /H2O system are outlined. It appears that designing an efficient and sustainable Fe 0 remediation system is a pure site-specific issue and that available data are not (really) comparable. In particular, it was made clear that Fe 0 barriers that have been successfully working for more than one decade were not designed according to any scientific basis. While the success or failure of implemented reactive walls can be rationalized, more systematic research is needed for a science-based design of efficient and sustainable Fe 0-based systems for environmental remediation.
Cited as:
Noubactep C. (2019): Metallic Iron for Environmental Remediation: Prospects and Limitations. Chap. 36, A Handbook of Environmental Toxicology: Human Disorders and Ecotoxicology. J.P.F. D’Mello (ed), CAB International, 531–544.
... However, after the experiments, parts of the filters were agglomerated by the products of the dezincification, mineral precipitations, and pore space cementation, which could have negative effects on the hydraulic conductivity after longer operation periods. Similar effects could be observed on Fe 0 filters in permeable reactive barriers where plugging occurred resulting from mineral precipitations (Hendersen and Demond, 2007;Johnson et al., 2008;Wilkin et al., 2002). This limitation might be overcome by using larger particles or a mixture with sand, but further investigations are needed here. ...
Brass shavings (CuZn45) were tested for their efficiency to remove Hg(II) from contaminated groundwater through amalgamation. The study was focused on long-term retention efficiency, the understanding of the amalgamation process and kinetics, and influences of filter surface alteration.
Column tests were performed with brass filters (thickness 3 to 9 cm) flushed with 1000 μg/L Hg solution for 8 hours under different flow rates (300 to 600 mL/h). Brass filters consistently removed >98% of Hg from solution independent of filter thickness and flow rate. In a long-term experiment (filter thickness 2 cm), Hg retention decreased from 96 to 92% within 2000 hours. Batch and column experiments for studying kinetics of Hg removal indicate ~100% Hg removal from solution within only 2 hours. Solid-phase mercury thermo-desorption analysis revealed that Hg(0) diffusion into the brass surface controls kinetics of mercury retention. Brass surface alteration could be observed, but did not influence Hg retention.
... Approximately 100 L of DI water containing 5000 mg/L bromide was injected in a shallow upgradient well of each PRB (AS2 and BS4) over a one-hour period. The amount of bromide tracer used here was determined by the method detailed by Johnson et al. (2008). Groundwater samples were collected periodically from downgradient wells over a two month period. ...
High concentrations of iron (Fe(II)) and manganese (Mn(II)) reductively dissolved from soil minerals have been detected in groundwater monitoring wells near many municipal solid waste landfills. Two in situ permeable reactive barriers (PRBs), comprised of limestone and crushed concrete, were installed downgradient of a closed, unlined landfill in Florida, USA, to remediate groundwater containing high concentrations of these metals. Influent groundwater to the PRBs contained mean Fe and Mn concentrations of approximately 30 mg/L and 1.62 mg/L, respectively. PRBs were constructed in the shallow aquifer (maximum depth 4.6 m below land surface) and groundwater was sampled from a network of nearby monitoring wells to evaluate barrier performance in removing these metals. PRBs significantly (p < 0.05) removed dissolved Fe and Mn from influent groundwater; Fe was removed from influent water at average rates of 91% and 95% (by mass) for the limestone and crushed concrete PRBs, respectively, during the first year of the study. The performance of the PRBs declined after 3 years of operation, with Fe removal efficiency decreasing to 64% and 61% for limestone and concrete PRBs, respectively. A comparison of water quality in shallow and deep monitoring wells showed a more dramatic performance reduction in the deeper section of the concrete PRB, which was attributed to an influx of sediment into the barrier and settling of particulates from the upper portions of the PRBs. Although removal of Fe and Mn from redox impacts was achieved with the PRBs, the short time frame of effectiveness relative to the duration of a full-scale remediation effort may limit the applicability of these systems at some landfills because of the construction costs required.
... ZVI has been shown to successfully treat water contaminated with heavy metals such as arsenic, chromium, copper, manganese, uranium, and zinc (3)(4)(5)(6). The contaminants removed from water by ZVI has been expanded to a broader range of pollutants, including freons, radionuclides, pesticides, phosphates, nutrients, explosives, viruses, and bacteria (7)(8)(9)(10). A major focus of this research has been the development of ZVI as a media for the treatment of contaminant plumes by permeable reactive barriers (PRBs) (11). ...
Three different granular zero-valent iron (ZVI) sources are assessed for the removal of Cu2+ and Zn2+ ions from aqueous solutions. For initial heavy metal concentrations of 50 µM the observed kinetics could be modelled via first order reaction rates. Differences in the observed kinetic rate of copper and zinc removal for different ZVI sources could not be explained by geometric or nitrogen based adsorption surface areas. Results obtained via varying both temperature and shaker rate indicate that in an environment which contains dissolved oxygen, ion diffusion and hence water flow rate will control the kinetics of copper and zinc removal by granular ZVI.
... As such, it is important to have an idea of the permeability loss over time for particular design of a PRB. It is evident from several literatures (e.g., Wilkin et al. 2002;Reardon 2005;Johnson et al. 2008) that the permeability loss occurs after the PRB has been installed in the subsurface. However, the data collected from field scale are difficult to use directly to determine the long-term permeability losses due to the fact that there are many uncontrolled parameters that affect the process. ...
Artificial neural networks (ANNs) were developed which enable evaluation of long-term permeability losses that occur in permeable reactive barriers (PRBs) used in groundwater remediation. The network architectures consist of non-changing input and output layer(s) while the optimal hidden layer types and structures were determined through trial-and-error. Fluid residence time within the PRB, pressure drop, inlet volumetric flow rate, dynamic viscosity of fluid, average porosity, average particle size and the length of the reactor were selected as the input parameters to estimate the output parameter, namely, permeability. Of all experimental data available for each ANN structure, 70% was used for training, 15% for validation and the remaining 15% for testing the ANN. The ANN structures were developed using a combination of soft computing techniques and mathematical association of varying physical parameters. Predictions obtained from the optimized ANN structures were compared with linear and non-linear regression models to assess their performance. The results indicate that ANN performs significantly better than the regression models and ANN modelling is a promising tool for the simulation and assessment of the permeability decline in PRBs.
... The most common remediation methods that have been previously studied are bioremediation, incineration, photodegradation, and advance oxidation processes. 45,46,[137][138][139][140][141][142][143][144] Advance oxidation processes (AOPs) have shown enormous potential for large scale treatment of recalcitrant, hazardous organic pollutants in water and soils. ...
Pollution poses serious threats to both the environmental and the organisms that depend on their environment for survival. Due to the toxicity of most contaminants, there is a dire need for remediation of polluted sites. Remediation studies were conducted on two high priority pollutants: 2,4,6-trinitrotoluene (TNT) and crude oil.
TNT was the most common explosive used in the 20th century. Continuous contamination has resulted in an urgent need for remediation. Fenton reagent provides an advanced oxidation process that is capable of remediating recalcitrant explosives, such as TNT. One drawback of Fenton chemistry is that the reaction requires acidic pH to prevent precipitation of iron. Our studies have investigated Fenton degradation of TNT at near neutral pH with several modifiers present: β-cyclodextrin, carboxymethyl-β-cyclodextrin, alcohols, and polyethylene glycol (PEG, MW 200, 400, or 600 g/mol). Fenton degradation was also carried out on other nitroaromatics to better understand the reaction mechanism with PEG 400. Further mechanistic studies investigated the production of nitrate and ammonium with and without PEG 400.
The Deepwater Horizon oil spill devastated the Gulf of Mexico and the surrounding wetlands. There are several mechanisms for degradation of oil released into aquatic environments. Bioremediation is one of the most important remediation methods; however degradation becomes stagnant in low nutrient waters. Furthermore, larger molecular weight alkanes and polycyclic aromatic hydrocarbons (PAHs) are not readily available for biodegradation. Transformation of these molecules often requires initial photodegradation. We have investigated the photochemical transformation of oil films with and without photocatalysts present. To better understand the photochemical transformations that occur to the Deepwater Horizon oil, we have conducted additional studies with dispersants present.
... This is the rule of thumb on which future research should be based in order to accelerate progress in knowledge. Moreover, available results from field Fe 0 filters[5,19,20,32,32,[108][109][110] have to be evaluated using Eq. 11 as well. ...
Filtration systems based on metallic iron (Fe0 filters) have been successfully used for water treatment over the past two decades. Relevant Fe0 filters expand from subsurface permeable reactive barriers (PRBs) to household filters. Fe0 filters systems are shown efficient for the remediation of biological and chemical contamination. Properly designing a Fe0 filter is finding a long-term balance between two major interdependent design parameters: (i) Fe0 reactivity, and (ii) filter permeability. Other relevant design parameters include (i) aqueous flow velocity, (ii) bed thickness, and (iii) water chemistry. Water chemistry includes nature and extent of contamination. To date, attempts to design more sustainable Fe0 filters have been mostly pragmatic as: (i) reactive Fe0 has failed to be considered as in-situ generator of contaminant collectors (and ‘secondary’ reducing agents), and (ii) the volumetric expansive nature of iron corrosion has been overlooked. On the other hand, valuable design criteria were available in the hydrometallurgical literature (cementation using elemental metals) prior to the advent of Fe0 filters. As a consequence the literature is full of seemingly controversial results which are easily conciliated by the physico-chemistry of the system. The present review is limited at identifying some misconceptions and demonstrating their proliferation. Tools for better analyses are recalled. Recent X-ray tomography data are used as illustration of how valuable data are insufficiently discussed. It is hoped that the present contribution will boost systematic research for the design of more sustainable Fe0 filters.
... This was attributed to secondary mineral precipitation reducing the permeability of the gate and promoting flow by-pass. Importantly the PRB was not keyed into an underlying aquitard in this instance (Johnson et al., 2008). ...
A permeable bio-reactive barrier (PRB) was installed at Casey Station, Antarctica in 2005/06 to intercept, capture and degrade petroleum hydrocarbons from a decade old fuel spill. A funnel and gate configuration was selected and implemented. The reactive gate was split into five separate cells to enable the testing of five different treatment combinations. Although different treatment materials were used in each cell, each treatment combination contained the following reactive zones: a zone for the controlled release of nutrients to enhance degradation, a zone for hydrocarbon capture and enhanced degradation, and a zone to capture excess nutrients. The materials selected for each of these zones had other requirements, these included; not having any adverse impact on the environment, being permeable enough to capture the entire catchment flow, and having sufficient residence time to fully capture migrating hydrocarbons. Over a five year period the performance of the PRB was extensively monitored and evaluated for nutrient concentration, fuel retention and permeability. At the end of the five year test period the material located within the reactive gate was excavated, total petroleum hydrocarbon concentrations present on the material determined and particle size analysis conducted. This work found that although maintaining media reactivity is obviously important, the most critical aspect of PRB performance is preserving the permeability of the barrier itself, in this case by maintaining appropriate particle size distribution. This is particularly important when PRBs are installed in regions that are subject to freeze thaw processes that may result in particle disintegration over time.
... Elemental iron (Fe 0 ) is a well known material for the abiotic removal of organic and inorganic contaminants from groundwater, soils, sediments, and waste streams1234567891011121314. Fe 0 is widely termed in the literature on permeable reactive barriers as zerovalent iron (ZVI) and is available as Fe 0 -based alloys (Fe 0 materials), mostly cast iron and low alloy steel. ...
In an attempt to characterize material intrinsic reactivity, iron dissolution from elemental iron materials (Fe0) was investigated under various experimental conditions in batch tests. Dissolution experiments were performed in a dilute solution of ethylenediaminetetraacetate (Na2-EDTA - 2 mM). The dissolution kinetics of eighteen Fe0 materials were investigated. The effects of individual operational parameters were assessed using selected materials. The effects of available reactive sites [Fe0 particle size (≤2.0 mm) and metal loading (2-64 g L–1)], mixing type (air bubbling, shaking), shaking intensity (0-250 min–1), and Fe0 pre-treatment (ascorbate, HCl and EDTA washing) were investigated. The data were analysed using the initial dissolution rate (kEDTA). The results show increased iron dissolution with increasing reactive sites (decreasing particle size or increasing metal loading), and increasing mixing speed. Air bubbling and material pre-treatment also lead to increased iron dissolution. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A unified experimental procedure for the investigation of processes in Fe0/H2O systems is suitable. Alternatively, a parameter (τEDTA) is introduced which could routinely used to characterize Fe0 reactivity under given experimental conditions.
... The processes occurring at the interface Fe 0 /H 2 O are of great interest for the use of metallic iron in environmental remediation (e.g. in Fe 0 /H 2 O systems). A great deal of work has been reported in this area during the past 20 years [1][2][3][4][5][6][7][8][9][10]. Since the seminal work of Matheson and Tratnyek [1], a substantial amount of literature concerning the removal mechanism of various contaminants in Fe 0 /H 2 O systems has been published. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... Iron-based alloys (metallic iron, elemental iron or Fe 0 materials) have been used as an abiotic contaminant reducing reagent for organic and inorganic groundwater contaminants for over 15 years [1][2][3][4][5][6][7][8][9][10][11][12][13]. In this context, Fe 0 materials are widely termed as zerovalent iron (ZVI) materials, contaminants have been denoted as reductates [14], and the bare surface of Fe 0 as reductant. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe0/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe0-materials (material A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe0/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min-1 and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (kEDTA). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on kEDTA from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe0 surface.
... Permeable reactive barriers of elemental iron (Fe 0 walls or remediation Fe 0 /H 2 O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1][2][3][4][5][6][7]. Since its development in 1990 by Canadian hydrogeologists numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [4]. ...
Metallic iron (Fe0) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe0 used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe0 as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe0 in the process of electron transfer at the Fe0/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe0/H2O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO2) by selectively removing metallic iron (Fe0) from reduced ilmenite (RI). This process involves an aqueous oxygen leaching step at near neutral pH. Oxygen leaching suffers from serious limitations imposed by limited mass transport rates of dissolved oxygen across the matrix of iron oxides from initial Fe0 oxidation. In a Fe0/H2O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O2) to the Fe0/H2O interface. Instead of this universal role of oxide layers on Fe0, improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe0/oxide/water interface.
... Two simple methods for material selection were recently introduced[70][71][72].Finally, it should be explicitly said that the approach considering remediation Fe 0 /H 2 O systems as primary filtration systems is not negating the complex chemical and physical processes (adsorption, co-precipitation, desorption, oxidation, reduction) occurring in it. On the contrary, this approach takes into account the fundamental fact that volumetric expansive iron corrosion needs free space to occur optimally and explains why so many species have been successfully removed in systems designed for individual compounds[1,13,50,73,74]. For example, Johnson et al.[74] reported decrease in sulfate, carbonate, and calcium concentration in a barrier designed to remove explosives from contaminated groundwater. ...
The use of metallic iron filters (Fe0 filters) has been discussed as a promising low-cost option for safe drinking water production at household level. Filter clogging due to the volumetric expansive nature of iron corrosion has been identified as the major problem of Fe0 filters. Mixing Fe0 and sand (yielding Fe0/sand filters) has been proposed as a tool to extent filter service life. However, no systematic discussion rationalizing Fe0:sand mixtures is yet available. This communication theoretically discussed suitable Fe0:sand proportions for efficient filters. Results suggested that Fe0/sand filters should not contain more that 50 vol-% Fe0 (25 wt-% when Fe0 is mixed with quartz). The actual Fe0 percentage in a filter will depend on its intrinsic reactivity.
... However, despite gaps in knowledge of the nature of metabolites, the state-of-the-art knowledge on the mechanism of contaminant removal by metallic iron (Fe 0 ) suggests that filtration on Fe 0 beds will be efficient at removing emerging contaminants and their metabolites. In fact, filtration on Fe 0 beds has been proven efficient for the quantitative removal of various contaminants (6)(7)(8)(9). Accordingly, regardless on the actual removal mechanism (hydrolysis, elimination, and reduction), a well-designed Fe 0 filter will successfully remove TCP and its metabolites from the aqueous phase as discussed below. ...
... At present it is suggested that the mechanism of permeability loss in Fe 0 PRBs is due to the accumulation of insoluble minerals within the PRB pore network [10,13]. [10,13,[20][21][22][23][24][25][26]. Another mechanism reported attributes the permeability loss to the build-up of H 2 gas, formed due to the hydrolysis of water during Fe 0 corrosion [11,27,28]. ...
Over the past 30 years the literature has burgeoned with in-situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe0) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe0 barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe0 corrosion in porous media is presented. Subsequent implications for the design of Fe0 filtration systems are modelled. The results suggest that: (i) for the pH range of natural waters (> 4.5), the high volumetric expansion of Fe0 during oxidation and precipitation dictates that Fe0 should be mixed with a non-expansive material; (ii) naturally-occurring solute precipitates have a negligible impact on permeability loss compared to Fe0 expansive corrosion; and (iii) the proliferation of H2 metabolising bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe0 PRB system as a physical (size-exclusion) water filter device.
... At present it is suggested that the mechanism of permeability loss in Fe 0 PRBs is due to the accumulation of insoluble minerals within the PRB pore network [10,13]. Relevant minerals include siderite (FeCO 3 ), aragonite (CaCO 3 ), and iron (hydr)oxides (e.g., Fe(OH) 2 , Fe(OH) 3 , FeOOH, Fe 2 O 3 , and Fe 3 O 4 ) [10,13,[20][21][22][23][24][25][26]. Another mechanism reported attributes the permeability loss to the build-up of H 2 gas, formed due to the hydrolysis of water during Fe 0 corrosion [11,27,28]. ...
Over the past 30 years the literature has burgeoned with in-situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe0) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe0 barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe0 corrosion in porous media is presented. Subsequent implications for the design of Fe0 filtration systems are modelled. The results suggest that: (i) for the pH range of natural waters (> 4.5), the high volumetric expansion of Fe0 during oxidation and precipitation dictates that Fe0 should be mixed with a non-expansive material; (ii) naturally-occurring solute precipitates have a negligible impact on permeability loss compared to Fe0 expansive corrosion; and (iii) the proliferation of H2 metabolising bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe0 PRB system as a physical (size-exclusion) water filter device.
... Recent theoretical works demonstrated that a Fe 0 -based filter should be considered as a system in which iron is corroded mostly by water and the micro-pollutants are sequestrated in the matrix of precipitation corrosion products [14,[34][35][36]. This view corroborates concordant reports regarding Fe 0 filters as a long-term sink for C, S, Ca, Si, Mg, and N [12,[37][38][39]. ...
Current knowledge of the basic principles underlying the design of Fe0 beds is weak. The volumetric expansive nature of iron corrosion was identified as the major factor determining the sustainability of Fe0 beds. This work attempts to systematically verify developed concepts. Pumice and sand were admixed to 200 g of Fe0 in column studies (50:50 volumetric proportion). Reference systems containing 100% of each material have been also investigated. The mean grain size of the used materials (in mm) were 0.28 (sand), 0.30 (pumice), and 0.50 (Fe0). The five studied systems were characterized (i) by the time dependent evolution of their hydraulic conductivity (permeability) and (ii) for their efficiency for aqueous removal of CuII, NiII, and ZnII (about 0.3 mM of each). Results showed unequivocally that (i) quantitative contaminant removal was coupled to the presence of Fe0, (ii) additive admixture lengthened the service life of Fe0 beds, and (iii) pumice was the best admixing agent for sustaining permeability while the Fe0/sand column was the most efficient for contaminant removal. The evolution of the permeability was well-fitted by the approach that the inflowing solution contained dissolved O2. The achieved results are regarded as starting point for a systematic research to optimize/support Fe0 filter design.
... Many works are available in literature on modeling PRB systems. Usually, numerical models are used to simulate groundwater flow and pollutant transport for different remediation scenarios (Gupta et al., 1999;Das, 2002;Elder et al., 2002;Burger et al., 2007;Johnson et al., 2008 ;Liu et al., 2011). Analytical models can be useful for preliminary evaluations, but they are not suitable to design full-scale systems (Craig et al., 2006). ...
Biobarriers (BBs) are a new type of in situ technology for the remediation of contaminated groundwater. In recent years, this remediation technique has been more and more used in place of traditional Pump & Treat systems or other in situ technologies both in the USA and Europe. This work reviews the main experiences of BBs. The literature contains reports about tests and application at different scales (laboratory, pilot and full scale), which have been analyzed according to the aim of the study, the operative conditions adopted, the filling material, the inoculation procedure, the electron acceptor and the nutrient delivery systems. Operative conditions were extremely varied. Lab scale experiments pointed out good results in terms of pollutant removal efficiency. Pilot scale tests and full-scale applications confirmed the results obtained at lab scale, but also pointed out the importance of design for a proficient remediation system. The experiences underlined some possible critical issues: (a) the filling material must ensure proper hydraulic properties, but it also must be capable of keeping biomass in the reactive zone; (b) inoculation is a critical step and measurements should be carried out to check the initial distribution of microorganisms and its evolution over time; (c) electron acceptor and nutrient supply is usually required, but oxygenation into anaerobic aquifers can be critical.
... A ZVI–PRB was installed at the site in November 2003. Details of the installation are given in Johnson et al. (2007). Briefly, a 15-m long by 4.5-m deep by 0.9-m wide (50 15 3 foot) thick trench containing 30% by weight granular iron (Peerless Abrasives and Metals, Detroit, MI) in local sand was emplaced in a trench perpendicular to groundwater flow using guar gum slurry to maintain the trench during installation. ...
Phylogenetic analyses of micro-organisms in groundwater samples from within and around a zero-valent iron (ZVI) permeable reactive barrier (PRB) identified several bacteria that could utilize H 2 produced dur-ing anaerobic ZVI corrosion and residual guar biopolymer used during PRB installation. Some of these bacteria are likely contributing to the removal of some groundwater constituents (i.e., sulfate). Bacteria concentrations increased from 10 1 cells mL 1 at 2 m upgradient to 10 2 cells mL 1 within the PRB and 10 4 cells mL 1 at 2 to 6 m downgradient. This trend possibly reflects increased substrate avail-ability through the PRB, although a corrosion-induced increase in pH beyond optimum levels within the iron layer (from pH 7 to 9.8) may have limited microbial colonization. Micro-organisms that were de-tected using quantitative PCR include (iron reducing) Geobacter sp. (putative methanogenic) Archaea, and (sulfate reducing) -proteobacteria such as Desulfuromonadales sp. Sequencing of DGGE bands also revealed the presence of uncultured dissimilatory metal reducers and Clostridia sp., which was dominant in a sample collected within the ZVI-PRB. These results suggest that indigenous microbial communities are likely to experience population shifts when ZVI-PRBs are installed to exploit several metabolic niches that evolve when ZVI corrodes. Whether such population shifts enhance ZVI-PRB performance requires further investigation.
... Two simple methods for material selection were recently introduced[70][71][72].Finally, it should be explicitly said that the approach considering remediation Fe 0 /H 2 O systems as primary filtration systems is not negating the complex chemical and physical processes (adsorption, co-precipitation, desorption, oxidation, reduction) occurring in it. On the contrary, this approach takes into account the fundamental fact that volumetric expansive iron corrosion needs free space to occur optimally and explains why so many species have been successfully removed in systems designed for individual compounds[1,13,50,73,74]. For example, Johnson et al.[74] reported decrease in sulfate, carbonate, and calcium concentration in a barrier designed to remove explosives from contaminated groundwater. ...
The use of metallic iron filters (Fe0 filters) has been discussed as a promising low-cost option for safe drinking water production at household level. Filter clogging due to the volumetric expansive nature of iron corrosion has been identified as the major problem of Fe0 filters. Mixing Fe0 and sand (yielding Fe0/sand filters) has been proposed as a tool to extent filter service life. However, no systematic discussion rationalizing Fe0/sand mixtures is yet available. This communication theoretically discussed suitable Fe0/sand proportions for efficient filters. Results suggested that Fe0/sand filters should not contain more that 50 vol% Fe0 (25 wt% when Fe0 is mixed with quartz). The actual Fe0 percentage in a filter will depend on its intrinsic reactivity.
... Applications of these innovative systems have included (i) groundwater remediation, (ii) drinking water treatment, and (iii) wastewater treatment. Successful quantitative removal of metals (e. g., Cd, Co, Cr, Cu, Pb, U, Zn), non metals (e. g., As, Mo, Se, Sn), anions (e. g., AsO4 3-, F -, MoO4 2-, NO3 -, PO4 3-, SeO4 2-, SO4 2-), organic dyes, organic compounds (e. g., benzene, chlorinated solvents, phenol, pesticides, toluene), bacteria, suspended solids, and viruses has been reported [16,[25][26][27][28][29][30]. Almost all studies dealing with pollutant removal were limited to proving the viability of Fe 0 /H 2 O systems for a few target pollutants and were not incorporated within a broad based understanding of Fe 0 remediation technology. ...
The availability of sustainable safe drinking water is one of Millennium Development Goals (MDGs). The world is on schedule to meet the MDG to ‘halve by 2015 the proportion of people without sustainable access to safe drinking water in 2000’. However, present technologies may still leave more than 600 million people without access to safe water in 2015. The objective of the present article is to present a concept for universal water filters primarily made of metallic iron (Fe0) and sand. The concept of Fe0/sand filters is based on a combination of: (i) recent developments in slow sand filtration and (ii) recent progress in understanding the process of contaminant removal in Fe0/H2O systems. The filters should be made up of more than 60% sand and up to 40% Fe0. The actual Fe0 proportion will depend on its intrinsic reactivity. The most important question to be answered regards the selection of the material to be used. The design of the filter can be derived from existing filters. It appears that Fe0/H2O based filters could be a technology with worldwide applicability.
... Iron-based alloys (metallic iron, elemental iron or Fe 0 materials ) have been used as an abiotic contaminant reducing reagent for organic and inorganic groundwater contaminants for over 15 years12345678910111213. In this context, Fe 0 materials are widely termed as zerovalent iron (ZVI) materials, contaminants have been denoted as reductates [14], and the bare surface of Fe 0 as reductant. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in 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.
An explosion occurs when a large amount of energy is suddenly released. This energy may come from an over-pressurized steam boiler, from the products of a chemical reaction involving explosive materials, or from a nuclear reaction that is uncontrolled. In order for an explosion to occur, there must be a local accumulation of energy at the site of the explosion, which is suddenly released. This release of energy can be dissipated as blast waves, propulsion of debris, or by the emission of thermal and ionizing radiation. Modern explosives or energetic materials are nitrogen-containing organic compounds with the potential for self-oxidation to small gaseous molecules (N
A series of dynamic-flow kinetic experiments were conducted to assess the removal rates of aqueous Cu2+ and Zn2+ ions by zero-valent iron (ZVI), a promising material for inclusion in cold-climate remediation applications. The influence of experimental parameters on contaminant removal rates, including aqueous flow rate, operating temperature, and the concentrations of ZVI, salt and dissolved oxygen, was investigated.
A mass transport model has been developed that accounts (i) aqueous-phase dispersion processes, (ii) film diffusion of contaminant ions to the reactive ZVI surface and (iii) the reactive removal mechanism itself. Regression to the experimental data indicated that when oxygen is present in the solution feed Cu2+ and Zn2+ removal processes were limited by film diffusion. In de-aerated solutions film diffusion still controls Cu2+ removal but a first-order surface reaction provides a better model for Zn2+ kinetics.
Using air as the equilibrium feed gas, the reactive proportion of the total surface area for contaminant removal was calculated to be 97% and 64% of the active spherically-assumed geometric area associated with ZVI media for Cu2+ and Zn2+, respectively.
Relative to a gas absorption area, determined in previous studies, the reactive proportion is less than 0.41% of the unreacted ZVI total surface area. These findings suggest that only part of the iron oxyhydroxide surface is reacting during ZVI based metal contaminant removal.
Permeable reactive barriers (PRBs) use solution-media interactions for contaminant removal from ground and surface waters. When located in a cold region subjected to freeze-thaw cycling, these liquid-solid phase interactions may be detrimental to PRB performance. This study presents a laboratory based assessment of contaminant removal using granular zero-valent iron (ZVI) under freeze-thaw conditions. Freeze-thaw induced changes to simulated PRBs, contained within Darcy boxes, subjected to 0, 21 and 42 freeze-thaw cycles were assessed using the flow of both reactive and conservative solutions. The reactive contaminants, Cu2 + and Zn2 + ions, were removed from the pore water during solution flow and freeze-thaw cycling. The hydraulic retention time within the reactive media as assessed by a conservative tracer, decreased by 15-18% after the first set of freeze-thaw cycling and remained constant after the second set of freeze-thaw cycling. A decrease in the uniformity of the particle size distribution and the agglomeration of particles were observed; however there was no change in the hydraulic conductivity within the variance associated with the calculation method. Analysis of the solid particles suggested the contaminant metals were not concentrated in the < 212 μm fines that were generated during the experiment. The results obtained suggest that ZVI is suitable for the inclusion in sequenced PRBs for the remediation of metal contaminants in cold regions.
Hydraulic performance of zero-valent iron and nano-sized zero-valent iron permeable reactive barriers – laboratory test. The hydrau-lic conductivity of zero-valent iron treatment zone of permeable reactive barriers (PRBs) may be decreased by reducing the porosity caused by gas production and solids precipitation. The study was undertaken in order to evaluate the infl uence of chloride and heavy metals on the hydraulic conductivity of ZVI and nZVI using hydraulic conductivity tests as well as continuous column tests. Results show that the lead retention in the solution had no impact for hydraulic con-ductivity in ZVI sample, on the other hand the calculated hydraulic conductivity losses in nZVI sample (from 4.10·10 –5 to 2.30·10 –5 m·s –1) were observed. Results also indicate that liquids con-taining the mixture of heavy metals may cause sig-nifi cant decrease in hydraulic conductivity (from 1.03·10 –4 to 1.51·10 –6 m·s –1). During the column tests, several number of clogging of the reactive material caused by iron hydroxides precipita-tion was observed over the course of injection of heavy metals solution. In contrast, the hydraulic conductivity of ZVI and nZVI is unaffected when they are permeated with chloride ions solution (k = 1.03·10 –4 m·s –1). Finally, the results indicate the need to take account of changes in the hydrau-lic conductivity of reactive materials for success-ful implementation of PRBs technology.
Granular Feo, used to reductively degrade a variety of contaminants in groundwater, corrodes in water to produce H2(g). A portion enters the Feo lattice where it is stored in trapping sites such as lattice defects and microcracks. The balance is dissolved by the groundwater where it may exsolve as a gas if its solubility is exceeded. Gas exsolution can reduce the effectiveness of the Feo treatment zone by reducing contact of the contaminant with iron surfaces or by diverting groundwater flow. It also represents a lost electron resource that otherwise could be involved in reductive degradation of contaminants. It is advantageous to select an iron for remediation purposes that captures a large proportion of the H2(g) it generates. This study examines various aspects of the H2(g) uptake process and has found 1) H2(g) does not have to be generated at the water/iron interface to enter the lattice. It can enter directly from the gas/water phases, 2) exposure of granular sponge iron to H2(g) reduces the dormant period for the onset of iron corrosion, 3) the large quantities of H2(g) generated by nano-Feo injected into a reactive barrier of an appropriate granular iron can be captured in the lattice of that iron, 4) Lattice-bound hydrogen represents an additional electron resource to Feo for remediation purposes and may be accessible using physical or chemical means.
The photochemical behavior of Deepwater Horizon oil collected from the surface of the Gulf of Mexico was studied. Thin oil films on water were subjected to simulated sunlight, and the resulting chemical and optical changes were observed. Polycyclic aromatic hydrocarbons (PAHs) showed substantial photodegradation, with larger PAHs being more rapidly decomposed. About 60% of the fluorescence at the excitation and emission maxima was observed with 12h of simulated solar irradiation equivalent to approximately 3d of sunlight. Synchronous scan fluorescence measurements showed 80-90% loss of larger PAHs with 12h of simulated solar irradiation. Absorbance of the oil decreased by only 20% over the same time period. Alkanes showed no significant photochemical losses. After irradiation, the toxicity of water in contact with the oil significantly increased, presumably due to the release of water soluble photoproducts that were toxic. Photocatalyst addition resulted in enhanced degradation rate for PAHs, and toxicity of the aqueous layer was altered in the presence of photocatalysts added to the oil film. Photochemistry is an important pathway for degradation of large PAHs, which are typically resistant to biodegradation.
This final technical report documents the demonstration of a zero-valent iron (ZVI) in situ treatment well (ISTW) to remove explosives from groundwater. The general purpose of the demonstration was to evaluate the efficacy of ZVI ISTW for treating explosives-contaminated groundwater.
The Monkstown zero-valent iron permeable reactive barrier (ZVI PRB), Europe's oldest commercially-installed ZVI PRB, had been treating trichloroethene (ICE) contaminated groundwater for about 10 years on the Nortel Network site in Northern Ireland when cores from the reactive zone were collected in December, 2006. Groundwater data from 2001-2006 indicated that TCE is still being remediated to below detection limits as the contaminated groundwater flows through the PRB. Ca and Fe carbonates, crystalline and amorphous Fe sulfides, and Fe (hydr)oxides have precipitated in the granular ZVI material in the PRB. The greatest variety of minerals is associated with a similar to 1-2 cm thick, slightly cemented crust on top (up-gradient influent entrance) of the ZVI section of the PRB and also with the discontinuous cemented ZVI material (similar to 23 cm thick) directly below it. The greatest presence of microbial communities also occurred in the up-gradient influent portion of the PRB compared to its down-gradient effluent section, with the latter possibly due to less favorable conditions (i.e., high pH, low oxygen) for microbial growth. The ZVI filings in the down-gradient effluent section of the PRB have a projected life span of >10 years compared with ZVI filings from the continuous to discontinuous cemented up-gradient ZVI section (upper similar to 25 cm) of the PRB, which may have a life span of only similar to 2-5 more years. Supporting Information from applied, multi-tracer testing indicated that restricted groundwater flow is occurring in the upper similar to 25 cm of the ZVI section and preferential pathways have also formed in this PRB over its 10 years of operation.
Zerovalent iron barriers have become a viable treatment for field-scale cleanup of various ground water contaminants. While contact with the iron surface is important for contaminant destruction, the interstitial pore water within and near the iron barrier will be laden with aqueous, adsorbed and precipitated Fe(II) phases. These freshly precipitated iron minerals could play an important role in transforming high explosives (HE). Our objective was to determine the transformation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and TNT (2,4,6-trinitrotoluene) by freshly precipitated iron Fe(II)/Fe(III) minerals. This was accomplished by quantifying the effects of initial Fe(II) concentration, pH, and the presence of aquifer solids (Fe(III) phases) on HE transformation rates. Results showed that at pH 8.2, freshly precipitated iron minerals transformed RDX, HMX, and TNT with reaction rates increasing with increasing Fe(II) concentrations. RDX and HMX transformations in these solutions also increased with increasing pH (5.8-8.55). By contrast, TNT transformation was not influenced by pH (6.85-8.55) except at pH values <6.35. Transformations observed via LC/MS included a variety of nitroso products (RDX, HMX) and amino degradation products (TNT). XRD analysis identified green rust and magnetite as the dominant iron solid phases that precipitated from the aqueous Fe(II) during HE treatment under anaerobic conditions. Geochemical modeling also predicted Fe(II) activity would likely be controlled by green rust and magnetite. These results illustrate the important role freshly precipitated Fe(II)/Fe(III) minerals in aqueous Fe(II) solutions play in the transformation of high explosives.
A new direct-push procedure has been developed for the purpose of conducting discrete-interval slug tests to define vertical
variations in hydraulic conductivity (K.) This approach is an extension of existing dual-tube methods developed for soil sampling.
In this procedure, nested rods (tubes) are simultaneously advanced to predetermined test intervals. The inner rods are then
removed and a screen is inserted into the formation for slug testing and possible water sampling. Once testing and sampling
are completed, the screen is retrieved, the inner rods reinserted, and the system is advanced to the next test interval. A
series of field tests were performed in a highly permeable sand and gravel aquifer to assess the effectiveness of this new
approach. Dual-tube profiling results were compared to multilevel slug tests conducted in conventional monitoring wells for
intervals in which hydraulic conductivity ranged from 175 ft/day to over 800 ft/day. An initial evaluation found that the
dual-tube profiling results were in good agreement (< or =12 percent difference) with K values obtained from multilevel slug
tests in the closest monitoring well. Two more-detailed profiles demonstrate that the dual-tube method can effectively delineate
small-scale vertical and horizontal variations in hydraulic conductivity. This field assessment shows that the dual-tube method
is an accurate and efficient procedure for obtaining information about spatial variations in hydraulic conductivity. This
information can be useful for selecting intervals for well installations, for assessment of various remediation alternatives,
and for identifying preferential flow paths and other features that can control contaminant movement in the subsurface. The
information is obtained without the need for permanent wells. Because this is a direct-push procedure, drill cuttings are
eliminated and the volume of development water generated is significantly reduced.
The working lifetime of permeable reactive barriers (PRBs) using Fe-0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe-0. Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe-0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe-0-sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to calcite and iron (oxyhydr)oxide. In the absence of flow information, the field sampling data were used to construct an SI-pH diagram, from which the extent of reaction with Fe-0, the potential for precipitate buildup, and relative residence time for the pore water were identified. Kinetic and heterogeneous flow effects were also discussed. To illustrate the application of chemical equilibrium modeling to the design and planning phase of PRBs, groundwater data from four PRB sites were analyzed. The analysis shows that up to 0.63 cm(3)/L solid could form in pore water using an average Fe-0 dissolution rate, leading to severe clogging of Fe-0 medium over a 10-yr period of operation.
This report discusses soil and ground-water sampling methods and procedures used to evaluate the long-term performance of permeable reactive barriers (PRBs) at two sites, Elizabeth City, NC, and the Denver Federal Center near Lakewood, CO. Both PRBs were installed in 1996 and have been monitored and studied since installation to determine their continued effectiveness for removing contaminants from ground water. An effective monitoring program requires appropriate soil and ground-water sampling techniques. For ground-water sampling, water quality indicator parameters must be monitored to determine when formation water has been accessed. Geochemical parameters include oxidation-reduction potential (ORP), pH, specific conductance, dissolved oxygen (DO), and turbidity. Field analytical methods are discussed along with interferences and issues which may arise when using certain electrodes or instruments in the field. Detailed field analytical procedures for hexavalent chromium, ferrous iron, alkalinity, hydrogen sulfide, and dissolved oxygen are described. Also included are laboratory methods for sample analyses for organics, cations, anions, and carbon. Sample collection methods, sample containers, preservation methods, and sample storage techniques are also discussed.
The U.S. Department of Defense (DOD) Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) sponsored a project to assess performance and longevity issues at DOD permeable reactive barrier (PRIB) sites. The goal of this project was to evaluate short- and long-term performance issues associated with permeable reactive barriers (PRBs) installed at several United States Department of Defense (DoD) sites, A PRB is a passive, in situ technology, in which natural groundwater flow brings contaminants into contact with a reactive or adsorptive material that removes the dissolved contaminants and protects down gradient receptors. Therefore, PRBs have potentially lower life cycle costs compared to an equivalent pump-and-treat system. The key regulatory driver for the technology is the proven ability of common barrier materials, such as elemental iron, to meet groundwater cleanup standards for many common contaminants, including chlorinated solvents and certain heavy metals. Regulatory interest in this project was driven by the two challenges involved in implementing PRBs, namely, their longevity and hydraulic performance. The Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) sponsored this project. The Naval Facilities Engineering Service Center (NFESC) was the lead agency for the DoD project. Eattelle, under contract to NFESC, planned and implemented the technical scope and has prepared this report to summarize the results.
This final technical report documents the demonstration of a zero-valent iron (ZVI) permeable reactive barrier (PRB) for the removal of explosives from groundwater. The demonstration was conducted at the Cornhusker Army Ammunition Plant (CAAP) near Grand Island, Nebraska. The primary objective of this project was to evaluate the cost and performance of the ZVI PRB Performance of the PRB was evaluated by monitoring groundwater concentrations of explosives downgradient of the PRB. Data obtained during the demonstration were used to assess the costeffectiveness of this approach for long-term removal of explosives from groundwater. The primary advantages of ZVI PRBs for groundwater remediation are: 1. No aboveground remediation equipment is required 2. Rapid conversion of groundwater to reducing conditions 3. Low operation and maintenance costs 4. Long-lasting (>20 years) in situ treatment 5. Cost-effective. The cost-effective use of ZVI PRBs may be limited by the depth to groundwater and the ability to install the PRB in some geologic media. However, at sites without these physical constraints, the approach can be highly effective.
Phylogenetic analyses of micro-organisms in groundwater samples from within and around a zero-valent iron (ZVI) permeable reactive barrier (PRB) identified several bacteria that could utilize H 2 produced dur-ing anaerobic ZVI corrosion and residual guar biopolymer used during PRB installation. Some of these bacteria are likely contributing to the removal of some groundwater constituents (i.e., sulfate). Bacteria concentrations increased from 10 1 cells mL 1 at 2 m upgradient to 10 2 cells mL 1 within the PRB and 10 4 cells mL 1 at 2 to 6 m downgradient. This trend possibly reflects increased substrate avail-ability through the PRB, although a corrosion-induced increase in pH beyond optimum levels within the iron layer (from pH 7 to 9.8) may have limited microbial colonization. Micro-organisms that were de-tected using quantitative PCR include (iron reducing) Geobacter sp. (putative methanogenic) Archaea, and (sulfate reducing) -proteobacteria such as Desulfuromonadales sp. Sequencing of DGGE bands also revealed the presence of uncultured dissimilatory metal reducers and Clostridia sp., which was dominant in a sample collected within the ZVI-PRB. These results suggest that indigenous microbial communities are likely to experience population shifts when ZVI-PRBs are installed to exploit several metabolic niches that evolve when ZVI corrodes. Whether such population shifts enhance ZVI-PRB performance requires further investigation.
Multicomponent reactive transport modeling was conducted for the
permeable reactive barrier at the Coast Guard Support Center near
Elizabeth City, North Carolina. The zero-valent iron barrier was
installed to treat groundwater contaminated by hexavalent chromium and
chlorinated solvents. The simulations were performed using the reactive
transport model MIN3P, applied to an existing site-specific conceptual
model. Reaction processes controlling the geochemical evolution within
and down gradient of the barrier were considered. Within the barrier,
the treatment of the contaminants, the reduction of other electron
acceptors present in the ambient groundwater, microbially mediated
sulfate reduction, the precipitation of secondary minerals, and
degassing of hydrogen gas were included. Down gradient of the barrier,
water-rock interactions between the highly alkaline and reducing pore
water emanating from the barrier and the aquifer material were
considered. The model results illustrate removal of Cr(VI) and the
chlorinated solvents by the reactive barrier and highlight that
reactions other than the remediation reactions most significantly affect
the water chemistry in the barrier. In particular, sulfate reduction and
iron corrosion by water control the evolution of the pore water while
passing through the treatment system. The simulation results indicate
that secondary mineral formation has the potential to decrease the
porosity in the barrier over the long term and illustrate that the
precipitation of minerals is concentrated in the upgradient portion of
the barrier. Two-dimensional simulations demonstrate how preferential
flow can affect the reduction of electron acceptors, the consumption of
the treatment material, and the formation of secondary minerals. In
addition, the model results indicate that deprotonation and the
adsorption of cations down gradient of the barrier can potentially
explain the observed pH buffering.
Multicomponent reactive transport modeling was conducted to analyze and quantify the acid neutralization reactions observed in a column experiment. Experimental results and the experimental procedures have been previously published. The pore water geochemistry was described by dissolution and precipitation reactions involving primary and secondary mineral phases. The initial amounts of the primary phases ankerite-dolomite, siderite, chlorite, and gypsum were constrained by mineralogical analyses of the tailings sample used in the experiment. Secondary gibbsite was incorporated into the model to adequately explain the changes in pH and concentration changes of Al in the column effluent water. The results of the reactive transport modeling show that the pH of the column effluent water can be explained by dissolution reactions of ankerite-dolomite, siderite, chlorite, and secondary gibbsite. The modeling results also show that changes in Eh can be explained by dissolution of ferrihydrite during the experiment. In addition, the modeling results show that the kinetically limited dissolution of chlorite contributes the largest mass of dissolved Mg and Fe (II) in the effluent water, followed by ankerite-dolomite, which contributes substantially less. In summary, reactive transport modeling based on detailed geochemical and mineralogical data was successful to quantitatively describe the changes in pH and major ions in the column effluent.
The dehalogenation of chlorinated solvents by zero-valence iron has recently become the subject of intensive research and development as a potentially cost-effective, passive treatment for contaminated groundwater through reactive barriers. Because of its successful application in the laboratory and other field sites, the X-625 Groundwater Treatment Facility (GTF) was constructed to evaluate reactive barrier technology for remediating trichloroethylene (TCE)-contaminated groundwater at the Portsmouth Gaseous Diffusion Plant (PORTS). The X-625 GTF was built to fulfill the following technical objectives: (1) to test reactive barrier materials (e.g., iron filings) under realistic groundwater conditions for long term applications, (2) to obtain rates at which TCE degrades and to determine by-products for the reactive barrier materials tested, and (3) to clean up the TCE-contaminated water in the X-120 plume. The X-625 is providing important field-scale and long-term for the evaluation and design of reactive barriers at PORTS. The X-625 GTS is a unique facility not only because it is where site remediation is being performed, but it is also where research scientists and process engineers can test other promising reactive barrier materials. In addition, the data collected from X-625 GTF can be used to evaluate the technical and economic feasibility of replacing the activated carbon units in the pump-and-treat facilities at PORTS.
This report briefly reviews issues regarding the implementation of the zero-valent iron permeable reactive barrier (PRB) technology at sites managed by the U.S. Department of Energy (DOE). Initially, the PRB technology, using zero-valent iron for the reactive media, was received with great enthusiasm, and DOE invested millions of dollars testing and implementing PRBs. Recently, a negative perception of the technology has been building. This perception is based on the failure of some deployments to satisfy goals for treatment and operating expenses. The purpose of this report, therefore, is to suggest reasons for the problems that have been encountered and to recommend whether DOE should invest in additional research and deployments. The principal conclusion of this review is that the most significant problems have been the result of insufficient characterization, which resulted in poor engineering implementation. Although there are legitimate concerns regarding the longevity of the reactive media, the ability of zero-valent iron to reduce certain chlorinated hydrocarbons and to immobilize certain metals and radionuclides is well documented. The primary problem encountered at some DOE full-scale deployments has been an inadequate assessment of site hydrology, which resulted in misapplication of the technology. The result is PRBs with higher than expected flow velocities and/or incomplete plume capture. A review of the literature reveals that cautions regarding subsurface heterogeneity were published several years prior to the full-scale implementations. Nevertheless, design and construction have typically been undertaken as if the subsurface was homogeneous. More recently published literature has demonstrated that hydraulic heterogeneity can cause so much uncertainty in performance that use of a passive PRB is precluded. Thus, the primary conclusion of this review is that more attention must be given to site-specific issues. Indeed, the use of a passive PRB requires an unusually comprehensive hydrologic characterization so that the design can be based on a thorough understanding of subsurface heterogeneity rather than on average values for hydraulic parameters. Scientists and engineers are capable of conducting the level of investigation required. However, design costs will increase, and the pre-design field work may demonstrate that a passive PRB is not suitable for a particular site. In such cases, an option to consider is hydraulic augmentation, such as pumping (in which the system is no longer passive) or gravity flow from drains. In these circumstances, operation of the treatment media is under known hydraulic conditions. These systems typically contain the treatment media in a vault or in drums. Most of the media problems in such systems have been related to the exclusion of air and can be addressed by better engineering design or by frequent maintenance. Finally, a number of outstanding issues require resolution for further application of this technology. Of particular interest to DOE is resolving the removal mechanisms for uranium and technetium. Few data are available for the latter, and for the former, the technical literature is contradictory. Determining the mechanisms has long-term cost implications; engineers must consider whether it is appropriate to remove or simply abandon a barrier that is no longer functioning. Other issues that are unresolved include determining how hydraulic performance is affected by the emplacement method and quantifying the effects of varying groundwater types on barrier longevity.
The U.S. Geological Survey's Modular Ground-Water Flow Model assumes that model nodes are in the center of cells and that transmissivity is constant within a cell . Based on these assumptions, the model calculates coefficients, called conductance, that are multiplied by head difference to determine flow between cells. Although these are common assumptions in finite-difference models, other assumptions are possible. A new option to the model program reads conductance as input data rather than calculating it. This option allows the user to calculate conductance outside of the model . The user thus has the flexibility to define conductance using any desired assumptions. For a water-table condition, horizontal conductance must change as water level varies. To handle this situation, the new option reads conductance divided by thickness (CDT) as input data. The model calculates saturated thickness and multiplies it by CDT to obtain conductance. Thus, the user is still free from the assumptions of centered nodes and constant transmissivity in cells. The model option is written in FORTRAN 77 and is fully compatible with the existing model. This report documents the new model option; it includes a description of the concepts, detailed input instructions, and a listing of the code.
The operating life of an Fe(0)-based permeable reactive barrier (PRB) is limited due to chemical reactions of Fe(0) in groundwater. The relative contributions from mineral precipitation, gas production, and microbial activity to the degradation of PRB performance have been uncertain. In this controlled field study, nitrate-rich, site groundwater was treated by Fe(0) in large-volume, flow-through columns to monitor the changes in chemical and hydraulic parameters over time. Tracer tests showed a close relationship between hydraulic residence time and pH measurements. The ionic profiles suggest that mineral precipitation and accumulation is the primary mechanism for pore clogging around the inlet of the column. Accumulated N(2) gas generated by biotic processes also affected the hydraulics although the effects were secondary to that of mineral precipitation. Quantitative estimates indicate a porosity reduction of up to 45.3% near the column inlet over 72 days of operation under accelerated flow conditions. According to this study, preferential flow through a PRB at a site with similar groundwater chemistry should be detected over approximately 1 year of operation. During the early operation of a PRB, pH is a key indicator for monitoring the change in hydraulic residence time resulting from heterogeneity development. If the surrounding native material is more conductive than the clogged Fe-media, groundwater bypass may render the PRB ineffective for treating contaminated groundwater.
The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17−33 mM HCO3-) and sulfate (7−20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3- ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.
The degradation of 2,4,6-trinitrotoluene (TNT) and other explosives by zero-valent iron (ZVI) is rapid and, as a result, potentially useful for both ex situ ground water “pump-and-treat” systems and in situ permeable reactive barriers. However, the usefulness of ZVI in either configuration may be limited by reaction-induced reduction in both hydraulic conductivity (K) and reactivity (as represented by the surface area–normalized rate constant, ksa). The impacts of dissolved oxygen and TNT on K and ksa are examined here using field and laboratory columns. The data suggest that K reduction in ZVI columns can be significant when dissolved oxygen is present. However, when TNT is present at approximately the same concentration (10 mg/L), it does not cause significant reduction in K. In contrast, TNT causes a significant reduction in ksa, while dissolved oxygen appears to have relatively little impact on the reactivity of the columns toward TNT.
The hydraulic and geochemical performance of a 366 m long permeable reactive barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluated. The funnel and gate system, which was installed in 1996 to intercept and remediate ground water contaminated with chlorinated aliphatic hydrocarbons (CAHs), contained four 12.2 m wide gates filled with zero-valent iron. Ground water mounding on the upgradient side of the PRB resulted in a tenfold increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRB. Water balance calculations for April 1997 indicate that about 75 % of the ground water moving toward the PRB from upgradient areas moved through the gates. The rest of the water either accumulated on the upgradient side of the PRB or bypassed the PRB. Chemical data from monitoring wells screened down-gradient, beneath, and at the ends of the PRB indicate that contaminants had not bypassed the PRB, except in a few isolated areas. Greater than 99 % of the CAH mass entering the gates was retained by the iron. Fifty-one percent of the CAH carbon entering one gate was accounted for in dissolved C1 and C2 hydrocarbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates displaced contaminated ground water at a distance of at least 3 m downgradient from the PRB by the end of 1997. Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate could reduce gate porosity by about 0.35 % per year. Results from this study indicate that funnel and gate systems containing zero-valent iron can effectively treat ground water contaminated with CAHs. However, the hydrologic impacts of the PRB on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.
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.
A small-scale field test was initiated in September 1994 to evaluate the in situ remediation of groundwater contaminated with chromate using a permeable reactive barrier composed of a mixture of zero-valent Fe, sand and aquifer sediment. The site used was an old chrome-plating facility located on a U.S. Coast Guard air base near Elizabeth City, North Carolina. Dissolved chromate concentrations were reduced to less than 0.01 mg/L via reduction from Cr(VI) to Cr(III) as a result of the corrosion of the Fe. As the Fe corrodes, pH increases, oxidation-reduction potential declines, dissolved oxygen is consumed, and Fe(II) is generated. Mineral phases formed as a result of the Fe corrosion include ferrous sulfides and various Fe oxides, hydroxides, and oxyhydroxides.
One of the newest and most promising remediation techniques for the treatment of contaminated groundwater and soil is the reactive barrier wall (commonly known as PRB for permeable reactive barrier or reactive barrier). Although a variety of treatment media and strategies are available, the most common technique is to bury granular iron in a trench so that contaminated groundwater passes through the reactive materials, the contaminants are removed and the water becomes `clean'. The principal advantages of the technique are the elimination of pumping, mass excavation, off-site disposal, and a very significant reduction in costs. The use of this technology is now becoming better known and implemented. Special construction considerations need to be made when planning the installation of reactive barriers or PRBs to ensure the design life of the installation and to be cost-effective. Geotechnical techniques such as slurry trenching, deep soil mixing, and grouting can be used to simplify and improve the installation of reactive materials relative to conventional trench and fill methods. These techniques make it possible to reduce the hazards to workers during installation, reduce waste and reduce costs for most installations. To date, most PRBs have been installed to shallow depths using construction methods such as open trenching and/or shored excavations. While these methods are usable, they are limited to shallow depths and more disruptive to the site's normal use. Geotechnical techniques are more quickly installed and less disruptive to site activities and thus more effective. Recently, laboratory studies and pilot projects have demonstrated that geotechnical techniques can be used successfully to install reactive barriers. This paper describes the factors that are important in designing a reactive barrier or PRB installation and discusses some of the potential problems and pitfalls that can be avoided with careful planning and the use of geotechnical techniques.
A computer program for simulating ground-water flow in three dimensions is presented. This report includes detailed explanations of physical and mathematical concepts on which the model is developed. Ground-water flow within the aquifer is simulated by using a block-centered finite-difference approach. The program is written in Fortran 77 and has a modular structure, which permits the addition of new packages to the program without modifying existing packages.
A pilot-scale permeable reactive barrier (PRB) consisting of granular iron was installed in May 1995 at an industrial facility in New York to evaluate the use of this technology for remediation of chlorinated volatile organic compounds (VOCs) in groundwater. The performance of the barrier was monitored over a 2-year period. Groundwater velocity through the barrier was determined using water level measurements, tracer tests, and in situ velocity measurements. While uncertainty in the measured groundwater velocity hampered interpretation of results, the VOC concentration data from wells in the PRB indicated that VOC degradation rates were similar to those anticipated from laboratory results. Groundwater and core analyses indicated that formation of carbonate precipitates occurred in the upgradient section of the iron zone, however, these precipitates did not appear to adversely affect system performance. There was no indication of microbial fouling of the system over the monitoring period. Based on the observed performance of the pilot, a full-scale iron PRB was installed at the site in December 1997.
As permeable reactive barriers containing zero-valent iron are becoming more widely used to remediate contaminated groundwaters, there remains much uncertainty in predicting their long-term performance. This study focuses on two factors affecting performance and lifetime of the granular iron media: plugging at the treatment zone entrance and precipitation in the bulk iron media. Plugging at the system entrance is due principally to mineral precipitation promoted by dissolved oxygen in the influent groundwater and is an issue in aerobic aquifers or in above-ground canister tests. Designs to minimize plugging in field applications where the groundwater is oxygenated include the use of larger iron particles and admixing sand of comparable size with the iron particles. Beyond the entrance zone, the groundwater in anaerobic and mineral precipitation leads to porosity losses in the bulk iron media, potentially reducing flow through the treatment zone. The nature of the mineral precipitation and the factors that affect extent of mineral precipitation have been examined by a variety of tools, including tracer tests, aqueous inorganic profiles, and surface analytical techniques. At short treatment times, porosity losses as measured by tracer tests are due mainly to Fe(OH)(2) precipitates and possible entrapment of a film of hydrogen gas on the iron surfaces. Over longer treatment times, precipitation of Fe(OH)(2) and FeCO(3) in low carbonate waters and of Fe(OH)(2), FeCO(3) and CaCO(3) in higher carbonate waters begin to dominate porosity losses. The control of pH within the iron media by addition of ferrous sulfide was shown not to reduce significantly calcium and carbonate precipitates, indicating that mineral precipitation is controlled by more than simple carbonate equilibrium considerations.
Data collected from a field study of in situ zero-valent iron treatment for TCE were analyzed in the context of coupled transport and reaction processes. The focus of this analysis was to understand the behavior of chemical components, including contaminants, in groundwater transported through the iron cell of a pilot-scale funnel and gate treatment system. A multicomponent reactive transport simulator was used to simultaneously model mobile and nonmobile components undergoing equilibrium and kinetic reactions including TCE degradation, parallel iron dissolution reactions, precipitation of secondary minerals, and complexation reactions. The resulting mechanistic model of coupled processes reproduced solution chemistry behavior observed in the iron cell with a minimum of calibration. These observations included the destruction of TCE and cis-1,2-DCE; increases in pH and hydrocarbons; and decreases in EH, alkalinity, dissolved O2 and CO2, and major ions (i.e., Ca, Mg, Cl, sulfate, nitrate). Mineral precipitation in the iron zone was critical to correctly predicting these behaviors. The dominant precipitation products were ferrous hydroxide, siderite, aragonite, brucite, and iron sulfide. In the first few centimeters of the reactive iron cell, these precipitation products are predicted to account for a 3% increase in mineral volume per year, which could have implications for the longevity of favorable barrier hydraulics and reactivity. The inclusion of transport was key to understanding the interplay between rates of transport and rates of reaction in the field.
Although progress has been made toward understanding the surface chemistry of granular iron and the mechanisms through which it attenuates groundwater contaminants, potential long-term changes in the solute transport properties of granular iron media have until now received relatively little attention. As part of column investigations of alterations in the reactivity of granular iron, studies using tritiated water (3H(2)O) as a conservative and non-partitioning tracer were periodically conducted to independently isolate transport-related effects on performance from those more directly related to surface reactivity. Hydraulic residence time distributions (HRTDs) within each of six 39-cm columns exposed to bicarbonate solutions were obtained over the course of 1100 days of operation. First moment analyses of the data revealed generally modest increases in mean pore water velocity (v) over time, indicative of decreasing water-filled porosity. Gravimetric measurements provided independent estimates of water-filled porosity that were initially consistent with those obtained from 3H(2)O tracer tests, although at later times, porosities derived from gravimetric measurements deviated from the tracer test results owing to mineral precipitation. The combination of gravimetric measurements and 3H(2)O tracer studies furnished estimates of precipitated mineral mass; depending on the assumed identity of the predominant mineral phase(s), the porosity decrease associated with solute precipitation amounted to 6-24% of the initial porosity. The accumulation of mineral and gas phases led to the formation of regions of immobile water and increased spreading of the tracer pulse. Application of a dual-region transport model to the 3H(2)O breakthrough curves revealed that the immobile water-filled region increased from initially negligible values to amounts ranging between 3% and 14% of the total porosity in later periods of operation. For the aged columns, mobile-immobile mass transfer coefficients (k(mt)) were generally in the range of 0.1-1.0 day(-1) and reflected a slow exchange of 3H(2)O between the two regions. Additional model calculations incorporating sorption and reaction suggest that although changes in HRTD can have an appreciable effect on trichloroethylene (TCE) transformation, the effect is likely to be minor relative to that stemming from passivation of the granular iron surface.
A method incorporating laboratory analysis of constituents that formed as reaction products was developed and used to determine the flux of groundwater through a zerovalent iron-based permeable reactive barrier (PRB) installed to treat U-contaminated groundwater. Concentrations of three nonvolatile constituents (Ca, U, and V) that formed as reaction products in the PRB were analyzed in 279 samples. Areal distributions of the reaction products indicate that groundwater flowed through all portions of the PRB and that nearly the entire volume of reactive material is treating the groundwater. Almost 9 t of calcium carbonate precipitated in the PRB during the first 2.7 yr of operation, but only 24 kg of combined U- and V-bearing minerals precipitated during the same period. Concentration gradients of Ca, U, and V dissolved in the groundwater indicate that a hydraulically upgradient portion of the PRB lost some reactivity during the first 2.7 yr of operation. Calculations that partially couple porosity changes to ZVI reactivity suggest that loss of reactivity may be more limiting than porosity reduction for long-term performance of the PRB. Calculations using groundwater concentration gradients and solid-phase concentrations indicate that the mean groundwater flux ranged from 11 to 24 L/min, considerably less than the design flux of 185 L/min. Flux values calculated with all three constituents were in good agreement. This method provides a more accurate determination of groundwater flux than is possible with flow sensor measurements, dissolved tracers, or Darcy's law computations.
Geochemical and microbiological factors that control long-term performance of subsurface permeable reactive barriers were evaluated at the Elizabeth City, North Carolina, and the Denver Federal Center, Colorado, sites. These ground water treatment systems use zero-valent iron filings (Peerless Metal Powders Inc.) to intercept and remediate chlorinated hydrocarbon compounds at the Denver Federal Center (funnel-and-gate system) and overlapping plumes of hexavalent chromium and chlorinated hydrocarbons at Elizabeth City (continuous wall system). Zero-valent iron at both sites is a long-term sink for carbon, sulfur, calcium, silicon, nitrogen, and magnesium. After about four years of operation, the average rates of inorganic carbon (IC) and sulfur (S) accumulation are 0.09 and 0.02 kg/m2/year, respectively, at Elizabeth City where upgradient waters contain <400 mg/L of total dissolved solids (TDS). At the Denver Federal Center site, upgradient ground water contains 1000 to 1200 mg/L TDS and rates of IC and S accumulation are as high as 2.16 and 0.80 kg/m2/year, respectively. At both sites, consistent patterns of spatially variable mineral precipitation and microbial activity are observed. Mineral precipitates and microbial biomass accumulate the fastest near the upgradient aquifer-Fe0 interface. Maximum net reductions in porosity due to the accumulation of sulfur and inorganic carbon precipitates range from 0.032 at Elizabeth City to 0.062 at the Denver Federal Center (gate 2) after about four years. Although pore space has been lost due the accumulation of authigenic components, neither site shows evidence of pervasive pore clogging after four years of operation.
Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.
Thermogravimetric analysis (TGA) combined with X-ray diffraction (XRD) was used to identify mineral phases and determine corrosion rates of granular iron samples from a 2-yr field column study. Similar to other studies, goethite, magnetite, aragonite, and calcite were found to be the major precipitated minerals, with Fe2(OH)2CO3 and green rust as minor phases. Based on TGA-mass spectrometry (MS) analysis, Fe0 corrodes at rates of 0.5-6.1 mmol kg(-1) d(-1) in the high NO3- (up to 13.5 mM) groundwater; this rate is significantly higher than previously reported. Porosity reduction was 40.6%-45.1% for the inlet sand/Fe0 interface and 7.4%-25.6% for effluent samples of two test columns. Normalized for treatment volumes, porosity loss values are consistent with studies that use high levels of SO4(2-) but are higher than those using low levels of corrosive species. Aqueous mass balance calculations yield corrosion rates similar to the TGA-MS method, providing an alternative to coring and mineralogical analysis. A severely corroded iron sample from the column simulating a 17-yr treatment throughput showed >75% porosity loss. Extensive porosity loss due to high levels of corrosive species in groundwater will have significant impact on long-term performance of permeable reactive barriers.
Long-term reactivity and permeability are critical factors in the performance of granular iron permeable reactive barriers (PRBs). Thus it is a topic of great practical importance, as well as scientific interest. In this study, four types of source solutions (distilled H2O, 10 mg/L TCE, 300 mg/L CaCO3, and 10 mg/L TCE + 300 mg/L CaCO3) were supplied to four columns containing a commercial granular iron material. In all four columns, gases accumulated to approximately 10% of the initial porosity and resulted in declines in permeability of approximately 50% to 80%. In the columns receiving CaCO3, carbonate precipitates accumulated to approximately 7% of the initial porosity, with no apparent decline in permeability. The data indicate that precipitates formed initially at the influent ends of the columns, reducing the reactivity of the iron in this region. As a consequence of the reduced reactivity, calcium and bicarbonate migrated further into the column, to precipitate in a region where the reactivity remained high. Thus precipitation occurred as a moving front through the columns. The results suggest improved methods for PRB design and rehabilitation, and also suggest improvements that are needed in the mathematical models developed for predicting long-term performance.
A 46 meter long, 7.3 meter deep and 0.6 meter wide reactive barrier was installed at the U.S. Coast Guard Support Center (USCG) in Elizabeth City, North Carolina, in June 1996. The reactive barrier was designed to remediate a hexavalent chromium [Cr(VI)] groundwater plume, in addition to treating portions of a larger and not yet fully characterized trichloroethylene (TCE) groundwater plume at the site. The barrier is composed of Peerless Metal and Abrasives of Detroit, Michigan (Peerless) granular iron and removes Cr(VI) and TCE from the groundwater via processes of reduction and precipitation, and reductive-dechlorination, respectively. In addition to nine large-screen compliance wells, a monitoring network of approximately 150 small-screen sampling points was installed in November 1996 to provide detailed information on changes in porewater geochemistry through the barrier. This network was sampled seven times between November 1996 and December 1998 at 3 to 6 month intervals: Novemb...
A 46 m long, 7.3 m deep, and 0.6 m wide permeable subsurface reactive wall was installed at the U.S. Coast Guard (USCG) Support Center, near Elizabeth City, North Carolina, in June 1996. The reactive wall was designed to remediate hexavalent chromium [Cr(VI)] contaminated ground water at the site, in addition to treating portions of a larger overlapping trichloroethylene (TCE) ground-water plume which has not yet been fully characterized. The wall was installed in approximately 6 hours using a continuous trenching technique, which simultaneously removed aquifer sediments and installed the porous reactive medium. The reactive medium was composed entirely of granular iron, with an average grain size (d 50 ) of 0.4 mm. The reactive medium was selected from various mixtures on the basis of reaction rates with Cr(VI), TCE and degradation products, hydraulic conductivity, porosity, and cost. The continuous wall configuration was chosen over a Funnel-and-Gate configuration, based on three-dim...
Reactive transport modeling has been conducted to describe the performance of the permeable reactive barrier at the U.S. Coast Guard Support Center near Elizabeth City, N.C. The reactive barrier was installed to treat groundwater contaminated by hexavalent chromium and chlorinated organic solvents. The conceptual model of the Elizabeth City site described in Volumes 1 and 2 of this document series (Blowes et al., 2000) provide the basis for the modeling study. The multicomponent reactive transport model MIN3P was used for the simulations. The essential reactions contained in the conceptual model are aqueous complexation reactions, combined reduction-corrosion reactions between the treatment material zero-valent iron and the contaminants or other electron acceptors dissolved in the ambient groundwater and the precipitation of secondary minerals within the reactive barrier. The simulations have been carried out along a cross-section through the barrier that corresponds to a transect of t...
corresponding author, professor, is at the Beaverton, OR 97006; (503) 748-1193; rjohnson@ebs.ogi 97006; thoms@ebs.ogi
- Biographical Sketches
- R L Johnson
- D R Ph
- B S O 'brien Johnson
- Research
Biographical Sketches R.L. Johnson, Ph.D., corresponding author, professor, is at the Department of Environmental & Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Rd., Beaverton, OR 97006; (503) 748-1193; rjohnson@ebs.ogi.edu. R.B. Thoms M.S., research scientist, is at the Department of Environmental & Biomolecular Systems, Oregon Health & Sci-ence University, 20000 NW Walker Rd., Beaverton, OR 97006; thoms@ebs.ogi.edu. R. O'Brien Johnson, B.S., research assistant, is at the Depart-ment of Environmental & Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Rd., Beaverton, OR 97006; johnsono@ebs.ogi.edu.
A field-scale test of trichloroethylene dechlorination using iron filings for the X-120/X749 ground-water plume. ORNL/TM-13410
- L Liang
- O R West
- N E Korte
- J D Goodlaxson
- D A Pickering
- J L Zutman
- F J Anderson
- C A Welch
- M J Pelfrey
- M J Dickey
Liang, L., O.R. West, N.E. Korte, J.D. Goodlaxson, D.A. Pickering, J.L. Zutman, F.J. Anderson, C.A. Welch, M.J. Pelfrey, and M.J. Dickey. 1997. A field-scale test of trichloroethylene dechlorination using iron filings for the X-120/X749 ground-water plume. ORNL/TM-13410. Oak Ridge, Tennessee: Oak Ridge National Laboratory.
is at Geosyntec Consultants, Guelph, Ontario, N1G 5G3 Canada; (519) 822-2230; tkrug@geosyntec
- T Krug
- P E R L Com
- Johnson
T. Krug, P.E., is at Geosyntec Consultants, Guelph, Ontario, N1G 5G3 Canada; (519) 822-2230; tkrug@geosyntec.com. R.L. Johnson et al./ Ground Water Monitoring & Remediation 28, no. 3: 47–55
corresponding author, professor, is at the
- Biographical Sketches
- R L Johnson
- Ph D Nw Walker Rd
Biographical Sketches
R.L. Johnson, Ph.D., corresponding author, professor, is at the
Department of Environmental & Biomolecular Systems, Oregon
Health & Science University, 20000 NW Walker Rd., Beaverton,
OR 97006; (503) 748-1193; rjohnson@ebs.ogi.edu.
R.B. Thoms M.S., research scientist, is at the Department of
Environmental & Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Rd., Beaverton, OR 97006;
thoms@ebs.ogi.edu.
is at Geosyntec Consultants
- T Krug
T. Krug, P.E., is at Geosyntec Consultants, Guelph, Ontario,
N1G 5G3 Canada; (519) 822-2230; tkrug@geosyntec.com.
Identification and quantification of mineral precipitation in Fe0 filings from a column study Environmental Science &
- W Kamolpornwijit
- L Linag
- G R Moline
- T Hart
- O R West
A field-scale test of trichloroethylene dechlorination using iron filings for the X-120/X749 groundwater plume
- L Liang
- O R West
- N E Korte
- J D Goodlaxson
- D A Pickering
- J L Zutman
- F J Anderson
- C A Welch
- M J Pelfrey
- M J Dickey
Liang, L., O.R. West, N.E. Korte, J.D. Goodlaxson, D.A. Pickering,
J.L. Zutman, F.J. Anderson, C.A. Welch, M.J. Pelfrey, and
M.J. Dickey. 1997. A field-scale test of trichloroethylene
dechlorination using iron filings for the X-120/X749 groundwater plume. ORNL/TM-13410. Oak Ridge, Tennessee: Oak
Ridge National Laboratory.
corresponding author, professor, is at the Department of Environmental & Biomolecular Systems, Oregon Health & Science University
- Biographical Sketches
- R L Johnson
Biographical Sketches
R.L. Johnson, Ph.D., corresponding author, professor, is at the
Department of Environmental & Biomolecular Systems, Oregon
Health & Science University, 20000 NW Walker Rd., Beaverton,
OR 97006; (503) 748-1193; rjohnson@ebs.ogi.edu.
R.B. Thoms M.S., research scientist, is at the Department of
Environmental & Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Rd., Beaverton, OR 97006;
research assistant, is at the Department of Environmental & Biomolecular Systems, Oregon Health & Science University
- R O'brien Johnson
R. O'Brien Johnson, B.S., research assistant, is at the Department of Environmental & Biomolecular Systems, Oregon Health
& Science University, 20000 NW Walker Rd., Beaverton, OR
97006; johnsono@ebs.ogi.edu.
Long-term performance of permeable reactive barriers using zero-valent iron: Geochemical and microbiological effects
- Wilken
An in-situ permeable reactive barrier for the treatment of hexavalent chromium and trichloroethylene in ground water. EPA/600/R-99/095c
- D W Blowes
- . U Mayer
Performance monitoring vol. 2. An in-situ permeable reactive barrier for the treatment of hexavalent chromium and trichloroethylene in ground water
- D W R W Blowes
- R W Puls
- C J Gillham
- T A Ptacek
- J G Bennett
- C J Bain
- . J Hanton-Fong Andc
- Paul
Evaluating the longevity and hydraulic performance of permeable reactive barriers at department of defense sites
- A B Gavaskar
- N Sass
- E Gupta
- J Drescher W.-S.Yoon
- J Sminchak
- Hicks
- Condit
Capstone report on the application monitoring and performance of permeable reactive barriers for ground-water remediation: Volume 1-Performance evaluations at two sites. EPA/600/R-03/045a
- R T Wilken
- . W Puls