Zero-valent Iron Permeable Reactive Barrier Research Articles

New insights on zero-valent iron permeable reactive barrier for Cr(VI) removal: The function of FeS reaction zone downstream in-situ generated by sulfate-reducing bacteria

The biogeochemical behavior downstream of the zero-valent iron permeable reactive barrier (ZVI-PRB) plays an enormous positive role in the remediation of contaminated-groundwater, but has been completely neglected for a long time. Therefore, this study conducted a 240-day SRB-enhanced ZVI-PRB column experiment, focusing on what exactly happens downstream of ZVI-PRB. Results show that biosulfidation of SRB inside ZVI-PRB prolonged the complete Cr(VI) removal longevity of ZVI-PRB from 38 days to at least 240 days. More importantly, unlike previous studies that focused on improving the performance of ZVI-PRB itself, this study found an in-situ generated FeS reduction reaction zone downstream of the ZVI-PRB. When the ZVI-PRB fails, the downstream reaction zone can continue to play a role in Cr(VI) removal. The maximum Cr(VI) removal capacity of the aquifer media from the reaction zone reached 155.1mg/kg, which was 39.7% of commercial ZVI capacity. The reduction zone was further confirmed to be predominantly FeS rather than FeS2. Biogeochemistry occurring within and downstream of ZVI-PRB leads to the formation of FeS. Gene sequencing revealed significantly higher SRB abundance downstream of ZVI-PRB than within the ZVI-PRB. The understanding of the downstream FeS reaction zone provides new insights for more effective remediation using ZVI-PRB.

Enhanced Biogenic Sulfidation of Zero-Valent Iron in Columns: Implications for Promoting Dechlorination in Permeable Reactive Barriers.

Biogenic sulfidation of zero-valent iron (ZVI) using sulfate reducing bacteria (SRB) has shown enhanced dechlorination rates comparable to those produced by chemical sulfidation. However, controlling and sustaining biogenic sulfidation to enhance in situ dechlorination are poorly understood. Detailed interactions between SRB and ZVI were examined for 4 months in column experiments under enhanced biogenic sulfidation conditions. SRB proliferation and changes in ZVI surface properties were characterized along the flow paths. The results show that ZVI can stimulate SRB activity by removing excessive free sulfide (S2-), in addition to lowering reduction potential. ZVI also hinders downgradient movement of SRB via electrostatic repulsion, restricting SRB presence near the upgradient interface. Dissolved organic carbon (e.g., >2.2 mM) was essential for intense biogenic sulfidation in ZVI columns. The presence of SRB in the upgradient zone appeared to promote the formation of iron polysulfides. Biogenic FeSx deposition increased the S content on ZVI surfaces ∼3-fold, corresponding to 3-fold and 2-fold improvements in the trichloroethylene degradation rate and electron efficiency in batch tests. Elucidation of SRB and ZVI interactions enhances sustained sulfidation in ZVI permeable reactive barrier.

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart–Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C–70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C–70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h−1 to 2.9 ± 2.2 h−1 (GM) and 0.1 ± 0.1 h−1 to 1.8 ± 0.0 h−1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h−1 to 5.1 ± 3.9 h−1 at GM. Activation energies for PCE degradation yielded 32 kJ mol−1 (GM) and 56 kJ mol−1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Investigating the sustainable performance of a nanoscale zerovalent iron permeable reactive barrier for removal of nitrate, sulfide, and arsenic

Abstract The quality of groundwater resources is at catastrophic risk. The proper performance of iron nanoparticles has made a permeable reactive barrier (PRB) an alternative to conventional filtration methods. The performance of nanozerovalent iron (nZVI) PRBs is limited by particle aggregation, instability, and phase separation, even at low iron concentrations. Therefore, the precipitation of reactive materials and a decrease in the longevity of PRB are fundamental challenges. A laboratory setup is used to compare the performance of bare nZVI and xanthan gum (XG)-nZVI + Mulch PRB to simultaneously remove nitrate, sulfide, and arsenic in groundwater. nZVI (average diameter of 35–55 nm) particles are used as reactive media. The objectives are (1) to develop a method for treating nitrate, sulfide, and arsenic simultaneously in groundwater using organic mulch and XG-nZVI; and (2) to evaluate the longevity performance of the XG-nZVI + Mulch and bare nanoparticles treatment system over 10 days. The results showed that the XG-nZVI + Mulch barrier's performance for eliminating NO3-, As, and S2− was generally improved compared to the bare nZVI barriers by 5.7, 19.2, and 10.9%, respectively. Finally, despite the need for long-term sustainability assessment, XG-nZVI PRB performance is impressive, and this stability promises to improve the longevity of nanoparticles while used in PRBs.

Assessment of the use of a zero-valent iron permeable reactive barrier for nitrate removal from groundwater in the alluvial plain of the Dagu River, China

Groundwater in a shallow aquifer used for potable supply in Qingdao, China, has been contaminated by nitrate by the excessive use of chemical fertilisers for vegetable production in the area. In this study, a representative site was selected to construct a non-continuous permeable reactive barrier (PRB) back-filled with a mixed reactive medium containing of zero-valent iron, activated carbon and medium- to coarse-grained sand. The PRB consists of alternate well and pillars and was constructed to provide a funnel and gate treatment system for two pumping wells. Groundwater monitoring during a trial of the system indicated that the concentrations of nitrate in monitoring wells within the treatment system rapidly declined (within a day) from about 120 mg/L to 20 mg/L. Reductions in nitrate concentrations were also observed in the pumping wells and downgradient monitoring wells, but changes in nitrate concentrations were moderated by mixing with contaminated groundwater. The PRB construction technique of using alternative wells and pillars combined with jet grouting was found to be effective for nitrate removal from groundwater at a small scale.

Isotopic evidence of nitrate degradation by a zero-valent iron permeable reactive barrier: Batch experiments and a field scale study

Permeable reactive barriers (PRBs) filled with zero-valent iron (ZVI) are a well-known remediation approach to treat groundwater plumes of chlorinated volatile organic compounds as well as other contaminants. In field implementations of ZVI-PRBs designed to treat these contaminants, nitrate consumption has been reported and has been attributed to direct abiotic nitrate reduction by ZVI or to denitrification by autochthonous microorganisms using the dissolved hydrogen produced from ZVI corrosion. Isotope tools have proven to be useful for monitoring the performance of nitrate remediation actions. In this study, we evaluate the use of isotope tools to assess the effect of ZVI-PRBs on the nitrate fate for the further optimization of full-scale applications. Laboratory batch experiments were performed using granular cast ZVI and synthetic nitrate solutions at pH 4–5.5 or nitrate-containing groundwater (pH = 7.0) from a field site where a ZVI-PRB was installed. The experimental results revealed nitrate attenuation and ammonium production for both types of experiments. In the field site, the chemical and isotopic data demonstrated the occurrence of ZVI-induced abiotic nitrate reduction and denitrification in wells located close to the ZVI-PRB. The isotopic characterization of the laboratory experiments allowed us to monitor the efficiency of the ZVI-PRB at removing nitrate. The results show the limited effect of the barrier (nitrate reduction of less than 15–20%), probably related to its non-optimal design. Isotope tools were therefore proven to be useful tools for determining the efficacy of nitrate removal by ZVI-PRBs at the field scale.

Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable Reactive Barrier: A Twenty-Two-Year Performance Evaluation.

This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10670 μg/L. Treatment efficiency ranged from 81 to >99%, and TCE concentrations from <1 μg/L to 165 μg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ13C values of methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zerovalent iron PRBs that competes with carbonate mineralization, and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO2(aq) to methane.

Investigating graphene oxide permeable reactive barriers for filtering groundwater contaminated from hydraulic fracturing

Hydraulic fracturing, or fracking, is a method of natural gas extraction which involves pumping a brine solution into the ground to create a fracture that will allow natural gas to rise. One of the major concerns surrounding this method of natural gas extraction is that wastewater enters the groundwater supply, thereby contaminating it. This wastewater contains toxic materials such as heavy metal ions, radionuclides and other salts and organic compounds in high concentrations. Some of these materials are carcinogenic and thus a concern to human life and the environment. The current solution involves the use of a zerovalent iron (ZVI) permeable reactive barrier (PRB) to filter out these toxic substances. However, it causes more fouling due to the accumulation of mineral precipitates and therefore is not very effective. A recent development in nanotechnology may allow us to develop a superior water filter to prevent groundwater contamination. Therefore, a novel PRB is suggested: featuring the use of solid graphene oxide (GO), a nanomaterial with a superior sorption ability is proposed as a replacement for the system. The proposed experiment will test the filtration capability of the GO-PRB as compared to the traditional ZVI-PRB. By emulating the process of groundwater contamination and flow using common materials found in fracking wastewater, we can determine how much more effective the GO-PRB is than the ZVIPRB.

A Non-dimensional Analysis of Permeability Loss in Zero-Valent Iron Permeable Reactive Barrier (PRB)

Zero-valent iron (ZVI) permeable reactive barrier (PRB) is a treatment wall filled with ZVI as a reactive material that is installed perpendicular to the groundwater flow in the subsurface. To aid design of these PRBs, a non-dimensional analysis of the permeability reduction has been carried out in this work where the dimensionless equation has been identified to correlate different variables. Additionally, the change in physical features of ZVI PRB has been identified using the inspection system of X-ray microcomputer tomography and it has shown that the particle size is expanding, thus reducing the permeability. The change in chemical composition that impacts the surface reactivity has been confirmed using X-ray diffraction, and the corroded products of maghemite and magnetite have been identified. Flow experiments have been conducted to observe and measure the changes in permeability, where the pressure at various points of the experimental rigs has been measured for the calculation of permeability values. The reduction in permeability could be observed from both small- and large-scale experiments. For example, the flow experiments indicated that the permeability value has been significantly reduced for coarse particle, e.g. in small-scale experiment, it reduced from 7.04E−8 to 3.09E−9 cm2. It can also be seen that the permeability is decreased by 95.6% for small scale (coarse particle) and by 79.5% for large scale.

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md−1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md−1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.

Influences of benzene or toluene on dechlorination of perchloroethene and trichloroethene in a simulated zero-valent iron permeable reactive barrier

Abstract Column studies were conducted to investigate the influence of benzene or toluene on the dechlorination of perchloroethene (PCE) and trichloroethene (TCE) in columns packed with zero-valent iron (ZVI) in order to simulate a permeable reactive barrier (PRB). Enhancive and inhibitive influences of benzene and toluene, respectively, on PCE and TCE reduction were observed within 10–80 pore volumes (PV) that flowed through the columns. However, such influences dissipated when the flow-through volume increased above 80 PV. The presence of benzene increased the mean dechlorination kobs of PCE and TCE by 7% and 6%, respectively; in contrast, the presence of toluene decreased the mean dechlorination kobs of PCE and TCE by 21% and 10%, respectively. We presumed that the more competitive adsorption between benzene and toluene in comparison to PCE and TCE on the ZVI particle surface might have caused the disparate influences. With a lower affinity for ZVI, benzene has no substantial influence on PCE and TCE adsorption on the ZVI particle surface. However, toluene has a higher affinity for ZVI and could compete with PCE and TCE by contacting the ZVI particle surface. Moreover, given benzene's higher polarity, it could also benefit electron transfer from ZVI to PCE and TCE.

Reaction Kinetics and Impacting Mechanism of Cr(Ⅵ) Removal in Fe0-PRB Systems

Zero-Valent Iron Permeable Reactive Barrier (Fe0-PRB) is a competitive and economical in-situ groundwater remediation technology in recent years, and high removal efficiencies of Cr(Ⅵ)-polluted groundwater have been realized. The present study focused on the impacting mechanism of Cr(Ⅵ) removal by Fe0-PRB. Environmental condition response was revealed from kinetic view, and kinetic expression describing the removal process was determined. In addition, the effect of groundwater chemical conditions was studied. It was found that calcium had little effect on Cr(Ⅵ) removal. Chloride might influence Cr(Ⅵ) removal through impacting electron transfer, and sulfate/magnesium might influence Cr(Ⅵ) removal through participating in reactions. Bicarbonate might influence Cr(Ⅵ) removal through combined effects of impacting electron transfer and participating in reactions. The results provide method and theory basis for process parameter optimization in Fe0-PRB systems.

Formation of chukanovite in simulated groundwater containing

ABSTRACTChukanovite (Fe2(OH)2CO3) is one of the secondary mineral precipitates on the surfaces of zero-valent iron (ZVI) barriers in groundwater containing carbonates. Synthesizing experiments were conducted in FeCl2, NaOH, and Na2CO3 solutions to investigate the effect of carbonate concentration on the formation of Fe2(OH)2CO3 and estimate the stability field of Fe2(OH)2CO3 on the potential–pH diagram. Results revealed that Fe2(OH)2CO3 is a unique product based on X-ray diffraction. The , OH−, and Fe2+ concentrations and the ratios (R = [Fe2+]/[OH−] and R′ = []/[OH−]) are important parameters in the formation of the Fe2(OH)2CO3. Fe2(OH)2CO3 was better formed in the R = 1.1, R′ = 0.9 system than in the R = 1.1, R′ = 0.7 system. The crystallization of Fe2(OH)2CO3was increased with the concentration of increased from 0.018 to 0.18 mol/L. The standard Gibbs free energy of the formation of Fe2(OH)2CO3 was −1151.1 ± 5.3 kJ/mol from the equilibrium conditions between Fe2(OH)2CO3 and Fe2+, . Potential–pH diagram of iron, including Fe2(OH)2CO3, was drawn in the Fe–C–H2O system. In this diagram, the stable domain of Fe2(OH)2CO3 was 7.87 < pH < 10.34, −740 mV < Eh < −400 mV, which can be converted into FeCO3 in low pH value in the 0.18mol/L carbonate solution. This work will aid in predicting the potential for mineral precipitation, as well as estimating the reactivity, porosity, and hydraulic performance of ZVI permeable reactive barriers.

Remediation of PCE contaminated clay soil by coupling electrokinetics with zero-valent iron permeable reactive barrier

In the present study, the electrokinetic technology was coupled with a permeable reactive barrier (PRB), composed of micro-scale Fe to treat a clay soil contaminated by perchloroethylene (PCE). A non-ionic surfactant, Triton X-100, was selected as the solubility enhancing agent. A series of batch tests, having 10-day duration, were carried out to evaluate the fate, transport and removal of PCE. This study also demonstrates the effectiveness of the non-uniform EK/ZVIPRB system for the treatment of the PCE-contaminated, low permeability soils. A bench scale system was used, which contained 400 g of PCE contaminated soil (0.6 g PCE kg−1 soil) and a PRB containing 30 g of iron particles. Seven lab scale tests were conducted using a potential gradient of 1 V cm−1 for studying the influence of different variables like surfactant, the inclusion of PRB in the soil and the effect of polarity reversal. Results showed that the combination of EK and ZVIPRB could enhance the overall removal of PCE from soils by 40 % as compared to EK alone. Among the variables, the surfactant was seen to be essential for the movement of the pollutant through the PRB, and the PCE removal improved significantly on reversing the electrode polarity in the EK-PRB test (73 % in anode, 64 % in cathode). The best results showed an 80 % PCE removal efficiency after 10 days.

The roles of a pillared bentonite on enhancing Se(VI) removal by ZVI and the influence of co-existing solutes in groundwater

The zero-valent iron permeable reactive barrier (ZVI-PRB) is a promising technology for in-situ groundwater remediation. However, its long-term performance often declined due to the blocked reactive sites by corrosion products and by interference of co-existing solutes. In order to address these issues, a pillared bentonite (Al-bent) was homogeneously mixed with ZVI for removing selenate (Se(VI)) from simulated groundwater in column experiments. The Se(VI) removal was enhanced because first Al-bent could facilitate the mass transfer of Se(VI) from solution to iron surface and accelerate Se(VI) reduction. XANES analysis indicated that Se(VI) was almost completely reduced to Se(0) and Se(-II) of less toxicity and solubility by the ZVI/Al-bent mixture, and the buffering effect of Al-bent could maintain the pH at a lower level that favored the Se(VI) removal. Besides, Al-bent could transfer the corrosion products away from iron surface, leading to the enhanced reactivity and longevity of ZVI. The inhibition on reactivity towards Se(VI) in both the single ZVI and the ZVI/Al-bent systems increased in the order of Cl−<NO3−<HCO3−<SO42−, and the removal efficiency decreased with the increasing HA concentration. However, the lower decrease of Se(VI) removal in the ZVI/Al-bent system indicates its resistance to the interference of these co-existing solutes in groundwater.

C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site

Compound-specific isotopic analysis of multiple elements (C, Cl, H) was tested to better assess the effect of a zero-valent iron-permeable reactive barrier (ZVI-PRB) installation at a site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). The focus was on (1) using 13C to evaluate natural chlorinated ethene biodegradation and the ZVI-PRB efficiency; (2) using dual element 13C-37Cl isotopic analysis to distinguish biotic from abiotic degradation of cis-dichloroethene (cis-DCE); and (3) using 13C-37Cl-2H isotopic analysis of cis-DCE and TCE to elucidate different contaminant sources. Both biodegradation and degradation by ZVI-PRB were indicated by the metabolites that were detected and the 13C data, with a quantitative estimate of the ZVI-PRB efficiency of less than 10% for PCE. Dual element 13C-37Cl isotopic plots confirmed that biodegradation was the main process at the site including the ZVI-PRB area. Based on the carbon isotope data, approximately 45% and 71% of PCE and TCE, respectively, were estimated to be removed by biodegradation. 2H combined with 13C and 37Cl seems to have identified two discrete sources contributing to the contaminant plume, indicating the potential of δ2H to discriminate whether a compound is of industrial origin, or whether a compound is formed as a daughter product during degradation.

Removal of zinc from contaminated groundwater by zero-valent iron permeable reactive barrier

The possibility of using zero-valent iron (ZVI) as permeable reactive barrier (PRB) to remove zinc from a contaminated groundwater was investigated. Batch equilibrium tests were carried out. The effects of many parameters such as contact time between adsorbate and adsorbent (0–240 min), initial pH of the solution (4–8), sorbent dosage (1–12 g/100 ml), initial metal concentration (50–250 mg/l), and agitation speed (0–250 rpm) were studied. The best values of these parameters that achieve the maximum removal efficiency of Zn+2 (=91%) were 3 h, 5, 10 g/100 ml, 50 mg/l, and 200 rpm, respectively. Langmuir isotherm model gives better fit for the sorption data of Zn+2 ion by ZVI than Freundlich model under the studied conditions. Finite difference method and COMSOL Multiphysics 3.5a software, which is based on finite element method, were used to simulate the one-dimensional equilibrium transport of zinc through sandy aquifer with and without presence of PRB. The predicted and experimental results proved that the PRB plays a potential role in the restriction of the contaminant plume migration. A reasonable agreement between these results was recognized with root mean squared error not exceeded the 0.1487.

Hydraulic performance of zero-valent iron and nano-sized zero-valent iron permeable reactive barriers – laboratory test

Abstract Hydraulic performance of zero-valent iron and nano-sized zero-valent iron permeable reactive barriers - laboratory test. The hydraulic 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 influence 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 conductivity 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 containing the mixture of heavy metals may cause significant 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 precipitation 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 hydraulic conductivity of reactive materials for successful implementation of PRBs technology.

Quantification of changes in zero valent iron morphology using X-ray computed tomography

Morphological changes within the porous architecture of laboratory scale zero valent iron (ZVI) permeable reactive barriers (PRBs), after exposure to different groundwater conditions, have been quantified experimentally for different ZVI/sand ratios (10%, 50% and 100%, W/W) with the aim of inferring porosity changes in field barriers. Column studies were conducted to simulate interaction with different water chemistries, a synthetic groundwater, acidic drainage and deionised (DI) water as control. Morphological changes, in terms of pore size and distribution, were measured using X-ray computed tomography (CT). CT image analysis revealed significant morphological changes in columns treated with different water chemistries. For example, 100% ZVI (W/W) columns had a higher frequency of small pores (0.6 mm) was observed in ZVI grains reacted with typical groundwater, resulting in a porosity of 27%, compared to 32% when exposed to DI water. In comparison, ZVI grains treated with the acidic drainage had higher porosity (44%) and larger average pore size (2.8 mm). 10% ZVI PRB barrier material had the highest mean porosity (56%) after exposure to any water chemistry whilst 100% ZVI (W/W) columns always had the lowest (34%) with the 50% ZVI (W/W) in between (40%). These results agree with previously published PRB field data and simultaneously conducted geochemical monitoring and mass balance calculation, indicating that both the geochemical and hydraulic environment of the PRB play an important role in determining barrier lifespan. This study suggests that X-ray CT image analysis is a powerful tool for studying the detailed inter pores between ZVI grains within PRBs.

Zero-Valent Iron/Biotic Treatment System for Perchlorate-Contaminated Water: Lab-Scale Performance, Modeling, and Full-Scale Implications

AbstractThe computer program AQUASIM was used to model biological treatment of perchlorate-contaminated water using zero-valent iron corrosion as the hydrogen gas source. The laboratory-scale column was seeded with an autohydrogenotrophic microbial consortium previously shown to degrade perchlorate. Consortium biokinetic parameters and data from column experiments were used to verify the model. The model was then used to simulate full-scale performance of an in-situ zero-valent iron permeable reactive barrier perchlorate-treatment system. Simulation results indicate full-scale field treatment systems have the potential to degrade significant concentrations of perchlorate in the presence of oxygen under a variety of operating conditions.