Calcite precipitation dominates the electrical signatures of zero valent iron columns under simulated field conditions

Calcium carbonate is a secondary mineral precipitate influencing zero valent iron (ZVI) barrier reactivity and hydraulic performance. We conducted column experiments to investigate electrical signatures resulting from concurrent CaCO₃ and iron oxides precipitation under simulated field geochemical conditions. We identified CaCO₃ as a major mineral phase throughout the columns, with magnetite present primarily close to the influent based on XRD analysis. Electrical measurements revealed decreases in conductivity and polarization of both columns, suggesting that electrically insulating CaCO₃ dominates the electrical response despite the presence of electrically conductive iron oxides. SEM/EDX imaging suggests that the electrical signal reflects the geometrical arrangement of the mineral phases. CaCO₃ forms insulating films on ZVI/magnetite surfaces, restricting charge transfer between the pore electrolyte and ZVI particles, as well as across interconnected ZVI particles. As surface reactivity also depends on the ability of the surface to engage in redox reactions via charge transfer, electrical measurements may provide a minimally invasive technology for monitoring reactivity loss due to CaCO₃ precipitation. Comparison between laboratory and field data shows consistent changes in electrical signatures due to iron corrosion and secondary mineral precipitation.     

Longevity of granular iron in groundwater treatment processes: changes in solute transport properties over time

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.     

The effect of silica on the degradation of organohalides in granular iron columns

Dissolved silica species are naturally occurring, ubiquitous groundwater constituents with corrosion-inhibiting properties. Their influence on the performance and longevity of iron-based permeable reactive barriers for treatment of organohalides was investigated through long-term column studies using Connelly iron as the reactive medium. Addition of dissolved silica (0.5 mM) to the column feed solution led to a reduction in iron reactivity of 65% for trichloroethylene (TCE), 74% for 1,1,2-trichloroethane (1,1,2-TCA), and 93% for 1,1,1-trichloroethane (1,1,1-TCA), compared to columns operated under silica-free conditions. Even though silica adsorption was a gradual process, the inhibitory effect was evident within the first week, with subsequent decreases in reactivity over 288 days being relatively minor. Lower concentrations of dissolved silica species (0.2 mM) led to a lesser decrease (70%) in iron reactivity toward 1,1,1-TCA. The presence of dissolved silica species produced a shift in TCE product distribution toward the more highly chlorinated product cis-dichloroethylene (cis-DCE), although it did not appear to alter products originating from the trichloroethanes. The major corrosion products identified were magnetite (Fe3O4) or maghemite (gamma-Fe2O3) and carbonate green rust ([Fe4(2+)Fe(2)3+(OH)12][CO(3).2H2O]). Iron carbonate hydroxide (Fe(II)1.8Fe(III)0.2(OH)2.2CO3) was only found in the silica-free column, indicating that silica may hinder its formation. A comparison with columns operated under the same conditions, but using Master Builder iron as the reactive matrix, showed that Connelly iron is initially less reactive, but performs better than Master Builder iron over 288 days.     

Investigation of gas production and entrapment in granular iron medium

A method for measuring gas entrapment in granular iron (Fe0) was developed and used to estimate the impact of gas production on porosity loss during the treatment of a high NO3- groundwater (up to approximately 10 mM). Over the 400-d study period the trapped gas in laboratory columns was small, with a maximum measured at 1.3% pore volume. Low levels of dissolved H2(g) were measured (up to 0.07+/-0.02 M). Free moving gas bubbles were not observed. Thus, porosity loss, which was determined by tracer tests to be 25-30%, is not accounted for by residual gas trapped in the iron. The removal of aqueous species (i.e., NO3-, Ca, and carbonate alkalinity) indicates that mineral precipitation contributed more significantly to porosity loss than did the trapped gases. Using the stoichiometric reactions between Fe0 and NO3-, an average corrosion rate of 1.7 mmol kg-1 d-1 was derived for the test granular iron. This rate is 10 times greater than Fe0 oxidation by H2O alone, based on H2 gas production. NO3- ion rather than H2O was the major oxidant in the groundwater in the absence of molecular O2. The N-mass balance [e.g., N2g and NH4+ and NO3-] suggests that abiotic reduction of NO3- dominated at the start of Fe0 treatment, whereas N2 production became more important once the microbial activity began. These laboratory results closely predict N2 gas production in a separated large column experiment that was operated for approximately 2 yr in the field, where a maximum of approximately 600 ml d-1 gas volumes was detected, of which 99.5% (v/v) was N2. We conclude that NO3- suppressed the production of H2(g) by competing with water for Fe0 oxidation, especially at the beginning of water treatment when Fe0 is highly reactive. Depends on the groundwater composition, gas venting may be necessary in maintaining PRB performance in the field.     

Immobilization of uranium and arsenic by injectible iron and hydrogen stimulated autotrophic sulphate reduction

The main object of the study was the development of a long-term efficient and inexpensive in-situ immobilization technology for uranium (U) and arsenic (As) in smaller and decentralized groundwater discharges from abandoned mining processing sites. Therefore, corrosion of grey cast iron (gcFe) and nano-scale iron particles (naFe) as well as hydrogen stimulated autotrophic sulphate reduction (aSR) were investigated. Two column experiments with sulphate reducing bacterias (SRB) (biotic gcFe , biotic naFe) and one abiotic gcFe-column experiment were performed. In the biotic naFe column, no particle translocation was observed and a temporary but intensive naFe corrosion indicated by a decrease in E(h), a pH increase and H(2) evolution. Decreasing sulphate concentrations and (34)S enrichment in the column effluent indicated aSR. Fe(II) retention could be explained by siderite and consequently FeS precipitation by geochemical modeling (PhreeqC). U and As were completely immobilised within the biotic naFe column. In the biotic gcFe column, particle entrapment in open pore spaces resulted in a heterogeneous distribution of Fe-enriched zones and an increase in permeability due to preferential flow. However, Fe(II) concentrations in the effluent indicated a constant and lasting gcFe corrosion. An efficient immobilization was found for As, but not for U.     

An in situ study of the effect of nitrate on the reduction of trichloroethylene by granular iron

The effect of nitrate on the reduction of TCE by commercial granular iron was investigated in column experiments designed to allow for the in situ monitoring of the iron surface film with Raman spectroscopy. Three column experiments were conducted; one with an influent solution of 100 mg/l nitrate+1.5 mg/l TCE, and two control columns, one saturated directly with 100 mg/l nitrate solution, the other pre-treated with Millipore water prior to the introduction of a 100 mg/l nitrate solution. In the presence of nitrate, TCE adsorbed onto the iron, but there was little TCE reduction to end-products ethene and ethane. The iron used (Connelly, GPM, Chicago) is a product typical of those used in permeable granular iron walls. The material is covered by an air-formed high-temperature oxidation film, consisting of an inner layer of Fe(3)O(4), and an outer, passive layer of Fe(2)O(3). In the control column pre-treated with Millipore water, the passive Fe(2)O(3) layer was removed upon contact with the water in a manner consistent with an autoreduction reaction. In the TCE+nitrate column and the direct nitrate saturation column, nitrate interfered with the removal of the passive layer and maintained conditions such that high valency protective corrosion species, including Fe(2)O(3) and FeOOH, were stable at the iron surface. The lack of TCE reduction is explained by the presence of these species, as they inhibit both mechanisms proposed for TCE reduction by iron, including catalytic hydrogenation, and direct electron transfer.     

Influence of solution composition and column aging on the reduction of nitroaromatic compounds by zero-valent iron

Granular iron is used in reactive permeable barriers for the reductive treatment of organic and inorganic groundwater contaminants. The technology is well established, however, its long-term performance and the importance of the groundwater composition are not yet well understood. Here, the influence of chloride, nitrate, silicate, and Aldrich humic acid on the reactivity of Master Builder iron was studied under anoxic conditions using small packed columns and 2-nitrotoluene (2-NT) as a model contaminant. After initially complete reduction of 2-NT to 2-aminotoluene (2-AT) in the column, possibly under mass-transfer controlled conditions, the reactivity of the iron was found to decrease substantially. In the presence of chloride, this decrease was slowed while exposure to silicate resulted in a very quick loss of iron reactivity. Nitrate was found to interfere strongly with the effect of chloride. These observations are interpreted in terms of corrosion inhibition/promotion and competition. Our results suggest that reactive barrier performance may be strongly affected by the composition of the treated groundwater.     

Effects of initial iron corrosion rate on long-term performance of iron permeable reactive barriers: Column experiments and numerical simulation

Column experiments and numerical simulation were conducted to test the hypothesis that iron material having a high corrosion rate is not beneficial for the long-term performance of iron permeable reactive barriers (PRBs) because of faster passivation of iron and greater porosity loss close to the influent face of the PRBs. Four iron materials (Connelly, Gotthart-Maier, Peerless, and ISPAT) were used for the column experiments, and the changes in reactivity toward cis-dichloroethene (cis-DCE) degradation in the presence of dissolved CaCO₃ were evaluated. The experimental results showed that the difference in distribution of the accumulated precipitates, resulting from differences in iron corrosion rate, caused a difference in the migration rate of the cis-DCE profiles and a significant difference in the pattern of passivation, indicating a faster passivation in the region close to the influent end for the material having a higher corrosion rate. For the numerical simulation, the accumulation of secondary minerals and reactivity loss of iron were coupled using an empirically-derived relationship that was incorporated into a multi-component reactive transport model. The simulation results provided a reasonable representation of the evolution of iron reactivity toward cis-DCE treatment and the changes in geochemical conditions for each material, consistent with the observed data. The simulations for long-term performance were also conducted to further test the hypothesis and predict the differences in performance over a period of 40 years under typical groundwater conditions. The predictions showed that the cases of higher iron corrosion rates had earlier cis-DCE breakthrough and more reduction in porosity starting from near the influent face, due to more accumulation of carbonate minerals in that region. Therefore, both the experimental and simulation results appear to support the hypothesis and suggest that reactivity changes of iron materials resulting from evolution of geochemical conditions should be considered in the design of iron PRBs.     

Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions

Long-term column experiments were conducted under different geochemical conditions to estimate the longevity of Fe 0 permeable reactive barriers (PRBs) treating hexavalent chromium (Cr(VI)). Secondary carbonate minerals were precipitated, and their effects on the performance, such as differences in the mechanism for Cr removal and the changes in system hydraulics, were assessed. Sequestration of Cr(VI) occurred primarily by precipitation of Fe(III)-Cr(III) (oxy)hydroxides. Trace amounts of Cr were observed in iron hydroxy carbonate presumably due to substitution of Cr3+ for Fe3+. The formation of Fe(III)-Cr(III) (oxy)hydroxide greatly decreased the reactivity of the Fe 0 and thus resulted in migration of the Cr removal front. Carbonate minerals did not appear to contribute to further passivation with regard to reactivity toward Cr removal; rather, the column receiving high contents of dissolved calcium carbonate showed slightly enhanced Cr removal by means of a higher corrosion rate of Fe 0 and because of sequestration by an iron hydroxy carbonate. Precipitation of carbonates, however, governed other geochemical parameters. The porosity and hydraulic conductivity in the column receiving high contents of dissolved calcium carbonate did not indicate a great loss in system permeability because the accumulation of carbonates declined as the Fe 0 was passivated over time. However, the accumulated carbonates and associated Fe(III)-Cr(III) (oxy)hydroxide could cause problems because the presence of these solids resulted in a decline in flow rate after about 1400 pore volumes of operation.     

Column studies on transport of deicing additive benzotriazole in a sandy aquifer and a zerovalent iron barrier

Benzotriazole (BTA), a chemical with wide industrial applications, is a typical additive in deicer/anti-icer used at airport. To achieve a better understanding of the transport behaviour and environmental fate of BTA, laboratory column studies have been performed on subsoil samples from Oslo Airport, Gardermoen. To explore possibilities for aquifer remediation, BTA behaviour was also studied in a column of granular zerovalent iron (Fe(0)). The subsoil column study demonstrates a very limited retardation of BTA. Consecutive loadings of BTA of the subsoil column showed no change of the break-through curve (BTC) and complete desorption was observed. The sorption behaviour of BTA to metallic iron (Fe(0)) was rather complex. Considerable retardation was observed in the Fe(0) column and repeated BTA loading resulted in an earlier break-through. Between 20% and 50% of the input concentration was retained permanently in the iron (Fe(0)) column. The BTA sorption to metallic iron was found to be enhanced by chloride which lowered the break-through concentration (i.e the C/C(0) plateau). The fraction of BTA remaining in the iron column was found to vary with the flow rate, indicating a time dependant multilayer sorption mechanism. The steady increase in the amount of adsorbed BTA to the iron column during loading corresponds to a rather strong bonding of 4-15 BTA layers to the iron surface. A very slow desorption of BTA was observed; even after flushing with 753 pore volumes of BTA free water, 7.5% of the BTA remained in the column. A geochemical model was developed based on PHREEQC-2 to simulate the sorption and transport of BTA in the tested materials. The BTA sorption was modelled with Freundlich sorption isotherms, as earlier determined in batch experiments. A slight adjustment of the Freundlich parameters was required to fit the observed column break-through. However, our model was not able to simulate the long-term retainment of BTA in the granular iron columns. The simulations confirm the high mobility of BTA in groundwater aquifers and suggest that zerovalent iron could be used to retain a BTA plume, although oxidation of the sorbent might reduce the long-term performance of such a remediation scheme and slow desorption has been observed.     

Modelling the long-term performance of zero-valent iron using a spatio-temporal approach for iron aging

Zero-valent iron (ZVI) permeable reactive barriers (PRBs) have become popular for the degradation of chlorinated ethenes (CEs) in groundwater. However, a knowledge gap exists pertaining to the longevity of ZVI. The present investigation addresses this situation by suggesting a numerical simulation model that is intended to be used in conjunction with field or column tests in order to describe long-term ZVI performance at individual sites. As ZVI aging processes are not yet completely understood and are still subject to research, we propose a phenomenological modelling technique instead of a common process-based approach. We describe ZVI aging by parameters that characterise the extent and rate of ZVI reactivity change depending on the propagation of the precipitation front through ZVI. We approximate degradation of CEs by pseudo-first order kinetics accounting for the formation of partially dechlorinated products, and describe ZVI reactivity change by scaling the degradation rate constants. Three independent modelling studies were carried out to test the suitability of the conceptual and numerical model to describe the observations of accelerated column tests. All three tests indicated that ZVI reactivity declined with an increasing number of exchanged pore volumes. Measured and modelled concentrations showed good agreement, thereby proving that resolving spatial as well as temporal changes in ZVI reactivity is reasonable.     

High-level arsenite removal from groundwater by zero-valent iron

The objectives of this study were to conduct batch and column studies to (i) assess the effectiveness of zero-valent iron for arsenic remediation in groundwater, (ii) determine removal mechanisms of arsenic, and (iii) evaluate implications of these processes with regard to the stability of arsenic and long-term remedial performance of the permeable reactive barrier (PRB) technology. A high concentration arsenic solution (50 mg l(-1)) was prepared by using sodium arsenite (arsenic (III)) to simulate groundwater at a heavily contaminated Superfund site in the USA. Batch studies indicate that the removal of arsenic is a two-step reaction with fast initial disappearance of arsenite followed by a slow subsequent removal process. Flow-through columns were conducted at a flow rate of 17 ml h(-1) under reducing conditions for 6.6 mo. Kinetic analysis suggested that arsenic removal behaves as a zero-order reaction at high arsenic concentrations. Arsenic removal rate constants decreased with time and arsenic breakthrough was observed in the column study. Arsenic removal capacity of zero-valent iron was determined to be approximately 7.5 mg As/g Fe. Carbonate green rust was identified from the analysis of surface precipitates; arsenite uptake by green rust may be a major mechanism responsible for arsenic remediation by zero-valent iron. Analysis of HCl-extractable arsenic from iron samples indicated that approximately 28% of arsenic was in the form of arsenate suggesting that a surface oxidation process was involved in the arsenic removal with zero-valent iron.     

Sulfur and oxygen isotope tracing in zero valent iron based In situ remediation system for metal contaminants

In the present study, controlled laboratory column experiments were conducted to understand the biogeochemical changes during the microbial sulfate reduction. Sulfur and oxygen isotopes of sulfate were followed during sulfate reduction in zero valent iron incubated flow through columns at a constant temperature of 20 ± 1 °C for 90 d. Sulfur isotope signatures show considerable variation during biological sulfate reduction in our columns in comparison to abiotic columns where no changes were observed. The magnitude of the enrichment in δ³⁴S values ranged from 9.4‰ to 10.3‰ compared to initial value of 2.3‰, having total fractionation δS between biotic and abiotic columns as much as 6.1‰. Sulfur isotope fractionation was directly proportional to the sulfate reduction rates in the columns. Oxygen isotopes in this experiment seem less sensitive to microbial activities and more likely to be influenced by isotopic exchange with ambient water. A linear relationship is observed between δ³⁴S and δ¹⁸O in biotic conditions and we also highlight a good relationship between δ³⁴S and sulfate reduction rate in biotic columns.     

Removal of chromium from synthetic plating waste by zero-valent iron and sulfate-reducing bacteria.

Experiments were conducted to evaluate the potential of zero-valent iron and sulfate-reducing bacteria (SRB) for reduction and removal of chromium from synthetic electroplating waste. The zero-valent iron shows promising results as a reductant of hexavalent chromium (Cr+6) to trivalent chromium (Cr+3), capable of 100% reduction. The required iron concentration was a function of chromium concentration in the waste stream. Removal of Cr+3 by adsorption or precipitation on iron leads to complete removal of chromium from the waste and was a slower process than the reduction of Cr+6. Presence SRB in a completely mixed batch reactor inhibited the reduction of Cr+6. In a fixed-bed column reactor, SRB enhanced chromium removal and showed promising results for the treatment of wastes with low chromium concentrations. It is proposed that, for waste with high chromium concentration, zero-valent iron is an efficient reductant and can be used for reduction of Cr+6. For low chromium concentrations, a SRB augmented zero-valent iron and sand column is capable of removing chromium completely.     

Preferential flow path development and its influence on long-term PRB performance: column study

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.     

Influence of Na2S on the degradation kinetics of CCl4 in the presence of very pure iron

This paper presents the results of kinetic studies to investigate the effect of FeS film formation on the degradation rate of CCl(4) by 99.99% pure metallic iron. The film was formed by submersing metallic iron grains in an oxygen free HCO(3)(-)/CO(3)(2-) electrolyte solution. When the grains had reached a quasi steady-state value of the corrosion potential, Na(2)S((aq)) was injected. Upon injection, a microm thick poorly crystalline FeS film formed immediately on the iron surface. Over time, the iron became strongly corroded and both the FeS film and the metallic iron grains began to crack leading to exposure of bare metallic iron to the solution. The effect of the surface film on the degradation rate of CCl(4) was investigated following four periods of aging, 1, 10, 30, and 60 days. Relative to the controls, the 1-day sulfide-aged iron showed a substantial decrease in rate of degradation of CCl(4.) However, over time, the rate of degradation increased and surpassed the degradation rate obtained in the controls. It has been proposed that CCl(4) is reduced to HCCl(3) by metallic iron by electron transfer. The FeS film is substantially less conducting than the bulk iron metal or non-stoichiometric magnetite and from the results of this study, greatly decreases the rate of CCl(4) degradation relative to iron that has not been exposed to Na(2)S. However, continued aging of the FeS film results in breakdown and stress-induced cracking of the film, followed by dissolution and cracking of the iron itself. The cracking of the bulk iron is believed to be a consequence of hydrogen embrittlement, which is promoted by sulfide. The increase in CCl(4) degradation rate, as the FeS films age, suggests that the process of hydrogen cracking increases the surface area available for charge transfer.     

Modelling of geochemical and isotopic changes in a column experiment for degradation of TCE by zero-valent iron

Zero-valent iron (ZVI) permeable-reactive barriers have become an increasingly used remediation option for the in situ removal of various organic and inorganic chemicals from contaminated groundwater. In the present study a process-based numerical model for the transport and reactions of chlorinated hydrocarbon in the presence of ZVI has been developed and applied to analyse a comprehensive data set from laboratory-scale flow-through experiments. The model formulation includes a reaction network for the individual sequential and/or parallel transformation of chlorinated hydrocarbons by ZVI, for the resulting geochemical changes such as mineral precipitation, and for the carbon isotope fractionation that occurs during each of the transformation reactions of the organic compounds. The isotopic fractionation was modelled by formulating separate reaction networks for lighter ((12)C) and heavier ((13)C) isotopes. The simulation of a column experiment involving the parallel degradation of TCE by hydrogenolysis and beta-elimination can conclusively reproduce the observed concentration profiles of all collected organic and inorganic data as well as the observed carbon isotope ratios of TCE and its daughter products.     

The addition of organic carbon and nitrate affects reactive transport of heavy metals in sandy aquifers

Organic carbon introduction in the soil to initiate remedial measures, nitrate infiltration due to agricultural practices or sulphate intrusion owing to industrial usage can influence the redox conditions and pH, thus affecting the mobility of heavy metals in soil and groundwater. This study reports the fate of Zn and Cd in sandy aquifers under a variety of plausible in-situ redox conditions that were induced by introduction of carbon and various electron acceptors in column experiments. Up to 100% Zn and Cd removal (from the liquid phase) was observed in all the four columns, however the mechanisms were different. Metal removal in column K1 (containing sulphate), was attributed to biological sulphate reduction and subsequent metal precipitation (as sulphides). In the presence of both nitrate and sulphate (K2), the former dominated the process, precipitating the heavy metals as hydroxides and/or carbonates. In the presence of sulphate, nitrate and supplemental iron (Fe(OH)(3)) (K3), metal removal was also due to precipitation as hydroxides and/or carbonates. In abiotic column, K4, (with supplemental iron (Fe(OH)(3)), but no nitrate), cation exchange with soil led to metal removal. The results obtained were modeled using the reactive transport model PHREEQC-2 to elucidate governing processes and to evaluate scenarios of organic carbon, sulphate and nitrate inputs.     

Zero-valent iron and iron oxide-coated sand as a combination for removal of co-present chromate and arsenate from groundwater with humic acid

The combination of zero-valent iron (Fe⁰) and iron oxide-coated sand (IOCS) was used to remove Cr(VI) and As(V) from groundwater in this study. The efficiency and the removal mechanism of Cr(VI) and As(V) by using this combination, with the influence of humic acid (HA), were investigated using batch experiments. Results showed that, compared to using Fe⁰ or IOCS alone, the Fe⁰-IOCS can perform better on the removal of both Cr(VI) and As(V). Metal extraction studies showed that As(V) was mainly removed by IOCS and iron corrosion products while Cr(VI) was mainly removed by Fe⁰ and its corrosion products. Competition was found between Cr(VI) and As(V) for the adsorption sites on the iron corrosion products. HA had shown insignificant effects on Cr(VI) removal but some effects on As(V) removal kinetics. As(V) was adsorbed on IOCS at the earlier stage, but adsorbed/coprecipitated with the iron corrosion products at the later stage.     

Nitrite reduction and formation of corrosion coatings in zerovalent iron systems

Batch tests were conducted to investigate nitrite reduction in a zerovalent iron (Fe0) system under various conditions. Nitrite at 1.4 mM initial concentration was slowly reduced to nitrogen gas in the first stage (days 1-6), which was mediated by an amorphous, Fe(II)-rich iron oxide coating. The second stage (days 7-14) featured a rapid reduction of nitrite to both ammonia and nitrogen gas and the formation of a more crystalline, magnetite form iron oxide coating. Water reduction by Fe0 occurred concurrently with nitrite reduction from the beginning and contributed significantly to the overall iron corrosion. Nitrite at 14 mM was found to passivate the surface of Fe0 grains with respect to nitrite reduction. Adding aqueous Fe2+ significantly accelerated reduction of nitrite by Fe0 to nitrogen gas with lepidocrocite as the main iron corrosion product. Substantially, though still substoichiometrically, 0.55 mol of Fe2+ were concomitantly consumed per 1.0 mol nitrite reduction, indicating that Fe0 was the main electron source. In the presence of Fe2+, nitrite reduction out-competed water reduction in terms of contributing to the overall iron corrosion. Results of this study help understand complicated interactions between water reduction and nitrite reduction, the roles of surface-bound Fe2+, and the evolution of the iron corrosion coating.