Understanding chromate reaction kinetics with corroding iron media using Tafel analysis and electrochemical impedance spectroscopy

The kinetics of chromate removal from contaminated water by zerovalent iron media are not well understood. This study investigated the reactions occurring on iron surfaces in chromate solutions in order to understand the removal kinetics and to assess the long-term ability of zerovalent iron for removing Cr(VI) from contaminated water. Tafel polarization analysis and electrochemical impedance spectroscopy were used to determine the corrosion rates and charge-transfer resistances associated with Cr(VI) removal by iron wires suspended in electrolyte solutions with initial Cr(VI) concentrations of 10,000 microg/L. The condition of the iron surfaces at the time of their exposure to chromate determined the effectiveness of the iron for chromate removal. Both iron coated with a water-formed oxide and initially oxide-free iron were effective for chromate removal. However, iron coated with an air-formed oxide was an order of magnitude less effective for removing soluble chromium. Although iron with the air-formed oxide was largely passivated with respect to chromate removal, its overall rate of corrosion was similar to that for iron with the other initial surface conditions. This indicates that water, but not chromate, was able to penetrate the air-formed oxide coating and access cathodic sites. For all initial surface conditions, addition of chromate decreased the corrosion rate by increasing the corrosion potential and the anodic charge transfer resistance. Although Cr(VI) is a strong oxidant rates of iron corrosion were not proportional to the aqueous Cr(VI) concentrations due to anodic control of iron corrosion. Under anodically controlled conditions, the rate of corrosion was limited by the rate at which Fe2+ could be released at anodic sites and not by the rate at which oxidants were able to accept electrons. This study shows that the zero order removal kinetics of Cr(VI) by iron media can be explained by anodic control of iron corrosion and the concomitant anodic control of Cr(VI) reduction.     

Understanding nitrate reactions with zerovalent iron using tafel analysis and electrochemical impedance spectroscopy

This study investigated the reaction mechanisms of nitrate (NO3-) with zerovalent iron (ZVI) media under conditions relevantto groundwatertreatment using permeable reactive barriers (PRB). Reaction rates of NO3- with freely corroding and with cathodically or anodically polarized iron wires were measured in batch reactors. Tafel analysis and electrochemical impedance spectroscopy (EIS) were used to investigate the reactions occurring on the iron surfaces. Reduction of NO3- by corroding iron resulted in near stoichiometric production of NO2-, which did not measurably react in the absence of added Fe(II). Increasing NO3- concentrations resulted in increasing corrosion currents. However, EIS and Tafel analyses indicated that there was little direct reduction of NO3- at the ZVI surface, despite the presence of water reduction. This behavior can be attributed to formation of a microporous oxide on the iron surfaces that blocked reduction of NO3- and NO2- but did not block water reduction. This finding is consistent with previous observations that NO3- impedes reduction of organic compounds by ZVI. Nitrite concentrations greater than 4 mM resulted in anodic passivation of the iron, but passivation was not observed with NO3- concentrations as high as 96 mM. This indicates that the passivating oxide preventing NO3- reduction was permeable toward cation migration. Since reaction with Fe(0) can be excluded asthe mechanism for NO3- and NO2- reduction, reaction with Fe(II)-containing oxides coating the iron surface is the most likely reaction mechanism. This suggests that short-term batch tests requiring little turnover of reactive sites on the iron surface may overestimate long-term rates of NO3- removal because the effects of passivation are not apparent in batch tests conducted with high initial Fe(II) to NO3- ratios.     

Chromium-removal processes during groundwater remediation by a zerovalent iron permeable reactive barrier

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values >1500 microg L(-1) in groundwater hydraulically upgradient of the PRB to values <1 microg L(-1) in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral-water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion.     

Understanding soluble arsenate removal kinetics by zerovalent iron media

Zerovalent iron filings have been proposed as a filter medium for removing arsenic compounds from potable water supplies. This research investigated the kinetics of arsenate removal from aqueous solutions by zerovalent iron media. Batch experiments were performed to determine the effect of the iron corrosion rate on the rate of As(V) removal. Tafel analyses were used to determine the effect of the As(V) concentration on the rate of iron corrosion in anaerobic solutions. As(V) removal in column reactors packed with iron filings was measured over a 1-year period of continuous operation. Comparison of As(V) removal by freely corroding and cathodically protected iron showed that rates of arsenate removal were dependent on the continuous generation of iron oxide adsorption sites. In addition to adsorption site availability, rates of arsenate removal were also limited by mass transfer associated with As(V) diffusion through iron corrosion products. Steady-state removal rates in the column reactor were up to 10 times faster between the inlet-end and the first sampling port than between the first sampling port and the effluent-end of the column. Faster removal near the influent-end of the column was due to a faster rate of iron oxidation in that region. The presence of 100 microg/L As(V) decreased the iron corrosion rate by up to a factor of 5 compared to a blank electrolyte solution. However, increasing the As(V) concentration from 100 to 20,000 microg/L resulted in no further decrease in the iron corrosion rate. The kinetics of arsenate removal ranged between zeroth- and first-order with respect to the aqueous As(V) concentration. The apparent reaction order was dependent on the availability of adsorption sites and on the aqueous As(V) concentration. X-ray absorption spectroscopy analyses showed the presence of iron metal, magnetite (Fe3O4), an Fe(III) oxide phase, and possibly an Fe(II,III) hydroxide phase in the reacted iron filings. These mixed valent oxide phases are not passivating and permit sustained iron corrosion and continuous generation of new sites for As(V) adsorption.     

Removal of chromium (VI) by acid-washed zero-valent iron under various groundwater geochemistry conditions

The hexavalent chromium (Cr(VI)) removal capacity of acid-washed zerovalent iron (AW-Fe0) was evaluated under different groundwater geochemistry conditions through column experiments. It was found that each gram of the AW-Fe0 could remove 0.65-1.76 mg of Cr(VI) from synthetic groundwater in the absence of bicarbonate (HCO3-), magnesium and/or calcium ions. Groundwater geochemistry was found to exert various degrees of impact on Cr(VI) removal by the AW-Fe0, in which HCO3- alone gave the mildest impact whereas the copresence of calcium and HCO3- exerted the greatest impact In comparison with the unwashed Fe0, the AW-Fe0 showed a poorer Cr(VI) removal capacity and was also more susceptible to the influence of the dissolved groundwater constituents on Cr(VI) removal,thereby indicating the unsuitability of using AW-Fe0 in permeable reactive barriers for remediation of Cr(VI)-contaminated groundwater. On the AW-Fe0 surface, where the indigenous iron precipitates were almost erased, trivalent chromium including chromium (III) oxides, hydroxides, and oxyhydroxides in irregular strip, chick footmark-liked or boulder-liked forms as well as Cr(III)-Cr(VI) mixed oxides were detected.     

Electrochemical and spectroscopic study of arsenate removal from water using zero-valent iron media

This study investigated the mechanisms involved in removing arsenate from drinking water supplies using zero-valent iron media. Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreased iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analyses indicated that there was no measurable reduction of As(V) to As(III). X-ray absorption spectroscopy analyses indicated that all arsenic associated with the zero-valent iron surfaces was in the oxidation state. Interatomic arsenic-iron distances determined from EXAFS analyses were consistent with bidentate corner-sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zero-valent iron media involves surface complexation only and does not involve reduction to metallic arsenic.     

Iron promoted reduction of chromate by dissimilatory iron-reducing bacteria

Chromate is a priority pollutant within the U.S. and many other countries, the hazard of which can be mitigated by reduction to the trivalent form of chromium. Here we elucidate the reduction of Cr(VI) to Cr(III) via a closely coupled, biotic-abiotic reductive pathway under iron-reducing conditions. Injection of chromate into stirred-flow reactors containing Shewanella alga strain BrY and iron (hydr)oxides of varying stabilities results in complete reduction to Cr(III). The maximum sustainable Cr(VI) reduction rate was 5.5 micrograms CrVI.mg-cell-1.h-1 within ferric (hydr)oxide suspensions (surface area 10 m2). In iron limited systems (having HEPES as a buffer), iron was cycled suggesting it acts in a catalytic-type manner for the bacterial reduction of Cr(VI). BrY also reduced Cr(VI) directly; however, the rate of direct (enzymatic) reduction is considerably slower than by Fe(II)(aq) and is inhibited within 20 h due to chromate toxicity. Thus, dissimilatory iron reduction may provide a primary pathway for the sequestration and detoxification of chromate in anaerobic soils and water.     

Influence of Anionic Cosolutes and pH on Nanoscale Zerovalent Iron Longevity: Time Scales and Mechanisms of Reactivity Loss toward 1,1,1,2-Tetrachloroethane and Cr(VI)

Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl(-), SO(4)(2-), and ClO(4)(-) suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO(3)(-) and HCO(3)(-) concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO(3)(-) suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction.     

Combined removal of chlorinated ethenes and heavy metals by zerovalent iron in batch and continuous flow column systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.     

Rate and extent of aqueous perchlorate removal by iron surfaces

The rate and extent of perchlorate reduction on several types of iron metal was studied in batch and column reactors. Mass balances performed on the batch experiments indicate that perchlorate is initially sorbed to the iron surface, followed by a reduction to chloride. Perchlorate removal was proportional to the iron dosage in the batch reactors, with up to 66% removal in 336 h in the highest dosage system (1.25 g mL(-1)). Surface-normalized reaction rates among three commercial sources of iron filings were similar for acid-washed samples. The most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial pH values. Surface mediation of the reaction is supported by the absence of reduction in batch experiments with soluble Fe2+ and also by the similarity in specific reaction rate constants (kSA) determined for three different iron types. Elevated soluble chloride concentrations significantly inhibited perchlorate reduction, and lower removal rates were observed for iron samples with higher amounts of background chloride contamination. Perchlorate reduction was not observed on electrolytic sources of iron or on a mixed-phase oxide (Fe3O4), suggesting that the reactive iron phase is neither pure zerovalent iron nor the mixed oxide alone. A mixed valence iron hydr(oxide) coating or a sorbed Fe2+ surface complex represent the most likely sites for the reaction. The observed reaction rates are too slow for immediate use in remediation system design, but the findings may provide a basis for future development of cost-effective abiotic perchlorate removal techniques.     

Field column study using zerovalent iron for mercury removal from contaminated groundwater

Passive in situ remediation technologies, for example, permeable reactive barriers, PRBs, are an attractive and less expensive alternative compared to conventional pump and treat systems for groundwater remediation. Field column experiments were conducted to evaluate the removal of dissolved mercury from groundwater using zerovalent iron as the reactive media. Two column tests were conducted over a 6-week period, which simulated 2 and 10 years of groundwater flow through a potential full-scale treatment system. The influent groundwater pH was 7.8-9.5. The groundwater was reduced with an Eh, corrected to the standard hydrogen electrode, ranging from 0 to 120 mV over the trial period. Prior to treatment the total mercury concentration of the groundwater was approximately 40 microg L(-1). Effluent from the 10-year simulation contained approximately 0.5 microg/L of mercury during the first 3 weeks and increased to as much as 4 microg L(-1) by the end of the testing period. Effluent from the 2-year simulation was generally < 0.1 microg L(-1). Profile sampling of the 2-year simulation suggests that most of the mercury removal occurred in the initial 50% of the 20 cm column. Mineralogical studies, conducted using SEM/EDS and X-ray absorption spectroscopy (XAS), confirm the accumulation of mercury onto a zerovalent iron surface in this 20-cm zone. These analyses indicate that mercury accumulated as a mercury sulfide with a stoichiometery similar to those of cinnabar and metacinnabar (HgS).     

Ferrate treatment for removing chromium from high-level radioactive tank waste

A method has been developed for removing chromium from alkaline high-level radioactive tank waste. Removing chromium from these wastes is critical in reducing the volume of waste requiring expensive immobilization and deep geologic disposition. The method developed is based on the oxidation of insoluble chromium(III) compounds to soluble chromate using ferrate. This method could be generally applicable to removing chromium from chromium-contaminated solids, when coupled with a subsequent reduction of the separated chromate back to chromium(III). The tests conducted with a simulated Hanford tank sludge indicate that the chromium removal with ferrate is more efficient at 5 M NaOH than at 3 M NaOH. Chromium removal increases with increasing Fe(VI)/Cr(II) molar ratio, but the chromium removal tends to level out for Fe(VI)/ Cr(III) greaterthan 10. Increasingtemperature leadsto better chromium removal, but higher temperatures also led to more rapid ferrate decomposition. Tests with radioactive Hanford tank waste generally confirmed the simulant results. In all cases examined, ferrate enhanced the chromium removal, with a typical removal of around 60-70% of the total chromium present in the washed sludge solids. The ferrate leachate solutions did not contain significant concentrations of transuranic elements, so these solutions could be disposed as low-activity waste.     

Natural occurrence of hexavalent chromium in the Aromas Red Sands Aquifer, California

To address increasing concerns of chromium contamination in the drinking water of Santa Cruz County, we designed a study to investigate the source(s) and spatial gradients of the chromium concentration and speciation in local aquifers. This study was catalyzed by a report (January 2001) bythe Soquel Creek Water District of elevated hexavalent chromium concentrations ranging from 6 to 36 microg L(-1), approaching the state's maximum concentration limit of 50 microg L(-1), in the Aromas Red Sands aquifer. To test the accuracy of those preliminary measurements, we collected groundwater using trace metal clean techniques from 11 sites in Santa Cruz County, including 10 from the aquifer with reportedly elevated chromium concentrations and 1 from an adjacent aquifer, the Purisima, and analyzed them fortotal chromium using inductively couple plasma mass spectrometry. Nine of the reportedly 10 contaminated sites had total chromium concentrations ranging from 5 to 39 microg L(-1), while one from the control site was below the limit of detection (0.01 microg L(-1)). We also measured the speciation of chromium at all sites using a solid supported membrane extraction coupled with graphite furnace atomic absorption spectrometry and determined that on average 84% of total chromium was Cr(VI). In addition to the groundwater analyses, a series of extractions were performed on sediment samples from both the Aromas Red Sands and Purisima aquifers. These tests were used to empirically characterize sediment trace metal (Cr, Fe, Mn) distributions in five phases providing information about the origin, availability, reactivity, and mobilization of these trace metals. Results from groundwater and sediment samples indicate that the chromium is naturally occurring in the Aromas Red Sands aquifer, possibly by Cr(III) mineral deposits being oxidized to Cr(VI) by manganese oxides in the aquifer.     

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.     

Evaluation of mixed valent iron oxides as reactive adsorbents for arsenic removal

The objective of this research was to determine if Fe(II)-bearing iron oxides generate ferric hydroxides at sufficient rates for removing low levels of arsenic in packed-bed reactors, while at the same time avoiding excessive oxide production that contributes to bed clogging in oxygenated waters. Column experiments were performed to determine the effectiveness of three media for arsenic removal over a range in empty bed contact times, influent arsenic concentrations, dissolved oxygen (DO) levels, and solution pH values. Corrosion rates of the media as a function of the water composition were determined using batch and electrochemical methods. Rates of arsenic removal were first order in the As(V) concentration and were greater for media with higher corrosion rates. As(V) removal increased with increasing DO levels primarily due to faster oxidation of the Fe2+ released by media corrosion. To obtain measurable amounts of arsenic removal in 15 mM NaCl electrolyte solutions containing 50 microg/L As(V), the rate of Fe2+ released by the media needed to be at least 15 times greater than the As(V) feed rate into the column. In waters containing 30 mg/L of silica and 50 microg/L of As(V), measurable amounts of arsenic removal were obtained only for Fe2+ release rates that were at least 200 times greater than the As(V) feed rate. Although all columns showed losses in hydraulic conductivity overthe course of 90 days of operation, the conductivity values remained high, and the losses could be reversed by backwashing the media. The reaction products produced by the media in domestic tap water had average As-to-Fe ratios that were approximately 25% higher than those for a commercially available adsorbent.     

Kinetics and structural constraints of chromate reduction by green rusts

Green rusts, ferrous-ferric iron oxides, occur in many anaerobic soils and sediments and are highly reactive, making them important phases impacting the fate and transport of environmental contaminants. Despite their potential importance in environmental settings, reactions involving green rusts remain rather poorly described. Chromate is a widespread contaminant having deleterious impacts on plant and animal health; its fate may in part be controlled by green rust. Here we examine chromate reduction by a series of green rust phases and resolve the reaction kinetics at pH 7. The overall kinetics of the reactions are well described by the expression d[Cr(VI)]/dt = -k[Cr(VI)][GR], and this model was successfully used to predict rates of reaction at varying chromium concentrations. The rates of reduction are controlled by the concentration of ferrous iron, surface area, and chemical structure of the green rust including layer spacing. On a mass basis, green rust (GR) chloride is the most rapid reductant of Cr(VI) followed by GRCO3 and GRSO4, with pseudo-first-order rate coefficients (k(obs)) (with respect to Cr(VI) concentration) ranging from 1.22 x 10(-3) to 3.7 x 10(-2) s(-1). Chromium(III)-substituted magnetite and lepidocrocite were identified as the major oxidation products. The nature of the oxidation products appears to be independent of the anionic class of green rust, but their respective concentrations display a dependence on the initial GR. The mole fraction of Fe(III) in the Cr(x),Fe(1-x)(OH)3 x nH2O reaction product ranged from 17% to 68%, leading to a highly stabilized (low solubility) phase.     

Enhanced redox conversion of chromate and arsenite in ice

The redox transformation of trace elements is critically affected by properties of the environmental media. While the environmentally important redox chemical reactions in aquatic environments have been extensively studied, those in the ice phase have been studied in only a few cases. In this work, chromium and arsenic species were selected as the model inorganic oxyanion contaminants for the study of redox chemical transformation in ice. We investigated (1) the reduction of hexavalent Cr(VI) (as chromate) by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) (as arsenite) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) by various organic acids (electron donors) was negligible in ambient aqueous solution but was significantly accelerated in ice. The simultaneous reduction of Cr(VI) and oxidation of As(III) in ice phase proceeded stoichiometrically, whereas their mutual conversion was insignificant in aqueous solution. The enhanced redox conversion of Cr(VI)/As(III) in ice is ascribed to the freeze concentration of both electron donors (e.g., organic acids, arsenites) and protons in the ice crystal grain boundaries. When the concentrations of both electron donors and protons were highly raised to an extreme, the removal rates of Cr(VI) in aqueous solution approached to those in ice. This specific combination of Cr(VI)/As(III) redox couple may provide an example that represents innumerable redox conversion reactions that could be greatly accelerated in ice/snow-covered or frozen environments.     

Use of copper shavings to remove mercury from contaminated groundwater or wastewater by amalgamation

The efficacy of copper shavings (Cu(0)) for the removal of Hg2+ from aqueous solution by amalgamation is demonstrated. Two kinds of copper shavings were investigated: (a) chemically processed shavings (Fluka) and (b) recycled shavings from scrap metal. Batch sorption experiments yielded very high retardation coefficients of 28 850-82 830 for the concentration range studied (1-10 000 microg/L Hg2+ dissolved in distilled water or in a 0.01 M CaCl2 matrix solution). Sorption data were well-described bythe Freundlich isotherm equation. Kinetic batch sorption experiments showed that 96-98% of Hg2+ was removed within 2 h. Column experiments were performed with a mercury solution containing 1000 microg/L Hg in a 0.01 M CaCl2 matrix with a flow rate of 0.5 m/d. No mercury breakthrough (c/c(0) = 0.5) could be detected after more than 2300 percolated pore volumes, and the high retardation coefficients determined in the batch studies could be confirmed. Copper was released from the shavings due to the amalgamation process and to copper corrosion by oxygen, resulting in concentrations of mobilized copper of 0.2-0.6 mg/L. Due to their high efficiency in removing Hg2+ from aqueous solution, the use of copper shavings for the removal of mercury from contaminated water is suggested, employing a sequential system of mercury amalgamation followed by the removal of mobilized copper by an ion exchanger such as zeolites. Possible applications could be in environmental technologies such as wastewater treatment or permeable reactive barriers for in situ groundwater remediation.     

Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions: the role of passivation

This study investigated Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions similar to those present at the Hanford site. Compared to acidic and neutral pH, chromium(VI) reduction by magnetite at high pH conditions is limited (<20% of potential reduction capacity), and the extent of reduction does not vary significantly with increasing NaOH concentration. This is due to the formation of maghemite, goethite, and/or Fe1-xCrxOOH, which may form a passivation layer on the magnetite surface, stopping further chromate reduction. Maghemite is formed in lower NaOH concentrations. The extent of goethite formation increases with NaOH concentration. Goethite may be formed through two mechanisms: (i) dissolution of magnetite leads to the precipitation of goethite and/or (ii) dissolution of newly formed maghemite intermediate, followed by precipitation of goethite. Extended X-ray absorption fine structure spectroscopy shows that Cr has a similar structural environment at alkaline pH as at acidic and circumneutral conditions.     

Solid waste removes toxic liquid waste: adsorption of chromium(VI) by iron complexed protein waste

The leather processing industry generates huge amounts of wastes, both in solid and liquid form. Fleshing from animal hides/skins is one such waste that is high in protein content. In this study, raw fleshing has been complexed with iron and is used for removal of chromium(VI). The effect of pH and the initial concentration of chromium(VI) on the removal of Cr(IV) by iron treated fleshing is presented. Iron treatment is shown to greatly improve adsorption of the fleshing for hexavalent chromium. The ultimate adsorption capacity of iron treated fleshing is 51 mg of chromium(VI) per gram of fleshing. That of untreated fleshing is 9 mg/g such that iron treatment increases the adsorption capacity of fleshing by 10-fold. The measured adsorption kinetics is well described by a pseudo-second-order kinetic model. The uptake of chromium(VI) by fleshing is best described by the Langmuir adsorption isotherm model. X-ray photoelectron spectroscopic (XPS) studies show that the iron is incorporated into the protein matrix. Shifts in XPS spectra suggest that dichromate binding occurs with iron at active adsorption sites and that iron treated fleshing removes chromium(VI) without reducing it to chromium(III).