Iron(III) modification of Bacillus subtilis membranes provides record sorption capacity for arsenic and endows unusual selectivity for As(V)

Bacillus subtilis is a spore forming bacterium that takes up both inorganic As(III) and As(V). Incubating the bacteria with Fe(III) causes iron uptake (up to ～0.5% w/w), and some of the iron attaches to the cell membrane as hydrous ferric oxide (HFO) with additional HFO as a separate phase. Remarkably, 30% of the Bacillus subtilis cells remain viable after treatment by 8 mM Fe(III). At pH 3, upon metalation, As(III) binding capacity becomes ～0, while that for As(V) increases more than three times, offering an unusual high selectivity for As(V) against As(III). At pH 10 both arsenic forms are sorbed, the As(V) sorption capacity of the ferrated Bacillus subtilis is at least of 11 times higher than that of the native bacteria. At pH 8 (close to pH of most natural water), the arsenic binding capacity per mole iron for the ferrated bacteria is greater than those reported for any iron containing sorbent. A sensitive arsenic speciation approach is thus developed based on the binding of inorganic arsenic species by the ferrated bacteria and its unusual high selectivity toward As(V) at low pH.     

Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility

Arsenic derived from natural sources occurs in groundwater in many countries, affecting the health of millions of people. The combined effects of As(V) reduction and diagenesis of iron oxide minerals on arsenic mobility are investigated in this study by comparing As(V) and As(III) sorption onto amorphous iron oxide (HFO), goethite, and magnetite at varying solution compositions. Experimental data are modeled with a diffuse double layer surface complexation model, and the extracted model parameters are used to examine the consistency of our results with those previously reported. Sorption of As(V) onto HFO and goethite is more favorable than that of As(III) below pH 5-6, whereas, above pH 7-8, As(II) has a higher affinity for the solids. The pH at which As(V) and As(III) are equally sorbed depends on the solid-to-solution ratio and type and specific surface area of the minerals and is shifted to lower pH values in the presence of phosphate, which competes for sorption sites. The sorption data indicate that, under most of the chemical conditions investigated in this study, reduction of As(V) in the presence of HFO or goethite would have only minor effects on or even decrease its mobility in the environment at near-neutral pH conditions. As(V) and As(III) sorption isotherms indicate similar surface site densities on the three oxides. Intrinsic surface complexation constants for As(V) are higher for goethite than HFO, whereas As(III) binding is similar for both of these oxides and also for magnetite. However, decrease in specific surface area and hence sorption site density that accompanies transformation of amorphous iron oxides to more crystalline phases could increase arsenic mobility.     

Arsenic(III) and arsenic(V) reactions with zerovalent iron corrosion products

Zerovalent iron (Fe0) has tremendous potential as a remediation material for removal of arsenic from groundwater and drinking water. This study investigates the speciation of arsenate (As(V)) and arsenite (As(III)) after reaction with two Fe0 materials, their iron oxide corrosion products, and several model iron oxides. A variety of analytical techniques were used to study the reaction products including HPLC-hydride generation atomic absorption spectrometry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray analysis, and X-ray absorption spectroscopy. The products of corrosion of Fe0 include lepidocrocite (gamma-FeOOH), magnetite (Fe3O4), and/or maghemite (gamma-Fe2O3), all of which indicate Fe(II) oxidation as an intermediate step in the Fe0 corrosion process. The in-situ Fe0 corrosion reaction caused a high As(III) and As(V) uptake with both Fe0 materials studied. Under aerobic conditions, the Fe0 corrosion reaction did not cause As(V) reduction to As(III) but did cause As(III) oxidation to As(V). Oxidation of As(III) was also caused by maghemite and hematite minerals indicating that the formation of certain iron oxides during Fe0 corrosion favors the As(V) species. Water reduction and the release of OH- to solution on the surface of corroding Fe0 may also promote As(III) oxidation. Analysis of As(III) and As(V) adsorption complexes in the Fe0 corrosion products and synthetic iron oxides by extended X-ray absorption fine structure spectroscopy (EXAFS) gave predominant As-Fe interatomic distances of 3.30-3.36 A. This was attributed to inner-sphere, bidentate As(III) and As(V) complexes. The results of this study suggest that Fe0 can be used as a versatile and economical sorbent for in-situ treatment of groundwater containing As(III) and As(V).     

A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development

A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (+/-1%), and As was completely recovered from HFO-doped gels (+/-4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.     

Arsenic removal with iron(II) and iron(III) in waters with high silicate and phosphate concentrations

Arsenic removal by passive treatment, in which naturally present Fe(II) is oxidized by aeration and the forming iron(III) (hydr)oxides precipitate with adsorbed arsenic, is the simplest conceivable water treatment option. However, competing anions and low iron concentrations often require additional iron. Application of Fe(II) instead of the usually applied Fe(III) is shown to be advantageous, as oxidation of Fe(II) by dissolved oxygen causes partial oxidation of As(III) and iron(III) (hydr)oxides formed from Fe(II) have higher sorption capacities. In simulated groundwater (8.2 mM HCO3(-), 2.5 mM Ca2+, 1.6 mM Mg2+, 30 mg/L Si, 3 mg/L P, 500 ppb As(III), or As(V), pH 7.0 +/- 0.1), addition of Fe(II) clearly leads to better As removal than Fe(III). Multiple additions of Fe(II) further improved the removal of As(II). A competitive coprecipitation model that considers As(III) oxidation explains the observed results and allows the estimation of arsenic removal under different conditions. Lowering 500 microg/L As(III) to below 50 microg/L As(tot) in filtered water required > 80 mg/L Fe(III), 50-55 mg/L Fe(II) in one single addition, and 20-25 mg/L in multiple additions. With As(V), 10-12 mg/L Fe(II) and 15-18 mg/L Fe(III) was required. In the absence of Si and P, removal efficiencies for Fe(II) and Fe(III) were similar: 30-40 mg/L was required for As(II), and 2.0-2.5 mg/L was required for As(V). In a field study with 22 tubewells in Bangladesh, passive treatment efficiently removed phosphate, but iron contents were generally too low for efficient arsenic removal.     

Coadsorption of arsenic(III) and arsenic(V) onto hydrous ferric oxide: effects on abiotic oxidation of arsenic(III), extraction efficiency, and model accuracy

Single solute adsorption and coadsorption of As(III) and As(V) onto hydrous ferric oxide (HFO), oxidation of As(III), and extraction efficiencies were measured in 0.2 atm O2. Oxidation was negligible for single-adsorbate experiments, but significant oxidation was observed in the presence of As(V) and HFO. Single-adsorbate As(III) or As(V) were incompletely extracted (0.5 M NaOH for 20 min), but all As was recovered in coadsorbate experiments. Single-adsorbate data were well-simulated using published surface complexation models, but those models (calibrated for single-adsorbate results) provided poor fits for coadsorbate experiments. An amended model accurately simulated single- and coadsorbate results. Model predictions of significant change in As(III) surface complex speciation in coadsorbate experiments was confirmed using zeta potential measurements. Our results demonstrate that mobility of arsenic in groundwater and removal in engineered treatment systems are more complicated when both As(III) and As(V) are present than anticipated based on single-adsorbate experimental results.     

Arsenic adsorption by Fe(III)-loaded open-celled cellulose sponge. Thermodynamic and selectivity aspects

Nowadays there is a great concern on the study of new adsorbent materials for either the removal or fixation of arsenic species because of their high toxicity and the health problems associated to such substances. The present paper reports a basic study of the adsorption of arsenic inorganic species from aqueous solutions using an open-celled cellulose sponge with anion-exchange and chelating properties (Forager Sponge). Consequences of preloading the adsorbentwith Fe(III) to enhance the adsorption selectivity are discussed and compared with the nonloaded adsorbent properties. The interactions of arsenic species with the Fe(III)-loaded adsorbent are accurately determined to clarify the feasibility of an effective remediation of contaminated waters. Arsenate is effectively adsorbed by the nonloaded and the Fe(III)-loaded sponge in the pH range 2-9 (maximum at pH 7), whereas arsenite is only slightly adsorbed by the Fe(III)-loaded sponge in the pH range 5-10 (maximum at pH 9), being that the nonloaded sponge is unable to adsorb As(III). The maximum sorption capacities are 1.83 mmol As(V)/g (pH approximately 4.5) and 0.24 mmol As(lII)/g (pH approximately 9.0) for the Fe(III)-loaded adsorbent. This difference is explained in terms of the different acidic behavior of both arsenic species. The interaction of the arsenic species with the Fe(III) loaded in the sponge is satisfactorily modeled. A 1:1 Fe:As complex is found to be formed for both species. H2AsO4- and H3AsO3 are determined to be adsorbed on Fe(III) with a thermodynamic affinity defined by log K = 2.5 +/- 0.3 and log K = 0.53 +/- 0.07, respectively. As(V) is, thus, found to be more strongly adsorbed than As(III) on the Fe(III) loaded in the sponge. A significant enhancement on As(V) adsorption selectivity by loading Fe(III) in the sponge is observed, and the effectiveness of the Fe(III)-loaded sponge for the As(V) adsorption is demonstrated, even in the presence of high concentrations of interfering anions (chloride, nitrate, sulfate, and phosphate).     

Iron(II)-catalyzed oxidation of arsenic(III) in a sediment column

Arsenic contamination in aquatic systems is a worldwide concern. Understanding the redox cycling of arsenic in sediments is critical in evaluating the fate of arsenic in aquatic environments and in developing sediment quality guidelines. The direct oxidation of inorganic trivalent arsenic, As(III), by dissolved molecular oxygen has been studied and found to be quite slow. A chemical pathway for As(III) oxidation has been proposed recently in which a radical species, Fe(IV), produced during the oxidation of divalent iron, Fe(II), facilitates the oxidation of As(III). Rapid oxidation of As(III) was observed (on a time scale of hours) in batch systems at pH 7 and 7.5, but the extent of As(III) oxidation was limited. The Fe(II)-catalyzed oxidation of As(III) is examined in a sediment column using both computational and experimental studies. A reactive-transport model is constructed that incorporates the complex kinetics of radical species generation and Fe(II) and As(III) oxidation that have been developed previously. The model is applied to experimental column data. Results indicate that the proposed chemical pathway can explain As(III) oxidation in sediments and that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.     

Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and coprecipitation with iron hydroxide

Acid mine drainage (AMD), which is caused by the biological oxidation of sulfidic materials, frequently contains arsenic in the form of arsenite, As(III), and/or arsenate, As(V), along with much higher concentrations of dissolved iron. The present work is directed toward the removal of arsenic from synthetic AMD by raising the pH of the solution by electrochemical reduction of H+ to elemental hydrogen and coprecipitation of arsenic with iron(III) hydroxide, following aeration of the catholyte. Electrolysis was carried out at constant current using two-compartment cells separated with a cation exchange membrane. Four different AMD model systems were studied: Fe(III)/As(V), Fe(III)/As(III), Fe(II)/As(V), and Fe(II)/As(III) with the initial concentrations for Fe(III) 260 mg/L, Fe(II) 300 mg/L, As(V), and As(III) 8 mg/L. Essentially quantitative removal of arsenic and iron was achieved in all four systems, and the results were independent of whether the pH was adjusted electrochemically or by the addition of NaOH. Current efficiencies were approximately 85% when the pH of the effluent was 4-7. Residual concentrations of arsenic were close to the drinking water standard proposed by the World Health Organization (10 microg/L), far below the mine waste effluent standard (500 microg/L).     

Thermodynamic stabilization of hydrous ferric oxide by adsorption of phosphate and arsenate

Hydrous ferric oxide (HFO) is an X-ray amorphous compound with a high affinity for anions under strongly or mildly acidic conditions. Because of the usually small particle size of HFO, the adsorption capacity is high and adsorption may significantly impact the thermodynamic properties of such materials. Here we show that adsorption of phosphate and arsenate stabilizes HFO by experimental determination of enthalpies of formation (by acid-solution calorimetry) and estimates of standard entropies for six phosphate- or arsenate-enriched HFO samples. At pH values lower than ～5, the phosphate-doped HFO is not only less soluble than ferrihydrite (anion-free HFO) but also crystalline FeOOH polymorphs feroxyhyte and lepidocrocite. The arsenate-doped HFO is also stabilized with respect to the ferrihydrite. Phosphate availability in soils can be controlled by the phosphate-enriched HFO which is many orders of magnitude less soluble than apatite or crystalline Fe(III) phosphates, for example strengite (FePO(4)·2H(2)O). Thermodynamic dissolution models for scorodite (FeAsO(4)·2H(2)O) and As-enriched HFO show that under mildly acidic or circumneutral conditions, scorodite dissolves, As-HFO precipitates, and a substantial amount of As(V) is released into the aqueous solution (at pH 7, log m(As) ～ -2.5). The data presented in this paper can be used to model the equilibrium concentration of Fe(III), P(V), or As(V) in soil solutions or in natural or anthropogenic sediments polluted by arsenic.     

Microbial reductive transformation of phyllosilicate Fe(III) and U(VI) in fluvial subsurface sediments

The microbial reduction of Fe(III) and U(VI) was investigated in shallow aquifer sediments collected from subsurface flood deposits near the Hanford Reach of the Columbia River in Washington State. Increases in 0.5 N HCl-extractable Fe(II) were observed in incubated sediments and (57)Fe M?ssbauer spectroscopy revealed that Fe(III) associated with phyllosilicates and pyroxene was reduced to Fe(II). Aqueous uranium(VI) concentrations decreased in subsurface sediments incubated in sulfate-containing synthetic groundwater with the rate and extent being greater in sediment amended with organic carbon. X-ray absorption spectroscopy of bioreduced sediments indicated that 67-77% of the U signal was U(VI), probably as an adsorbed species associated with a new or modified reactive mineral phase. Phylotypes within the Deltaproteobacteria were more common in Hanford sediments incubated with U(VI) than without, and in U(VI)-free incubations, members of the Clostridiales were dominant with sulfate-reducing phylotypes more common in the sulfate-amended sediments. These results demonstrate the potential for anaerobic reduction of phyllosilicate Fe(III) and sulfate in Hanford unconfined aquifer sediments and biotransformations involving reduction and adsorption leading to decreased aqueous U concentrations.     

Arsenic sequestration by ferric iron plaque on cattail roots

Typha latifolia (cattail) sequesters arsenic within predominantlyferric iron root coatings, thus decreasing mobility of this toxic element in wetland sediments. Element-specific XRF microtomographic imaging illustrated a high spatial correlation between iron and arsenic in root plaques, with little arsenic in the interior of the roots. XANES analyses demonstrated that the plaque was predominantly ferric iron and contained approximately 20% As(III) and 80% As(V), which is significant because the two oxidation states form species that differ in toxicity and mobility. For the first time, spatial distribution maps of As oxidation states were developed, indicating that As(III) and As(V) are both fairly heterogeneous throughoutthe plaque. Chemical extractions showed that As was strongly adsorbed in the plaque rather than coprecipitated. Iron and arsenic concentrations ranged from 0.03 to 0.8 g Fe g(-1) wet plaque and 30 to 1200 microg As g(-1) wet plaque, consistent with a mechanism of As adsorption onto Fe(III) oxyhydroxide plaque. Because this mechanism decreases the concentrations of both As(III) and As(V) in groundwater, we propose that disruption of vegetation could increase the concentrations of mobile arsenic.     

Compost-based permeable reactive barriers for the source treatment of arsenic contaminations in aquifers: column studies and solid-phase investigations

The bulk of arsenic (As) at contaminated sites is frequently associated with iron (hydr)oxides. Various studies ascribe increasing dissolved As concentrations to the transformation of iron (hydr)oxides into iron sulfides, which is initiated by dissolved sulfide. We investigated whetherthis processes can be utilized as a source treatment approach using compost-based permeable reactive barriers (PRB), which promote microbial sulfate reduction. Arsenic-bearing aquifer sedimentfrom a contaminated industrial site showed a decrease in As content of <10% after 420 days of percolation with sulfide-free artificial groundwater. In contrast, water that had previously passed through organic matter and exhibited sulfide concentrations of 10-30 mg/L decreased As content in the sediment by 87% within 360 days. X-ray diffraction showed no arsenic sulfides, but XANES spectra (X-ray absorption near edge structure) and associated linear combinations revealed that adsorbed arsenate of the original sediment was in part reduced to arsenite and indicated the formation of minor amounts of a substance that contains As and sulfur. The speciation of dissolved As changed from initially As(V)-dominated to As(III)-dominated after sulfide flushing was started, which increases the mobility of As. Because sulfide can be supplied not only by compost-based PRBs but also by direct injection, sulfide flushing has a wide range of application for the source treatment of arsenic.     

Intraparticle reduction of arsenite (As(III)) by nanoscale zerovalent iron (nZVI) investigated with In Situ X-ray absorption spectroscopy

While a high efficiency of contaminant removal by nanoscale zerovalent iron (nZVI) has often been reported for several contaminants of great concern, including aqueous arsenic species, the transformations and translocation of contaminants at and within the nanoparticles are not clearly understood. By analysis using in situ time-dependent X-ray absorption spectroscopy (XAS) of the arsenic core level for nZVI in anoxic As(III) solutions, we have observed that As(III) species underwent two stages of transformation upon adsorption at the nZVI surface. The first stage corresponds to breaking of As-O bonds at the particle surface, and the second stage involves further reduction and diffusion of arsenic across the thin oxide layer enclosing the nanoparticles, which results in arsenic forming an intermetallic phase with the Fe(0) core. Extended X-ray absorption fine-structure (EXAFS) data from experiments conducted at different iron/arsenic ratios indicate that the reduced arsenic species tend to be enriched at the surface of the Fe(0) core region and had limited mobility into the interior of the metal core within the experimental time frame (up to 22 h). Therefore, there was an accumulation of partially reduced arsenic at the Fe(0)/oxide interface when a relatively large arsenic content was present in the solid phase. These results illuminate the role of intraparticle diffusion and reduction in affecting the chemical state and spatial distribution of arsenic in nZVI materials.     

Microbially catalyzed nitrate-dependent oxidation of biogenic solid-phase Fe(II) compounds

The potential for microbially catalyzed NO3(-)-dependent oxidation of solid-phase Fe(II) compounds was examined using a previously described autotrophic, denitrifying, Fe(II)-oxidizing enrichment culture. The following solid-phase Fe(II)-bearing minerals were considered: microbially reduced synthetic goethite, two different end products of microbially hydrous ferric oxide (HFO) reduction (biogenic Fe3O4 and biogenic FeCO3), chemically precipitated FeCO3, and two microbially reduced iron(III) oxide-rich subsoils. The microbially reduced goethite, subsoils, and chemically precipitated FeCO3 were subject to rapid NO3(-)-dependent Fe(II) oxidation. Significant oxidation of biogenic Fe3O4 was observed. Very little biogenic FeCO3 was oxidized. No reduction of NO3- or oxidation of Fe(II) occurred in pasteurized cultures. The molar ratio of NO3- reduced to Fe(II) oxidized in cultures containing chemically precipitated FeCO3, and one of the microbially reduced subsoils approximated the theoretical stoichiometry of 0.2:1. However, molar ratios obtained for oxidation of microbially reduced goethite, the other subsoil, and the HFO reduction end products did not agree with this theoretical value. These discrepancies may be related to heterotrophic NO3- reduction coupled to oxidation of dead Fe(III)-reducing bacterial biomass. Our findings demonstrate that microbally catalyzed NO3(-)-dependent Fe(II) oxidation has the potential to significantly accelerate the oxidation of solid-phase Fe(II) compounds by oxidized N species. This process could have an important influence on the migration of contaminant metals and radionuclides in subsurface environments.     

Preloading hydrous ferric oxide into granular activated carbon for arsenic removal

Arsenic is of concern in water treatment because of its health effects. This research focused on incorporating hydrous ferric oxide (HFO) into granular activated carbon (GAC) for the purpose of arsenic removal. Iron was incorporated into GAC via incipient wetness impregnation and cured at temperatures ranging from 60 to 90 degrees C. X-ray diffractions and arsenic sorption as a function of pH were conducted to investigate the effect of temperature on final iron oxide (hydroxide) and their arsenic removal capabilities. Results revealed that when curing at 60 degrees C, the procedure successfully created HFO in the pores of GAC, whereas at temperatures of 80 and 90 degrees C, the impregnated iron oxide manifested a more crystalline form. In the column tests using synthetic water, the HFO-loaded GAC prepared at 60 degrees C also showed higher sorption capacities than media cured at higher temperatures. These results indicated that the adsorption capacity for arsenic was closely related to the form of iron (hydr)oxide for a given iron content For the column test using a natural groundwater, HFO-loaded GAC (Fe, 11.7%) showed an arsenic sorption capacity of 26 mg As/g when the influent contained 300 microg/L As. Thus, the preloading of HFO into a stable GAC media offered the opportunity to employ fixed carbon bed reactors in water treatment plants or point-of-use filters for arsenic removal.     

Removal mechanism of As(III) by a novel Fe-Mn binary oxide adsorbent: oxidation and sorption

A novel Fe-Mn binary oxide adsorbent was developed for effective As(III) removal, which is more difficult to remove from drinking water and much more toxic to humans than As(V). The synthetic adsorbent showed a significantly higher As(III) uptake than As(V). The mechanism study is therefore necessary for interpreting such result and understanding the As(III) removal process. A control experiment was conducted to investigate the effect of Na2SO3-treatment on arsenic removal, which can provide useful information on As(III) removal mechanism. The adsorbent was first treated by Na2SO3, which can lower its oxidizing capacity by reductive dissolution of the Mn oxide and then reacted with As(V) or As(III). The results showed that the As(V) uptake was enhanced while the As(III) removal was inhibited after the pretreatment, indicating the important role of manganese dioxide during the As(III) removal. FTIR along with XPS was used to analyze the surface change of the original Fe-Mn adsorbent and the pretreated adsorbent before and after reaction with As(V) or As(III). Change in characteristic surface hydroxyl groups (Fe-OH, 1130, 1048, and 973 cm(-1)) was observed by the FTIR. The determination of arsenic oxidation state on the solid surface after reaction with As(III) revealed that the manganese dioxide instead of the iron oxide oxidized As(III) to As(V). The iron oxide was dominant for adsorbing the formed As(V). An oxidation and sorption mechanism for As(III) removal was developed. The relatively higher As(III) uptake may be attributed to the formation of fresh adsorption sites at the solid surface during As(III) oxidation.     

Microbes enhance mobility of arsenic in pleistocene aquifer sand from Bangladesh

Dissimilatory metal-reducing bacteria can mobilize As, but few studies have studied such processes in deeper orange-colored Pleistocene sands containing 1-2 mg kg(-1) As that are associated with low-As groundwater in Bangladesh. To address this gap, anaerobic incubations were conducted in replicate over 90 days using natural orange sands initially containing 0.14 mg kg(-1) of 1 M phosphate-extractable As (24 h), >99% as As(V), and 0.8 g kg(-1) of 1.2 M HCl-leachable Fe (1 h at 80 °C), 95% as Fe(III). The sediment was resuspended in artificial groundwater, with or without lactate as a labile carbon source, and inoculated with metal-reducing Shewanella sp. ANA-3. Within 23 days, dissolved As concentrations increased to 17 μg L(-1) with lactate, 97% as As(III), and 2 μg L(-1) without lactate. Phosphate-extractable As concentrations increased 4-fold to 0.6 mg kg(-1) in the same incubations, even without the addition of lactate. Dissolved As levels in controls without Shewanella, both with and without lactate, instead remained <1 μg L(-1). These observations indicate that metal-reducers such as Shewanella can trigger As release to groundwater by converting sedimentary As to a more mobilizable form without the addition of high levels of labile carbon. Such interactions need to be better understood to determine the vulnerability of low-As aquifers from which drinking water is increasingly drawn in Bangladesh.     

Spectroscopic evidence for Fe(II)-Fe(III) electron transfer at the iron oxide-water interface

Using the isotope specificity of 57Fe M?ssbauer spectroscopy, we report spectroscopic observations of Fe(II) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(II)). M?ssbauer spectra of Fe(II) adsorbed to rutile (TiO2) and aluminum oxide (Al2O3) show only Fe(II) species, whereas spectra of Fe(II) reacted with goethite (alpha-FeOOH), hematite (alpha-Fe2O3), and ferrihydrite (Fe5HO8) demonstrate electron transfer between the adsorbed Fe(II) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(III) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(II) is present. This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(II) in the presence of oxide surfaces.     

Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.