Iron and arsenic cycling in intertidal surface sediments during wetland remediation

The accumulation and behavior of arsenic at the redox interface of Fe-rich sediments is strongly influenced by Fe(III) precipitate mineralogy, As speciation, and pH. In this study, we examined the behavior of Fe and As during aeration of natural groundwater from the intertidal fringe of a wetland being remediated by tidal inundation. The groundwater was initially rich in Fe(2+) (32 mmol L(-1)) and As (1.81 μmol L(-1)) with a circum-neutral pH (6.05). We explore changes in the solid/solution partitioning, speciation and mineralogy of Fe and As during long-term continuous groundwater aeration using a combination of chemical extractions, SEM, XRD, and synchrotron XAS. Initial rapid Fe(2+) oxidation led to the formation of As(III)-bearing ferrihydrite and sorption of >95% of the As(aq) within the first 4 h of aeration. Ferrihydrite transformed to schwertmannite within 23 days, although sorbed/coprecipitated As(III) remained unoxidized during this period. Schwertmannite subsequently transformed to jarosite at low pH (2-3), accompanied by oxidation of remaining Fe(2+). This coincided with a repartitioning of some sorbed As back into the aqueous phase as well as oxidation of sorbed/coprecipitated As(III) to As(V). Fe(III) precipitates formed via groundwater aeration were highly prone to reductive dissolution, thereby posing a high risk of mobilizing sorbed/coprecipitated As during any future upward migration of redox boundaries. Longer-term investigations are warranted to examine the potential pathways and magnitude of arsenic mobilization into surface waters in tidally reflooded wetlands.     

Low molecular weight carboxylic acids in oxidizing porphyry copper tailings

The distribution of low molecular weight carboxylic acids (LMWCA) was investigated in pore water profiles from two porphyry copper tailings impoundments in Chile (Piuquenes at La Andina and Cauquenes at El Teniente mine). The objectives of this study were (1) to determine the distribution of LMWCA, which are interpreted to be the metabolic byproducts of the autotroph microbial community in this low organic carbon system, and (2) to infer the potential role of these acids in cycling of Fe and other elements in the tailings impoundments. The speciation and mobility of iron, and potential for the release of H+ via hydrolysis of the ferric iron, are key factors in the formation of acid mine drainage in sulfidic mine wastes. In the low-pH oxidation zone of the Piuquenes tailings, Fe(III) is the dominant iron species and shows high mobility. LMWCA, which occur mainly between the oxidation front down to 300 cm below the tailings surface at both locations (e.g., max concentrations of 0.12 mmol/L formate, 0.17 mmol/L acetate, and 0.01 mmol/L pyruvate at Piuquenes and 0.14 mmol/L formate, 0.14 mmol/L acetate, and 0.006 mmol/L pyruvate at Cauquenes), are observed at the same location as high Fe concentrations (up to 71.2 mmol/L Fe(II) and 16.1 mmol/L Fe(III), respectively). In this zone, secondary Fe(III) hydroxides are depleted. Our data suggest that LMWCA may influence the mobility of iron in two ways. First, complexation of Fe(III), through formation of bidentate Fe(III)-LMWCA complexes (e.g., pyruvate, oxalate), may enhance the dissolution of Fe(III) (oxy)hydroxides or may prevent precipitation of Fe(III) (oxy)hydroxides. Soluble Fe(III) chelate complexes which may be mobilized downward and convert to Fe(II) by Fe(III) reducing bacteria. Second, monodentate LMWCA (e.g., acetate and formate) can be used by iron-reducing bacteria as electron donors (e.g., Acidophilum spp.), with ferric iron as the electron acceptor. These processes may, in part, explain the low abundances of secondary Fe(III) hydroxide precipitates below the oxidation front and the high concentrations of Fe(II) observed in the pore waters of some low-sulfide systems. The reduction of Fe(III) and the subsequent increase of iron mobility and potential acidity transfer (Fe(II) oxidation can result in the release of H+ in an oxic environment) should be taken in account in mine waste management strategies.     

Arsenic removal from Bangladesh tube well water with filter columns containing zerovalent iron filings and sand

Arsenic removal is often challenging due to high As(III), phosphate, and silicate concentrations and low natural iron concentrations. Application of zerovalent iron is promising, as metallic iron is widely available. However, removal mechanisms remained unclear and currently used removal units with iron have not been tested systematically, partly due to their large size and long operation time. This study investigated smaller filter columns with 3-4 filters, each containing 2.5 g of iron filings and 100-150 g of sand. At a flow rate of 1 L/h, these columns were able to treat 75-90 L of well water with 440 microg/L As, 1.8 mg/L P, 4.7 mg/L Fe, 19 mg/L Si, and 6 mg/L dissolved organic carbon (DOC) to below 50 microg/L As(tot), without addition of an oxidant. As(III) was oxidized in parallel to oxidation of corrosion-released Fe(II) by dissolved oxygen and sorbed on the forming hydrous ferric oxides (HFO). The open filter columns prevented anoxic conditions. DOC did not appear to interfere with arsenic removal. Manganese was reduced after a slight initial increase from 0.3 mg/L to below 0.1 mg/L. About 100 mg of Fe(0)/L of water was required, 3-5 times less than that for larger units with sand and iron turnings.     

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).     

Spectroscopic evidence for interfacial Fe(II)-Fe(III) electron transfer in a clay mineral

Interfacial electron transfer has been shown to occur between sorbed Fe(II) and structural Fe(III) in Fe oxides, but it is unknown whether a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we used the isotopic specificity of (57)Fe Mo?ssbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mo?ssbauer spectra of NAu-2 reacted with aqueous (56)Fe(II) (which is invisible to (57)Fe Mo?ssbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mo?ssbauer spectra using aqueous (57)Fe(II) showed that sorbed Fe(II) is oxidized upon sorption to the clay and pXRD patterns indicate that the oxidation product is lepidocrocite. Spectra collected at different temperatures indicate that reduction of structural Fe(III) by sorbed Fe(II) induces electron delocalization in the clay structure. Our results also imply that interpretation of room temperature and 77 K Mo?ssbauer spectra may significantly underestimate the amount of Fe(II) in Fe-bearing clays. These findings provide compelling evidence for abiotic reduction of Fe-bearing clay minerals by sorbed Fe(II), and require us to reframe our conceptual model for interpreting biological reduction of clay minerals, as well as contaminant reduction by reduced clays.     

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.     

Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. BoFeN1

The mobility of toxic metals and the transformation of organic pollutants in the environment are influenced and in many cases even controlled by iron minerals. Therefore knowing the factors influencing iron mineral formation and transformation by Fe(II)-oxidizing and Fe(III)-reducing bacteria is crucial for understanding the fate of contaminants and for the development of remediation technologies. In this study we followed mineral formation by the nitrate-reducing Fe(II)-oxidizing strain Acidovorax sp. BoFeN1 in the presence of the crystalline Fe(III) (oxyhydr)oxides goethite, magnetite and hematite added as potential nucleation sites. M?ssbauer spectroscopy analysis of minerals precipitated by BoFeN1 in (57)Fe(II)-spiked microbial growth medium showed that goethite was formed in the absence of mineral additions as well as in the presence of goethite or hematite. The presence of magnetite minerals during Fe(II) oxidation induced the formation of magnetite in addition to goethite, while the addition of humic substances along with magnetite also led to goethite but no magnetite. This study showed that mineral formation not only depends on the aqueous geochemical conditions but can also be affected by the presence of mineral nucleation sites that initiate precipitation of the same underlying mineral phases.     

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).     

Fe(II) sorption on hematite: new insights based on spectroscopic measurements

We collected M?ssbauer spectra of 57Fe(II) interacting with 56hematite (alpha-Fe2O3) over a range of Fe(II) concentrations and pH values to explore whether a sorbed Fe(II) species would form. Several models of Fe(II) sorption (e.g., surface complexation models) assume that stable, sorbed Fe(II) species form on ligand binding sites of Fe(III) oxides and other minerals. Model predictions of changes in both speciation and concentration of sorbed Fe(II) species are often invoked to explain Fe(II) sorption patterns, as well as rates of contaminant reduction and microbial respiration of Fe(III) oxides. Here we demonstrate that, at low Fe(II) concentrations, sorbed Fe(II) species are transient and quickly undergo interfacial electron transfer with structural Fe(III) in hematite. At higher Fe(II) concentrations, however, we observe the formation of a stable, sorbed Fe(II) phase on hematite that we believe to be the first spectroscopic confirmation for a sorbed Fe(II) phase forming on an iron oxide. Low-temperature M?ssbauer spectra suggest that the sorbed Fe(II) phase contains varying degrees of Fe(II)-Fe(II) interaction and likely contains a mixture of adsorbed Fe(II) species and surface precipitated Fe(OH)2(s). The transition from Fe(II)-Fe(III) interfacial electron transfer to formation of a stable, sorbed Fe(II) phase coincides with the macroscopically observed change in isotherm slope, as well as the estimated surface site saturation suggesting that the finite capacity for interfacial electron transfer is influenced by surface properties. The spectroscopic demonstration of two distinctly different sorption endpoints, that is an Fe(III) coating formed from electron transfer or a stable, sorbed Fe(II) phase, challenges us to reconsider our traditional interpretations and modeling of Fe(II) sorption behavior (as well as, we would argue, of any other redox active sorbate-sorbent couple).     

Arsenic mobilization through microbially mediated deflocculation of ferrihydrite

This study examined the potential impact of microbially mediated reduction of Fe in the Fe(III)-(hydr)oxide mineral ferrihydrite on the mobility of As in natural waters. In microcosm experiments, the obligately anaerobic bacterium Geobacter metallireducens reduced on average 10% of the Fe(III) in ferrihydrite with varying sorbed As(V) surface coverages, which resulted in deflocculation of initially micron-sized As-bearing ferrihydrite aggregates to nanometersized colloids. No reduction of As(V) to As(III) was observed in microcosm samples. Measurement of Fe and As within operationally defined particulate, colloidal, and dissolved fractions of microcosm slurry samples revealed that little Fe or As was released from ferrihydrite as dissolved species. Microbially induced deflocculation of ferrihydrite in the presence of G. metallireducens was correlated with more negative zeta potential of ferrihydrite nanoparticles suggesting that G. metallireducens mediated As mobilization through alteration of ferrihydrite surface charge. TEM analysis and solution chemistry conditions suggested formation of a magnetite surface layer through topotactic recrystallization of ferrihydrite (2LFH) driven by sorbed Fe(II). The formation of nanometer-sized As-bearing colloids through microbially mediated reduction of Fe-(hydr)oxides has the potential to increase human As exposure by enhancing As mobility in natural waters and hindering As removal during subsequent drinking water treatment.     

Cr(III) is indirectly oxidized by the Mn(II)-oxidizing bacterium Bacillus sp. strain SG-1

Manganese oxides are the only known oxidants of Cr(III) in the environment, and predictions of the fate of Cr(III) have been based on Cr(III) oxidation rates with well-characterized Mn(III, IV) oxide minerals. Our research, however, indicates that the presence of Mn(II)-oxidizing bacteria may accelerate these rates through the production of very reactive Mn oxides or intermediates formed in the oxidation process. Experiments with the Mn(II)-oxidizing Bacillus sp. strain SG-1 show that this bacterium can accelerate Cr(III) oxidation compared to both abiotic and biologically produced Mn oxides. Initial rates of Cr(III) oxidation by biogenic oxides were approximately 7 times faster than Cr(III) oxidation rates by equivalent amounts of synthetic delta-MnO2 and 25 times faster by SG-1 spores with Mn(II). Cr(III) oxidation by SG-1 is not direct; Mn is required, but only in small amounts, indicating that it is recycled. Cr(III) oxidation is inhibited above 5 microM dissolved Mn(II),while Mn(II) oxidation is not, suggesting that the processes are controlled by different mechanisms. These results illustrate the need to consider bacterial activity and the concentration of Mn when predicting the potential for Cr(III) oxidation.     

Spectroscopic evidence for ternary complex formation between arsenate and ferric iron complexes of humic substances

Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ~10). The local (<5 ?) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5-12.5 μmol As(V)/g HS (25-70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binuclear ((2)C) and monodentate mononuclear ((1)V) complex stabilized by H-bonds (R(As-Fe) = 3.30 ?). The analysis of Fe K-edge EXAFS spectra revealed that Fe in Fe(III)-HS complexes was predominantly present as oligomeric Fe(III) clusters at neutral pH. Shell-fit results complied with a structural motif in which three corner-sharing Fe(O,OH)(6) octahedra linked by a single μ(3)-O bridge form a planar Fe trimer. In these complexes, the average Fe-C and Fe-Fe bond distances were 2.95 ? and 3.47 ?, respectively. Our study provides the first spectroscopic evidence for ternary complex formation between As(V) and Fe(III)-HS complexes, suggesting that this binding mechanism is of fundamental importance for the cycling of oxyanions such as As(V) in organic-rich, oxic soils and sediments.     

XAS and XMCD evidence for species-dependent partitioning of arsenic during microbial reduction of ferrihydrite to magnetite

Poorly crystalline Fe(III) oxyhydroxides, ubiquitously distributed as mineral coatings and discrete particles in aquifer sediments, are well-known hosts of sedimentary As. Microbial reduction of these phases is widely thought to be responsible for the genesis of As-rich reducing groundwaters found in many parts of the world, most notably in Bangladesh and West Bengal, India. As such, it is important to understand the behavior of As associated with ferric oxyhydroxides during the early stages of Fe(lll) reduction. We have used X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) to elucidate the changes in the bonding mechanism of As(III) and As(V) as their host Fe(III) oxyhydroxide undergoes bacterially induced reductive transformation to magnetite. Two-line ferrihydrite, with adsorbed As(III) or As(V), was incubated under anaerobic conditions in the presence of acetate as an electron donor, and Geobacter sulfurreducens, a subsurface bacterium capable of respiring on Fe(lll), but not As(V). In both experiments, no increase in dissolved As was observed during reduction to magnetite (complete upon 5 days incubation), consistent with our earlier observation of As sequestration by the formation of biogenic Fe(III)-bearing minerals. XAS data suggested that the As bonding environment of the As(III)-magnetite product is indistinguishable from that obtained from simple adsorption of As(lll) on the surface of biogenic magnetite. In contrast, reduction of As(V)-sorbed ferrihydrite to magnetite caused incorporation of As5+ within the magnetite structure. XMCD analysis provided further evidence of structural partitioning of As5+ as the small size of the As5+ cation caused a distortion of the spinel structure compared to standard biogenic magnetite. These results may have implications regarding the species-dependent mobility of As undergoing anoxic biogeochemical transformations, e.g., during early sedimentary diagenesis.     

Role for Fe(III) minerals in nitrate-dependent microbial U(IV) oxidation

Microbiological reduction of soluble U(VI) to insoluble U(IV) is a means of preventing the migration of that element in groundwater, but the presence of nitrate in U(IV)-containing sediments leads to U(IV) oxidation and remobilizaton. Nitrite or iron(III) oxyhydroxides may oxidize U(IV) under nitrate-reducing conditions, and we determined the rate and extent of U(IV) oxidation by these compounds. Fe(III) oxidized U(IV) at a greater rate than nitrite (130 and 10 microM U(IV)/day, respectively). In aquifer sediments, Fe(III) may be produced during microbial nitrate reduction by oxidation of Fe(II) with nitrite, or by enzymatic Fe(II) oxidation coupled to nitrate reduction. To determine which of these mechanisms was dominant, we isolated a nitrate-dependent acetate- and Fe(ll)-oxidizing bacterium from a U(VI)- and nitrate-contaminated aquifer. This organism oxidized U(IV) at a greater rate and to a greater extent under acetate-oxidizing (where nitrite accumulated to 50 mM)than under Fe(II)-oxidizing conditions. We showthatthe observed differences in rate and extent of U(IV) oxidation are due to mineralogical differences between Fe(III) produced by reaction of Fe(II) with nitrite (amorphous) and Fe(III) produced enzymatically (goethite or lepidocrocite). Our results suggest the mineralogy and surface area of Fe(III) minerals produced under nitrate-reducing conditions affect the rate and extent of U(IV) oxidation. These results may be useful for predicting the stability of U(IV) in aquifers.     

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.     

Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O2

The objective of this study was to identify the rate and mechanism of abiotic oxidation of ferrous iron at the water-ferric oxide interface (heterogeneous oxidation) at neutral pH. Oxidation was conducted at a low partial pressure of O2 to slow the reactions and to represent very low dissolved oxygen (DO) conditions that can occur at oxic/anoxic fronts. Hydrous ferric oxide (HFO) was partially converted to goethite after 24 h of anoxic contact with Fe(II), consistent with previous results. This resulted in a significant decrease in sorption of Fe(II). No conversion to goethite was observed after 25 min of anoxic contact between HFO and Fe(II). O2 was then introduced into the chamber and sparged (transfer half-time of 1.6 min) into the previously anoxic suspension, and the rate of oxidation of Fe(II) and the distribution between sorbed and dissolved Fe(II) were measured with time. The concentration of sorbed Fe(II) remained steady during each experiment, despite removal of all measurable dissolved Fe(II) in some experiments. The rate of oxidation of Fe(II) was proportional to the concentration of DO and both sorbed and dissolved Fe(II) up to a surface density of 0.02 mol Fe(II) per mol Fe(III), i.e., approximately 0.2 Fe(II) per nm2 of ferric oxide surface area. This result differs from previous studies of heterogeneous oxidation, which found that the rate was proportional to sorbed Fe(II) and DO but did not find a dependence on dissolved Fe(II). Most previous experiments were autocatalytic; i.e., the initial concentration of ferric oxide was low or none, and sorbed Fe(II) was not measured. The results were consistentwith an anode/cathode mechanism, with O2 reduced at electron-deficient sites with strongly sorbed Fe(II) and Fe(II) oxidized at electron-rich sites without sorbed Fe(II). The pseudo-first-order rate constants for oxidation of dissolved Fe(II) were about 10 times faster than those previously predicted for heterogeneous oxidation of Fe(II).     

Impacts of Shewanella putrefaciens strain CN-32 cells and extracellular polymeric substances on the sorption of As(V) and As(III) on Fe(III)-(hydr)oxides

We investigated the effects of Shewanella putrefaciens cells and extracellular polymeric substances on the sorption of As(III) and As(V) to goethite, ferrihydrite, and hematite at pH 7.0. Adsorption of As(III) and As(V) at solution concentrations between 0.001 and 20 μM decreased by 10 to 45% in the presence of 0.3 g L(-1) EPS, with As(III) being affected more strongly than As(V). Also, inactivated Shewanella cells induced desorption of As(V) from the Fe(III)-(hydr)oxide mineral surfaces. ATR-FTIR studies of ternary As(V)-Shewanella-hematite systems indicated As(V) desorption concurrent with attachment of bacterial cells at the hematite surface, and showed evidence of inner-sphere coordination of bacterial phosphate and carboxylate groups at hematite surface sites. Competition between As(V) and bacterial phosphate and carboxylate groups for Fe(III)-(oxyhydr)oxide surface sites is proposed as an important factor leading to increased solubility of As(V). The results from this study have implications for the solubility of As(V) in the soil rhizosphere and in geochemical systems undergoing microbially mediated reduction and indicate that the presence of sorbed oxyanions may affect Fe-reduction and biofilm development at mineral surfaces.     

Arsenate and arsenite adsorption and desorption behavior on coprecipitated aluminum:iron hydroxides

Although arsenic adsorption/desorption behavior on aluminum and iron (oxyhydr)oxides has been extensively studied, little is known about arsenic adsorption/desorption behavior by bimetal Al:Fe hydroxides. In this study, influence of the Al:Fe molar ratio, pH, and counterion (Ca2+ versus Na+) on arsenic adsorption/desorption by preformed coprecipitated Al:Fe hydroxides was investigated. Adsorbents were formed by initial hydrolysis of mixed Al3+/ Fe3+ salts to form coprecipitated Al:Fe hydroxide products. At Al:Fe molar ratios < or = 1:4, Al3+ was largely incorporated into the iron hydroxide structure to form a poorly crystalline bimetal hydroxide; however, at higher Al:Fe molar ratios, crystalline aluminum hydroxides (bayerite and gibbsite) were formed. Although approximately equal As(V) adsorption maxima were observed for 0:1 and 1:4 Al:Fe hydroxides, the As(III) adsorption maximum was greater with the 0:1 Al: Fe hydroxide. As(V) and As(III) adsorption decreased with further increases in Al:Fe molar ratio. As(V) exhibited strong affinity to 0:1 and 1:4 Al:Fe hydroxides at pH 3-6. Adsorption decreased at pH > 6.5; however, the presence of Ca2+ compared to Na+ as the counterion enhanced As( retention by both hydroxides. There was more As(V) and especially As(III) desorption by phosphate with an increase in Al:Fe molar ratio.     

Advances in the detection of as in environmental samples using low energy X-ray fluorescence in a scanning transmission X-ray microscope: arsenic immobilization by an Fe(II)-oxidizing freshwater bacteria

Speciation and quantitative mapping of elements, organic and inorganic compounds, and mineral phases in environmental samples at high spatial resolution is needed in many areas of geobiochemistry and environmental science. Scanning transmission X-ray microscopes (STXMs) provide a focused beam which can interrogate samples at a fine spatial scale. Quantitative chemical information can be extracted using the transmitted and energy-resolved X-ray fluorescence channels simultaneously. Here we compare the relative merits of transmission and low-energy X-ray fluorescence detection of X-ray absorption for speciation and quantitative analysis of the spatial distribution of arsenic(V) within cell-mineral aggregates formed by Acidovorax sp. strain BoFeN1, an anaerobic nitrate-reducing Fe(II)-oxidizing β-proteobacteria isolated from the sediments of Lake Constance. This species is noted to be highly tolerant to high levels of As(V). Related, As-tolerant Acidovorax-strains have been found in As-contaminated groundwater wells in Bangladesh and Cambodia wherein they might influence the mobility of As by providing sorption sites which might have different properties as compared to chemically formed Fe-minerals. In addition to demonstrating the lower detection limits that are achieved with X-ray fluorescence relative to transmission detection in STXM, this study helps to gain insights into the mechanisms of As immobilization by biogenic Fe-mineral formation and to further the understanding of As-resistance of anaerobic Fe(II)-oxidizing bacteria.     

Arsenate and arsenite removal by zerovalent iron: kinetics, redox transformation, and implications for in situ groundwater remediation

Batch tests were performed utilizing four zerovalent iron (Fe0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 degrees C for up to 5 days in the dark with 41.5 mL of 2 mg L(-1) As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L(-1) in 4 days with the exception of Aldrich Fe0. More As(III) was sorbed than As(V) by Peerless Fe0 in the initial As concentration range between 2 and 100 mg L(-1). No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe0, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe0 when As(II) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (22-25%) was achieved at 30 and 60 days on the Peerless Fe0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe0 is probably responsible for the coexistence of both As(V) and As(III) on Fe0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.