Identification of iron-cyanide complexes in contaminated soils and wastes by Fourier transform infrared spectroscopy

Persistent cyanide species in soil are mainly iron-cyanide (Fe-CN) complexes. They originate from anthropogenic inputs of compounds such as Fe4[Fe(II)(CN)6]3 in deposited gas-purifier wastes (GPW) or K2Zn3[Fe(II)(CN)6]2 in deposited blast-furnace sludge (BFSI). Fe-CN species in de-inking sludge (DIS) and sewage farm soils (SF) are still unknown. We investigated 35 soil and waste samples from 15 European sites and five synthesized Fe-CN-containing compounds by Fourier transform infrared spectroscopy. Furthermore, we determined the contents of total and adsorbed CN and pH. In all samples (pH 2.2-9.5), adsorbed Fe-CN complexes were negligible. In GPW and DIS samples, Fe4[Fe(II)(CN)6]3 and KFe[FeII(CN)6] were the only Fe-CN compounds (total CN contents 0.4-449 g kg(-1)). Several BFSI samples were free of CN. The spectra of other BFSI samples partially indicated dissolution of the characteristic compound K2Zn3[Fe(II)(CN)6]2, resulting in a loss of CN (contents < 12.3 g kg(-1)). Distinctive changes in BFSI with respect to CN speciation occurred within relatively short periods, <20 years. Dissolution of K2Zn3[Fe(II)(CN)6]2 was followed by dissociation of free CN from [Fe(CN)6](3-/4-) and adsorption of [Fe(II)(CN)6](4-) on organic surfaces. In DIS and SF samples, Fe4[Fe(11)(CN)6]3 and K2Zn3[Fe(II)(CN)6]2 (SF only) were identified.     

Reduction of aqueous chromate by Fe(II)/Fe(III) carbonate green rust: kinetic and mechanistic studies

This work describes the heterogeneous reaction between FeII in carbonate green rust and aqueous chromate, in NaHCO3 solutions at 25 degrees C, and at pH values of 9.3-9.6. Evidence for reduction of CrVI to CrIII and concomitant solid-state oxidation of lattice FeII to FeIII was found from FeII titration and from structural analysis of the solids using FTIR, XRD, SEM, and XPS methods. Results indicate the formation of ferric oxyhydroxycarbonate and the concomitant precipitation of CrIII monolayers at the surface of the iron compound that induce passivation effects and progressive rate limitations. The number of CrIII monolayers formed at the completion of the reaction depends on [FeII]t=0, the molar concentration of FeII(solid) at t=0; on [n(o)]t=0, the molar concentration of reaction sites present at the surface of the solid phase at t=0; and on [CrVI]t=0, the molar concentration of CrVI at t=0. Kinetic data were modeled using a model based on the formation of successive CrIII monolayers, -(d[CrVI]/dt) = sigma(1)j k(i)[S] [CrVI]([n(i - 1)] - [n(i)]) with k(i)[S] (in s(-1) L mol(-1)), the rate coefficient of formation of CrIII monolayer i, and [n(i)] and [n(i - 1)], the molar concentration of CrIII precipitated in monolayer i and monolayer i - 1, respectively. Good matching curves were obtained with kinetic coefficients: k(1)[S] = 5-8 x 10(-4), k(2)[S] = 0.5-3 x 10(-5), and k(3)[S] about 1.7 x 10(-6) s(-1) m(-2) L. The CrVI removal efficiency progressively decreases along with the accumulation of CrIII monolayers at the surface of carbonate green rust particles. In the case of thick green rust particles resulting from the corrosion of iron in permeable reactive barriers, the quantity of FeII readily accessible for efficient CrVI removal should be rather low.     

In situ redox flexibility of FeII-III Oxyhydroxycarbonate green rust and fougerite

Bacterial activity is commonly thought to be directly responsible for denitrification in soils and groundwater. However, nitrate reduction in low organic sediments occurs abiotically by FeII ions within the fougerite mineral (IMA 2003-057), giving the bluish-green color of gleysols. Fougerite, the mineral counterpart of FeII-III oxyhydroxycarbonate, FeII6(1-x)FeIII6xO12H2(7-3x)CO3, provides a unique in situ redox flexibility, which can adapt x = {[FeIII]/[Fetotal]} between 1/3 and 2/3 as shown using M?ssbauer spectroscopy. Chemical potential and Eh-pH diagrams for this system were determined from electrode potential monitored during deprotonation of hydroxycarbonate FeII4FeIII2(OH)12CO3 to assess the possibility of reducing pollutants in the field. Bioreduction of ferric oxyhydroxides in anoxic groundwater yields dissolved FeII, whereas HCO3- anions produced from organic matter are incorporated into fougerite layered double oxyhydroxide structure. Thus, fougerite is the solid-state redox mediator acting as electron shuttle that helps bacterial activity for reducing nitrate by coupling dissimilatory FeIII reduction and oxidation of FeII with reduction of NO3-. It is proposed that this system could be used in the remediation of soils and nitrified waters.     

Zinc adsorption effects on arsenite oxidation kinetics at the birnessite-water interface

Arsenite is more toxic and mobile than As(V) in soil and sediment environments, and thus it is advantageous to explore factors that enhance oxidation of As(III) to As(V). Previous studies showed that manganese oxides, such as birnessite (delta-MnO2), directly oxidized As(III). However, these studies did not explore the role that cation adsorption has on As(III) oxidation. Accordingly, the effects of adsorbed and nonadsorbed Zn on arsenite (As(III)) oxidation kinetics at the birnessite-water interface were investigated using batch adsorption experiments (0.1 g L(-1); pH 4.5 and 6.0; I= 0.01 M NaCl). Divalent Zn adsorption on synthetic delta-MnO2 in the absence of As(II) increased with increasing pH and caused positive shifts in electrophoretic mobility values at pH 4-6, indirectly suggesting inner-sphere Zn adsorption mechanisms. Arsenite was readily oxidized on birnessite in the absence of Zn. The initial As(III) oxidation rate constant decreased with increasing pH from 4.5 to 6.0 and initial As(III) concentrations from 100 to 300 microM. Similar pH and initial As(III) concentration effects were observed in systems when Zn was present (i.e., presorbed Zn prior to As(III) addition and simultaneously added Zn-As(III) systems), but As(III) oxidation reactions were suppressed compared to the respective control systems. The suppression was more pronounced when Zn was presorbed on the delta-MnO2 surfaces as opposed to added simultaneously with As(III). This study provides further understanding of As(III) oxidation reactions on manganese oxide surfaces under environmentally applicable conditions where metals compete for reactive sites.     

Reduction of oxamyl and related pesticides by FeII: influence of organic ligands and natural organic matter

The reduction of oxamyl and related oxime carbamate pesticides (OCPs; methomyl and aldicarb) by FeII is an important pathway for the degradation of these compounds in soil and groundwater. A series of batch kinetic experiments was carried out to assess the effects that selected carboxylate and aminocarboxylate ligands have on these reactions. In the absence of FeII, no OCP reduction by the ligands is observed. In the presence of FeII, the rate of OCP reduction varies by several orders of magnitude and can be described by the expression k(red) = [FeII]sigma(i)k(i)alpha(i) where k(red) is the observed pseudo-first-order rate constant for OCP reduction, [FeII] is the total FeII concentration, alpha(i) is the fraction of each FeII species in solution, and k(i) is the second-order rate constant for OCP reduction by each FeII species. The reactivity of individual FeII species is dependent upon the standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (E(H)o) and the availability of inner-sphere FeII coordination sites for bonding with Lewis base donor groups within the OCP structure. A linear free energy relationship is proposed. Kinetic measurements demonstrate that natural organic matter from the Great Dismal Swamp facilitates OCP reduction by FeII in the same manner as the individual organic ligands. Results from this study improve our understanding of the pathways and rates of pesticide degradation in reducing subsurface environments, especially those rich in organic matter.     

Reduction of the pesticides oxamyl and methomyl by FeII: effect of pH and inorganic ligands

This work examines the effect that pH and selected inorganic ligands have on the kinetics of reactions between FeII and two structurally related oxime carbamate pesticides, oxamyl and methomyl. In anoxic solutions containing FeII, these compounds degrade by parallel elimination and reduction pathways. Rates of FeII-independent carbamate elimination (EIcb mechanism) are proportional to [OH-], increasing 10-fold for each unit increase in pH. In homogeneous solution, rates of carbamate reduction by 0.5 mM FeII are relatively constant at pH <7, but increase dramatically between pH 7 and pH 8.3. At pH >8.3, Fe(OH)2(s) precipitation occurs, and carbamates react with both solution-phase and solid-phase FeII. Carbamate reduction by FeII is not significantly affected by the presence of chloride, bromide, nitrate, perchlorate, and sulfate. In contrast, increased rates of carbamate reduction are observed in solutions containing fluoride, carbonate, and phosphate. Kinetic measurements are interpreted in terms of changing FeII speciation according to the expression kred = [FeII]sigmaikialphai, where k(red) is the pseudo-first-order rate constant for carbamate reduction, [FeII] is the total FeII concentration, and ki and alphai are the second-order rate constant and fractional concentration of each FeII species, respectively. It follows that the overall kinetics of carbamate reduction is a function of the identity and concentration of individual FeII species present in solution as well as the inherent reactivity of each species with carbamates. The magnitude of ki is related to the standard one-electron reduction potential (E(H) degrees) of the corresponding FeIII/FeII redox couple.     

Arsenic(III) oxidation by birnessite and precipitation of manganese(II) arsenate

Solution chemical techniques were used to investigate the oxidation of As(III) to As(V) in 0.011 M arsenite suspension of well-crystallized hexagonal birnessite (H-birnessite, 2.7 g L(-1)) at pH 5. Products of the reaction were studied by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), atomic force microscopy (AFM), and X-ray absorption near-edge structure spectroscopy (XANES). In the initial stage (first 74 h), chemical results have been interpreted quantitatively, and the reaction is shown to proceed in two steps as suggested by previous authors: 2>Mn(IV)O2 + H3AsO3 + H2O --> 2>Mn(III)OOH + H2AsO4- + H+ and 2>Mn(III)OOH + H3AsO3 + 3H+ --> 2Mn2+ + H2AsO4- + 2H2O. The As(III) depletion rate was lower (0.02 h(-1)) than measured in previous studies because of the high crystallinity of the H-birnessite sample used in this study. The surface reaction sites are likely located on the edges of H-birnessite layers rather than on the basal planes. The ion activity product of Mn(II) and As(V) reached after 74 h reaction time was the solubility product of a protonated manganese arsenate, having a chemical composition close to that of krautite as identified by XANES and EDS. Krautite precipitation reaction can be written as follows: Mn2+ + H2AsO4- + H2O = MnHAsO4 x H2O + H+ log Ks approximately -0.2. Equilibrium was reached after 400 h. The manganese arsenate precipitate formed long fibers that aggregated at the surface of H-birnessite. The oxidation reaction transforms a toxic species, As(III), to a less toxic aqueous species, which further precipitates with Mn2+ as a mixed As-Mn solid characterized by a low solubility product.     

Abiotic reduction of nitroaromatic compounds by aqueous iron(ll)-catechol complexes

Complexation of iron(ll) by catechol and thiol ligands leads to the formation of aqueous species that are capable of reducing substituted nitroaromatic compounds (NACs) to the corresponding anilines. No reactions of NACs are observed in FelI-only or ligand-only solutions. In solutions containing FeII and tiron, a model catechol, rates of NAC reduction are heavily dependent on pH, ligand concentration, and ionic strength. Observed pseudo-first-order rate constants (k(obs)) for 4-chloronitrobenzene reduction vary by more than 6 orders of magnitude, and the variability is well described by the expression k(obs) = k(FeL2)(6-) [FeL2(6-)], where [FeL2(6-)] is the concentration of the 1:2 FeII-tiron complex and kFeL2(6-) is the bimolecular rate constant for 4-chloronitrobenzene reaction with this species. The high reactivity of this FeII species is attributed to the low standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (EH0 = -0.509 V vs NHE). The relative reactivity of different NACs can be described by a linear free-energy relationship (LFER) with the one-electron reduction potentials of the NACs, EH1'(ArNO2). The experimentally derived slope of the LFER indicates that electron transfer is rate determining. These findings suggest that FeII-organic complexes may play an important, previously unrecognized, role in the reductive transformation of persistent organic contaminants.     

Ferrate(VI) oxidation of weak-acid dissociable cyanides

Cyanide is commonly found in electroplating, mining, coal gasification, and petroleum refining effluents, which require treatment before being discharged. Cyanide in effluents exists either as free cyanide or as a metal complex. The kinetics of the oxidation of weak-acid dissociable cyanides by an environmentally friendly oxidant, ferrate(VI) (Fe(VI)O4(2-), Fe(VI)), were studied as a function of pH (9.1-10.5) and temperature (15-45 degrees C) using a stopped-flow technique. The weak-acid dissociable cyanides were Cd(CN)4(2-) and Ni(CN)4(2-), and the rate-laws for the oxidation may be -d[Fe(VI)]/dt = k[Fe(VI)][M(CN)4(2-)]n where n = 0.5 and 1 for Cd(CN)4(2-) and Ni(CN)4(2-), respectively. The rates decreased with increasing pH and were mostly related to a decrease in concentration of the reactive protonated Fe(VI) species, HFeO4(-). The stoichiometries with Fe(VI) were determined to be: 4HFeO4(-) + M(CN)4(2-) + 6H2O --> 4Fe(OH)3 + M(2+) + 4NCO(-) + O2 + 4OH(-). Mechanisms are proposed that agree with the observed reaction rate-laws and stoichiometries of the oxidation of weak-acid dissociable cyanides by Fe(VI). Results indicate that Fe(VI) is effective in removing cyanide in coke oven plant effluent, where organics are also present.     

Iron(VI) and iron(V) oxidation of copper(I) cyanide

Copper(Il) cyanide (Cu(CN)4(3-)) in the gold mine industry presentsthe biggest concern in cyanide management because it is much more stable than free cyanide. Cu(CN)4(3-) is highlytoxic to aquatic life; therefore, environmentally friendly techniques are required for the removal of Cu(CN)4(3-) from gold mine effluent. The oxidation of Cu(CN)4(3-) by iron-(VI) (FeVIO4(2-), Fe(VI)) and iron(V) (FeVO4(3-), Fe(V)) was studied using stopped-flow and premix pulse radiolysis techniques. The stoichiometry with Fe(VI) was determined to be 5HFeO(4-) + Cu(CN)4(3-) + 8H2O - > 5Fe(OH)3 + Cu2+ + 4CNO- +3/202 + 6OH-. The rate law for the oxidation of Cu(CN)4(3-) by Fe(VI) was found to be first-order with each reactant. The rates decreased with increasing pH and were mostly related to a decrease in concentration of reactive protonated Fe(VI) species, HFeO4-. A mechanism is proposed that agrees with the observed reaction stoichiometry and rate law. The rate constant for the oxidation of Cu(CN)4(3-) by Fe(V) was determined at pH 12.0 as 1.35 +/- 0.02 x 10(7) M(-1) s(-1), which is approximately 3 orders of magnitude larger than Fe(VI). Results indicate that Fe(VI) is highly efficient for removal of cyanides in gold mill effluent.     

In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination

In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.     

Speciation of iron and sulfate in acid waters: aqueous clusters to mineral precipitates

Acid mine drainage (AMD) contaminates surface water bodies, groundwater, soils, and sediments at innumerable locations around the world. AMD usually originates by weathering of pyrite (FeS2) and is rich in Fe and sulfate. In this study, we investigated speciation of FeII, FeIII, and SO4 in acid waters by Fourier transform infrared and X-ray absorption spectroscopy. The molalities of sulfate (15 mmol/ kg) and iron (10, 20, and 50 mmol/kg), and pH (1, 2, and 3) were chosen to mimic the concentration of ions in AMD waters. Sulfate and FeII either associate in outer-sphere complexes or do not associate at all. In contrast, sulfate interacts strongly with FeIII. The predominating species in FeIII-SO4 solutions are hydrogen-bonded complexes; inner-sphere complexes account only for 10+/-10% of the total sulfate. Our results show that the mode of interaction between FeIII and sulfate is similar in aqueous phase and in nanocrystalline precipitate schwertmannite (approximately FeO(OH)3/4(SO4)1/8). Because of this similarity, schwert-mannite should be the phase that controls solubility and availability of FeIII, SO4, and indirectly also other components in the AMD solutions.     

Oxidative transformation of triclosan and chlorophene by manganese oxides

The antibacterial agents triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) and chlorophene (4-chloro-2-(phenylmethyl)phenol) show similar susceptibility to rapid oxidation by manganese oxides (delta-MnO2 and MnOOH) yielding Mn(II) ions. Both the initial reaction rate and adsorption of triclosan to oxide surfaces increase as pH decreases. The reactions are first-order with respect to the antibacterial agent and MnO2. The apparent reaction orders to H+ were determined to be 0.46 +/- 0.03 and 0.50 +/- 0.03 for triclosan and chlorophene, respectively. Dissolved metal ions (Mn(II), Zn(II), and Ca(II)) and natural organic matter decrease the reaction rate by competitively adsorbing and reacting with MnO2. Product identification indicates that triclosan and chlorophene oxidation occurs at their phenol moieties and yields primarily coupling and p-(hydro)quinone products. A trace amount of 2,4-dichlorophenol is also produced in triclosan oxidation, suggesting bond-breaking of the ether linkage. The experimental results support the mechanism that after formation of a surface precursor complex of the antibacterial agent and the surface-bound Mn(IV), triclosan and chlorophene are oxidized to phenoxy radicals followed by radical coupling and further oxidation to form the end products. Compared to several structurally related substituted phenols (i.e., 2-methyl-4-chlorophenol, 2,4-dichlorophenol, 3-chlorophenol, and phenol), triclosan and chlorophene exhibit comparable or higher reactivities toward oxidation by manganese oxides. The higher reactivities are likely affected by factors including electronic and steric effects of substituents and compound hydrophobicity. Once released into the environment, partitioning of triclosan and chlorophene to soils and sediments is expected because of their relatively hydrophobic nature. Results of this study indicate that manganese oxides in soils will facilitate transformation of these antibacterial agents.     

Microscopic observations of reductive manganite dissolution under oxic conditions

At oxic/anoxic transition zones, manganese release from (hydr)oxide minerals into aqueous solution is a dynamic balance between mineral dissolution and Mn2+(aq) oxidation and precipitation, which are processes respectively promoted by organic reductants and molecular oxygen. We employ a flow-through atomic force microscope reactor (AFM-R) to investigate the reductive dissolution of the [010] surface of manganite (gamma-MnOOH) across a range of pH values and ascorbic acid concentrations in aqueous solutions equilibrated with atmospheric CO2 and O2. The apparent dissolution rate increases with higher ascorbic acid concentrations and lower pH values. Concurrent changes in surface morphology show that dissolution proceeds at low pH via etching and step retreat, while at high pH dissolution is concurrent with precipitation. The precipitates are characterized ex situ by X-ray photoelectron spectroscopy (XPS) and found to be Mn(III)-oxide. The onset of precipitation is consistent with an analysis of the thermodynamic driving forces for the reactions of a two-step mechanism. In the first step, Mn2+ is released to aqueous solution by reduction of gamma-MnOOH in reaction with ascorbic acid. This step is thermodynamically favorable under all conditions employed. In the second step, which leads to precipitation, surface adsorbed Mn2+ is oxidized by O2 to yield a Mn(III)-oxide precipitate. This step is thermodynamically possible only at pH > 5 for our experimental conditions. When the second step is active, the apparent dissolution rate equals the intrinsic dissolution rate minus the precipitation rate. Analysis of the growth rates observed in AFM indicates the precipitation rate reaches 71% of the intrinsic dissolution rate under some reactor conditions. Comparison of our gamma-MnOOH results to literature reports for Mn2+ oxidation on gamma-FeOOH indicates gamma-MnOOH is a more effective surface catalyst by a factor of 10(8).     

Reactivity of Pb(II) at the Mn(III,IV) (oxyhydr)oxide--water interface

In this study, the reactivity of lead (Pb(II)) on naturally occurring Mn(III,IV) (oxyhydr)oxide minerals was evaluated using kinetic, thermodynamic, and spectroscopic investigations. Aqueous Pb(II) was more strongly adsorbed to birnessite (delta-MnO1.7) than to manganite (gamma-MnOOH) under all experimental conditions. The isoteric heat of Pb adsorption (delta HT) or birnessite was 94 kJ mol-1 at a surface loading of 1.1 mmol g-1, and decreased with increasing adsorption density. This indicated that adsorption was an endothermic process and that birnessite possessed heterogeneous sites of reactivity for Pb. X-ray absorption fine structure (XAFS) spectra revealed that Pb was adsorbed as inner-sphere complexes on both birnessite and manganite with no evidence to suggest oxidation as an operative sorption mechanism. Lead appeared to coordinate to vacancy sites in the birnessite layer structure with concurrent release of Mn to solution, which resulted in a greater number of second shell Mn scatterers in Pb-birnessite when compared to Pb-manganite samples. The difference in Pb coordination apparently explained the contrasting desorption behavior between the two Mn minerals. These results have significant implications for Pb partitioning in soil environments containing solid-phase Mn(III,IV) (oxyhydr)oxides.     

Kinetics of Cr(VI) reduction by carbonate green rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).     

Arsenite oxidation by a poorly crystalline manganese-oxide. 2. Results from X-ray absorption spectroscopy and X-ray diffraction

Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (δ-MnO?) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the δ-MnO? structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the δ-MnO? structure appears to increase as the reaction proceeds. Binding of As(V) to δ-MnO? also changes as Mn(III) becomes more prominent in the δ-MnO ? structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline δ-MnO? is greatly affected by Mn oxidation state in the δ-MnO? structure.     

Interaction of inorganic arsenic with biogenic manganese oxide produced by a Mn-oxidizing fungus, strain KR21-2

In batch culture experiments we examined oxidation of As(III) and adsorption of As(III/V) by biogenic manganese oxide formed by a manganese oxide-depositing fungus, strain KR21-2. We expected to gain insight into the applicability of Mn-depositing microorganisms for biological treatment of As-contaminated waters. In cultures containing Mn2+ and As(V), the solid Mn phase was rich in bound Mn2+ (molar ratio, approximately 30%) and showed a transiently high accumulation of As(V) during the early stage of manganese oxide formation. As manganese oxide formation progressed, a large proportion of adsorbed As(V) was subsequently released. The high proportion of bound Mn2+ may suppress a charge repulsion between As(V) and the manganese oxide surface, which has structural negative charges, promoting complex formation. In cultures containing Mn2+ and As(III), As(III) started to be oxidized to As(V) after manganese oxide formation was mostly completed. In suspensions of the biogenic manganese oxides with dissolved Mn2+, As(III) oxidation rates decreased with increasing dissolved Mn2+. These results indicate that biogenic manganese oxide with a high proportion of bound Mn2+ oxidizes As(III) less effectively than with a low proportion of bound Mn2+. Coexisting Zn2+, Ni2+, and Co2+ also showed similar effects to different extents. The present study demonstrates characteristic features of oxidation and adsorption of As by biogenic manganese oxides and suggests possibilities of developing a microbial treatment system for water contaminated with As that is suited to the actual situation of contamination.     

Competitive microbially and Mn oxide mediated redox processes controlling arsenic speciation and partitioning

The speciation and partitioning of arsenic (As) in surface and subsurface environments are controlled, in part, by redox processes. Within soils and sediments, redox gradients resulting from mass transfer limitations lead to competitive reduction-oxidation reactions that drive the fate of As. Accordingly, the objective of this study was to determine the fate and redox cycling of As at the interface of birnessite (a strong oxidant in soil with a nominal formula of MnO(x), where x ≈ 2) and dissimilatory As(V)-reducing bacteria (strong reductant). Here, we investigate As reduction-oxidation dynamics in a diffusively controlled system using a Donnan reactor where birnessite and Shewanella sp. ANA-3 are isolated by a semipermeable membrane through which As migrates. Arsenic(III) injected into the reaction cell containing birnessite is rapidly oxidized to As(V). Arsenic(V) diffusing into the Shewanella chamber is then reduced to As(III), which subsequently diffuses back to the birnessite chamber, undergoing oxidation, and establishing a continuous cycling of As. However, we observe a rapid decline in the rate of As(III) oxidation owing to passivation of the birnessite surface. Modeling and experimental results show that high [Mn(II)] combined with increasing [CO(3)(2-)] from microbial respiration leads to the precipitation of rhodochrosite, which eventually passivates the Mn oxide surface, inhibiting further As(III) oxidation. Our results show that despite the initial capacity of birnessite to rapidly oxidize As(III), the synergistic effect of intense As(V) reduction by microorganisms and the buildup of reactive metabolites capable of passivating reactive mineral surfaces-here, birnessite-will produce (bio)geochemical conditions outside of those based on thermodynamic predictions.     

Electron energy-loss safe-dose limits for manganese valence measurements in environmentally relevant manganese oxides

Manganese (Mn) oxides are among the strongest mineral oxidants in the environment and impose significant influence on mobility and bioavailability of redox-active substances, such as arsenic, chromium, and pharmaceutical products, through oxidation processes. Oxidizing potentials of Mn oxides are determined by Mn valence states (2+, 3+, 4+). In this study, the effects of beam damage during electron energy-loss spectroscopy (EELS) in the transmission electron microscope have been investigated to determine the "safe dose" of electrons. Time series analyses determined the safe dose fluence (electrons/nm(2)) for todorokite (10(6) e/nm(2)), acid birnessite (10(5)), triclinic birnessite (10(4)), randomly stacked birnessite (10(3)), and δ-MnO(2) (<10(3)) at 200 kV. The results show that meaningful estimates of the mean Mn valence can be acquired by EELS if proper care is taken.