Sb(III) and Sb(V) sorption onto Al-rich phases: hydrous Al oxide and the clay minerals kaolinite KGa-1b and oxidized and reduced nontronite NAu-1

We have studied the immobilization of Sb(III) and Sb(V) by Al-rich phases - hydrous Al oxide (HAO), kaolinite (KGa-1b), and oxidized and reduced nontronite (NAu-1) - using batch experiments to determine the uptake capacity and the kinetics of adsorption and Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy to characterize the molecular environment of adsorbed Sb. Both Sb(III) and Sb(V) are adsorbed in an inner-sphere mode on the surfaces of the studied substrates. The observed adsorption geometry is mostly bidentate corner-sharing, with some monodentate complexes. The kinetics of adsorption is relatively slow (on the order of days), and equilibrium adsorption isotherms are best fit using the Freundlich model. The oxidation state of the structural Fe within nontronite affects the adsorption capacity: if the clay is reduced, the adsorption capacity of Sb(III) is slightly decreased, while Sb(V) uptake is increased significantly. This may be a result of the presence of dissolved Fe(II) in the reduced nontronite suspensions or associated with the structural rearrangements in nontronite due to reduction. These research findings indicate that Sb can be effectively immobilized by Al-rich phases. The increase in Sb(V) uptake in response to reducing structural Fe in clay can be important in natural settings since Fe-rich clays commonly go through oxidation-reduction cycles in response to changing redox conditions.     

Iron-mediated oxidation of antimony(III) by oxygen and hydrogen peroxide compared to arsenic(III) oxidation

Antimony is used in large quantities in a variety of products, though it has been declared as a pollutant of priority interest by the Environmental Protection Agency of the United States (USEPA). Oxidation processes critically affect the mobility of antimony in the environment since Sb(V) has a greater solubility than Sb(lll). In this study, the cooxidation reactions of Sb(lIl) with Fe(ll) and both O2 and H2O2 were investigated and compared to those of As(III). With increasing pH, the oxidation rate coefficients of Sb(lll) in the presence of Fe(ll) and O2 increased and followed a similar pH trend as the Fe(ll) oxidation by O2. Half-lives of Sb(lll) were 35 and 1.4 h at pH 5.0 and pH 6.2, respectively. The co-oxidation with Fe(ll) and H2O2 is about 7000 and 20 times faster than with Fe(ll) and O2 at pH 3 and pH 7, respectively. For both systems, *OH radicals appear to be the predominant oxidant below approximately pH 4, while at more neutral pH values, other unknown intermediates become important. The oxidation of As(lll) follows a similar pH trend as the Sb(lll) oxidation; however, As(lll) oxidation was roughly 10 times slower and only partly oxidized in most of the experiments. This study shows that the Fe(ll)-mediated oxidation of Sb(Ill) can be an important oxidation pathway at neutral pH values.     

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.     

Effect of aqueous Fe(II) on arsenate sorption on goethite and hematite

Biogeochemical iron cycling often generates systems where aqueous Fe(II) and solid Fe(III) oxides coexist. Reactions between these species result in iron oxide surface and phase transformations, iron isotope fractionation, and redox transformations of many contaminant species. Fe(II)-induced recrystallization of goethite and hematite has recently been shown to cause the repartitioning of Ni(II) at the mineral-water interface, with adsorbed Ni incorporating into the iron oxide structure and preincorporated Ni released back into aqueous solution. However, the effect of Fe(II) on the fate and speciation of redox inactive species incompatible with iron oxide structures is unclear. Arsenate sorption to hematite and goethite in the presence of aqueous Fe(II) was studied to determine whether Fe(II) causes substantial changes in the sorption mechanisms of such incompatible species. Sorption isotherms reveal that Fe(II) minimally alters macroscopic arsenate sorption behavior except at circumneutral pH in the presence of elevated concentrations (10?3 M) of Fe(II) and at high arsenate loadings, where a clear signature of precipitation is observed. Powder X-ray diffraction demonstrates that the ferrous arsenate mineral symplesite precipitates under such conditions. Extended X-ray absorption fine structure spectroscopy shows that outside this precipitation regime arsenate surface complexation mechanisms are unaffected by Fe(II). In addition, arsenate was found to suppress Fe(II) sorption through competitive adsorption processes before the onset of symplesite precipitation. This study demonstrates that the sorption of species incompatible with iron oxide structure is not substantially affected by Fe(II) but that such species may potentially interfere with Fe(II)-iron oxide reactions via competitive adsorption.     

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).     

XAS study of iron and arsenic speciation during Fe(II) oxidation in the presence of As(III)

The speciation of As and Fe was studied during the oxidation of Fe(II)-As(III) solutions by combining XAS analysis at both the Fe and As K-edges. Fe(II) and As(III) were first hydrolyzed to pH 7 under anoxic conditions; the precipitate was then allowed to oxidize in ambient air for 33 h under vigorous stirring. EXAFS analysis at the As K-edge shows clear evidence of formation of inner-sphere complexes between As(III) and Fe(II), i.e., before any oxidation. Inner-sphere complexes were also observed when Fe became sufficiently oxidized, in the form of edge-sharing and double-corner linkages between AsIIIO3 pyramids and FeIIIO6 octahedra. XAS analyses at the Fe K-edge reveal that the presence of As(III) in the solution limits the polymerization of Fe(II) and the formation of green rust and inhibits the formation of goethite and lepidocrocite. Indeed, As(III) accelerates the Fe(II) oxidation kinetics and leads to the formation of nanosized Fe-As subunits of amorphous aggregates. These observations, rather than a presumed weaker affinity of As(III) for iron oxyhydroxides, might explain why As(III) is more difficult to remove than As(V) by aerating reducing groundwater.     

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.     

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.     

Kinetics and mechanisms for reactions of Fe(II) with iron(III) oxides

Uptake of Fe(II) onto hematite (alpha-Fe2O3), corundum (alpha-Al2O3), amorphous ferric oxide (AFO), and a mixture of hematite and AFO was measured. Uptake was operationally divided into adsorption (extractable by 0.5 N HCl within 20 h) and fixation (extractable by 3.0 N HCl within 7 d). For 0.25 mM Fe(II) onto 25 mM iron(III) hematite at pH 6.8: (i) 10% of Fe(II) was adsorbed within 1 min; (ii) 20% of Fe(II) was adsorbed within 1 d; (iii) uptake slowly increased to 24% of Fe(II) during the next 24 d, almost all adsorbed; (iv) at 30 d, the uptake increased to 28% of Fe(II) with 6% of total Fe(II) fixed; and (v) uptake slowly increased to 30% of Fe(II) by 45 d with 10% of total Fe(II) fixed. Similar results were observed for 0.125 mM Fe(II) onto 25 mM iron(III) hematite, except that percent of adsorption and fixation were increased. There was adsorption but no fixation for 0.25 mM Fe(II) onto corundum [196.2 mM Al(III)] at pH 6.8, for 0.125 mM Fe(II) onto 25 mM iron(III) hematite at pH 4.5, and for 0.25 mM Zn(II) onto 25 mM iron(III) hematite at pH 6.8. A small addition of AFO to the hematite suspension increased Fe(II) fixation when 0.25 mM Fe(II) was reacted with 25 mM iron(III) hematite and 0.025 mM Fe(III) AFO at pH 6.8. Reaction of 0.125 mM Fe(II) with 2.5 mM Fe(III) AFO resulted in rapid adsorption of 30% of added Fe(II), followed by conversion of AFO to goethite and a decrease in adsorption without Fe(II) fixation. The fixation of Fe(II) by hematite at pH 6.8 is consistent with interfacial electron transfer and the formation of new mineral phases. We propose that electron transfer from adsorbed Fe(II) to structural Fe(III) in hematite results in oxidation of Fe(II) to AFO on the surface of hematite and that solid-phase contact among hematite, AFO, and structural Fe(II) produces magnetite (Fe3O4). The unique interactions of Fe(II) with iron(III) oxides would be environmentally important to understand the fate of redox-sensitive chemicals.     

Photoinduced oxidation of arsenite to arsenate in the presence of goethite

The photochemistry of an aqueous suspension of goethite in the presence of arsenite (As(III)) was investigated with X-ray absorption near edge structure (XANES) spectroscopy and solution-phase analysis. Irradiation of the arsenite/goethite under conditions where dissolved oxygen was present in solution led to the presence of arsenate (As(V)) product adsorbed on goethite and in solution. Under anoxic conditions (absence of dissolved oxygen), As(III) oxidation occurred, but the As(V) product was largely restricted to the goethite surface. In this circumstance, however, there was a significant amount of ferrous iron release, in stark contrast to the As(III) oxidation reaction in the presence of dissolved oxygen. Results suggested that in the oxic environment ferrous iron, which formed via the photoinduced oxidation of As(III) in the presence of goethite, was heterogeneously oxidized to ferric iron by dissolved oxygen. It is likely that aqueous reactive oxygen species formed during this process led to the further oxidation of As(III) in solution. Results from the current study for As(III)/goethite also were compared to results from a prior study of the photochemistry of As(III) in the presence of another iron oxyhydroxide, ferrihydrite. The comparison showed that at pH 5 and 2 h of light exposure the instantaneous rate of aqueous-phase As(V) formation in the presence of goethite (12.4 × 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 × 10(-6) M s(-1) m(-2)). It was proposed that this increased rate of ferrous iron oxidation in the presence of goethite and dissolved oxygen was the primary reason for the higher As(III) oxidation rate when compared to the As(III)/ferrihydrite system. The surface area-normalized pseudo-first-order rate constant, for example, associated with the heterogeneous oxidation of Fe(II) by dissolved oxygen in the presence of goethite (1.9 × 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 × 10(-7) L s(-1) m(-2)) at a solution pH of 5.     

Solubility and toxicity of antimony trioxide (Sb2O3) in soil

Antimony trioxide (Sb2O3) is a widely used chemical that can be emitted to soil. The fate and toxicity of this poorly soluble compound in soil is insufficiently known. A silt-loam soil (pH 7.0, background 0.005 mmol Sb kg(-1)) was amended with Sb2O3 at various concentrations. More than 70% of Sb in soil solution was present as Sb(V) (antimonate) within 2 days. The soil solution Sb concentrations gradually increased between 2 and 35 days after Sb2O3 amendment but were always below that of soils amended with the more soluble SbCl3 at the lower Sb concentrations. The soil solution Sb concentrations in freshly amended SbCl3 soils (7 days equilibration) were equivalent to those in Sb2O3-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled by a combination of sorption on the soil surface, Sb precipitation at the higher doses, and slow dissolution of Sb2O3, the latter being modeled with a half-life ranging between 50 and 250 days. Toxicity of Sb to plant growth (root elongation of barley, shoot biomass of lettuce) or to nitrification was found in soil equilibrated with Sb2O3 (up to 82 mmol Sb kg(-1)) for 31 weeks with 10% inhibition values at soil solution Sb concentrations of 110 microM Sb or above. These concentrations are equivalent to 4.2 mmol Sb per kg soil (510 mg Sb kg(-1)) at complete dissolution of Sb2O3 in this soil. No toxicity to plant growth or nitrification was evident in toxicity tests starting one week after soil amendment with Sb2O3, whereas clear toxicity was found in a similar test using SbCl3. However, these effects were confounded by a decrease in pH and an increase in salinity. It is concluded that the Sb(V) toxicity thresholds are over 100-fold larger than background concentrations in soil and that care must be taken to interpret toxicity data of soluble Sb(III) forms due to confounding factors.     

Kinetic studies on Sb(III) oxidation by hydrogen peroxide in aqueous solution

Knowledge of antimony redox kinetics is crucial in understanding the impact and fate of Sb in the environment and optimizing Sb removal from drinking water. The rate of oxidation of Sb(III) with H2O2 was measured in 0.5 mol L(-1) NaCl solutions as a function of [Sb(III)], [H2O2], pH, temperature, and ionic strength. The rate of oxidation of Sb(III) with H2O2 can be described by the general expression: -d[Sb(III)]/dt= k[Sb(III)][H2O2][H+](-1) with log k = -6.88 (+/- 0.17) [kc min(-1)]. The undissociated Sb(OH)3 does not react with H2O2: the formation of Sb(OH)4- is needed for the reaction to take place. In a mildly acidic hydrochloric acid medium, the rate of oxidation of Sb(III) is zeroth order with respect to Sb(III) and can be described by the expression -d[Sb(III)]/dt = k[H2O2][H+][Cl-] with log k = 4.44 (+/- 0.05) [k. L2 mol(-2) min(-1)]. The application of the calculated rate laws to environmental conditions suggests that Sb(III) oxidation by H2O2 may be relevant either in surface waters with elevated H2O2 concentrations and alkaline pH values or in treatment systems for contaminated solutions with millimolar H2O2 concentrations.     

Effect of oxide formation mechanisms on lead adsorption by biogenic manganese (hydr)oxides, iron (hydr)oxides, and their mixtures

The effects of iron and manganese (hydr)oxide formation processes on the trace metal adsorption properties of these metal (hydr)oxides and their mixtures was investigated by measuring lead adsorption by iron and manganese (hydr)oxides prepared by a variety of methods. Amorphous iron (hydr)oxide formed by fast precipitation at pH 7.5 exhibited greater Pb adsorption (gamma(max) = 50 mmol of Pb/mol of Fe at pH 6.0) than iron (hydr)oxide formed by slow, diffusion-controlled oxidation of Fe(II) at pH 4.5-7.0 or goethite. Biogenic manganese(III/IV) (hydr)oxide prepared by enzymatic oxidation of Mn(II) by the bacterium Leptothrix discophora SS-1 adsorbed five times more Pb (per mole of Mn) than an abiotic manganese (hydr)oxide prepared by oxidation of Mn(II) with permanganate, and 500-5000 times more Pb than pyrolusite oxides (betaMnO2). X-ray crystallography indicated that biogenic manganese (hydr)oxide and iron (hydr)oxide were predominantly amorphous or poorly crystalline and their X-ray diffraction patterns were not significantly affected by the presence of the other (hydr)oxide during formation. When iron and manganese (hydr)oxides were mixed after formation, or for Mn biologically oxidized with iron(III) (hydr)oxide present, observed Pb adsorption was similar to that expected for the mixture based on Langmuir parameters for the individual (hydr)oxides. These results indicate that interactions in iron/manganese (hydr)oxide mixtures related to the formation process and sequence of formation such as site masking, alterations in specific surface area, or changes in crystalline structure either did not occur or had a negligible effect on Pb adsorption by the mixtures.     

Photochemical oxidation of As(III) in ferrioxalate solutions

Photochemical reactions involving aqueous Fe(III) complexes are known to generate free radical species such as OH* that are capable of oxidizing numerous inorganic and organic compounds. Recent work has shown that As(III) can be oxidized to As(V) via photochemical reactions in ferric-citrate solutions; however, the mechanisms of As(III) oxidation and the potential importance of photochemical oxidation in natural waters are poorly understood. Consequently, the objectives of this study were to evaluate oxidation rates of As(III) in irradiated ferrioxalate solutions as a function of pH, identify mechanisms of photochemical As(III) oxidation, and evaluate the oxidation of As(III) in a representative natural water containing dissolved organic C (DOC). The oxidation of As(III) was studied in irradiated ferrioxalate solutions as a function of pH (3-7), As(III), Fe(III), and 2-propanol concentration. Rates of As(III) oxidation (0.5-254 microM h(-1)) were first-order in As(III) and Fe(III) concentration and increased with decreasing pH. Experiments conducted at pH 5.0 using 2-propanol as an OH* scavenger in light and dark reactions suggested that OH* is the important free radical responsible for As(III) oxidation. Significant rates of As(III) oxidation (4-6 microM h(-1)) were also observed in a natural water sample containing DOC, indicating that photochemical oxidation of As(III) may contribute to arsenic (As) cycling in natural waters.     

Formation of metal-arsenate precipitates at the goethite-water interface

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 micromol m(-2), respectively, and Zn surface saturation was absent at pH 4 and approximately 1.0 micromol m(-2) at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 +/- 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.     

pH dependence of Fenton reagent generation and As(III) oxidation and removal by corrosion of zero valent iron in aerated water

Corrosion of zerovalent iron (ZVI) in oxygen-containing water produces reactive intermediates that can oxidize various organic and inorganic compounds. We investigated the kinetics and mechanism of Fenton reagent generation and As(III) oxidation and removal by ZVI (0.1m2/g) from pH 3-11 in aerated water. Observed half-lives for the oxidation of initially 500 microg/L As(III) by 150 mg Fe(0)/L were 26-80 min at pH 3-9. At pH 11, no As(III) oxidation was observed during the first two hours. Dissolved Fe(III) reached 325, 140, and 6 microM at pH 3, 5, and 7. H2O2 concentrations peaked within 10 min at 1.2, 0.4, and < 0.1 microM at pH 3, 5, and 7, and then decreased to undetectable levels. Addition of 2,2'-bipyridine (1-3 mM), prevented Fe(II) oxidation by O2 and H2O2 and inhibited As(III)oxidation. 2-propanol (14 mM), scavenging OH-radicals, quenched the As(III) oxidation at pH 3, but had almost no effect at pH 5 and 7. Experimental data and kinetic modeling suggest that As(III) was oxidized mainly in solution by the Fenton reaction and removed by sorption on newly formed hydrous ferric oxides. OH-radials are the main oxidant for As(III) at low pH, whereas a more selective oxidant oxidizes As(III) at circumneutral pH.     

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.     

Sorption of Am(III) onto 6-line-ferrihydrite and its alteration products: investigations by EXAFS

For the long-term performance assessment of nuclear waste repositories, knowledge about the interactions of actinide ions with mineral surfaces such as iron oxides is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at these surfaces and on their incorporation into the mineral structure. In this study the interaction of Am(III) with 6-line-ferrihydrite (6LFh) was investigated by EXAFS spectroscopy. At low pH values (pH 5.5), as well at higher pH values (pH 8.0), Am(III) sorbs as a bidentate corner-sharing species onto the surface. Investigations of the interaction of Am(III) with Fh coated silica colloids prove the sorption onto the iron coating and not onto the silica substrate. Hence, the presence of Fh, even as sediment coating, is the dominant sorption surface. Upon heating, Fh is transformed into goethite and hematite as shown by TEM and IR measurements. The results of the fit to the EXAFS data indicate the release of sorbed Am(III) at pH 5.5 during the transformation and likely a partial incorporation of Am into the Fh transformation products at pH 8.0.     

Molecular-scale structure of uranium(VI) immobilized with goethite and phosphate

The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4-7 in the presence of ~100 μM total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4-7, formed bidentate edge-sharing, ≡ Fe(OH)(2)UO(2), and bidentate corner-sharing, (≡ FeOH)(2)UO(2), surface complexes with respective U-Fe coordination distances of ~3.45 and ~4.3 ?. In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U(VI)-phosphate-Fe(III) oxide ternary surface complex is suggested as the dominant species at pH 4 and total U(VI) of 10 μM or less on the basis of the linear combination fitting, a P shell indicated by EXAFS, and the simultaneous enhancement of U(VI) and phosphate uptake on goethite. A structural model for the ternary surface complex was proposed that included a single phosphate shell at ~3.6 ? (U-P) and a single iron shell at ~4.3 ? (U-Fe). While the data can be explained by a U-bridging ternary surface complex, (≡ FeO)(2)UO(2)PO(4), it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present.     

Abiotic degradation of pentachloronitrobenzene by Fe(III): reactions on goethite and iron oxide nanoparticles

Pentachloronitrobenzene is a fungicide that is degraded in anoxic soils and sediments through unknown processes that are often thought to be biologically mediated. The present research describes the kinetics for the abiotic reduction of this compound in aqueous Fe(II)/goethite systems at near-neutral pH values. The results provide evidence for a rate-affecting surface-association process rather than a direct (i.e., kinetically second-order) reaction with an adsorbed Fe(II) species. It is therefore likely that the iron oxide surface participates directly in the reaction. Furthermore, reduction is observed in the apparent presence of trace amounts of suspended iron oxide nanoparticles, formed in situ by the oxidation of Fe(II). Given that Fe(III) colloids and other nanoscale phases may occur in natural sediments, such abiotic reactions could significantly influence the environmental fate of nitroaromatic compounds.