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.     

Observation of surface precipitation of arsenate on ferrihydrite

X-ray diffraction and Raman spectroscopy were used in this study to characterize arsenate phases in the arsenate-ferrihydrite sorption system. Evidence has been obtained for surface precipitation of ferric arsenate on synthetic ferrihydrite at acidic pH (3-5) underthe following experimental conditions: sorption density of As/Fe approximately 0.125-0.49 and arsenic equilibrium concentration of <0.02-440 mg/L. Surface precipitation occurred under apparently undersaturated (in the bulk solution phase) conditions, and probably involved initial uptake of arsenate by surface complexation followed by transition to ferric arsenate formation on the surface as indicated by XRD analysis. At basic pH (i.e., pH 8), however, no ferric arsenate was observed in arsenate-ferrihydrite samples at a sorption density of As/Fe approximately 0.125-0.30 and an arsenic equilibrium concentration of 2.0-1100 mg/ L. At pH 8, arsenate is sorbed on ferrihydrite predominantly via surface adsorption, and the XRD patterns resemble basically that of ferrihydrite.     

Molecular-scale characterization of uranium sorption by bone apatite materials for a permeable reactive barrier demonstration

Uranium binding to bone charcoal and bone meal apatite materials was investigated using U L(III)-edge EXAFS spectroscopy and synchrotron source XRD measurements of laboratory batch preparations in the absence and presence of dissolved carbonate. Pelletized bone char apatite recovered from a permeable reactive barrier (PRB) at Fry Canyon, UT, was also studied. EXAFS analyses indicate that U(VI) sorption in the absence of dissolved carbonate occurred by surface complexation of U(VI) for sorbed concentrations < or = 5500 microg U(VI)/g for all materials with the exception of crushed bone char pellets. Either a split or a disordered equatorial oxygen shell was observed, consistent with complexation of uranyl by the apatite surface. A second shell of atoms at a distance of 2.9 A was required to fit the spectra of samples prepared in the presence of dissolved carbonate (4.8 mM total) and is interpreted as formation of ternary carbonate complexes with sorbed U(VI). A U-P distance at 3.5-3.6 A was found for most samples under conditions where uranyl phosphate phases did not form, which is consistent with monodentate coordination of uranyl by phosphate groups in the apatite surface. At sorbed concentrations > or = 5500 microg U(VI)/g in the absence of dissolved carbonate, formation of the uranyl phosphate solid phase, chernikovite, was observed. The presence of dissolved carbonate (4.8 mM total) suppressed the formation of chernikovite, which was not detected even with sorbed U(VI) up to 12,300 microg U(VI)/g in batch samples of bone meal, bone charcoal, and reagent-grade hydroxyapatite. EXAFS spectra of bone char samples recovered from the Fry Canyon PRB were comparable to laboratory samples in the presence of dissolved carbonate where U(VI) sorption occurred by surface complexation. Our findings demonstrate that uranium uptake by bone apatite will probably occur by surface complexation instead of precipitation of uranyl phosphate phases under the groundwater conditions found at many U-contaminated sites.     

Sorption and desorption of arsenic to ferrihydrite in a sand filter

Elevated arsenic concentrations in drinking water occur in many places around the world. Arsenic is deleterious to humans, and consequently, As water treatment techniques are sought. To optimize arsenic removal, sorption and desorption processes were studied at a drinking water treatment plant with aeration and sand filtration of ferrous iron rich groundwater at Elmevej Water Works, Fensmark, Denmark. Filter sand and pore water were sampled along depth profiles in the filters. The sand was coated with a 100-300 microm thick layer of porous Si-Ca-As-contaning iron oxide (As/Fe = 0.17) with locally some manganese oxide. The iron oxide was identified as a Si-stabilized abiotically formed two-line ferrihydrite with a magnetic hyperfine field of 45.8 T at 5 K. The raw water has an As concentration of 25 microg/L, predominantly as As(II). As the water passes through the filters, As(III) is oxidized to As(V) and the total concentrations drop asymptotically to a approximately 15 microg/L equilibrium concentration. Mn is released to the pore water, indicating the existence of reactive manganese oxides within the oxide coating, which probably play a role for the rapid As(III) oxidation. The As removal in the sand filters appears controlled by sorption equilibrium onto the ferrihydrite. By addition of ferrous chloride (3.65 mg of Fe(II)/L) to the water stream between two serially connected filters, a 3 microg/L As concentration is created in the water that infiltrates into the second sand filter. However, as water flow is reestablished through the second filter, As desorbs from the ferrihydrite and increases until the 15 microg/L equilibrium concentration. Sequential chemical extractions and geometrical estimates of the fraction of surface-associated As suggest that up to 40% of the total As can be remobilized in response to changes in the water chemistry in the sand filter.     

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.     

Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous diatomite

In this study, a simplified and effective method was tried to immobilize iron oxide onto a naturally occurring porous diatomite. Experimental resultsfor several physicochemical properties and arsenic edges revealed that iron oxide incorporated into diatomite was amorphous hydrous ferric oxide (HFO). Sorption trends of Fe (25%)-diatomite for both arsenite and arsenate were similar to those of HFO, reported by Dixit and Hering (Environ. Sci. Technol. 2003, 37, 4182-4189). The pH at which arsenite and arsenate are equally sorbed was 7.5, which corresponds to the value reported for HFO. Judging from the number of moles of iron incorporated into diatomite, the arsenic sorption capacities of Fe (25%)-diatomite were comparable to or higher than those of the reference HFO. Furthermore, the surface complexation modeling showed that the constants of [triple bond]SHAsO4- or [triple bond]SAsO4(2-) species for Fe (25%)-diatomite were larger than those reference values for HFO or goethite. Larger differences in constants of arsenate surface species might be attributed to aluminum hydroxyl ([triple bond]Al-OH) groups that can work better for arsenate removal. The pH-controlled differential column batch reactor (DCBR) and small-scale column tests demonstrated that Fe (25%)-diatomite had high sorption speeds and high sorption capacities compared to those of a conventional sorbent (AAFS-50) that is known to be the first preference for arsenic removal performance in Bangladesh. These results could be explained by the fact that Fe (25%)-diatomite contained well-dispersed HFO having a great affinity for arsenic species and well-developed macropores as shown by scanning electron microscopy (SEM) and pore size distribution (PSD) analyses.     

Influence of calcium carbonate on U(VI) sorption to soils

The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.     

Modeling precipitation and sorption of elements during mixing of river water and porewater in the Coeur d'Alene River basin

Reddish brown flocs form along the edge of the Coeur d'Alene River when porewater drains into river water during the annual lowering of water level in the basin. The precipitates are efficient scavengers of dissolved elements and have characteristics that may make metals associated with them bioavailable. This work characterizes the geochemistry of the porewater and models the formation and composition of the flocs. Porewater is slightly acidic, has suboxic to anoxic characteristics, tends to have higher alkalinity, and contains elevated concentrations of many constituents relative to river water. Laboratory mixing experiments involving porewater and river water were done to produce the precipitates. Thermodynamic predictions using PHREEQC indicate that predicted amounts of ferrihydrite and gibbsite agree with removal of Fe and Al. Predictions of element removal by adsorption onto ferrihydrite are consistent with observed removal using a combination of surface complexation constants for the generalized two-layer model (As and Se), alternative surface constants derived from experiments at high sorbate-to-sorbent ratios (Cd, Co, Cu, Ni, Pb, and Zn), and adjusted surface constants to fit experimental data (Cr, Mo, and Sb). This new set of surface complexation constants needs further testing in other contaminated systems.     

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.     

Methyl arsenic adsorption and desorption behavior on iron oxides

In virtually all Earth surface environments, methylated forms of arsenic can be found. Because of the widespread distribution and toxicity of methyl-arsenic compounds, their adsorption by soil minerals is of considerable interest. The objective of this study was to compare the adsorption and desorption behavior of methylarsonic acid (MMAsV), methylarsonous acid (MMAsIII), dimethylarsinic acid (DMAsV), dimethylarsinous acid (DMAsIII), arsenate (iAsV), and arsenite (iAsIII) on iron oxide minerals (goethite and 2-line ferrihydrite) by means of adsorption isotherms, adsorption envelopes, and desorption envelopes (using sulfate and phosphate as desorbing ligands). MMAsIII and DMAsIII were not appreciably retained by goethite or ferrihydrite within the pH range of 3 to 11, while iAsIII was strongly adsorbed to both iron oxides. MMAsV and iAsV were adsorbed in higher amounts than DMAsV on goethite and ferrihydrite at all pH values studied. MMAsV and iAsV exhibited high adsorption affinities on both goethite and ferrihydrite from pH 3 to 10, while DMAsV was adsorbed only at pH values below 8 by ferrihydrite and below 7 by goethite. All arsenic compounds were desorbed more efficiently by phosphate than sulfate. MMAsV, iAsV, and DMAsV each exhibited adsorption characteristics suggesting specific adsorption on both goethite and ferrihydrite. Increased methyl substitution resulted in both decreased adsorbed arsenic at low arsenic concentrations in solution and increased ease of arsenic release from the iron oxide surface.     

Natural organic matter affects arsenic speciation and sorption onto hematite

Arsenic mobility in natural environments is controlled primarily by sorption onto metal oxide surfaces, and the extent of this sorption may be influenced strongly by the presence of other dissolved substances that interact with surfaces or with arsenic itself. Natural organic matter (NOM), a prevalent constituent of natural waters, is highly reactive toward both metals and surfaces and is thus a clear candidate to influence arsenic mobility. The objectives of this study were therefore to reveal the influences of diverse NOM samples on the sorption of arsenic onto hematite, a model metal oxide, as well as to reveal influences of arsenic on the sorption of NOM, using conditions and concentrations relevant to natural freshwater environments. Of the six NOM samples tested, four formed aqueous complexes with arsenate and arsenite. The extent of complexation varied with the NOM origin and, in particular, increased with the cationic metal (primarily Fe) content of the NOM sample. In addition, every NOM sample showed active redox behavior toward arsenic species, indicating that NOM may greatly influence redox as well as complexation speciation of arsenic in freshwater environments. When NOM and As were incubated together with hematite, NOM dramatically delayed the attainment of sorption equilibrium and diminished the extent of sorption of both arsenate and arsenite. Consistent with this result, when NOM and As were introduced sequentially, all NOM samples displaced sorbed arsenate and arsenite from hematite surfaces, and arsenic species similarly displaced sorbed NOM from hematite in significant quantities. Competition between NOM and As for sorption thus appears to be a potentially important process in natural waters, suggesting that NOM may play a greater role in arsenic mobility than previously recognized. In addition, in all sorption experiments, arsenite was consistently desorbed or prevented from sorbing to a greater extent than arsenate, indicating that interactions with NOM may also partially explain the generally greater mobility of arsenite in natural environments.     

Modeling metal removal onto natural particles formed during mixing of acid rock drainage with ambient surface water

Studies have examined partitioning of trace metals onto natural particles to better understand the fate and transport of trace metals in the environment, but few studies have compared model predictions with field results. We evaluate the application of an empirical modeling approach, using surface complexation parameters available in the literature, to complex natural systems. In this work, the equilibrium speciation computer program PHREEQC was used along with the diffuse double-layer surface complexation model to simulate metal removal onto natural oxide particles formed during the mixing of acid rock drainage with ambient surface water. End-member solutions sampled in the Coeur d'Alene (CdA) Mining District in September 1999 from the Bunker Hill Mine and the South Fork Coeur d'Alene (SFCdA) River were filtered and mixed in several ratios. Solution chemistry was determined for end-members and mixed solutions, and X-ray diffraction (XRD) was used to determine the mineralogy of precipitate phases. Predicted amounts of Fe precipitates were in good agreement with measured values for particulate Fe. Surface area and reactive site characteristics were used along with surface complexation constants for ferrihydrite (Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; John Wiley & Sons: New York, 1990) to predict ion sorption as a function of mixing fraction. Comparisons of model predictions with field results indicate that Pb and Cu sorption are predicted well by the model, while As, Mo, and Sb sorption are less well-predicted. Additional comparisons with particulate metal and Fe data collected from the CdA Mining District in 1996 and 1997 suggest that sorption on particulate Fe, including amorphous iron oxides and schwertmannite, may be described using universal model parameters.     

Predominance of aqueous Tl(I) species in the river system downstream from the abandoned Carnoulès mine (Southern France)

Thallium concentration reached up to 534 μg L(-1) in the Reigous acid mine drainage downstream from the abandoned Pb-Zn Carnoule?s mine (Southern France). It decreased to 5.44 μg L(-1) in the Amous River into which the Reigous creek flows. Tl(I) predominated (>98% of total dissolved Tl) over Tl(III), mainly in the form of Tl(+). Small amounts of Tl(III) evidenced in Reigous Creek might be in the form of aqueous TlCl(2)(+). The range of dissolved to particulate distribution coefficients log K(d) = 2.5 L kg(-1) to 4.6 L kg(-1) indicated low affinity of Tl for particles, mainly ferrihydrite, formed in the AMD-impacted watershed. The low retention of Tl(+) on ferrihydrite was demonstrated in sorption experiments, the best fit between experimental and modeled data being achieved for surface complexation constants log K(ads) = -2.67 for strong sites and log K(ads) = -3.76 for weak sites. This new set of constants allowed reasonable prediction of the concentrations of aqueous and particulate Tl resulting from the mixing of water from Reigous Creek and the Amous River water during laboratory experiments, together with those measured in the Amous River field study.     

Laboratory investigations of enhanced sulfate reduction as a groundwater arsenic remediation strategy

Landfills have the potential to mobilize arsenic via induction of reducing conditions in groundwater and subsequent desorption from or dissolution of arsenic-bearing iron phases. Laboratory incubation experiments were conducted with materials from a landfill where such processes are occurring. These experiments explored the potential for induced sulfate reduction to immobilize dissolved arsenic in situ. The native microbial community at this site reduced sulfate in the presence of added acetate. Acetate respiration and sulfate reduction were observed concurrent with dissolved iron concentrations initially increasing from 0.6 microM (0.03 mg L(-1)) to a maximum of 111 microM (6.1 mg L(-1)) and subsequently decreasing to 0.74 microM (0.04 mg L(-1)). Dissolved arsenic concentrations initially covaried with iron but subsequently increased again as sulfide accumulated, consistent with the formation of soluble thioarsenite complexes. Dissolved arsenic concentrations subsequently decreased again from a maximum of 2 microM (148 microg L(-1)) to 0.3 microM (22 microg L(-1)), consistent with formation of sulfide mineral phases or increased arsenic sorption at higher pH values. Disequilibrium processes may also explain this second arsenic peak. The maximum iron and arsenic concentrations observed in the lab represent conditions most equivalent to the in situ conditions. These findings indicate that enhanced sulfate reduction merits further study as a potential in situ groundwater arsenic remediation strategy at landfills and other sites with elevated arsenic in reducing groundwater.     

Effects of dissolved carbonate on arsenic adsorption and mobility

The effects of high aqueous carbonate concentrations on arsenic mobility and transport in the subsurface were studied in synthetic iron oxide-coated sand column experiments. Elevated aqueous carbonate concentrations in groundwater have been studied and linked, by some authors, to increased aqueous As concentrations in natural waters. This study found that increasing carbonate concentrations had relatively little effect on As(V) adsorption to the iron oxide-coated sand surface at pH 7. The adsorption of As(V) decreased marginally when the CO2(g) partial pressure increased from 10(-3.5) to 10(-1.8) atm, despite a 50-fold increase in total dissolved carbonate (0.072 to 3.58 mM). Increasing the CO2(g) partial pressure to 10(-10) atm resulted in only a slight decrease in As(V) adsorption and increase in mobility, despite a >300-fold increase in total dissolved carbonate (to 22.7 mM). When compared to phosphate, a known competitive anion, carbonate mobilized less adsorbed As(V) than was mobilized by phosphate, even when present in much higher concentrations than phosphate. This was also true for an experiment with lower pore water velocity and an experiment where As(II) was introduced instead of As(V). Our experiments conclude that while carbonate anions do compete with As for adsorption to iron oxide-coated sand, the competitive effect is relatively small with regard to the total concentration of adsorbed As and the potential competitive effects of 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).     

Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation

The speciation of U(VI) sorbed to synthetic hydroxyapatite was investigated using a combination of U LIII-edge XAS, synchrotron XRD, batch uptake measurements, and SEM-EDS. The mechanisms of U(VI) removal by apatite were determined in order to evaluate the feasibility of apatite-based in-situ permeable reactive barriers (PRBs). In batch U(VI) uptake experiments with synthetic hydroxyapatite (HA), near complete removal of dissolved uranium (>99.5%) to <0.05 microM was observed over a range of total U(VI) concentrations up to equimolar of the total P in the suspension. XRD and XAS analyses of U(VI)-reacted HA at sorbed concentrations < or = 4,700 ppm U(VI) suggested that uranium(VI) phosphate, hydroxide, and carbonate solids were not present at these concentrations. Fits to EXAFS spectra indicate the presence of Ca neighbors at 3.81 A. U-Ca separation, suggesting that U(VI) adsorbs to the HA surfaces as an inner-sphere complex. Uranium(VI) phosphate solid phases were not detected in HA with 4700 ppm sorbed U(VI) by backscatter SEM or EDS, in agreement with the surface complexation process. In contrast, U(VI) speciation in samples that exceeded 7000 ppm sorbed U(VI) included a crystalline uranium(VI) phosphate solid phase, identified as chernikovite by XRD. At these higher concentrations, a secondary, uranium(VI) phosphate solid was detected by SEM-EDS, consistent with chernikovite precipitation. Autunite formation occurred at total U:P molar ratios > or = 0.2. Our findings provide a basis for evaluating U(VI) sorption mechanisms by commercially available natural apatites for use in development of PRBs for groundwater U(VI) remediation.     

Lead sorption onto ferrihydrite. 1. A macroscopic and spectroscopic assessment

In this research, traditional macroscopic studies were complemented with XAS analyses to elucidate the mechanisms controlling Pb(II) sorption onto ferrihydrite as a function of pH, ionic strength, and adsorbate concentrations. Analyses of XANES and XAFS studies demonstrate that Pb(II) ions predominantly sorb onto ferrihydrite via inner-sphere complexation, not retaining their primary hydration shell upon sorption. At higher pH values (pH > or = 5.0), edge-sharing bidentate complexes are mainly formed on the oxide surface with two Fe atoms located at approximately 3.34 A. In contrast, XAS studies on Pb(II) sorption onto ferrihydrite, at pH 4.5, reveal two distinct Pb-Fe bond average radial distances of 3.34 and 3.89 A, suggestive of a mixture of monodentate and bidentate sorption complexes present at the oxide surface. Interestingly, at constant pH, the configuration of the sorption complex is independent of the adsorbate concentration. Hence, Pb(II) sorption to a highly disordered adsorbent such as ferrihydrite can be described by one average type of mechanism. Overall, this information will aid scientists and engineers in improving the current models that predict and manage the fate of toxic metals, such as Pb(II), in the aquatic and soil environments.     

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).     

Influence of sediment bioreduction and reoxidation on uranium sorption

The influence of sediment bioreduction and reoxidation on U(VI) sorption was studied using Fe(II) oxide-containing saprolite from the U.S. Department of Energy (DOE) Oak Ridge site. Bioreduced sediments were generated by anoxic incubation with a metal-reducing bacterium, Shewanella putrefaciens strain CN32, supplied with lactate as an electron donor. The reduced sediments were subsequently reoxidized by air contact. U(VI) sorption was studied in NaNO3-HCO3 electrolytes that were both closed and open to atmosphere and where pH, U(VI), and carbonate concentration were varied. M?ssbauer spectroscopy and chemical analyses showed that 50% of the Fe(III)-oxides were reduced to Fe(II) that was sorbed to the sediment during incubation with CN32. However, this reduction and subsequent reoxidation of the sorbed Fe(II) had negligible influence on the rate and extent of U sorption or the extractability of sorbed U by 0.2 mol/L NaHCO3. Various results indicated that U(VI) surface complexation was the primary process responsible for uranyl sorption by the bioreduced and reoxidized sediments. A two-site, nonelectrostatic surface complexation model best described U(VI) adsorption under variable pH, carbonate, and U(VI) conditions. A ferrihydrite-based diffuse double layer model provided a better estimation of U(VI) adsorption without parameter adjustment than did a goethite-based model, even though a majority of the Fe(III)-oxides in the sediments were goethite. Our results highlight the complexity of the coupled U-Fe redox system and show that sorbed Fe(II) is not a universal reductant for U(VI) as commonly assumed.