Individual and competitive adsorption of arsenate and phosphate to a high-surface-area iron oxide-based sorbent

Individual and competitive adsorption of arsenate and phosphate were studied on a high-surface-area Fe/Mn-(hydr)oxide sorbent with surface and bulk properties similar to those of two-line ferrihydrite. It has maximum adsorption densities of 0.42 micromol As m(-2) at neutral pH and 1.24 micromol As m(-2) at pH 3. A surface complexation model (SCM) that used the diffuse double layer model was developed that could simulate single and binary sorbate adsorption over pH 4-9. The predominant adsorbed arsenate and phosphate species were modeled as bidentate binuclear surface complexes at low pH and as monodentate complexes at high pH. The model initially overpredicted the inhibition of arsenate adsorption by the presence of phosphate. The overprediction was resolved by separating surface sites into two types: ones to which both arsenate and phosphate bind and a smaller number to which only phosphate binds. The modified model predicted the competitive adsorption of arsenate and phosphate over pH 4-9 at total As concentrations of 6.67 and 80.1 microM and a total P concentration of 129 and 323 microM. The model may be used to predict arsenic adsorption to the sorbent for a given water source based on solution chemistry.     

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.     

Sorption of uranium(VI) onto ferric oxides in sulfate-rich acid waters

The mechanisms of the uranium(VI) sorption on schwertmannite and goethite in acid sulfate-rich solutions were studied by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The samples were prepared under N2 atmosphere and initial uranium(VI) concentrations of 1 x 10(-5) (pH 6.5) to 5 x 10(-5) M (pH 4.2). The ionic strength was adjusted using 0.01 M Na2SO4 or 0.01 M NaClO4, respectively. The EXAFS structural parameters for uranium(VI) sorbed on goethite in sulfate-rich, acid and near-neutral solutions indicate that uranium(VI) forms an inner-sphere, mononuclear, bidentate surface complex. This complex is characterized by a uranium-ferric-iron distance of approximately 3.45 A. Uranium(VI) sorbed onto schwertmannite in acid and sulfate-rich solution is coordinated to one or two sulfate molecules with a uranium-sulfur distance of 3.67 A. The EXAFS results indicate formation of binuclear, bidentate surface complexes and partly of mononuclear, monodentate surface complexes coordinated to the structural sulfate of schwertmannite. The formation of ternary uranium(VI)-sulfate surface complexes could not be excluded because of the uncertainty in assigning the sulfate either to the bulk structure or to adsorption reactions. The uranium(VI) adsorption onto schwertmannite in perchlorate solution occurs predominantly as a mononuclear, bidentate complexation with ferric iron due to the release of sulfate from the substrate.     

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.     

Arsenate sorption on lithium/ aluminum layered double hydroxide intercalated by chloride and on gibbsite: sorption isotherms, envelopes, and spectroscopic studies

The objective of this study was to provide fundamental knowledge of arsenate sorption on lithium/aluminum layered double hydroxide intercalated by chloride (Li/Al LDH-Cl) and further to reveal the contribution of exposed positive charge surface of Li/Al LDH-CI created by intercalating LiCl into Al(OH)3 layers to arsenate sorption. Therefore, sorption isotherms, envelopes and extended X-ray absorption fine structure (EXAFS) technique were employed to examine the reaction of arsenate on Li/Al LDH-Cl and on gibbsite. Based on an isotherm study, the sorption maximum of Li/Al LDH-Cl for arsenate was approximately six times higher than that of gibbsite. Sorption envelopes of arsenate on Li/Al LDH-Cl displayed a pH-sensitive behavior from pH 4.0 to 7.0, but it was insensitive to pH above pH 7.0, approaching to the pHpzc of Li/Al LDH-Cl (7.22). This transformation with shifted pHs illustrated that there were two types of reaction sites within Li/Al LDH-Cl that participate in arsenate sorption; one is pH-sensitive and the other is not. From EXAFS analysis, arsenate sorbed on Li/Al LDH, reacted not only with Al in the edges of Al-(OH)3 layers, but also with Li located in the vacant octahedral sites within Al(OH)3 layers; however, the decreasing intensity of As(V)-Al shells with increasing pH represented there were fewer As(V)-Al complex existed at higher pH, i.e., the complex between arsenate and Al is pH-sensitive. The superior sorption capability of Li/Al LDH-Cl to that of gibbsite could be attributed to the intercalated Li cations which served as the permanent sorption sites and made the surface of Al(OH)3 have high affinity to arsenate.     

Arsenic leachability in water treatment adsorbents

Arsenic leachability in water treatment adsorbents was studied using batch leaching tests, surface complexation modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy. Spent adsorbents were collected from five pilot-scale filters that were tested for removal of arsenic from groundwater in Southern New Jersey. The spent media included granular ferric hydroxide (GFH), granular ferric oxide, titanium dioxide, activated alumina, and modified activated alumina. The As leachability determined with the Toxicity Characteristic Leaching Procedure (TCLP, 0.1 M acetate solution) was below 180 microg L(-1) for all spent media. The leachate As concentration in the California Waste Extraction Test (0.2 M citrate solution) was more than 10 times higher than that in the TCLP and reached as high as 6650 microg L(-1) in the spent GFH sample. The EXAFS results indicate that As forms inner-sphere bidentate binuclear surface complexes on all five adsorbent surfaces. The As adsorption/desorption behaviors in each media were described with the charge distribution multisite complexation model. This study improved the understanding of As bonding structures on adsorptive media surfaces and As leaching behavior for different adsorbents.     

Coprecipitation of arsenate with metal oxides: nature, mineralogy, and reactivity of aluminum precipitates

Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (<4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.     

Phosphate adsorption on aluminum-impregnated mesoporous silicates: surface structure and behavior of adsorbents

Phosphorus from excess fertilizers and detergents ends up washing into lakes, creeks, and rivers. This overabundance of phosphorus causes excessive aquatic plant and algae growth and depletes the dissolved oxygen supply in the water. In this study, aluminum-impregnated mesoporous adsorbents were tested for their ability to remove phosphate from water. The surface structure of the materials was investigated with X-ray diffraction (XRD), a N2 adsorption-desorption technique, Fourier transform-infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) to understand the effect of surface properties on the adsorption behavior of phosphate. The mesoporous materials were loaded with Al components by reaction with surface silanol groups. In the adsorption test, the Al-impregnated mesoporous materials showed fast adsorption kinetics as well as high adsorption capacities, compared with activated alumina. The uniform mesopores of the Al-impregnated mesoporous materials caused the diffusion rate in the adsorption process to increase, which in turn caused the fast adsorption kinetics. High phosphate adsorption capacities of the Al-impregnated mesoporous materials were attributed to not only the increase of surface hydroxyl density on Al oxide due to well-dispersed impregnation of Al components but also the decrease in stoichiometry of surface hydroxyl ions to phosphate by the formation of monodentate surface complexes.     

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.     

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy

Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.     

Modeling the mass-action expression for bidentate adsorption

The number of bidentate binding sites on a pristine surface (i.e., sites comprising two adjacent monodentate sites plus the space between them) can be substantially larger than the maximum number of bidentate molecules that can adsorb to the surface. When bidentate sorption occurs, the number of available bidentate sites decreases in response to direct occupation of some sites, but an even more significant loss results from the fact that several unoccupied sites immediately surrounding each adsorbed molecule can also become unavailable. Recognition of this phenomenon allows development of a model for the adsorption process that matches simulated data from Monte Carlo (MC) modeling extremely well. The model also explains the observation that, on a given surface with a given fractional occupation, the number of available bidentate sites depends on whether the occupied sites are populated by monodentate or bidentate adsorbed species. A model developed more than 60 years ago but not widely recognized by modern adsorption modelers also fits the MC simulations very well. The simulated data are also reasonably approximated by assuming that the number of available bidentate sites on a surface is proportional to the square of the number of available monodentate sites, although the fit is not as good as with the models mentioned above. By contrast, approximating the number of available sites as proportional to the number of monodentate sites to the first power yields predictions that do not match the simulations. The results have implications for modeling of both multidentate adsorption reactions and monovalent-divalent ion exchange.     

Adsorption of arsenate onto ferrihydrite from aqueous solution: influence of media (sulfate vs nitrate), added gypsum, and pH alteration

Mineral processing effluents generated in hydrometallurgical industrial operations are sulfate based; hence it is of interest to investigate the effect sulfate matrix solution ("sulfate media") has on arsenate adsorption onto ferrihydrite. In this work, in particular, the influence of media (SO4(2-) vs NO3-), added gypsum, and pH alteration on the adsorption of arsenate onto ferrihydrite has been studied. The ferrihydrite precipitated from sulfate solution incorporated a significant amount of sulfate ions and showed a much higher adsorption capacityfor arsenate compared to nitrateferrihydrite at pH 3-8 and initial Fe/As molar ratios of 2, 4, and 8. Adsorption of arsenate onto sulfate-ferrihydrite involved ligand exchange with SO4(2-) ions that were found to be more easily exchangeable with increasing pH. Added gypsum to the adsorption system significantly enhanced the uptake of arsenate by ferrihydrite at pH 8. Equilibration treatment at acidic pH and addition of gypsum markedly improved the stability of adsorbed arsenate on ferrihydrite when pH was elevated. Comparison of arsenate adsorption onto ferrihydrite to coprecipitation of arsenate with iron(III) showed the latter process to lead to higher arsenic removal.     

Bidentate complexation modeling of heavy metal adsorption and competition on goethite

Accurate sorption modeling is critical for environmental risk assessment and development of sound remedial technologies. Adsorption to iron oxide phases is one of the important sorption processes regulating the bioavailability and toxicity of metal ions in natural systems. In this study, we used spectroscopically derived bidentate surface species to constrain surface complexation modeling in addressing Ni(ll) and Zn(ll) adsorption and competition on goethite surfaces. The 2-pK(a) triple layer model successfully predicted adsorption in single adsorbate systems. The curvature in adsorption isotherms was accurately depicted using two types of sites: high affinity and low affinity, and mononuclear bidentate surface complexes. A constrained set of parameters was found for each metal (log K(L) = -6.63 and log K(H) = -2.45 for Ni, log K(L) = -3.92 and log K(H) = 2.14 for Zn) that successfully described adsorption over a large range of experimental conditions, covering 6 to 7 orders of magnitude in concentration, ionic strength from 10(-3) to 10(-2), and environmentally relevant pH range between 4 and 6.5. Adsorption competition was predicted using the bidentate surface species with parameters calibrated using single adsorbate data.     

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.     

Extended X-ray absorption fine structure analysis of arsenite and arsenate adsorption on maghemite

Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may form a coating on magnetite surfaces. Such a sorption reaction could also favor arsenic immobilization at redox boundaries in groundwaters. The nature of arsenic adsorption complexes on maghemite particles, at near-neutral pH under anoxic conditions, was investigated using X-ray absorption fine structure (XAFS) spectroscopy at the As K-edge. X-ray absorption near edge structure spectra indicate that As(III) does notoxidize after 24 h in any of the sorption experiments, as already observed in previous studies of As(III) sorption on ferric (oxyhydr)oxides under anoxic conditions. The absence of oxygen in our sorption experiments also limited Fenton oxidation of As(III). Extended XAFS (EXAFS) results indicate that both As(III) and As(V) form inner-sphere complexes on the surface of maghemite, under high surface coverage conditions (approximately 0.6 to 1.0 monolayer), with distinctly different sorption complexes for As(III) and As(V). For As(V), the EXAFS-derived As-Fe distance (approximately 3.35 +/- 0.03 A) indicates the predominance of single binuclear bidentate double-corner complexes (2C). For As(III), the distribution of the As-Fe distance suggests a coexistence of various types of surface complexes characterized by As-Fe distances of approximately 2.90 (+/-0.03) A and approximately 3.45 (+/-0.03) A. This distribution can be interpreted as being due to a dominant contribution from bidentate binuclear double-corner complexes (2C), with additional contributions from bidentate mononuclear edge-sharing (2E) complexes and monodentate mononuclear corner-sharing complexes (1V). The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the maghemite-water interface.     

Investigation of chromate coordination on ferrihydrite by in situ ATR-FTIR spectroscopy and theoretical frequency calculations

Chromate mobility, reactivity, and bioavailability in soil environments are affected by adsorption reactions on iron oxide minerals, but the adsorption mechanisms remain controversial. In this study, we employed in situ attenuated total reflectance Fourier transform infrared spectroscopy and theoretical frequency calculations to characterize chromate adsorption on 2-line ferrihydrite. The effects of pH, aqueous chromate concentration, ionic strength, and deuterium exchange were investigated. Results suggest the formation of monodentate and bidentate surface complexes. It was determined that monodentate complexes are dominant at low surface coverage and pH ≥ 6.5 and that bidentate complexes form at high surface coverage and pH < 6. Deuterium exchange experiments indicated that the inner-sphere complexes are not protonated. Difference spectra revealed that monodentate complexes are particularly susceptible to ionic strength effects under acidic conditions.     

Arsenate adsorption structures on aluminum oxide and phyllosilicate mineral surfaces in smelter-impacted soils

A clearer understanding of arsenic (As) retention and transport in forest soils impacted by copper smelter emissions may reduce risks to human health and provide insight into As behavior in the vadose zone. On Vashon-Maury Island in Puget Sound, As is predominantly associated with the fine (< 63 microm) fraction of surficial soils. X-ray diffraction of oriented samples from the < 2 microm size fraction indicate that clinochlore isthe dominant phyllosilicate. X-ray absorption spectroscopy (XAS) was employed to examine As oxidation state and local coordination environment in impacted soil samples. Arsenic is present as As(V) in tetrahedral coordination with oxygen, associated with aluminum (Al) octahedra in bidentate binuclear (bridging) structures with As-Al distances of 3.15 - 3.16 angstroms. Including multiple scattering (MS) paths derived from the arsenate tetrahedron in esperanzaite significantly improved the match between XAS fine structure (EXAFS) data and models generated from theoretical phase and amplitude functions. The data are interpreted to indicate arsenate adsorption onto poorly crystalline aluminum oxyhydroxides and/or the edges of clinochlore interlayer hydroxyl sheets with constrained geometries causing MS to be important This implies that As initially released from the smelter as particulate As(III) and As(V) oxides was oxidized, dissolved, and adsorbed onto soil minerals and colloids; no evidence for relic arsenic oxide was observed. Physical transport of arsenic oxide particles and As adsorbed on soil colloids may account for limited downward migration of As within the soil column. The oxidizing and mildly acidic pH conditions in the upper vadose zone promote stable sorption complexes; barring substantial changes in soil chemistry, As is not expected to experience significant mobilization.     

Effects of dissolved carbonate on arsenate adsorption and surface speciation at the hematite--water interface

Effects of dissolved carbonate on arsenate [As(V)] reactivity and surface speciation at the hematite-water interface were studied as a function of pH and two different partial pressures of carbon dioxide gas [P(CO2) = 10(-3.5) atm and approximately 0; CO2-free argon (Ar)] using adsorption kinetics, pseudo-equilibrium adsorption/titration experiments, extended X-ray absorption fine structure spectroscopic (EXAFS) analyses, and surface complexation modeling. Different adsorbed carbonate concentrations, due to the two different atmospheric systems, resulted in an enhanced and/or suppressed extent of As(V) adsorption. As(V) adsorption kinetics [4 g L(-1), [As(V)]0 = 1.5 mM and I = 0.01 M NaCl] showed carbonate-enhanced As(V) uptake in the air-equilibrated systems at pH 4 and 6 and at pH 8 after 3 h of reaction. Suppressed As(V) adsorption was observed in the air-equilibrated system in the early stages of the reaction at pH 8. In the pseudo-equilibrium adsorption experiments [1 g L(-1), [As(V)]0 = 0.5 mM and I = 0.01 M NaCI], in which each pH value was held constant by a pH-stat apparatus, effects of dissolved carbonate on As(V) uptake were almost negligible at equilibrium, but titrant (0.1 M HCl) consumption was greater in the air-equilibrated systems (P(CO2) = 10(-3.5) atm) than in the CO2-free argon system at pH 4-7.75. The EXAFS analyses indicated that As(V) tetrahedral molecules were coordinated on iron octahedral via bidentate mononuclear ( 2.8 A) and bidentate binuclear (approximately equal to 3.3 A) bonding at pH 4.5-8 and loading levels of 0.46-3.10 microM m(-2). Using the results of the pseudo-equilibrium adsorption data and the XAS analyses, the pH-dependent As(V) adsorption under the P(CO2) = 10(-3.5) atm and the CO2-free argon system was modeled using surface complexation modeling, and the results are consistent with the formation of nonprotonated bidentate surface species at the hematite surfaces. The results also suggest that the acid titrant consumption was strongly affected by changes to electrical double-layer potentials caused by the adsorption of carbonate in the air-equilibrated system. Overall results suggest that the effects of dissolved carbonate on As(V) adsorption were influenced by the reaction conditions [e.g., available surface sites, initial As(V) concentrations, and reaction times]. Quantifying the effects of adsorbed carbonate may be important in predicting As(V) transport processes in groundwater, where iron oxide-coated aquifer materials are exposed to seasonally fluctuating partial pressures of CO2(g).     

Surface complexation modeling of U(VI) sorption to Hanford sediment with varying geochemical conditions

A series of U(VI) sorption experiments with varying pH, ionic strength, concentrations of dissolved U(VI), and alkalinity was conducted to provide a more realistic database for U(VI) sorption onto near-field vadose zone sediments at the proposed Integrated Disposal Facility (IDF) on the Hanford Site, Washington. The distribution coefficient (Kd) for U(VI) in a leachate that is predicted to result from the weathering of vitrified wastes disposed in the IDF is 0 mL/g due to the high sodium and carbonate concentrations and high pH of the glass leachate. However, when the pH and alkalinity of the IDF sediment native pore water increases during mixing with the glass leachate, U(VI) uptake is observed and the value of the U(VI) Kd increases 4.3 mL/g, because of U(VI) coprecipitation with newly formed calcite. A nonelectrostatic, generalized composite approach for surface complexation modeling was applied and a combination of two U(VI) surface species, monodentate (SOUO2+), and bidentate (SO2UO2(CO3)2-), simulated the measured U(VI) sorption data very well. The generalized composite surface complexation model, when compared to the constant or single-valued Kd model, more accurately predicted U(VI) sorption under the varying geochemical conditions expected at the IDF.     

Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite, and pyrite: an X-ray absorption spectroscopy study

The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5-6.5) interact with the surfaces of goethite, lepidocrocite, mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 A for As(III) and 1.69 A for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As-O: 1.69-1.76 A) with further sulfur (approximately 3.1 A) and iron (3.4-3.5 A) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, along with poorly crystalline arsenic sulfide (the only product at low As(V) concentrations). All oxidized samples show primary coordination to four oxygens at 1.7 A, indicating As(V); these arsenates may show both outer sphere complexation with residual mackinawite and inner sphere complexation with new oxyhydroxides. These experiments help to clarify our understanding of As mobility in near-surface environments.