Arsenate and arsenite adsorption onto Al-containing ferrihydrites. Implications for arsenic immobilization after neutralization of acid mine drainage

Natural ferrihydrites (Fh) often contain impurities such as aluminum, especially in acid mine drainage, and these impurities can potentially impact the chemical reactivity of Fh with respect to metal (loid) adsorption. In the present study, we have investigated the influence of aluminum on the sorption properties of ferrihydrite with respect to environmentally relevant aqueous arsenic species, arsenite and arsenate. We have conducted sorption experiments by reacting aqueous As(III) and As(V) with synthetic Al-free and Al-bearing ferrihydrite at pH 6.5. Our results reveal that, when increasing the Al:Fe molar ratio in Fh, the sorption density dramatically decreased for As(III), whereas it increased for As(V). Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy analysis at the As K-edge indicated that the As^IIIO₃ pyramid binds to FeO₆ octahedra on both Al-free Fh and Al-bearing Fh, by forming bidentate mononuclear edge-sharing (²E) and bidentate binuclear corner-sharing (²C) surface complexes characterized by As–Fe distances of 2.9 Å and 3.4 Å, respectively. The decrease in As(III) sorption density with increasing Al:Fe ratio in Fh could thus be explained by a low affinity of the As(OH)₃ molecule for Al surface sites compared to Fe ones. In contrast, on the basis of available literature on As(V) adsorption mechanisms, we suggest that, in addition to inner-sphere ²C arsenate surface complexes, outer-sphere arsenate surface complexes forming hydrogen bonds with both Al–OH and Fe–OH surface sites could explain the enhancement of As(V) sorption onto aluminous Fh relative to Al-free Fh, as observed in the present study. The presence of aluminum in Fh may thus enhance the mobility of arsenite with respect to arsenate in Acid Mine Drainage impacted systems, while mixed Al:Fe systems could present an alternative for arsenic removal from impacted waters, provided that As(III) would be oxidized to As(V).     

Sequestration of arsenate from aqueous solution using 2-line ferrihydrite: equilibria,kinetics, and X-ray absorption spectroscopic analysis

Arsenic(V), as the arsenate (AsO₄ ^3?) ion and its conjugate acids, has a strong affinity on Fe, Mn, and Al (oxyhydr)oxides and clay minerals. Removal of arsenate from aqueous solution by poorly crystalline ferrihydrite (hydrous ferric oxide) via a combination of macroscopic (equilibria and kinetics of sorption) and X-ray absorption spectroscopic studies was investigated. The removal of arsenate significantly decreased with increasing pH and sorption maxima of approximately 1.994 mmol/g (0.192 mol_As/mol_Fe) were achieved at pH 2.0. The Langmuir isotherm is most appropriate for arsenate sorption over the wide range of pH, indicating that arsenate sorption preferentially takes place at relatively homogenous and monolayer sites rather than heterogeneous and multilayer surfaces. The kinetic study demonstrated that arsenate sorption onto 2-line ferrihydrite is considerably fast, and sorption equilibrium was achieved within the reaction time of 2 h. X-ray absorption near-edge structure spectroscopy indicates no change in oxidation state of arsenate following interaction with the ferrihydrite surfaces. Extended X-ray absorption fine structure spectroscopy supports the efficient removal of arsenate by the 2-line ferrihydrite through the formation of highly stable inner-sphere surface complexes, such as bidentate binuclear corner-sharing (²C) and bidentate mononuclear edge-sharing (²E) complexes.     

Arsenate and antimonate adsorption competition on 6-line ferrihydrite monitored by infrared spectroscopy

Arsenate and antimonate are water-soluble toxic mining waste species which often occur together and can be sequestered with varying success by a hydrous ferric oxide known as ferrihydrite. The competitive adsorption of arsenate and antimonate to thin films of 6-line ferrihydrite has been investigated using primarily adsorption/desorption kinetics monitored by in situ attenuated total reflectance infrared (ATR-IR) spectroscopy on flowed solutions containing 10⁻³ and 10⁻⁵ mol L⁻¹ of both species at pH 3, 5, and 7. ICP-MS analysis of arsenate and antimonate adsorbed to 6-line ferrihydrite from 10⁻³ mol L⁻¹ mixtures in batch adsorption experiments at pH 3 and 7 was carried out to calibrate the relative surface concentrations giving rise to the IR spectral absorptions. The kinetic data from 10⁻³ and 10⁻⁵ mol L⁻¹ mixtures showed that at pH 3 antimonate achieved a greater surface concentration than arsenate after 60 min adsorption on 6-line ferrihydrite. However, at pH 7, the adsorbed arsenate surface concentration remained relatively high while that of adsorbed antimonate was much reduced compared with pH 3 conditions. Both species desorbed slowly into pH 3 solution while at pH 7 most adsorbed arsenate showed little desorption and adsorbed antimonate concentration was too low to register its desorption behaviour. The nature of arsenate which is almost irreversibly adsorbed to 6-line ferrihydrite remains to be clarified.     

Arsenic immobilization in water and soil using acid mine drainage sludge

Adsorption onto Fe-containing minerals is a well-known remediation method for As-contaminated water and soil. In this study, the use of acid mine drainage sludge (AMDS) to adsorb As was investigated. AMDS is composed of amorphous particles and so has a large surface area (251.2 m² g⁻¹). Here, adsorption of both arsenite and arsenate was found to be almost 100%, under various initial AMDS dosages, with the arsenate adsorption rate being faster. The optimum pH for As adsorption onto AMDS was pH 7.0 and the maximum adsorption capacities for arsenite and arsenate were 58.5 mg g⁻¹ and 19.7 mg g⁻¹ AMDS, respectively. In addition, experiments revealed that AMDS dosages decreased As release from contaminated soil. Therefore, the AMDS used in this study was confirmed to be a suitable candidate for immobilizing both arsenite and arsenate in contaminated soils.     

Influence of arsenic on iron sulfide transformations

The association of arsenate, As(V), and arsenite, As(III), with disordered mackinawite, FeS, was studied in sulfide-limited (Fe:S = 1:1) and excess-sulfide (Fe:S = 1:2) batch experiments. In the absence of arsenic, the sulfide-limited experiments produce disordered mackinawite while the excess-sulfide experiments yield pyrite with trace amounts of mackinawite. With increasing initially added As(V) concentrations the transformation of FeS to mackinawite and pyrite is retarded. At S:As = 1:1 and 2:1, elemental sulfur and green rust are the end products. As(V) oxidizes S(-II) in FeS and (or) in solution to S(0), and Fe(II) in the solid phase to Fe(III). Increasing initially added As(III) concentrations inhibit the transformation of FeS to mackinawite and pyrite and no oxidation products of FeS or sulfide, other than pyrite, were observed. At low arsenic concentrations, sorption onto the FeS surface may be the reaction controlling the uptake of arsenic into the solid phase. Inhibition of iron(II) sulfide transformations due to arsenic sorption suggests that the sorption sites are crucial not only as sorption sites, but also in iron(II) sulfide transformation mechanisms.     

Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids

A mixing of metal-loaded acid mine drainage with shallow groundwater or surface waters usually initiates oxidation and/or hydrolysis of dissolved metals such as iron (Fe) and aluminum (Al). Colloidal particles may appear and agglomerate with increasing pH. Likewise chemical conditions may occur while flooding abandoned uranium mines. Here, the risk assessment of hazards requires reliable knowledge on the mobility of uranium (U). A flooding process was simulated at mesocosm scale by mixing U-contaminated acid mine water with near-neutral groundwater under oxic conditions. The mechanism of U-uptake by fresh precipitates and the molecular structure of U bonding were determined to estimate the mobility of U(VI). Analytical and spectroscopic methods such as Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy at the Fe K-edge and the U L_III-edge, and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy were employed. The freshly formed precipitate was identified as colloidal two-line ferrihydrite. It removed U(VI) from solution by sorption processes, while surface precipitation or structural incorporation of U was not observed. EXAFS data suggest a mononuclear inner-sphere, edge-sharing complex of U(VI) with ferrihydrite in the absence of dissolved carbonate. By employing a novel EXAFS analysis method, Monte Carlo Target Transformation Factor Analysis, we could for the first time ascertain a 3-D configuration of this sorption complex without the necessity to invoke formation of a ternary complex. The configuration suggests a slightly tilted position of the adsorbed unit relative to the edge-sharing Fe(O, OH)₆ octahedra. In the presence of dissolved carbonate and at pH ∼8.0, a distal carbonate O-atom at ∼4.3 Å supports formation of ternary U(VI)-carbonato surface complexes. The occurrence of these complexes was also confirmed by ATR-FTIR. However, in slightly acidic conditions (pH 5-6) in equilibrium with atmospheric CO₂, the U(VI) sorption on ferrihydrite was dominated by the binary complex species Fe(O)₂UO₂, whereas ternary U(VI)-carbonato surface complexes were of minor relevance. While sulfate and silicate were also present in the mine water, they had no detectable influence on U(VI) surface complexation. Our experiments demonstrate that U(VI) forms stable inner-sphere sorption complexes even in the presence of carbonate and at slightly alkaline pH, conditions which previously have been assumed to greatly accelerate the mobility of U(VI) in aqueous environments. Depending on the concentrations of U(VI) and carbonate, the type of surface complexes may change from binary uranyl-ferrihydrite to ternary carbonato-uranyl-ferrihydrite complexes. These different binding mechanisms are likely to influence the binding stability and retention of U(VI) at the macroscopic level.     

Arsenate and chromate incorporation in schwertmannite

High concentrations of Cr (up to 812 ppm) and As (up to 6740 ppm) were detected in precipitates of the mineral schwertmannite in areas influenced by acid mine drainage. Schwertmannite may act as well as a natural filter for these elements in water as well as their source by releasing the previously bound elements during its dissolution or mineral-transformation. The mechanisms of uptake and potential release for the species arsenate and chromate were investigated by performing synthesis and stability experiments with schwertmannite.Schwertmannite, synthesized in solutions containing arsenate in addition to sulphate, was enriched by up to 10.3 wt% arsenate without detectable structural changes as demonstrated by powder X-ray diffraction (XRD). In contrast to arsenate, a total substitution of sulphate by chromate was possible in sulphate-free solutions. Thereby, the chromate content in schwertmannite could reach 15.3 wt%.To determine the release of oxyanions from schwertmannite over time, synthetic schwertmannite samples containing varying amounts of sulphate, chromate and arsenate were kept at a stable pH of either 2 or 4 over 1 year in suspension. At several time intervals Fe and the oxyanions were measured in solution and alterations of the solid part were observed by XRD and Fourier-Transform infrared (FT-IR) spectroscopy. At pH 2 schwertmannite partly dissolved and the total release of arsenate (24%) was low in contrast to chromate (35.4–57.5%) and sulphate (67–76%). Accordingly, the ionic activity product (log IAP) of arsenated schwertmannite was lowest (13.5), followed by the log IAP for chromated schwertmannite (16.2–18.5) and the log IAP for regular (=non-substituted) schwertmannite (18). At pH 4 schwertmannite transformed to goethite, an effect which occurred at the fastest rate for regular schwertmannite (=arsenate- and chromate-free), followed by chromate and arsenate containing schwertmannite. Both chromate and more evidently arsenate have a stabilizing effect on the schwertmannite structure, because they retarded the dissolution and transformation reactions.These kinetic investigations as well as crystallographic considerations demonstrated that the strength of the Fe(III) complexes with the anions controls the formation process and the stability of schwertmannite: with increasing affinity of the oxyanions to form complexes with Fe(III), the strength of the resulting binding and thus the stability and substitution preference increases.     

Quantifying arsenate partitioning in aquatic systems: Narrowing the laboratory-real world gap with kinetic sediment extractions and the Diffuse Layer Model

Many studies have proposed that silicic acid and phosphate (P^V) can displace arsenic sorbed to iron oxides leading to elevated As concentrations in aquatic systems. While surface complexation models are adept at quantifying sorption to synthetic oxides in laboratory systems their application to complex natural systems remains challenging. In this study we provide a systematic approach to developing a robust use of models for understanding As^V distribution in natural systems in which hydrated iron oxides are the main adsorptive phase. The Waikato River provides a useful laboratory for this work because it contains high H₄SiO₄, As^V and P^V loadings due to geothermal and agricultural inputs. A 15 min oxalate extraction and a 48 h ethylenediaminetetraacetic acid (EDTA) extraction of river sediment contained the same ratios of As:Fe, P:Fe and Si:Fe. Both of these extracts target the poorly ordered iron oxide phases (typically ferrihydrite) and by following the release of elements over time in the EDTA extraction it was possible to demonstrate that the extracted As, P, and Si were associated with the ferrihydrite. This demonstrates for the first time that a single oxalate extraction can quantify ferrihydrite sorbed H₄SiO₄, As and P^V and provides a basis to quantify the role of these ligands in inhibiting As^V sorption to sediments. The measured concentrations of ferrihydrite sorbed As^V, P^V and H₄SiO₄ for the Waikato River suspended sediment allow for the informed selection of appropriate model parameters for applying the Diffuse Layer Model to the system. In this way it was possible to quantify the effect of the individual components in the river water on As^V sorption. This study provides an explanation for the observation that the proportion of sorbed As in the Waikato River is generally significantly lower than that observed in rivers closer to the world average concentrations. More generally the study provides a method to quantify the role of individual water chemistry components on As^V distribution in natural systems.     

Metal binding to dissolved organic matter and adsorption to ferrihydrite in shallow peat groundwaters: Application to diamond exploration in the James Bay Lowlands,Canada

The speciation and solubility of kimberlite pathfinder metals (Ni, Nd, Ba and K) in shallow peat groundwaters is investigated over the Yankee, Zulu and Golf kimberlites in the Attawapiskat region, James Bay Lowlands, Canada. The purpose of this study is to examine the relationship between dissolved organic matter (DOM) complexation with kimberlite pathfinder metals and determine the spatial distribution of those metals in shallow peat groundwaters along sampling transects over subcropping kimberlites. Nickel, Nd, Ba and K complexation with DOM and the adsorption of these metals onto ferrihydrite were calculated using Visual MINTEQ 3.0 and the NICA-Donnan database. Calculations predict almost 100% of soluble Nd, Ni and Ba form complexes with DOM at sampling sites with little to no contribution from upwelling groundwater (i.e., dissolved organic C (DOC) concentrations = 40–132 mg/L, pH = 3.9–5.5, and log ionic strength ??3). In only the most ombrotrophic peat groundwater conditions does a majority fraction of K bind to DOM. By contrast, under conditions with large contributions from upwelling groundwaters (i.e., DOC concentrations ?40 mg/L, pH = 5.5–6.5, and log ionic strength = ?3 to ?2), as little as 10% of Nd and Ni, and 0% K and Ba are predicted to complex with DOM. The modeling calculations suggest the dominant control on metal–DOM complexation, particularly with respect to Ni and Nd, is competitive effects for DOM binding sites due to elevated ionic strength where there is evidence of strong groundwater upwelling. Visual MINTEQ modeling of metal adsorption on ferrihydrite surfaces predicts that under strong upwelling conditions, Ni and Nd are scavenged from solution due to increased ferrihydrite precipitation and decreased fractions of metals complexed with DOM. Analytical geochemical data are consistent with model predictions of metal adsorption on ferrihydrite. Total dissolved Ni and Nd concentrations at sites of strong upwelling are up to five times lower than waters with little to no upwelling and log ferrihydrite saturation indices (logSI_ferr) indicate precipitation (values up to 5) at sites of strong groundwater upwelling. Where the majority of Ni and Nd complex with DOM and ferrihydrite is highly under saturated (logSI_ferr = ?18 to ?5), the concentrations of total Ni and Nd are elevated compared to other sites along sampling transects. Metal complexation with DOM effectively inhibits metal scavenging from solution via adsorption and/or from forming secondary mineral precipitates. Also, because alkaline earth metals do not compete strongly with Ni and Nd for adsorption sites on ferrihydrite surfaces, but do compete strongly for insoluble organic sites, Ni and Nd are more likely to adsorb onto ferrihydrite.     

The influence of citric acid,EDTA, and fulvic acid on U(VI) sorption onto kaolinite

Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 × 10^?6–1 × 10^?4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO₃), and pCO₂ (0–5%) in the presence or absence of 1 × 10^?2–1 × 10^?4 M citric acid, 1 × 10^?2–1 × 10^?4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 × 10^?6–1 × 10^?4 M U(VI) in 0.01 M NaNO₃ were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 × 10^?5 and 1 × 10^?4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 × 10^?6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH ～3 to 7 and decreases from pH ～7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO₂ greatly diminishes U(VI) sorption between pH ～5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH.     

Fe(III) mobilisation by carbonate in low temperature environments: Study of the solubility of ferrihydrite in carbonate media and the formation of Fe(III) carbonate complexes

The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO₂ concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO₂ varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO₃ and Fe(CO₃)₃³⁻, with formation constants log^*β°_1,1,1 = 10.76 ± 0.38 and log β°_1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO₂ and at T = (25 ± 1) °C, as log^*K_s,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO₂-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.     

Infrared spectroscopic and X-ray diffraction characterization of the nature of adsorbed arsenate on ferrihydrite

Fourier transformed infrared (FTIR) spectroscopy was used to characterize arsenate-ferrihydrite sorption solids synthesized at pH 3-8. The speciation of sorbed arsenate was determined based on the As-O stretching vibration bands located at 650-950 cm⁻¹ and O-H stretching vibration bands at 3000-3500 cm⁻¹. The positions of the As-O and O-H stretching vibration bands changed with pH indicating that the nature of surface arsenate species on ferrihydrite was strongly pH dependent. Sorption density and synthesis media (sulfate vs. nitrate) had no appreciable effect. At acidic pH (3, 4), ferric arsenate surface precipitate formed on ferrihydrite and constituted the predominant surface arsenate species. X-ray diffraction (XRD) analyses of he sorption solids synthesized at elevated temperature (75 °C), pH 3 clearly showed the development of crystalline ferric arsenate (i.e. scorodite). In neutral and alkaline media (pH 7, 8), arsenate sorbed as a bidentate surface complex (in both protonated FeO₂As(O)(OH)⁻ and unprotonated forms). For the sorption systems in slightly acidic media (pH 5, 6), both ferric arsenate and surface complex were probably present on ferrihydrite. It was further determined that the incorporated sulfate in ferrihydrite during synthesis was substituted by arsenate and was more easily exchangeable with increasing pH.     

Effect of groundwater fulvic acid on the adsorption of arsenate by ferrihydrite and gibbsite

Adsorption studies have been conducted at pH 4, 6 and 8 to assess the effect of fulvic acid on arsenate adsorption to ferrihydrite and gibbsite. The studies compared the adsorption of arsenate on the mineral surfaces in the absence of fulvic acid, to those cases where increasing concentrations of fulvic acid (0.3–60 μM) were added to the mineral–arsenate suspensions. Experiments where arsenate was added to mineral–fulvate suspensions were also conducted. The results suggest that arsenate adsorption on both gibbsite and ferrihydrite decreases with increasing concentrations of fulvic acid. This effect was highest at pH 4, and decreased at pH 6 and 8. Ferrous ion concentrations were very low during the ferrihydrite experiments and support the view that fulvic acid can both displace arsenate from and inhibited its adsorption to mineral surfaces. The experiments also indicated that the amount of arsenate adsorbed was lower if fulvic acid was added before rather than after arsenate. This may reflect the relative size of arsenate and fulvic acid molecules and their ability to penetrate the crystal matrices of the minerals.     

Assessing Cd,Co, Cu,Ni, and Pb Sorption on montmorillonite using surface complexation models

Cadmium, Co, Cu, Ni and Pb adsorption is measured on montmorillonite as a function of pH (3–11), ionic strength (0.001–0.1 M NaNO₃), and sorbate concentration (0.1–10 μM metal on 0.5 g/L solid). Sorption of all metals shows strong dependence on ionic strength and sorbate concentration, as well as a break in the slope of the edge, indicative of a 2-site interaction with montmorillonite. The resulting adsorption edges are used to parameterize diffuse layer surface complexation models (DLMs) for each metal. A 2-site DLM with a bidentate variable charge surface hydroxyl site and a bidentate permanent charge exchange site produced good fits for the individual experiments, but lacked the robustness to accurately predict adsorption across the entire experimental range. Other models, such as CCM, TLM, or CD-MUSIC may be required for more accurate predictions across broad ranges of solution conditions.     

Characterisation of hydrous ferric oxides derived from iron-rich groundwaters and their contribution to the suspended sediment of streams

When Fe(II) bearing groundwaters surface in streams, particulate authigenic Fe-rich material is produced by oxidation. Such freshly precipitated Fe minerals may be transported as suspended sediment and have a profound impact on the fate of trace metals and nutrients in rivers. The objective of this study was to monitor changes in mineralogy and composition of authigenic material from its source to streams of increasing order. Groundwaters, surface waters, and suspended sediment in streams of different order were sampled in the Kleine Nete catchment (Belgium), a lowland with Fe-rich groundwaters (3.5–53.8 mg Fe/L; pH 6.3–6.9). Fresh authigenic material (>0.45 μm) was produced by oxidising filtered (<0.45 μm) groundwater and surface water. This material contained, on average, 44% Fe, and smaller concentrations of C, P, and Ca. Iron EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy showed that the Fe was present as poorly crystalline hydrous ferric oxides with a structure similar to that of ferrihydrite. The Fe concentration in the suspended sediment samples decreased to 36–40% (stream order 2), and further to 18–26% (stream order 4 and 5). Conversely, the concentrations of organic C, Ca, Si, and trace metals increased with increasing stream order, suggesting mixing of authigenic material with suspended sediment from a different source. The Fe speciation in the suspended sediment was similar to that in fresh authigenic material, but more Fe–Fe interactions were observed, i.e. it was increasingly hydrolysed, suggesting ageing reactions. The suspended sediment in the streams of order 4 and 5 is estimated to contain between 31% and 59% of authigenic material, but more data are needed to refine this estimate. The authigenic material is an important sink for P in these streams which may alleviate the eutrophication risk in this catchment.     

Seasonal variations of arsenic at the sediment–water interface of Poyang Lake,China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8 μg/L in sediments were higher than those (5.3–15.7 μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32 mg/kg and between 7.84 and 30.52 mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88 mg/m² a and 0.79 mg/m² a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.     

A predictive model (ETLM) for As(III) adsorption and surface speciation on oxides consistent with spectroscopic data

Arsenic(III) adsorption reactions are thought to play a critical role in the mobility of arsenic in the environment. It is the nature of the As(III) surface species that must be known on a wide variety of minerals and over a range of pH, ionic strength and surface coverage in order to be able to predict adsorption behavior. EXAFS and XANES spectroscopic studies have identified bidentate, binuclear inner-sphere surface species and/or an outer-sphere species, but only a few oxides have been examined. These results need to be integrated with a predictive surface complexation model in order to ascertain the environmental conditions under which the different surface species may be important on a wide range of solids. In the present study, the surface species information from XAFS and XANES studies has been built into a recent extension of the triple-layer model (ETLM) for the formation of inner-sphere complexes of anions that takes into account the electrostatics of water dipole desorption during ligand exchange reactions. The ETLM has been applied to regress surface titration, proton coadsorption, and As(III) adsorption data over extensive ranges of pH, ionic strength, electrolyte type and surface coverage for magnetite, goethite, gibbsite, amorphous hydrous alumina, hydrous ferric oxide (HFO), ferrihydrite, and amorphous iron oxide. Two principal reactions forming inner- and outer-sphere As(III) surface species,

     

Assessing the solubility controls on vanadium in groundwater,northeastern San Joaquin Valley,CA

The solubility controls on vanadium (V) in groundwater were studied due to concerns over possible harmful health effects of ingesting V in drinking water. Vanadium concentrations in the northeastern San Joaquin Valley ranged from <3 μg/L to 70 μg/L with a median of 21 μg/L. Concentrations of V were highest in samples collected from oxic groundwater (49% > 25 μg/L) and lowest in samples collected from anoxic groundwater (70% < 0.8 μg/L). In oxic groundwater, speciation modeling (SM) using PHREEQC predicted that V exists primarily as the oxyanion H₂VO₄⁻. Adsorption/desorption reactions with mineral surfaces and associated oxide coatings were indicated as the primary solubility control of V⁵⁺ oxyanions in groundwater. Environmental data showed that V concentrations in oxic groundwater generally increased with increasing groundwater pH. However, data from adsorption isotherm experiments indicated that small variations in pH (7.4–8.2) were not likely as an important a factor as the inherent adsorption capacity of oxide assemblages coating the surface of mineral grains. In suboxic groundwater, accurate SM modeling was difficult since Eh measurements of source water were not measured in this study. Vanadium concentrations in suboxic groundwater decreased with increasing pH indicating that V may exist as an oxycationic species [e.g. V(OH)₃⁺]. Vanadium may complex with dissolved inorganic and organic ligands under suboxic conditions, which could alter the adsorption behavior of V in groundwater. Speciation modeling did not predict the existence of V-inorganic ligand complexes and organic ligands were not collected as part of this study. More work is needed to determine processes governing V solubility under suboxic groundwater conditions. Under anoxic groundwater conditions, SM predicts that aqueous V exists as the uncharged V(OH)₃ molecule. However, exceedingly low V concentrations show that V is sparingly soluble in anoxic conditions. Results indicated that V may be precipitating as V³⁺- or mixed V³⁺/Fe³⁺-oxides in anoxic groundwater, which is consistent with results of a previous study. The fact that V appears insoluble in anoxic (Fe reducing) redox conditions indicates that the behavior of V is different than arsenic (As) in aquifer systems where the reductive dissolution of Fe-oxides with As adsorbed to the surface is a well-documented mechanism for increasing As concentrations in groundwater. This hypothesis is supported by the relation of V to As concentrations in oxic versus anoxic redox conditions.Sequential extraction procedures (SEP) applied to aquifer material showed that the greatest amount of V was recovered by the nitric acid (HNO₃) extract (37–71%), followed by the oxalate-ascorbic acid extract (19–60%) and the oxalate extract (3–14%). These results indicate that V was not associated with the solid phase as an easily exchangeable fraction. Although the total amount of V recovered was greatest for the HNO₃ extract that targets V adsorbed to sorption sites of crystalline Al, Fe and Mn oxides, the greatest V saturation of sorption sites appeared to occur on the amorphous and poorly crystalline oxide solid phases targeted by the oxalate and oxalate-ascorbic acid extracts respectively. Adsorption isotherm experiments showed no correlation between V sorption and any of the fractions identified by the SEP. This lack of correlation indicates the application of an SEP alone is not adequate to estimate the sorption characteristics of V in an aquifer system.     

Sorption and precipitation of Co(II) in Hanford sediments and alkaline aluminate solutions

Sorption and precipitation of Co(II) in simplified model systems related to the Hanford site high-level nuclear waste tank leakage were investigated through solution studies, geochemical modeling, and X-ray absorption fine structure (XAFS) spectroscopy. Studies of Co(II) sorption to pristine Hanford sediments (ERDF and Sub), which consist predominantly of quartz, plagioclase, and alkali feldspar, show an adsorption edge centered at pH ≈ 8.0 for both sediments studied, with sorption >99% above pH ≈ 9.0. Aqueous SiO₂ resulting from dissolution of the sediments increased in concentration with increasing pH, though the systems remained undersaturated with respect to quartz. XAFS studies of Co(II) sorption to both sediment samples reveal the oxidation of Co(II) to Co(III), likely by dissolved O₂, although this oxidation was incomplete in the Sub sediment samples. The authors propose that Fe(II) species, either in aqueous solution or at mineral surfaces, partially inhibited Co(II) oxidation in the Sub sediment samples, as these sediments contain significantly higher quantities of Fe(II)-bearing minerals which likely partially dissolved under the high-pH solution conditions. In alkaline solutions, Al precipitated as bayerite, gibbsite, or a mixture of the two at pH > 7; an amorphous gel formed at pH values less than 7. Aqueous Co concentrations were well below the solubility of known Co-bearing phases at low pH, suggesting that Co was removed from solution through an adsorption mechanism. At higher pH values, Co concentrations closely matched the solubility of a Co-bearing hydrotalcite-like solid. XAFS spectra of Co(II) sorbed to Al-hydroxide precipitates are similar to previously reported spectra for such hydrotalcite-like phases. The precipitation processes observed in this study can significantly reduce the environmental hazard posed by ⁶⁰Co in the environment.     

Calcium–phosphate treatment of contaminated soil for arsenic immobilization

The application of As-based herbicides at several industrial sites has resulted in numerous localized areas of As-contaminated soil. In this study, an As-contaminated soil (As = 278 mg/kg) collected from an industrial site located in the southeastern USA was subjected to inorganic phosphate (P_i) treatments. Although P_i treatments have been previously used for flushing As from contaminated soils, in this study, contaminated soil was amended with P_i to study the possible immobilization of As through a co-precipitation mechanism. Specifically, the P_i amendment was aimed at simultaneous flushing of As from the soil with orthophosphoric acid and co-precipitating it as Ca–phosphate–arsenate phases. Bench-scale P_i treatment experiments were performed at different pH conditions, with and without the addition of Ca. Sorption of P_i on BH soil in the presence or absence of additional Ca was determined, along with the associated mobilization of As from the soil. A significant amount of the HNO₃-digestible As (up to 55% at pH 4, 10–15% at pH 8, and ∼30% at pH 11) was released from the contaminated soil during the P_i sorption experiments. This increased mobility of As after the addition of P_i resulted from the competitive desorption of As from the soil. Although P_i sorption at high pH (>8) was largely controlled by precipitation, As did not co-precipitate with P_i. Aqueous geochemical modeling indicated that the lack of As co-precipitation during P_i-only treatment primarily resulted from the deficiency of Ca in the system. When additional Ca (16.9 mmol) was supplied along with P_i (3.38 mmol), the mobility of As decreased significantly at circum-neutral to high solution pH. Geochemical modeling suggested that the leachable As in the soil was potentially precipitated as As-bearing Ca–P_i phases. X-ray diffraction analysis of precipitates separated from the treated soil and from the synthetic leachate confirmed that the formation of a poorly crystalline carbonate apatite phase occurred as a consequence of the treatment. The results of this study support the potential application of Ca–P_i treatment for remediation of As-contaminated soil at environmentally relevant pH conditions.