The molecular structure of ions retained on mineral surfaces is needed to accurately model their sorption process and to determine their stability. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used in this study to deduce the local coordination environment of two environmental contaminants, arsenate and chromate, on the mineral goethite (R-FeOOH). Based on the oxyanion-Fe distances, it was concluded that three different surface complexes exist on goethite for both oxyanions: a monodentate complex, a bidentate-binuclear complex, and a bidentate-mononuclear complex. At low surface coverages, the monodentate complex was favored while at higher coverages the bidentate complexes were more prevalentsthe bidentate-binuclear complex appears to be in the greatest proportion at these highest surface coverages. Therefore, modeling efforts for chromate or arsenate retention on goethite need to consider a monodentate complex at very low coverages, both the monodentate and bidentate complexes at intermediate coverages, and predominantly the bidentate complexes at very high coverages.

The kinetics of arsenate and chromate adsorption/ desorption on goethite (R-FeOOH) were investigated using a pressure-jump (p-jump) relaxation technique. Information provided by this technique was used to elucidate the fate of arsenate and chromate in natural environments. Chemical relaxations resulting from rapidly induced pressure changes were monitored via conductivity detection. The adsorption/desorption of these oxyanions on goethite involved a double relaxation event. The proposed mechanism for the adsorption of arsenate and chromate on goethite is a two-step process resulting in the formation of an inner-sphere bidentate surface complex. The first step, associated with the fast τ values, involved an initial ligand exchange reaction of aqueous oxyanion species H 2 AsO 4or HCrO 4 -with OH ligands at the goethite surface forming an inner-sphere monodentate surface complex. The subsequent step, associated with the slow τ values, involved a second ligand exchange reaction, resulting in the formation of an inner-sphere bidentate surface complex. Overall, the results suggest that chromate may be the more mobile of the two oxyanions in soil systems.

The environmental fate of arsenic (As) is of utmost importance as the public and political debate continues with the USEPA's recent proposal to tighten the As drinking water standard from 50 to 10 μg L−1 In natural systems, the presence of dissolved organic C (DOC) may compete with As for adsorption sites on mineral surfaces, hence increasing its potential bioavailability. Accordingly, the adsorption of arsenate [As (V)] and arsenite [As (III)] on goethite (α‐FeOOH) was investigated in the presence of either a peat humic acid (Hap), a Suwannee River Fulvic Acid (FA) (International Humic Substances Society, St. Paul, MN), or citric acid (CA). Adsorption edges and kinetic experiments were used to examine the effects of equimolar concentrations of organic adsorbates on As adsorption. Adsorption edges were conducted across a pH range of 3 to 11, while the kinetic studies were conducted at pH 6.5 for As (V) and pH 5.0 for As (III). Both Hap and FA decreased As (V) adsorption, while CA had no effect. Humic acid reduced As (V) between pH 6 and 9 by ≈27%. Fulvic acid inhibited As (V) adsorption between pH 3 and 8 by a maximum of 17%. Arsenite adsorption was decreased by all three organic acids between pH 3 and 8 in the order of CA > FA ≈ Hap. The different pH regions in which Hap and FA decreased As (V) adsorption suggest that more than one functional group on these complex organic polymers may be responsible for binding to the α‐FeOOH surface. Similarly, the relative surface affinity of the As(III or V) species and that of the competing organic ligand as a function of pH may play a major role in the outcome of As adsorption on α‐FeOOH. The results of these experiments suggest that DOC substances are capable of increasing the bioavailability of As in soil and water systems in which the dominant solid phase is a crystalline iron oxide.

The adsorption of As(V) and As(III) on synthetic two-line ferrihydrite in the presence and absence of a peat humic acid (HAp), Suwannee River fulvic acid (FA), or citric acid (CA) was investigated. Previous work with goethite has demonstrated the ability of dissolved organic carbon (DOC) to decrease As(V) and As(III) adsorption. The results obtained demonstrate that As(V) adsorption on ferrihydrite was decreased only in the presence of CA. Arsenate decreased the adsorption of all organic acids except HAp. Both FA and CA reduced As(III) adsorption on ferrihydrite, while HAp had no effect. Fulvic and citric acid adsorption on ferrihydrite was decreased in the presence of As(III); however, FA and CA adsorption increased at lower pH, which was consistent with decreased As(III) adsorption. Peat humic acid did not decrease As(III) adsorption, and we believe that the adsorption process of HAp and As(III) and As(V) on ferrihydrite are independent of each other. Previously, we observed that As(V) adsorption on goethite decreased in the presence of HAp > FA > CA, while As(III) adsorption on goethite was decreased similarly to that on ferrihydrite in the presence of CA > FA approximately HAp, yet As(III) adsorption on ferrihydrite was greater than on goethite. The observed differences between this study and the earlier study on goethite are believed to be an intricate function of ferrihydrite's surface characteristics, which affect the mechanisms of adsorption and hence the affinity of organic acids such as HAp, FA, and CA for the ferrihydrite surface. As such, the adsorption of DOCs to ferrihydrite are assumed to be less favorable and to occur with a fewer number of ligands, resulting in lower surface coverage of weaker bond strength.

No abstract

Due to the acute toxicity of As, mobilization of even a small fraction of As into surface and ground waters used as a source for drinking water represents a substantial risk to human health. Here we evaluate the mobilization of arsenite [As(III)] from the Fe‐(hydr)oxide mineral goethite (α‐FeOOH) through competitive displacement by silicic acid, a naturally occurring and ubiquitous inorganic ligand. The adsorption behaviors of silicic acid and As(III) on goethite were investigated at environmentally relevant pH (3–11). Single ion adsorption and zeta‐potential data were collected at silica concentrations characteristic of natural waters (3–30 mg L−1) and initial solution As(III) concentrations representative of high levels of contamination (3.75–7.5 mg L−1). Competitive adsorption scenarios with either Si or As(III) sorbed first to the goethite surface, followed by equilibration with the other sorbate, were also examined. No competitive displacement of either oxyanion was observed at total sorbate concentrations less than reactive surface site density, regardless of pH or addition scenario. However, at total sorbate concentrations greater than reactive surface site density, As(III) adsorption was reduced by 10 to 15% over the entire pH range regardless of addition scenario, resulting in aqueous concentrations well in excess of current (10 μg L−1) drinking water maximum contaminant levels. Surface complexation modeling of single ion adsorption and zeta‐potential data using the Charge Distribution Multisite Surface Complexation (CD‐MUSIC) model was used to calculate an appropriate set of surface adsorption equilibrium constants for As(III) and silicic acid adsorption, which was used to describe the competitive adsorption scenarios. Comparison of competitive adsorption data and CD‐MUSIC model predictions, at total sorbate concentration greater than reactive surface site density of goethite, suggest that silica is competitively displaced by As(III).

The potential toxicity and availability of As in the environment is dependent on several factors including redox potential, pH, and the presence of ligands that can compete for adsorption sites on mineral surfaces. Silicic acid is a ligand ubiquitous in natural systems and strongly chemisorbs to Fe oxides. However, there are relatively few studies examining its influence on As adsorption on Fe oxides. This study examined the influence of silicic acid (0.10 and 1.0 mM) on the adsorption kinetics of arsenite and arsenate (0.10 mM) on goethite over a range of common soil pH values (4, 6, and 8). The rate of arsenic (III and V) and silicic acid adsorption was greatest at pH values near their pK1 value. However, silicic acid sorption was characterized by biphasic kinetics with rapid adsorption followed by a much slower adsorption reaction. The rate and total quantity of arsenite adsorption decreased in the presence of silicic acid at all pH values and concentrations of silicic acid. Approximately 40% less arsenite was adsorbed in the presence of 1.0 mM silicic acid at all pH values. At 0.10 mM, silicic acid had less of an effect on arsenite adsorption. In contrast, only the rate and not the total quantity of arsenate was reduced in the presence of silicic acid. The rate of arsenate adsorption decreased as pH and silicic acid concentration increased. This was attributed to a decrease in the goethite's surface potential upon specific adsorption of silicic acid and deprotonation of the arsenate molecule creating an unfavorable electrostatic field. These results demonstrate the importance of evaluating As speciation, reaction kinetics, and the influence of naturally occurring ligands on the adsorption of As on variable charge surfaces.

ality and crystallography of mineral surfaces involved, and the total concentration of the metal or oxyanion Sorptive interactions at the solid-water interface strongly influence present. However, most nutrient oxyanions and trace the bioavailability of many important nutrient oxyanions and trace metals occur at relatively low concentrations in natural contaminant metals in both natural and engineered settings. Recently, the charge-distribution multisite complexation (CD-MUSIC) model environments (micromolar or less) precluding precipitahas been developed to model ion adsorption behavior on variabletion. Therefore, adsorption processes ultimately control charge minerals. Although this model shows great promise, its use has the phase distribution and potential bioavailability of been limited by lack of incorporation into commonly used computer these ions (McBride, 1994). codes. In this study formulation of the CD-MUSIC model in the sur-Because trace metals including Fe, Cu, Zn, and oxyface complexation modeling program FITEQL 4.0 is described, and anions such as P i have central functions in many biodemonstrated using Cu 2؉ -and orthophosphate (P i )-goethite adsorpchemical pathways, and occur at relatively low concention data. Mass-action and mass-balance expressions for Cu 2؉ and P i trations in pristine environments, organisms generally adsorption on goethite were developed using a combination of monotake up inorganic nutrient ions using transport systems dentate and bidentate surface species. Pauling's rules were used to despecific for a particular chemical form. For example, termine the charge of surface adsorption sites and adsorption site density (nm Ϫ2 ) was calculated from crystallographic considerations. both microorganisms and plants generally take-up P and Electrostatic component coefficients in the mass-balance expressionsCu as free ions from solution-P i (Frossard et al., 1995) were adjusted to reflect the actual charge of goethite adsorption sites, and Cu 2ϩ (Deighton and Goodman, 1995). However, Cu, thereby satisfying both the local charge balance for adsorbed species Zn, and other trace metals may become toxic to microand the global charge balance of the system as a whole. FITEQL 4.0 organisms, phytoplankton, and plants at concentrations was used to determine the best-fit equilibrium constants for the Cu 2؉ as low as 10 Ϫ9 M, with toxicity and free metal ion activity and P i surface adsorption mass-action expressions, and the associated well correlated (Deighton and Goodman, 1995). Therespeciation of adsorbed ions. The speciation of adsorbed Cu 2؉ ions was fore, determining the distribution and speciation of metdominated by a single monodentate surface species; whereas, two monoals and oxyanions between adsorbed and solution phases dentate and one bidentate surface species were required to adequately is essential for evaluating their potential bioavailability describe P i adsorption. Formulating the CD-MUSIC model as outlined here provides a thermodynamically, electrostatically, and crystallo-i...