Mobilization of Arsenite by Competitive Interaction with Silicic Acid

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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).