Contamination of soil and groundwater by arsenic (As) is a global problem adversely affecting over 150 million people who consume contaminated drinking water on a daily basis. Additionally, agricultural practices can mobilize As in irrigation drainage water leading to large-scale redistribution in soils. The objectives of this study were to investigate the adsorption of arsenate (As(V)) and arsenite (As(III)) on a series of coastal California soils derived from sedimentary materials from the Golden Gate National Recreation Area south of Fort Funston, CA. The soil materials were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), and acid extraction to determine mineralogical and elemental properties. Batch equilibrium and kinetics experiments were employed to determine the rate and extent of As(III) and As(V) adsorption in the range of 0-60 mg/L As(III) and As(V), with measurements made by microwave plasma-atomic emission spectroscopy (MP-AES). The elemental and mineralogical composition of the soils showed large amounts of amorphous Fe (3-5% w:w), as determined by XRD, XRF, and wet chemical extraction followed by MP-AES analyses. Adsorption generally increased with increased Fe concentration. The results of the adsorption kinetics studies showed first-order rate constants in the range of 0.048-1.28 min-1 and 0.037-1.72 min-1 for As(III) and As(V), respectively, with As(V) generally adsorbing at a faster rate than As(III). A significant arsenic-induced pH effect occurred, where As(V) adsorbed more strongly below pH 7 with the As(III) species becoming more strongly adsorbed above pH 7, and at higher total As concentrations. A surface complexation model was used to describe the adsorption of As(III) and As(V) at varying suspension pH allowing for a detailed understanding of As(III) and As(V) adsorption in soil. Our results suggest that the total Fe concentration, pH, and As(III)/(V) speciation are important factors for predicting the overall mobility of As in the soil-water environment.

Modeling Arsenic(III) and (V) adsorption on soil: Implications for water treatment and agriculture

248th National Meeting (2014)

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