Anion Sorption on a Calcareous, Montmorillonitic Soil—Arsenic

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Abstract
Arsenic sorption on a calcareous, montmorillonitic soil was investigated as a function of solution pH (2–11) at two initial As concentrations (2.0 and 20 mmol m−3). Arsenite sorption could not be determined because oxidation of arsenite occurred during the experiment. Arsenate sorption increased with increasing pH, exhibited a maximum near pH 10.5, and decreased at higher pH. Arsenate sorption as a function of pH was studied on reference minerals representative of the dominant inorganic constituents of the soil: montmorillonite, kaolinite, and calcite. Arsenate sorption on the two clay minerals increased at low pH, exhibited a peak near pH 5, and decreased at higher pH. Arsenate sorption on the calcite increased from pH 6 to 10, peaked between pH 10 and 12, and decreased above pH 12. The constant capacitance model was able to describe arsenate sorption on kaolinite over the entire pH range studied and on montmorillonite and soil below pH 9. The soil sorption maximum near pH 10.5 was much reduced after removal of carbonates, indicating that carbonates play a major role in arsenate sorption above pH 9. Arsenate sorption on soil and calcite was unaffected by the presence of equimolar selenite concentrations.

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