Anion Sorption on a Calcareous, Montmorillonitic Soil‐Selenium

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Abstract
Selenite and selenate sorption on a calcareous, montmorillonitic soil was investigated as a function of solution pH (2–11) at two initial Se concentrations (1.9 mmol m−3 and 19 mmol m−3. Selenate sorption was not observed at any pH value for either initial Se concentration. Selenite sorption exhibited a maximum near pH 3, a sharp decline to pH 6, and a sorption plateau above pH 7. Selenite sorption as a function of pH was studied on reference minerals representative of the dominant mineral constituents of the soil: montmorillonite, kaolinite, and calcite. Selenite sorption on the clay minerals increased at low pH, exhibited a peak near pH 5, and decreased at higher pH. Selenite sorption on calcite increased from pH 6 to 8, peaked between pH 8 and 9, and decreased above pH 9. The constant capacitance model was able to describe selenite sorption on montmorillonite and kaolinite over the entire pH range studied and on the soil below pH 7. The soil sorption plateau above pH 7 virtually disappeared after removal of calcite, indicating that calcite plays an important role in selenite sorption of calcareous soils.