The adsorption of selenite by five alluvial soils from the San Joaquin Valley, California, was investigated with respect to the effects of soil composition and pH. Soil samples were prepared in the form of Na‐saturated pastes. Adsorption experiments were carried out in a 50 mol NaCl m−3 background at initial selenite concentrations comparable with those found in natural soil systems. The samples were equilibrated under N2‐stirring in a constant‐temperature apparatus with pH and redox conditions monitored. Selenite adsorption by the soils decreased uniformly with increasing pH in the range 4 to 9 and was independent of soil series above pH 6.

The adsorption of selenate by four alluvial soils from the San Joaquin Valley, CA, was investigated under conditions of varying pH and equilibrating solution composition. Soil samples were prepared in the form of Na‐saturated pastes. Adsorption experiments were carried out in 50 mol m−3 NaCl or NaClO4 background solution at initial selenate concentrations comparable to those found in field soils (5 mmol kg−1). The pH value and electrode potential (Eh) were monitored while the reaction mixtures were equilibrated using CO2‐free air for stirring in a constant temperature apparatus. In contrast to the results obtained previously for selenite, positive selenate adsorption by the four soils was not detected over the pH range of 5.5 to 9 in either the NaCl or NaClO4 electrolyte background. Selenate was found to behave in a manner similar to sulfate in these soils, but was not negatively adsorbed to the extent exhibited by chloride.

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The adsorption envelope for selenite on two alluvial soils from the San Joaquin Valley, California, was investigated with respect to variations in solution composition. Sodium‐saturated soil pastes were mixed with solutions containing sulfate, phosphate, and Ca ions in a background electrolyte of 50 mol NaCl m−3. The effects of increasing chloride concentration were investigated at 100 mol NaCl m−3. No discernible change in the amount of selenite adsorbed was found as a result of increased chloride concentration or through the addition of 16 mmol Na2SO4 kg−1. In contrast, an initial concentration of phosphate comparable to that of selenite resulted in a decrease of selenite adsorbed by approximately one‐half. The addition of 100 mol CaCl2 m−3 resulted in significant increases in selenite adsorption on both soils above pH 5, which could be attributed to precipitation or an increase of surface charge through Ca adsorption.

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