Selenite Adsorption on Alluvial Soils: III. Chemical Modeling

Selenite adsorption envelopes for five alluvial soils from the San Joaquin Valley, CA, were described with the constant capacitance model, which is based on the ligand exchange mechanism of anion adsorption. The adsorption envelope data covered the pH range 4 to 9 and referred to 1:25 (w/w) soil suspensions in a 50 mol m −3 NaCl background. The initial selenite concentrations used were 2 and 8 mmol m −3 . Model application consisted of determining, for one of the five soils, the intrinsic surface complexation constants for the species, SHSeO 0 3 , SSeO ‐ 3 , and S 2 SeO 0 3 , (S represents a surface Lewis acid site) subject to constraints of mass and charge balance and assumed values of the surface acid‐base reaction constants and capacitance density parameter. The binuclear complex S 2 SeO 0 3 was treated independently of the two monodentate surface species with respect to mass and charge balance. Optimization of the model with the adsorption envelope for a Panoche soil at an initial selenite concentration of 2 mmol m −3 was used to predict the adsorption envelopes for the four other soils at the same initial selenite concentration, as well as those for three soils at an initial selenite concentration of 8 mmol m −3 . The model predicted adsorption envelopes whose qualitative agreement with the experimental data was satisfactory, but whose quantitative agreement ranged from excellent to poor.