Measurement and modeling of surface charge and cation binding in agricultural soil

Abstract

Models of metal adsorption have typically been developed and tested for soil components rather than whole soils and at cation concentrations higher than usual environmental conditions. This study investigates whether the non-ideal consistent competitive adsorption (NICA) model can be applied to ion binding in whole soils at low total metal concentrations. Surface charge was measured for 18 agricultural soils from southern Quebec over the pH range 3.5 to 8. The adsorption of Ca, Cd, Cu, Pb and Zn was also measured for three total metal concentrations of 2, 5 and 10 mg L-1 at pH 6 in a 0.005 M Ca(ClO4)2 solution. NICA model parameters were solved for each soil using the surface charge and adsorption data and a non-linear least squares fitting routine. Two types of binding sites were identified: the first type had a pKa near 4 while the second type had a pKa near 7.5. The first type of binding site contributed the greater proportion of the variable charge over most of the pH range, and the second site was unimportant at the lower pHs. The surface charge was accurately described by the NICA model with a mean R2 of 0.995. A mean of the surface charge parameters describing H+ binding accounted for more than 95% of the variable charge on the soils. For individual soils, the NICA model gave a fit to the experimental data with mean R2 values for Ca, Cd, Cu, Pb and Zn of 1.00, 0.906, 0.879 (two concentrations only), 0.825 and 0.918 respectively. When mean adsorption parameters, instead of values determined for each soil individually, are used, the model gave mean R2s for Ca, Cd, Cu, Pb and Zn of 0.938, 0.941, 0.998 (two concentrations only), 0.978 and 0.935. It seems that the NICA model can be used to describe the surface charge and adsorption of cations by whole soils. The mean adsorption parameters appear to describe the adsorption behavior of the soil nearly as well as the individually fitted parameters. This implies that mean parameters for these agricultural soils may provide satisfactory predictions for the adsorption behavior of similar soils. Key words: NICA model, NICCA, surface charge, cation binding, agricultural soil