Cadmium adsorption capacity of selected Ontario soils

The retention of Cd by selected Ontario soils with a range of soil properties was investigated. Batch adsorption experiments were carried out at the actual (unadjusted) pH of the soil and at a range of pH values adjusted by the addition of acid or base. For all soils, Cd adsorption increased with increasing pH and with increasing Cd solution concentration. The adsorption data was fitted, by a linear least squares technique, to the Langmuir adsorption isotherm. Maximum adsorption, q(max), at unadjusted soil pH values ranged from less than 8 mmol kg(-1) for the Fox sandy soil to 64.8 mmol kg(-1) for the Hanbury heavy clay soil. Calculated Cd adsorption maxima were regressed against measured soil properties to determine the most important properties involved in the adsorption of Cd. Multiple linear regression analysis revealed the best model to be q(max) = 8.33 + 0.67 (organic carbon) + 4.37 (inorganic - poorly crystalline Fe, Fe-pc). Surface complexation modeling indicates that humic surfaces account for adsorption at pH values beginning at approximately 3.5 and that hydrous ferric oxide surfaces account for Cd adsorption at pH values greater than 7.