Cadmium Sorption to Soil Separates Containing Layer Silicates and Iron and Aluminum Oxides

Ion exchange to fixed charge sites and surface complexation to hydroxylated sites are recognized to affect Cd aqueous concentrations in soil; however, the individual contribution of these reactions to Cd sorption in mineralogically complex material has not been documented. The objective of this study was to isolate the contributions of fixed charge and hydroxylated sites to Cd sorption in two Ultisol subsurface horizons that contained kaolinite, 2:1 layer silicates, and crystalline Fe and Al oxides. Cadmium sorption was measured on clay-sized isolates (<2.0 µm) from the soil materials in NaClO₄ electrolyte at ionic strength (I) = 0.1, 0.01, and 0.001 mol L⁻¹ with pH ranging from 4 to 9 and Cd concentrations ranging from 2 × 10⁻⁸ to 3 × 10⁻⁵ mol L⁻¹. Comparative experiments were also performed with smectite (SWy-1), kaolinite (KGa-1), and crystalline and amorphous Fe oxides to evaluate the potential contribution of ion exchange and surface complexation reactions on these phases to Cd sorption in the isolates. Ion exchange of Cd²⁺ on layer silicates dominated Cd sorption within the isolates when I was ≤0.01 mol L⁻¹ and the pH was less than ≈6.5. Sorption of Cd on hydroxylated sites on the edges of the layer silicates or on Fe or Al oxides, and/or exchange of CdOH⁺ on the fixed-charge sites of the layer silicates, dominated Cd sorption at I = 0.1 mol L⁻¹, and when the pH was greater than ≈6.5 at any I. Below pH 6.5, the magnitude of the sorption was controlled by the cation-exchange capacity of the isolates, which, in turn, was related to their relative proportions of kaolinite and 2:1 layer silicates. By comparing the Cd sorption on the isolates to that on (i) dithionite-citrate-bicarbonate (DCB) treated islates and (ii) DCB-treated isolates augmented with Fe oxides, it was concluded that crystalline Fe oxides were not significant sorbents of Cd. Instead, the Fe oxides appeared to be associated with the layer silicates by a particle interaction that blocked access of Cd to fixed-charge sites on the layer silicates. Contributions of the Geosciences Dep., Battelle Pacific Northwest Laboratories.