RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

A multisite model was applied to Cd-sorption data across a range in pH and at ionic strength (I) = 0.1 and 0.01 on specimen layer silicates and on untreated and dithionite-citrate-bicarbonate (DCB) treated isolates from two Utisols. The two isolates contained different mass concentrations of kaolinite, 2:1 layer silicates, Al-substituted crystalline Fe oxides, and gibbsite. The multisite model included the exchange of Cd²⁺ on fixed-charge sites on the layer silicates, complexation to hydroxyl sites on the layer silicates and Fe oxides, and electrostatic interaction between positively charged sites on Fe oxides and fixed-charge sites on the layer silicates. Site concentrations for the specimen layer silicates and DCB-treated and untreated isolates were estimated from cation-exchange capacity (CEC) data and N₂-specific surface area. Equilibrium constants for specific surface reactions were estimated from Cd-sorption data on the specimen layer silicates, KGa-1 and SWy-1, and the DCB-treated soil isolates. The combined modeling results indicated that (i) Cd²⁺ ion exchange to fixed-charge sites was the dominant sorption reaction for pH < 6.5 and I = 0.01, (ii) mass action with Na suppressed ion exchange at III