INDIVIDUAL SORBENT CONTRIBUTIONS TO CADMIUM SORPTION ON ULTISOLS OF MIXED MINERALOGY

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 silicate, 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 re. actions 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 I = 0.01, (iii) Cd complexation to hydroxyl sites produced strong pH dependency in Cd sorption and was responsible for all the Cd sorption at I = 0.01 and above pH 6.5 at I = 0.01, and (iv) Fe oxides decreased Cd sorption by blocking access of Cd to exchange sites. The modeling results were unable to determine whether Fe oxides were important sorbents for Cd. Either with or without Cd sorption on Fe oxides, this model was able to describe the Cd sorption on the soil isolates reasonably well.