RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The contribution of edge-complexation reactions to metal ion sorption by smectites has not been well documented through experimentation or modeling. A model containing fixed-charge (X⁻) and hydroxylated edge sites (AlOH and SiOH) was applied to sorption data for Cd on SWy-1 smectite and a smectitic soil isolate in Na⁺, Ca²⁺, and mixed electrolytes to evaluate the potential contributions of ion exchange and surface coordination to metal cation binding. Sorption modeling was performed with FITEQL. Edge reactions were described using the triple-layer model (TLM) and an inner sphere Cd complex on AlOH sites and outer sphere complexes on SiOH sites. The complexation reactions were parameterized using (i) Cd sorption data from silica and alumina and (ii) site concentration estimates based on cation-exchange capacity (X⁻) and particle size measurements (AlOH and SiOH). Ion-exchange reactions were described in half-reaction format; exchange constants were fit using Cd sorption data in either Na⁺ or Ca²⁺ electrolyte. The sorption data was qualitatively described across ranges in pH and electrolyte concentration using a combination of ion exchange (CdX₂) and edge (AlOCd⁺) complexes. Complexation of Cd to edge SiOH was calculated to be unimportant. The calculated contribution of AlOCd⁺ complexes increased with increases in pH and ionic strength. Good predictions of Cd sorption in mixed electrolytes were obtained using constants derived from single-electrolyte systems. Edge sorption reactions were computed to be more important for the soil smectite than for SWy-1 because of its smaller particle size. However, the slope and magnitude of sorption on the soil isolate was inadequately described by the model. Competitive sorption reactions on the edge sites and binding to organic material are additional factors that must be considered to adequately describe Cd sorption to smectitic soil clays.