MINERAL SURFACE REACTIVITY AND SORPTION MODELING OF PB2+ AND CD2+ ON KAOLINITE AND GIBBSITE

Distinguishing reactive surface area, a measure of the functional groups that participate in molecular-scale reactions, from geometric surface area is a known problem in the accurate description of environmental processes involving mineral surfaces. In this study, we combine surface complexation modeling of aqueous lead (Pb²⁺) and cadmium (Cd²⁺) sorption onto kaolinite and gibbsite with spectroscopic probes (NMR, XAS) and periodic density functional theory (DFT) calculations to better quantify active, molecular-scale binding sites at surfaces. Batch sorption experiments with kaolinite or gibbsite were performed at different metal (PbCl₂ and/or CdCl₂) and background electrolyte (CaCl₂) concentrations for single and binary metal ion solutions. Samples were equilibrated for 24 hours at constant pH, centrifuged, and supernatant solutions removed. Filtered solutions were analyzed by ICP-OES and the adsorbed fraction of metal was calculated by difference. Single ion adsorption experiments were conducted on kaolinite with Pb²⁺ or Cd²⁺ (0.05 mM) in electrolyte solutions of 0.5, 5.0, and 50 mM. Surface coverages of Pb²⁺ varied from 0.0041 to 0.047 µmol/m² over pH 3.0-6.0, and Cd²⁺ from 0.0019 to 0.044 µmol/m² over pH 3.5-8.0. Lack of dependence of metal uptake on electrolyte concentration suggests that Ca²⁺ is not competing strongly for sites at these surface coverages. At higher coverage, competition among Pb²⁺, Cd²⁺, and Ca²⁺ is predicted by modeling. As constrained by DFT calculations, the postulated set of surface reactions includes mono- and bi-dentate Pb²⁺, Cd²⁺, and Ca²⁺ complexes on the (001) and (100) faces of kaolinite and gibbsite. Periodic DFT results were used to estimate the relative energies of each surface complex and interatomic distances. The Charge Distribution MUlti-SIte Complexation (CD-MUSIC) model implemented in PHREEQC, parameter estimation, and surface complex stoichiometry as constrained by DFT calculations are used to derive complexation constants. In combination with NMR and XAS analyses of the reacted solids, reactive surface area can be described quantitatively. Because reactive surface area relates to intrinsic properties of the mineral surface, it should not be scale dependent and thus can be confidently applied to field-scale sorption behavior.