Surface Complexation and Reactivity of Kaolinite and Gibbsite with Cadmium: A multi-method approach

Determination of energetically favorable surface complexes that form at the mineral-solution interface is important for understanding the surface reactivity of common minerals such as kaolinite and gibbsite and the behavior of toxic pollutants such as Cd2+. In this study, experimental Cd LIII X-ray Absorption Near-Edge Structure (XANES) was combined with periodic density functional theory (DFT), theoretical XANES calculations of the Cd LIII edge, and surface complexation modeling to characterize Cd2+ complexes sorbed to surfaces. Cd LIII spectra were collected for Cd2+ reference compounds, aqueous solutions, and sorption samples. Linear combination fitting of sorption sample XANES spectra was performed with known compounds and a sorption sample spectrum at low Cd surface coverage (IS-Kaol) that was interpreted to represent a mononuclear inner-sphere complex. With increasing surface coverage, spectra showed a systematic decrease in the fraction of IS-Kaol reference, indicating that as surface coverage increases, the proportion of bidentate complexes decreases. In linear combination fits, a component interpreted as a dimeric component (CdAlO4(s) or Cd(OH)2) increased with surface coverage, suggesting that dimerization or surface reformation may be occurring when surface coverage is high. A postulated set of Cd2+ surface reactions on the (100) face of kaolinite and gibbsite were compiled as constrained by DFT calculations and Cd LIII XANES. The Charge Distribution MUlti-SIte Complexation (CD-MUSIC) model was used in PhreePlot with this constrained set of reactions and used to derive complexation constants. Sorption on gibbsite was fit with a mononuclear bidentate reaction at low surface coverage (1 Cd/300 nm2), and with bidentate and dimer reactions at high coverage (1 Cd/3 nm2). Sorption on kaolinite was fit with a bidentate reaction at low surface coverage (1 Cd/150 nm2), and with bidentate, monodentate, and outer-sphere reactions at high surface coverage (1 Cd/1.5 nm2). The results indicate that mononuclear inner-sphere complexes form first on both kaolinite and gibbsite. With increased surface coverage, dimer complexes form on gibbsite, and outer-sphere complexes form on kaolinite, which is consistent with results from linear combination fits