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 L_III X-ray Absorption Near-Edge Structure (XANES) was combined with periodic density functional theory (DFT), theoretical XANES calculations of the Cd L_III edge, and surface complexation modeling to characterize Cd²⁺ complexes sorbed to surfaces. Cd L_III 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 L_III 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.

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