Mn loading on montmorillonite surfaces for Cd stabilization: Insights from density functional theory calculations and surface complexation modeling

The latest works have been devoted to the stabilization mensuration for heavy metals and indicate that clay minerals can promote Cd(II) precipitation by favoring the retention of Mn(II). The assessment however has been tempered due to lacking the information about the molecular-level surface complexation structure and Cd nucleation process on clay sur- faces. In this study, microscopic mechanisms for adsorption and stabilization of Cd at mont- morillonite interfaces with or without Mn loading were leveraged by combining surface complexation model (SCM) evaluations and density functional theory (DFT) calculations. Mn(II) substitution resulted in increases in surface acidity equilibrium constant (pKa) a ) by about 1 unit and complexation constant (lgK(SOCd K (SOCd + )) ) of Cd(II) by about 0.15 units at clay surface, and Mn(II) adion can provide extra active sites (i.e., OH-- groups) for complexing Cd(II) via hydrolysis. DFT calculations revealed Mn(II) and Cd(II) adions bind on base sur- faces by isomorphic substitutions through weak long-range interactions, whereas on edge surfaces by surface complexation with strong but short-range connections. Adsorption en- ergy calculations and electrostatic distribution showed heterogenous nucleation with sub- sequent cations on clay surfaces was thermodynamically favored, the stabilization process underwent the steps of the adsorption-hydrolysis-precipitation. The derived results pro- vide a quantitative basis for understanding the precipitation and heterogenous nucleation of cations on clay surfaces in surficial environments. (c) 2025 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.