Mechanistic understanding of antimony(V) complexation on montmorillonite surfaces: Insights from first-principles molecular dynamics

Abstract

Clay minerals play a vital role in retaining antimony (Sb) by surface complexation, which can largely attenuate its mobility and availability. However, a profound understanding of the environmental process of Sb on clay-water interface is insufficient. In this study, aiming to quantitatively revealing the complexation mechanism of Sb(V) onto montmorillonite at the molecular scale, we performed first-principles molecular dynamics (FPMD) simulations to systematically investigate the related structural and energetic information. In interlayer space, [Sb(OH)6]- anion can bind with Ca2+/Na+/Al3+/Fe3+ as stable inner-sphere complexes, and the Sb-Al/Fe complex is much more favorable than Sb-Na/Ca complex. On edge surfaces, the ligand exchange processes on possible sites in a wide range of pH have been examined. At (010) edge, [Sb(OH)6]− was adsorbed as bidentate complex on Al-(H2O)(H2O) and Mg-(H2O)(H2O) and monodentate complex on Al-(H2O)(OH), and at (110) edge [Sb(OH)6]− formed monodentate complex on Al-(H2O) and Mg-(H2O). The desorption free energies indicated that the Sb-Al bidentate complex is the most stable and favorable edge species, followed by the Sb-Mg bidentate complex. Overall, this work provides the molecular-scale insights into the fate of antimony in natural environments, and the derived fundamental data contribute to improving the experiment-based surface complexation models (SCM).