Toward a mechanistic understanding of cesium adsorption to todorokite: A molecular dynamics simulation study

A mechanistic understanding of cesium (Cs) adsorption to soil mineral phases is essential for effective mitigation of Cs mobility in the subsurface environment. Todorokite, a common tunnel-structured manganese oxide in soil, exhibits sorption capacity for Cs comparable to the capacities of clay minerals. However, the adsorption sites and molecular species of Cs+ adsorbed to todorokite remain uncertain in comparison with those of clay minerals. In this study, we explored adsorption of Cs+ to hydrated todorokite surfaces via atomistic molecular dynamics (MD) simulations. We performed the first MD simulations based on atomic pair potentials for Mn-oxide edge surfaces interfaced with an aqueous solution. MD simulations predicted that Cs+ forms only inner-sphere (IS) complexes within todorokite tunnels; however, Cs+ forms both IS and outer-sphere (OS) complexes at the external (010) and (100)/(001) external surfaces. On the (010) surface, the positions between IS and OS complexes of Cs+ were interchangeable during MD simulations. Detailed molecular structures of IS and OS Cs+ surface complexes are compared to those of Cs+ in an aqueous solution. The current MD simulation results can be used as an atomistic structural proxy for spectroscopic analysis of adsorbed metal speciation and surface complexation modeling of metal adsorption to Mn oxides.