Radioactive cesium (Cs) contamination is a serious threat to the critical zone. Mechanistic understanding of Cs adsorption to mineral surfaces is essential in the accurate prediction of Cs transport in soil and sediment. Todorokite, a tunnel-structured manganese oxide, exhibits a high sorption capacity for Cs+ which is comparable to the phyllosilicate sorption capacity. In experiments, adsorption isotherm data have been described with one- or two-site conceptual models, but with no spectroscopic evidence yet as to the adsorption sites. Using molecular dynamics (MD) simulations based on atomic pair potentials, we have explored plausible adsorption sites for Cs+ on todorokite surfaces. According to our exhaustive conformational search, Cs+ cations adsorb mainly to the corners of intra-tunnels and the step edges of lateral (100)/ (001) surfaces at the point of zero net charge. Previous X-ray absorption spectroscopy and X-ray diffraction studies indicated formation of Cs+ outer-sphere (OS) surface complexes with todorokite and birnessite. We present our MD simulation results that predicted formation of Cs+ inner-sphere (IS) surface complexes not only inside the tunnels of todorokite but also in the interlayer of birnessite, which is further supported by a series of geometry optimizations based on density functional theory. We also present MD simulation results that, on the lateral edge surfaces of todorokite, both IS and OS surface complexes can occur, while the Cs–O distances and coordination numbers are similar among the surface complexes.

Atomistic computer-simulation insights to the cesium adsorption sites on todorokite surface

ACS Spring 2022

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