The mechanism of cesium ions immobilization in the nanometer channel of calcium silicate hydrate: a molecular dynamics study.

The cement-based matrices are preferred candidates in disposing nuclear waste due to the immobilization role of the calcium-silicate-hydrate (C-S-H) gel. To better understand the immobilization mechanism of cementitious materials, molecular dynamics was utilized to investigate the intensity distribution, local structure and dynamics properties of Cs+ ions in the vicinity of the calcium silicate surface. The strong inner-sphere adsorbed cesium ions were restricted by coordinated oxygen atoms in bridging and pair silicate tetrahedron and water molecules were fixed in the silicate channel by H-bonds network. On the other hand, the adsorption of chloride ion, repulsed by the negatively charged silicate surface, is mainly attributed to the formation of the cation-anion ionic pair near the interface. As compared with those of the solvated ions in the solution, the relaxation time of water in the hydration shell of adsorbed Cs+ is significantly increased and the diffusion coefficient of adsorbed Cs+ is dramatically reduced. Furthermore, based on the intensity profile and resident-time analysis, the adsorption capacities of monovalent cations on the C-S-H surface increase with decrease in the ionic radius, following the sequence of Na+ ≫ K+ > Cs+. This study provides a molecular-level understanding of the immobilization mechanism of different ions in the C-S-H gel pores.