Structure and properties of montmorillonite containing Ca2+, Sr2+, and Ba2+ cations simultaneously

Abstract

Most raw montmorillonites (MMT) used for radioactive waste disposal (HLW) contain Ca in the interlayer space. When HLW and MMT come into contact, various ions, such as Sr2+ and Ba2+, enter the mineral. Therefore, the article presents the results of modeling, based on molecular dynamics, for four different models for the replacement of part of Ca2+ with Sr2+ and Ba2+ ions in Ca-MMT.The general trend of MMT swelling does not change significantly after the introduction of Sr2+ and Ba2+ ions. The diffusion coefficient of water molecules increases with an increase in the number of H2O layers between mineral layers or the presence of a certain amount of large radius ions in the series Ca-MMT, Ca-Ba-MMT, Ca-Sr-MMT, Ca-Sr-Ba-MMT. It shown that in the state of single-layer clay hydration, all the studied cations form outer-sphere complexes. During two-layer clay hydration, Ca2+ tends to form outer-sphere complexes, while Sr2+ and Ba2+ ions predominantly form inner-sphere complexes. With a further increase in the number of water molecules, the interaction of Ca2+ with the basal surface of the MMT mineral layer increases.