A ~(133)Cs, ~(29)Si, AND ~(27)A1 MAS NMR SPECTROSCOPIC STUDY OF CS ADSORPTION BY CLAY MINERALS: IMPLICATIONS FOR THE DISPOSAL OF NUCLEAR WASTES

摘要

A deep geological repository in stable plutonic rock is being considered for the long-term storage of spent nuclear fuel in Canada, In this study, we examine the adsorption of Cs~+ on clay minerals that may be used as engineered barriers in a repository or that are associated with fracture zones in the host rock. Ion-exchange experiments show that montmorillonite adsorbs more Cs than vermiculite, both 2:1 layer clay minerals. In the disordered structure of vermiculite, Cs was found to adsorb preferentially close to tetrahedral sites, where Al substitution for Si produced a local charge imbalance. Kaolinite, a 1:1 clay mineral, adsorbs very little cesium on its external surfaces. Elevated temperature and pressure, similar to those that would occur in an underground repository, do not affect the amount of Cs~+ adsorbed by montmorillonite. Cesium-133 MAS NMR spectroscopy indicates that Cs~+ is adsorbed in the interlayer of montmorillonite in two stages and in two environments: the layer of H_2O molecules, where Cs~+ is surrounded by H_2O molecules, and the layer closer to the clay surface, where Cs~+ is more tightly bonded to the basal oxygen atoms of the tetrahedra. The first stage of cesium adsoiption is a rapid uptake in both the basal oxygen layer and the H_2O molecule layer, in a ratio of 2:1. The H_2O molecule layer reached its capacity immediately, and the amount of cesium adsorbed remains constant up to seven days. However, the adsorption in the basal oxygen layer increases after one hour to a ratio of 5:1 with respect to the H_2O molecule layer, and remains at that level for the rest of the adsorption time.