強アルカリ性・高Ca濃度下でのモンモリロナイトの溶解に関する分子軌道法解析

抄録

In the geological repository of radioactive waste containing transuranic nuclides (TRU waste), after a long period of time, there is a concern that bentonite making up artificial barrier is dissolved through contact with strong alkaline water of high Ca content. The first-principle quantum chemical calculation was performed using the cluster model of montmorillonite which is the main component of bentonite. Under strong alkaline condition, octahedral Al had higher solubility than octahedral Mg had. In particular, unsaturated Si-O-Al bond connecting tetrahedral and octahedral sheets at the edges of (110) and (010) planes, was suggested to easily cleavage. The dissolution of montmorillonite through the dissociation of Si-OH and Al-OH_2 groups was shown to be less likely to occur. On one hand, the adsorption of OH^- on a Si atom caused dissolution of adjacent Si and Al atoms in addition to the Si atom. At least in the early stages of dissolution, the difference in exchangeable cation species (Na^+ or CaOH^+) was suggested not to affect the dissolution behavior of montmorillonite. Thus, when evaluating the long-term stability of bentonite in the geological repository, structural factors particularly the cation composition of the octahedral sheet of montmorillonite should be taken into account.