Colloidal properties of different smectite clays: Significance for the bentonite barrier erosion and radionuclide transport in radioactive waste repositories

Abstract

Compacted bentonite is envisaged as engineered barrier in high-level radioactive waste (HLRW) repositories and, to assess its functionality at a long term, many clay properties must be investigated. Bentonite erosion poses a potential problem, because a significant mass loss would jeopardize the performance of the barrier; in addition clay colloids generated from the eroded material might play a role on radionuclide transport. In this study, the intrinsic characteristics of colloids obtained from different smectitic clays (mass, size and surface charge) were analysed, focusing on their relation with the main physicochemical properties of the bulk clays: the smectite content, the composition of interlayer cations, the total layer charge and its distribution between the tetrahedral and octahedral layer. Results demonstrated that the predominance of Na as interlayer cation highly favours colloid detachment and the formation of colloids with the smallest size and the highest electrophoretic mobilities; the contrary occurs when Ca prevails (less detachment, larger colloids and lower mobilities). Besides, apart from the Na/Ca content, also the layer charge and charge distribution were shown to affect the intrinsic colloidal properties. For example,he increase of tetrahedral charge (τ) provokes stronger interactions between the TOT smectite layers, because cations (especially divalent) can be better bound to the clay surface. A τ increase causes the formation of larger aggregates with less probability of detachment. However, if the main exchange ion is Na, which strongly favours clay dispersion, high τ gives rise to an increased mass loss as, in fact, the released particles are bigger. The results were discussed in a generalised form, concluding on the potential erodibility of different smectites and the implications of this erosion on radionuclide migration in a HLRW repository.