Bentonite barrier performance affected by precipitation of secondary minerals: Diffusivities of water and iodide ions in the presence of calcium carbonate

Abstract

Precipitation of secondary minerals potentially affects the long-term barrier performance of bentonite, including hydraulic conductivity and diffusivity. In this regard, diffusion pathways of anions are expected to be preferentially clogged with secondary minerals, that may contribute to the containment of anionic radionuclides. However, experimental evidence is lacking because of the slow transport of reactants and complex alterations in the compacted bentonite. In this study, bentonite sample clogged with CaCO3 was prepared by an electrochemical technique. Then, through-diffusion experiments of HDO (deuterium water) and I− ions were conducted to elucidate the effect of CaCO3 on their effective diffusivities. Overall effective diffusivities were determined from through-diffusion experiments, and local effective diffusivities at the CaCO3 layer were determined from concentration gradients of tracers inside bentonite after reaching steady states of through-diffusion. A CaCO3 layer consisting of vaterite and calcite was precipitated in Na-bentonite; correspondingly, total porosity was maximally reduced from 0.57 to 0.35 at the CaCO3 layer. The overall effective diffusivity slightly decreased in the presence of CaCO3, while the local effective diffusivity at the CaCO3 layer considerably decreased for both HDO and I− ions. The impact of CaCO3 on the local effective diffusivity of I− ions was greater than that of HDO. The results suggest that long-term precipitation of secondary minerals originating from alterations of bentonite may decrease the diffusivity of radionuclides, particularly anions.