1. Bentonite barrier performance affected by precipitation of secondary minerals: Diffusivities of water and iodide ions in the presence of calcium carbonate.
- Author
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Tanaka, Shingo
- Subjects
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CALCIUM carbonate , *CALCIUM ions , *MINERALS in water , *BENTONITE , *MINERAL waters , *VATERITE - 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 CaCO 3 was prepared by an electrochemical technique. Then, through-diffusion experiments of HDO (deuterium water) and I− ions were conducted to elucidate the effect of CaCO 3 on their effective diffusivities. Overall effective diffusivities were determined from through-diffusion experiments, and local effective diffusivities at the CaCO 3 layer were determined from concentration gradients of tracers inside bentonite after reaching steady states of through-diffusion. A CaCO 3 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 CaCO 3 layer. The overall effective diffusivity slightly decreased in the presence of CaCO 3 , while the local effective diffusivity at the CaCO 3 layer considerably decreased for both HDO and I− ions. The impact of CaCO 3 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. • The through-diffusion of HDO and I− ions was conducted using bentonite clogged with CaCO 3. • Not only overall D e , but local D e at CaCO 3 were determined from internal concentrations. • The local D e of both HDO and I− ions considerably decreased in the presence of CaCO 3. • The impact of CaCO 3 on the diffusion of I− ions was greater than HDO owing to anion exclusion. • Long-term mineral precipitation may contribute to a lower diffusivity of anions. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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