Investigation of the effect of biotite distribution on anion diffusion in crystalline rock

Matrix diffusion in crystalline rocks is a key process for the retardation of radionuclide transport for geological disposal. Crystalline rocks mainly consist of quartz and alkaline feldspar grains and phyllosilicate minerals such as biotite. The biotite, randomly or regularly, distributed in the rock (foliation) could influence the diffusion of anion radionuclides due to the anion exclusion effect caused by its negatively charged surface. However, there is little information available about the effect of the distribution of biotite on anion exclusion in the matrix. In this study, through-diffusion tests with tritiated water (HTO) and anions (Cl or I), together with micro X-ray CT analysis, were conducted on crystalline rocks containing biotite to obtain their effective diffusion coefficient (De) and determine the biotite content and foliation. The anion exclusion effect was evaluated by the ratio of De of anion and HTO. Cylindrical rock samples (20 L5 mm) were used, collected from Olkiluoto (Finland), Grimsel (Switzerland) and three Japanese rocks (Hazu, Kofu, Inada). The biotite content in these rocks was determined by X-ray CT to be about 30, 17, 15, 10 and 5%, respectively. The Grimsel and Olkiluoto samples had foliation orientated biotite particles. Through-diffusion tests were conducted with HTO and 36Cl or 125I as tracers. To investigate the effects of the foliation, the diffusion tests for the Olkiluoto and Grimsel samples were performed in both directions, parallel and perpendicular to the foliation. The De of both Cl and I was lower than HTO in all samples, demonstrating the anion exclusion effect. However, the ratio of De of anion and HTO (De,anion/De,HTO) increased with increasing the biotite content. On the other hand, the De,anion/De,HTO for the Olkiluoto and Grimsel samples largely depended on the foliation direction, and the values for the foliation along the direction of diffusion were several times higher than that perpendicular to the direction of diffusion. This result suggests that both the biotite content and foliation direction could control the anion diffusion. This study was performed as part of The project for validating near-field assessment methodology in geological disposal (FY2018-2020) supported by the Ministry of Economy, Trade and Industry of Japan.