Deposition rates of colloidal silicic acid onto the surface of silicate mineral

The performance assessment of the repository system must also consider the high pH groundwater (pH 13) produced by the contact with cement material which is an essential material for the construction of repository system in Japan. In the limited space of the near field, the soluble-silicic-acid would be supersaturated by mixing the high pH groundwater (plume) with its surrounding natural groundwater (pH approx. =8). In this study, the dynamic behaviors of soluble and colloidal (polymeric) silicic-acid were examined in the presence of silicate mineral. The initial concentration of soluble silicic-acid was set to some given values in the range of from 3.0x10-3 to 6.0x10-3 M (pH>10). Then, the pH value of the solution was adjusted to 8 in the presence of the solid phase (amorphous silica). This study monitored the concentrations of both soluble silicic-acid and colloidal silicic-acid, by using silicomolybdenum-yellow method and ICP-AES. The results showed that both the initial concentration of supersaturated soluble silicic-acid and the specific surface area of solid phase strongly affect the deposition rate of silicic acid onto the surface of the solid phase. In the relatively low concentration of supersaturated soluble-silicic-acid in the range of from 3.0 mM to 6.0 mM, the soluble silicic acid rather than the colloidal form controlled the deposition rate of the silicic acid apparently. These results were quite different from that of the higher range such as 10 mM already reported. Moreover, the decrease rates of soluble and colloidal silicic acids in logarithm form showed linearity in the dependences on the specific surface area. The rate constants would be useful to evaluate the altered surface area of flow-paths around the repository, by considering the groundwater flow velocity, while the altered surface affects the sorption behaviors of radionuclides onto the solid phase. (author)