The influence of pH on the sorption and diffusion of 99TcO4- in Beishan granite

In order to deal with the potential highly increasing amount of high-level radioactive wastes and spent nuclear fuels along with the ambitious development of nuclear power plants, China needs to construct a geologic repository in the middle of the 21st century. After about 15 years of basic research, including site survey, radionuclide migration, Beishan area in Northwest China's Gansu Province has been considered as the most promising candidate place. The Beishan granite has been highly considered as one of the potential host rocks, and some geochemical and hydrogeologic work has been carried out with the granite. In terms of pre-safety assessment of a potential site, the sorption, diffusion and migration of important radionuclides which tend to be released from the potential repository in the rock/water system play an important role and must be thoroughly investigated with the site specific rock/water characteristics. Due to the potential mobility of technetium under repository conditions, the investigation of the diffusion of technetium in different potential rocks has been studied, while few work focused on the rock surface characteristics. Since the diffusion/migration of radionuclides in the potential host rocks of geologic repositories is mainly through the pores/fractures of the host rocks, the surface characteristics of the rocks should play non-negligible role. In this paper, the influence of pH in the rock/water system on the diffusion of (TcO4-)-Tc-99 in Beishan granite is investigated with through-diffusion experiments and batch sorption experiments. Experimental results indicate that the effective diffusion coefficients (D-e) of Tc-99 in Beishan granite varied from 9.7 x 10(-13) to 1.3 x 10(-12) m(2)/s, and the distribution coefficients (K-d) of Tc-99 in Beishan granite decreased when the pH of the rock/water system increased which may due to the decreasing of the density of positively charged sites on the granite surface.