Effective diffusivity of the uranyl ion in a granite from Inada, Ibaraki, Japan

The effective diffusivity of uranium(VI) in Inada granite has been determined by through-diffusion. Experiments were performed at room temperature (20–25°C) in a 0.1 mol 1−1 KCl solution where uranium is present predominantly as the poorly sorbing UO22+. An effective diffusivity (De) of (3.6 ± 1.6) × 10−14 m2 s−1 was obtained, close to that for uranine (nonsorbing organic tracer), but one order of magnitude lower than those obtained for Sr2+ and NpO2+, and two orders of magnitude lower than that obtained for I−. According to well established theory, a proportional relationship exists between De and the diffusivity in the bulk of the solution (Dv). The effective diffusivity obtained in granite was not proportional to Dv. This agrees with results obtained for effective diffusivity in a Swedish granite. The ratio De/Dv was found to be not constant but increased with De or Dv. This result suggests a limit to the application of the theory.