Retention of uranium(VI) by laumontite, a fracture-filling material of granite

Abstract

Retention of U(VI) by laumontite, a fracture-filling material of granite was investigated by conducting dynamic and batch sorption experiments in a glove-box using a granite core with a natural fracture. The hydrodynamic properties of the granite core were obtained from the elution curve of a non-sorbing tracer, Br−. The elution curve of U(VI) showed a similar behavior to Br−. This reveals that the retention of U(VI) by the fracture-filling material was not significant when migrating through the fracture at a given condition. From the dynamic sorption experiment, the retardation factor Ra and the distribution coefficient Ka of U(VI) were obtained as about 2.9 and 0.16 cm, respectively. The distribution coefficient (Kd) of U(VI) onto laumontite obtained by conducting a batch sorption experiment resulted in a small value of 2.3±0.5 mL/g. This low Kd value agreed with the result of the dynamic sorption experiment. For the distribution of uranium on the granite surface investigated by an X-ray image mapping, the fracture region filled with laumontite showed a relatively lower content of uranium compared to the surrounding granite surface. Thus, the low retention of U(VI) by the fracture-filling material can be explained by following two mechanisms. One is that U(VI) exists as anionic uranyl hydroxides or uranyl carbonates at a given groundwater condition and the other is the remarkably low sorption capacity of the laumontite for U(VI).