Sorption and diffusion of Eu in sedimentary rock in the presence of humic substance

Seida, Yoshimi; Terashima, Motoki  ; Tachi, Yukio  ; Iijima, Kazuki  ; Nakazawa, Toshiyuki*; Yamada, Norikazu*; Yui, Mikazu

Diffusion and sorption behaviors of Eu in sedimentary rock were investigated in the presence of humic substance in the present study. Sedimentary rock obtained from Horonobe URL test site in Hokkaido, Japan, (accessible pore diameter is around several to several ten nanometer) was used. The Diffusion behaviors of Eu were examined based on reservoir depletion method coupled with analysis of inner distribution of the radioactive elements in the rock. Time sequence of concentration of each radioactive element as well as the humic substance in the reservoir was obtained as a function of concentration and molecular size of humic substance. Coexistence of humic substance reduced the depletion of Eu in the reservoir, indicating complexation between the radioactive elements and the humic substance. On the contrary, obvious decrease of humic substance in the reservoir was not observed in the system. This observation suggests that the radioactive elements became hard to diffuse into the sedimentary rock due to an increase of their size through complexation with the humic substance. The sorption of Eu was reduced with increase of the humic substance although the sorption of the humic substance was not influenced by the existence of Th or Eu. The diffusion and sorption of Eu were found to be reduced in the presence of humic substance.