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

Sorption and diffusion behaviors of Eu in sedimentary rock in the presence of humic substance were investigated. The sedimentary rock collected from 500 m depth of HDB-6 bore hole at horonobe URL site of Japan and Aldrich humic acid (HA) were used in the present study. Sorption behaviors of Eu and the HA on the sedimentary rocks with and without the rock organic matter (ROM) were elucidated as a function of HA concentration. The HA reduced the sorption of Eu on the rock with the increase of HA. Eu and HA sorption on the rock with the ROM was larger than on the rock after removing the ROM, indicating that the ROM plays an important role on the sorption of Eu and HA. The diffusion of Eu in the presence of HA was examined as a function of HA concentration and molecular weight of the HA (∝ 150 kDa or below 10 kDa) by means of a reservoir depletion test method with the intact rock core of the sedimentary rock. Depletion of Eu concentration in the reservoir was reduced with the increase of HA concentration. On the other hand, slight depletion of HA concentration in the reservoir was observed, indicating that the larger HA molecule diffused less into the rock. From the depletion curve and indiffusion profile of Eu in the rock, the effective diffusion coefficient, De, and distribution coefficient, Kd, in the intact system were estimated based on the profile fitting of the diffusion data with the conventional simple diffusion-sorption model. It was elucidated that the HA reduced the Kd and De of Eu in the intact system with the increase of HA. The contribution of the HA with smaller molecular weight to both the Eu sorption and diffusion was examined. (orig.)