The effect of alkaline alteration on sorption properties of sedimentary rock
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The effect of alkaline alteration on sorption properties of sedimentary rock Satoko Shimoda1, Toshiyuki Nakazawa1, Hiroyasu Kato1, Yukio Tachi2, Yoshimi Seida2 1 Mitsubishi Materials Corporation, 1002-14, Mukoyama, Naka, Ibaraki, 311-0102, Japan 2 Japan Atomic Energy Agency, 4-33, Muramatsu, Tokai, Ibaraki, 319-1194, Japan ABSTRACT The potential effect of high pH plume caused by cementitious materials must be evaluated in the performance assessment for HLW geological disposal. Alkaline plume would lead to change sorption properties of host rock by primary mineral dissolution, secondary mineral precipitation and sequential change of pore water chemistry. In this study, the effect of alkaline alteration on sorption of Cs, Ni and Th was investigated using rock samples from the Horonobe Underground Research Laboratory. Crushed rock samples were reacted in high pH alkaline solution at 90 oC for 45 days, 95 days and 1,383 days, respectively. As a result of sample analysis, it was supposed that zeolitic mineral was precipitated as secondary mineral. The cation exchange capacity slightly increased in comparison with the unaltered sample. Distribution coefficients (Kd) of Cs, Ni and Th on unaltered and altered rock sample were measured by batch sorption experiment in synthetic groundwater. Kd of Cs increased with the alteration period. These results show that secondary minerals contribute to the increase in Cs sorption. By contrast, Kd of Ni and Th decreased with the alteration period. This change might be caused by dissolution of clay minerals and amorphous silicates controlling Ni and Th sorption by surface complexation. These results imply that effects of alkaline alteration on Kd of rocks depend on the dissolution/precipitation of minerals, their surface properties and sorption mechanisms. INTRODUCTION Sorption of radionuclides in host rock is one of the key processes in the performance assessment for HLW geological disposal [1]. The sorption properties of rocks have been investigated using key radionuclides under a wide variety of geochemical conditions, and distribution coefficients (Kd) setting approaches have been developed [2]. Cementitious materials will be used as tunnel support in the HLW disposal facility. High pH plume arising from dissolution of these materials will spread into a near field host rock. Alkaline plume would lead to mineral dissolution in the rock and secondary mineral precipitation, and cause to change in migration properties of the radionuclides. Therefore, it is important to understand the rock alteration and its effect on radionuclide sorption for the long-term performance assessment [1, 3, 4]. In this study, the mineral properties of the rock altered by alkaline solution and their sorption properties of Cs, Ni and Th were investigated using sedimentary rock samples from the Horonobe Underground Research Laboratory (URL). EXPERIMENTAL Rock sample preparation Sedimentary rock (siliceous mudstone) was sampled from about 510 m depth (Wakkanai formation) of the borehole HDB-6 at the Horonobe URL.
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