Sorption site capacities for Cs on the sedimentary rock in Horonobe area

• The radionuclides sorb to minerals by the uptake on the sorption sites.
• The sorption site capacity was estimated from the Cs sorption isotherms.
• The isotherms were obtained on sedimentary rock sampled in Horonobe area in Japan.
• Illite content was estimated by the alkylammonium treatment.

The sorption behavior of radionuclides depends on the content of minerals in geological media. The sorption of radionuclides on minerals has been interpreted as the uptake on the sorption sites on mineral surfaces. However, conventional investigations such as X-ray diffraction analysis cannot avoid large errors in quantification of minerals. Furthermore, the discrepancies of sorption behavior have been often found even on the same kind of minerals. Therefore, the sorption site capacity cannot be effectively estimated by the quantification of minerals. In this study, the sorption site on sedimentary rock sampled in Horonobe area, where the Horonobe Underground Research Center, JAEA, is located, was estimated from the Cs sorption isotherms obtained by sorption experiments. To deduce the fitting parameters, illite content estimated from the amount of extracted K by alkylammonium treatment and smectite content estimated from the cation exchange capacity measurement were introduced to the fitting procedure. The result shows that the sorption site capacities of smectite and illite in the sedimentary rock in Horonobe area are 1.3–1.7 × 10−4 and 1.4–4.0 × 10−5 eq/g, respectively.