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Physico-chemical characteristics of purified Na-smectite and protonation/deprotonation behavior of smectite surface in NaCl media

Shibutani, Tomoki; Kohara, Yukitoshi*; Oda, Chie; Kubota, Mitsuru*; not registered; Shibata, Masahiro

The physico-chemical characteristics of purified Na-smectite and protonation/deprotonation behavior of smectite surface in different concentrations of NaCl solutions were studied to identify the reaction mechanism of smectite-water interaction for performance assessment of HLW geological disposal system. The Na-smectite was purified from Kunipia F (obtained from Kunimine Industries Co. Ltd. Japan). In this smectite, small amount of quartz was detected as an impurity by X-ray diffraction. As a result of XRD and chemical analysis of the smectite, it was found that exchangeable sites in smectite inter-layer were occupied by Na+. Cation exchange capacity (CEC) was measured as 1.108meq g$$^{-1}$$ by using ammonium acetate. The N$$_{2}$$-BET surface area of smectite was 50$$sim$$58m$$^{2}$$ g$$^{-1}$$. Protonation/deprotonation behavior of smectite was studied for 0.01, 0.1 and 0.5M NaCl by using titration and back titration method. Total amount of H$$^{+}$$ consumption increased with decreasing pH for all NaCl concentrations, and the titration curves in these solutions showed similar trend in the pH range of 6-11. On the other hand, total amount of H$$^{+}$$ consumption increased with decreasing NaCl concentration in the pH range of 2-6. The dominant sorption mechanism of H$$^{+}$$ on smectite was different between pH$$<$$6 and pH$$>$$6, and it can be considered that H$$^{+}$$ was sorbed on the same site as Na$$^{+}$$ for pH$$<$$6 and different site from Na$$^{+}$$ for pH$$>$$6. The prediction of protonation/deprotonation behavior of smectite for 0.01, 0.1 and 0.5M NaCl was carried out based on ion exchange and surface complexation models. The sorption sites were assumed as inter-layer and crystal edge site. The site concentrations for ion exchange and surface complexation were calculated from CEC. The reaction constants were consequently calculated by fitting of experimental results as follows. ...

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