Categorization of Cement Hydrates by Radionuclide Sorption Mechanism
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Categorization of Cement Hydrates by Radionuclide Sorption Mechanism
K. Noshita1, T. Nishi1, T. Yoshida1, H. Fujihara2, N. Saito2 and S. Tanaka3 1 Hitachi Ltd., 7-2-1 Omika, Hitachi, Ibaraki 319-1221, Japan 2 Tokyo Electric Company, 1-3 Uchisaiwai, Chiyoda, Tokyo 100-0011, Japan 3 Tokyo University, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
ABSTRACT A categorization of cement components was attempted from the viewpoints of their chemical constituents and sorption behaviors. In this study, we assumed that the cement components are categorized into four groups, calcium silicate compounds, hydroxides, calcium aluminate compounds, and others. The sorption behaviors were compared among the four groups by batch sorption experiments carried out for 12 kinds of cement components using the radionuclides cesium, iodine, inorganic carbon, and nickel as different chemical species. The characteristic sorption trends were obtained in each group. We found that the sorption mechanism is closely related to the chemical constituents and surface species. Categorization of these cement of components will contribute to the understanding of sorption mechanisms.
INTRODUCTION Cementitious materials have been widely used as a means of containing radioactive waste and as an engineered barrier surrounding repository sites. Their choice was based on the high sorption abilities of cement components for many radionuclides. However, there are many variable factors which may affect the sorption ability in the actual environment as shown in Figure 1. To predict the sorption ability under various conditions, it is necessary to understand the sorption mechanisms. Although a number of investigations have been done, the mechanisms are still not well understood well due to the variety of cement components [1,2].
Figure 1. Variable factors of sorption ability In this study, to clarify the relationship between chemical constituents and sorption behaviors, we assumed that the cement components could be categorized into four groups, calcium silicate compounds, hydroxides, calcium aluminate compounds and others. To compare the sorption behaviors among the four groups, we carried out batch sorption experiments using radionuclides cesium, iodine, inorganic carbon and nickel, as different chemical species.
EXPERIMENTAL Selection of Radionuclides Table 1 shows the calculated chemical species of dissolved radionuclides under common repository conditions (pH=12.6, Eh=-600mV, CO32-=1.35x10-4M, at 298K). The calculations were carried out using HARPHRQ[3] ver.2 and HATCHES[4] ver.9 database, excluding Mo, Tc, Pd, Eu, Ac, Pa and Cm, which were estimated by chemical analogy. In this study, four radionuclide species, cationic Cs+, anionic I-, oxyanionic CO32-, and hydrolyzed Ni(OH)2(aq) were selected from each group to investigate the general sorption behaviors of cement components.
Table 1. Calculated Chemical Species Repository Conditions* Chemical species
Formulas
Elements
Cation
Mn+
Cs, Sr, Ra
Anion
n-
M
Cl, I, Se
Oxyanion
MOxn-
C
Hydrolyzed io
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