Sorption Behavior of Eu(III) into CSH Gel in Imitated Saline Groundwater - 12145
The sorption behavior of Eu(III) (europium (III)) into CSH (Calcium Silicate Hydrate) gel without dried processes was examined in imitated saline groundwater by using the spectro-fluorometer, Raman spectrophotometer and ICP-AES (Inductively Coupled Plasma- Atomic Emission Spectrometry). Ca/Si ratio was set to 0.4, 0.8, 1.2 and 1.6, and NaCl concentration was also set to 0.6, 0.06 and 0.006. The synthesis of each sample was conducted in a glove box saturated with nitrogen gas. The sealed sample tubes were gently shaken with 120 strokes/min. The time-period to contact Eu(III) with the CSH gel was set to 60 days. The fluorescence emission spectra suggested the incorporation of Eu{sup 3+} into CSH gel in the high Ca/Si ratio samples. On the other hand, from the decay behavior of fluorescence emission spectra, even in the low Ca/Si ratio samples, sorption behavior of Eu{sup 3+} into CSH gel was confirmed. Besides, the Raman spectra showed that the degree of polymerization of Si-O in CSH gel was raised with increasing Na ions concentration. These results suggest that the CSH gel, formed as secondary mineral, would retard the migration of radionuclides even in saline groundwater. Considering the inflow of saline groundwater into repository, this study examined the interaction between CSH gel (without dry processes) and Eu{sup 3+} by using the fluorescence emission spectra, the decay behavior of fluorescence and the Raman spectra. As a result, the fluorescence emission spectra of the sample of more than 0.8 Ca/Si ratio confirmed the intensity split into two peaks around 618 nm (5D0→7F2 transition). Furthermore, even in relatively low Ca/Si ratio samples, the fluorescence lifetimes both of the surface sorption sample and the co-precipitated samples exceeded that of the filtrate sample. These suggested that Eu{sup 3+} is not only hydrolyzed to form Eu(OH){sub 3} colloid, but is also stably incorporated into CSH gel (in Ca/Si ratio>1.2) or is forming complex on the surface of solid phase (in Ca/Si<0.8) in the co-presence of Na ions. On the other hand, the concentrations of Na, Ca and Si after 60 days in the solution filtrated through 0.2 μm membrane filter showed the ion exchange of Ca and Eu apparently in the samples of low Ca/Si ratio. Besides, in the samples with high Ca/Si ratio, Ca and Si concentrations in the solution were raised with increment of initial Na concentration. While Na ions may slightly increase the solubility of CSH gel, the CSH samples were stable, mostly maintaining the initial Ca/Si ratio synthesized. Moreover, the Raman spectra showed that the degree of polymerization of silicate chain in CSH samples increased with increasing Na concentration. Such an immobilized Na into the structure of CSH gel did not obstruct the incorporation of Eu{sup 3+} into CSH gel. The results mentioned above suggested that CSH gel (formed as a secondary mineral around the repository) also can retard the migration of radionuclides even if the repository and its surrounding are saturated by saline groundwater. (authors)