Abstract
We studied the Cm(III) retention by calcium silicate hydrate (C-S-H), portlandite (Ca(OH)2) and their alteration products calcite, vaterite, and aragonite in high ionic strength carbonate-containing solutions representing specific formation waters. For this, we synthesized C-S-H gels with calcium to silicon (C/S) ratios of 1.0 and 2.0 in the absence and presence of Cm(III), resulting in Cm(III)-free and Cm(III) doped C-S-H gel, respectively. For phase identification purposes we applied X-ray diffraction (XRD) while for the identification of the Cm(III)/C-S-H binding mode we applied site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS). The stability of Cm(III) doped phases under repository-relevant conditions was evaluated by studying the time-dependent release of Cm(III) from the Cm(III) doped C-S-H gel into leaching solutions containing 0.02 M NaHCO3 or 2.5 M NaCl/0.02 M NaHCO3 over 60 d. Speciation changes of Cm(III) due to leaching were followed with TRLFS while C-S-H structure alterations and secondary phase formation were monitored with XRD. From the results it could be concluded that Cm(III) is not mobilized by aqueous carbonate but either remains incorporated in the C-S-H structure and portlandite or becomes partially re-immobilized into secondary CaCO3 phases. The presence of NaCl led to an accelerated conversion of metastable secondary CaCO3 phases into calcite.
Highlights
Concrete will be used as part of the multi-barrier system in a deep geological nuclear waste repository to ensure mechanical stability and sealing of disposal tunnels against formation water
Tits et al ascribed the high uncertainty in the retardation coefficients (Rd) values to an extremely strong sorption of Eu(III) on the calcium silicate hydrate (C-S-H) phase and a possible incomplete phase separation during centrifugation leading to the presence of colloids in the supernatant
Since the Cm(III) speciation on solid C-S-H particles present in the supernatant should be identical to that on the solid, it is expected that the incomplete phase separation has no influence on recorded X-ray diffraction (XRD) patterns or TRLFS spectra
Summary
Concrete will be used as part of the multi-barrier system in a deep geological nuclear waste repository to ensure mechanical stability and sealing of disposal tunnels against formation water. Centration of the pore water an increased release of Ca(OH)[2] and the formation of secondary CaCO3 phases can influence the concrete stability, pH of the pore water, and actinide retention by the concrete material[14,15,16] In detail, this has been shown in a recent study investigating U(VI) doped C-S-H gel exposed to saline carbonate solutions under alkaline conditions, where a significant release of previously incorporated U(VI) from C-S-H gel was observed in addition to a carbonate-induced alteration of the C-S-H structure[14]. As representative for the trivalent actinides we have chosen the luminescent cation Cm(III), which enables laser-induced luminescence spectroscopic investigations of the actinide retention mode in the studied systems
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