Stability of U(VI) doped calcium silicate hydrate gel in repository-relevant brines studied by leaching experiments and spectroscopy

Wolter, Jan-Martin; Schmeide, Katja; Weiß, Stephan; Bok, Frank; Brendler, Vinzenz; Stumpf, Thorsten

doi:10.1016/j.chemosphere.2018.11.074

Cited by 5 publications

(7 citation statements)

References 46 publications

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“…As previously shown by XRD (Fig. 1) and TGA 14 investigations, a C-S-H phase with a C/S ratio of 2.0 contains around 18 mass% portlandite in addition to the C-S-H phase. Thus, the additional peak is assumed to arise from Cm(III) association with portlandite.…”

Section: Resultssupporting

confidence: 76%

“…These phases show characteristic XRD reflections of the C-S-H structure at 29.1, 32, 42, 46, 50 and 55°2Θ (Fig. 1), identical to those observed for U(VI) doped C-S-H gel 14 . For the C-S-H sample with a C/S ratio of 2.0, in addition, prominent portlandite reflections were detected (Fig.…”

Section: Resultssupporting

confidence: 67%

“…Thermogravimetric analysis (TGA) of a U(VI) doped C-S-H sample with a C/S ratio of 2.0 by Wolter et al . 14 , synthesized applying a comparable procedure, showed that at this C/S ratio around 18% of the mass is contributed by portlandite.

Figure 1XRD patterns of Cm(III)-free C-S-H samples with C/S ratios of 1.0 and 2.0 in comparison to C-S-H and portlandite references.…”

Section: Resultsmentioning

confidence: 94%

“…For clay deposits in North Germany 10,11 as well as for sedimentary bedrocks in Japan 12 and Canada 13 , taken into account as potential sites for nuclear waste repositories, high ionic strengths pore waters containing up to 4 M Na + , 2.8 M Cl − , 0.1 M S and 0.06 M HC are reported. In dependence on ionic strength and carbonate concentration of the pore water an increased release of Ca(OH) 2 and the formation of secondary CaCO 3 phases can influence the concrete stability, pH of the pore water, and actinide retention by the concrete material 14–16 . 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 .…”

Section: Introductionmentioning

confidence: 99%

“…In dependence on ionic strength and carbonate concentration of the pore water an increased release of Ca(OH) 2 and the formation of secondary CaCO 3 phases can influence the concrete stability, pH of the pore water, and actinide retention by the concrete material 14–16 . 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 . Whether such mobilization will occur also for actinides in other oxidation states should, thus, be investigated in detail, to allow predictions of the long-term safety of nuclear waste repositories.…”

Section: Introductionmentioning

confidence: 99%

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Cm(III) retention by calcium silicate hydrate (C-S-H) gel and secondary alteration phases in carbonate solutions with high ionic strength: A site-selective TRLFS study

Wolter

Schmeide

Huittinen

et al. 2019

Sci Rep

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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.

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Section: Resultssupporting

confidence: 76%

Section: Resultssupporting

confidence: 67%

Figure 1XRD patterns of Cm(III)-free C-S-H samples with C/S ratios of 1.0 and 2.0 in comparison to C-S-H and portlandite references.…”

Section: Resultsmentioning

confidence: 94%