Retention of Sn in reducing cement systems

Tin is an important element in nuclear waste disposal, with 126Sn (t1/2 = 1.1∙106 a) being one of the seven long-lived fission products [1]. Other isotopes with shorter half-lives, e.g., 121mSn (t1/2 = 55 a) or 125Sn-125 (t1/2 = 0.0264 a) [2], are also present in the waste and become relevant in the context of low and intermediate level waste (L/ILW). Cementitious materials are widely used in underground and near-surface repositories for L/ILW for conditioning of the waste, as well as backfill-, container- and liner materials. Cement mainly consists of calcium silicate hydrated phases (C-S-H), portlandite (Ca(OH)2), calcium aluminates types phases (AFt, AFm) and non-hydrated clinker minerals. Cementitious materials buffer the pore water composition in the hyperalkaline pH region (10 ≤ pH ≤ 13.5) with moderate Ca concentrations (10–4 M ≤ [Ca] ≤ 0.02 M). C-S-H phases are considered as the main sink of cationic radionuclides and metal ions [2]. Sn is primarily expected in the +IV oxidation state within the stability field of water, although on-going discussions on the uncertainties affecting ΔfG°(Sn4+) and the corresponding impact on the E°(Sn4+/Sn2+) may suggest the possible stabilization of Sn(II) in the very reducing conditions expected in underground repositories [1]. ... mehrIn this context, the retention of Sn in cementitious environments and reducing conditions was investigated with a combination of solubility experiments, sorption studies and advanced spectroscopic techniques.
Undersaturation solubility experiments with SnO(cr) and SnO2(cr) were performed within 9 ≤ pHm ≤ 12.8 and 0.1 M ≤ I ≤ 3.5 M in NaCl-NaOH and CaCl2-Ca(OH)2 systems. Solubility results for Sn(II) systems are in good agreement with data available in the literature for SnO(cr) and Sn6O4(OH)4(cr). Solubility data obtained for Sn(IV) are in moderate agreement with recent experimental work by Rai et al. and the calculated solubility limit of SnO2(cr) [1]. Batch sorption experiments resulted in fast and strong uptake of Sn(IV) by C-S-H phases and cement, in line with the sorption behaviour reported for other strongly hydrolysing M(IV) metal ions, e.g., Th(IV) [3] or Pu(IV) [4]. A slightly weaker uptake of Sn by C-S-H phases was observed under very reducing conditions. This behaviour could be explained by (i) the oxidative sorption of Sn, with Sn(IV) prevailing in the solid phase and Sn(II) in the aqueous phase, or (ii) a strong sorption of Sn(II), in line with the strong hydrolysis reported for this +II metal ion [5]. The focus of on-going work targets the mechanistic understanding of Sn retention by cementitious systems. The presence of natural Sn in cementitious materials hints that isotopic exchange may be considered as plausible mechanism for the uptake of radioactive isotopes of Sn in repository systems. XAFS measurements were conducted at the INE-beamline of the Karlsruhe Research Accelerator (KARA) to gain insight on the redox speciation of Sn in cementitious reducing systems. The last results of this on-going work will be presented in this contribution.

We acknowledge the KIT light source for provision of instruments at their beamlines and we would like to thank the Institute for Beam Physics and Technology (IBPT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA).



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[3] J. Tits, E. Wieland, Actinide Sorption by Cementitious Materials. PSI-report 18 (2), (2018).
[4] A. Tasi, X. Gaona, Th. Rabung, D. Fellhauer, J. Rothe, K. Dardenne, J. Lützenkirchen, M. Grivé, E. Colàs, J. Bruno, K. Källstrom, M. Altmaier, H. Geckeis. Plutonium retention in the isosaccharinate – cement system. Applied Geochemisty 126, 104862 (2021).
[5] H. Gamsjäger, T. Gajda, J. Sangster, S. K. Saxena, W. Voigt. Chemical Thermodynamics of Tin Vol. 12. OECD, (2012).