Solubility, redox behavior and sorption of Sn(II) and Sn(IV) in cement systems

$^{126}$Sn is a long-lived fission product (t1/2 = 1.1∙106 a) produced in low yield (~ 0.065%) in nuclear reactors. A small inventory of this nuclide is accordingly present in high level radioactive waste (HLW). In addition to 126Sn, the shorter-lived isotopes 119mSn (t1/2 = 0.802 a), 121mSn (t1/2 = 55 a) and 125Sn (t1/2 = 0.0264 a) are also expected in low and intermediate level radioactive waste (L/ILW). Cementitious materials are ubiquitous in underground and near-surface repositories for L/ILW. Hydrated cement is an heterogeneous material mainly consisting of calcium silicate hydrated (C-S-H) phases, portlandite (Ca(OH)2) and calcium aluminate-type phases (AFt, AFm). C-S-H phases have been described as main sink of cationic radionuclides and metal ions [1]. Although tin is primarily expected in the +IV oxidation state, uncertainties on the fG°(Sn4+) and the implications on the E°(Sn4+/Sn2+) are currently motive of debate [2].
Undersaturation solubility experiments with Sn(II) and Sn(IV) were performed within 9 ≤ pHm ≤ 12.8 and 0.1 M ≤ I ≤ 3.5 M in NaCl-NaOH and CaCl2-Ca(OH)2 systems. Batch sorption experiments were conducted with a combination of 112Sn (stable) and 113Sn (t1/2 = 115.09 d) isotopes under Ar atmosphere at T = (22 ± 2) °C. ... mehrC-S-H phases with Ca:Si ratios of 0.6, 1.07, 1.25 and 1.5 were considered as sorbing materials. Tin concentration was varied as∙10-6 M ≤ [Sn]0 ≤∙10-4 M for Sn(IV) and 10-6 M ≤ [Sn]0 ≤∙10-3 M for Sn(II)), with solid-to-liquid ratios of 5 and 2 g/L for Sn(IV) and Sn(II), respectively.
Solubility data determined for Sn(II) are in line with data available in the literature for SnO(cr) and Sn6O4(OH)4(cr), involving the formation of Sn(OH)3– at pHm > 9. Solubility data determined for Sn(IV) are in moderate agreement with the recent experimental work by Rai et al. and the calculated solubility limit of SnO2(cr) [2]. Sorption experiments confirmed a fast and strong sorption of Sn(IV) on C-S-H phases, in agreement with previous studies with M(IV) metal ions, e.g., Th(IV) [3]. Sorption experiments under strongly reducing conditions resulted also in high distribution ratios, though manifestly lower than those determined for the Sn(IV) system. These observations can be rationalized assuming the predominance of Sn(II) under reducing conditions, but considering that this metal ion is characterized by a much stronger hydrolysis than other M(II) ions. Moreover, the results obtained in this work hint that the E°(Sn4+/Sn2+) selected in the NEA-TDB [4] might be underestimated, and thus that Sn(II) may play a relevant role in the chemistry of Sn under repository conditions. XAFS measurements planned for this system will be discussed in this contribution, and are expected to provide additional insights for the mechanistic understanding of Sn sorption and redox behaviour under alkaline reducing systems.

References
[1] M. Ochs et al., Springer, 301pp, 2016.
[2] D. Rai, Journal of Solution Chemistry, 2022, 51, 1169-1186.
[3] J. Tits, E. Wieland, PSI Bericht 18-02, 2018.
[4] H. Gamsjäger et al., NEA-TDB Publications Vol. 12, OECD, 2012.