Hydro geochemistry of Sn and sorption on bentonite

Amongst the radionuclides present in radioactive wastes Sn is an element which, under its isotopic form 126Sn, is susceptible to present a long term risk because of its long half life (ca. 100 000 years). For a realistic risk assessment of nuclear waste storage in geological formations it is therefore essential to fully understand its expected behavior, under conditions of near field and far field containment. Up to day the hydro-geochemistry of Sn is poorly known, especially under conditions of low solubility (i.e., neutral pH and low chloride content). The chemistry of tin and the thermodynamic constants associated to relevant hydrolysis, complexation, redox and precipitation reactions were compiled by Seby et al. (2001). With very few exceptions, the known hydro-geochemistry of tin is based on conditions of simple laboratory experiments involving no adsorbing solid. Under the form Sn(0), precipitation is expected to control the transport and associated retardation of tin. As Sn(II) and especially Sn(IV), adsorption is expected to be the retarding mechanism. (authors)