Complexation of Sn with Boom Clay natural organic matter under nuclear waste repository conditions

摘要

Sn-126 is a long-lived fission product and as such a waste relevant radioisotope. Understanding its fate under deepgeological repository conditions is a requirement for a complete safety assessment. Yet, due to a limited knowledge of its hydrogeochemical properties, the solubility and speciation of tin in a nuclear waste repository remain uncertain. In Belgium, Boom Clay (BC) is investigated as a potential host rock for high-level and inter mediate level waste disposal. It contains a significant amount of dissolved organic matter (DOM), which was shown to strongly influence the mobility of several radionuclides and notably of tetravalent actinides. By chemical analogy, the speciation, sorption and mobility of Sn(IV) in BC is also assumed to be controlled by its binding to DOM. However, the Sn-DOM complexation has never been quantified under relevant repository conditions. In this work, the complexation of Sn with BC DOM was investigated under present-day BC conditions and in synthetic seawater as representative of more saline conditions. The ultrafiltration technique was used to quantify the amount of Sn bound to DOM. The two investigated repository conditions yielded nearly identical binding constants, evidencing a low influence of salinity on the level of binding of Sn to BC DOM. BC DOM forms strong complexes with Sn that would significantly increase Sn solubility in BC pore water. The investigated range of Sn and DOM concentrations (DOMtot/Sn-tot = 2.8 x 10(2) to 5.7 x 10(3)) also showed that the binding constants are dependent on the metal loading (5.0 < logK((Sn-DOM)) < 6.0). A two-site Langmuir isotherm was used to describe this dependency and highlighted the binding of Sn on both strong and weak sites present on DOM. Overall, the experimental data show that the binding of Sn with BC DOM controls the Sn speciation under BC repository conditions, which reveals the importance of taking it into account in safety assessment calculations.