Selenium uptake onto natural pyrite

Curti, E.*; Aimoz, L.*; Kitamura, Akira  

Se is a major dose-determining redox-sensitive nuclide in safety analysis of radioactive waste disposal sites. In aqueous solutions, selenium forms soluble anionic species (SeO and SeO) that hardly sorb on negatively charged surfaces of common host-rock minerals. However, Se is known to have a strong affinity with sulphides and interacts with pyrite, a common minor mineral of argillaceous rocks being considered as host rocks for radioactive waste repositories in Switzerland etc. In this study, we present micro- and bulk X-ray spectroscopy data (-XRF, -XANES, and EXAFS) showing that, under nearly anoxic conditions, dissolved SeO and SeO sorb directly onto the pyrite surface and are subsequently reduced to Se with increasing ageing time (up to 8 months). These results suggest that the mobility of Se released from radioactive waste could efficiently be reduced through uptake on the pyrite surface followed by transformation into a sparingly soluble reduced form.