New selenium solution speciation method by ion chromatography plus gamma counting and its application to FeS2-controlled reducing conditions

Selenium is a redox sensitive element. In reducing conditions its solubility is controlled by the formation of metallic Se and in the presence of Fe 21 also by the precipitation of FeSe or FeSe2. However very few data concerning this species in geochemical reducing environments is found in literature, particularly due to insufficient measuring methods. The assessment to what extent Se-79 is a critical radionuclide for the geological disposal of High-Level Radioactive Waste, depends on its actual speciation in storage conditions. Therefore a new method based on ion chromatography of radiolabelled Se-75 solutions followed by gamma-ray counting was developed to accurately measure selenium species with different degrees of oxidation (selenate (SeO42-) and selenite (SeO32-)) in solution. This method was then tested in laboratory conditions which mimic the reducing environment in Boom Clay. Different amounts of ground pyrite (< 125 mum) were contacted with Synthetic Boom Clay Water (essentially 10(-2) M NaHCO3) and spiked with different amounts of (SeO32-)-Se-75 and SeO42-. The batches were allowed to equilibrate over different time periods (up to two months) before analysing. The experiments were carried out in an oxygen-depleted glove box (99.6% N-2, 0.4% CO2). The kinetics of the redox reactions in the pyrite systems prevented the complete reduction of selenite (SeO32-) and especially selenate (SeO42-) on a limited time scale, probably due to the limited redox capacity of the studied systems.