Jacqueline Vancluysen

Reduction of Se(IV) in Boom Clay: XAS solid phase speciation.

The geochemical fate of selenium is of key importance for todays society due to its role as a highly toxic essential micronutrient and as a significant component of high level radioactive waste (HLRW) originating from the operation of nuclear reactors. Understanding and prediction of the long-term behavior of Se in natural environments requires identification of the in situ speciation of selenium. This article describes an XAS-based investigation into the solid phase speciation of Se upon interaction of Se(IV) with Boom Clay, a reducing, complex sediment selected as model host rock for clay-based deep geological disposal of HLRW in Belgium and Europe. Using a combination of long-term batch sorption experiments, linear combination XANES analysis and ITFA-based EXAFS analysis allowed for the first time to identify Se0 as the dominant solid phase speciation of Se in Boom Clay systems equilibrated with Se(IV).

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

Summary Selenium is a redox sensitive element. In reducing conditions its solubility is controlled by the formation of metallic Se and in the presence of Fe2+ 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 79Se 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 75Se 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 µm) were contacted with Synthetic Boom Clay Water (essentially 10−2 M NaHCO3) and spiked with different amounts of 75SeO32− 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% N2, 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.

Cation exchange properties of zeolites in hyper alkaline aqueous media.

Construction of multibarrier concrete based waste disposal sites and management of alkaline mine drainage water requires cation exchangers combining excellent sorption properties with a high stability and predictable performance in hyper alkaline media. Though highly selective organic cation exchange resins have been developed for most pollutants, they can serve as a growth medium for bacterial proliferation, impairing their long-term stability and introducing unpredictable parameters into the evolution of the system. Zeolites represent a family of inorganic cation exchangers, which naturally occur in hyper alkaline conditions and cannot serve as an electron donor or carbon source for microbial proliferation. Despite their successful application as industrial cation exchangers under near neutral conditions, their performance in hyper alkaline, saline water remains highly undocumented. Using Cs(+) as a benchmark element, this study aims to assess the long-term cation exchange performance of zeolites in concrete derived aqueous solutions. Comparison of their exchange properties in alkaline media with data obtained in near neutral solutions demonstrated that the cation exchange selectivity remains unaffected by the increased hydroxyl concentration; the cation exchange capacity did however show an unexpected increase in hyper alkaline media.