Selenite reduction in boom clay: effect of FeS2, clay minerals and dissolved organic matter

In Belgium, the Boom clay layer is considered as the candidate host rock for the disposal of high-level radioactive waste (HLRW). For this disposal, Selenium 79 is considered to be a critical radionuclide and responsible for the highest dose to man over a period of tens of thousands of years. The behaviour and reactivity of Se thereby depend on its speciation and on its complex biogeochemical transformations. 79Se is thought to occur in, and be released from the solid waste matrix in a variety of redox states, including Se oxyanions such as SeO32- or SeO42-. The composition of the solid and liquid phases of Boom clay was published before. In this paper, the reduction of Se oxyanions was investigated by adding appropriate amounts of SeO32- in over-saturation with respect to the proclaimed thermodynamical solubility of reduced Se solid phases (SeO, FeSe, FeSe2), to a number of systems which represent Boom clay geochemical conditions. The range of systems is chosen in order to incorporate in an increasing way the different Se competing organic and inorganic phases present in the Boom clay matrix. (authors)