The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Se_aq) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Se_aq followed by slower selenite reduction. Eventually, almost all solid-bound Se^IV became reduced to Se⁰ in all experiments. The progress of Se_aq removal and Se^IV reduction to Se⁰ could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se⁰ is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by Fe^II structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks.

Supplementary Material: X-ray diffractograms of separated clay-size, silt-size and total BC material are available as Supplementary Material. Also provided are particle size distributions of all materials and extra information on XANES and EXAFS results, Se concentrations through time for experiments with standard clay minerals and figures of the sensitivity analysis of the kinetic model. The information is available at https://doi.org/10.6084/m9.figshare.c.4363826