In situ reactivity and sorption of selenium in the Opalinus Clay

To study the fate of selenium oxyanions in the Opalinus Clay and the effect of a selenium reduc-ing microbial community, Se(VI) and Se(IV) reactivity was assessed in batch tests in the presence of Opalinus Clay, both under biotic and abiotic conditions. Furthermore, in situ Se reactivity and diffusion were investigated in a borehole in the Opalinus Clay, with the same microbial commu-nity as for the batch tests. PHREEQC was used to simulate Se(VI) reduction, Se(IV) sorption and solubility, and diffusion processes in the tests. Results show that under abiotic conditions, no sig-nificant reaction with Se(VI) occurs, while Se(IV) sorbs either directly or after initial reduction. Under biotic conditions, both Se(VI) and Se(IV) reduction is observed and sorption of reduced Se species to the clay occurs. Similarly, the Se(VI) injected in situ not only diffused from the borehole, but was also reduced, thereby producing reduced Se species, which can either sorb directly to the clay or undergo additional reduction prior to sorption. Based on this study, microbial activity is required to transform Se(VI) into more reduced and sorbing Se species. In the absence of Se re-ducing microbes, nonreactive transport of Se(VI) is expected in Opalinus Clay.