Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Selenium immobilization is of high importance due to its high toxicity and the mobility of the soluble oxyanion selenite. Fe-bearing clay minerals are major redox-active ingredients of Earth’s critical zone and constitute an important component of the barrier in (radioactive and other) waste repositories, having the potential to reduce selenite to insoluble forms under anaerobic condition. Our research focuses on a systematical investigation of the kinetics of selenite sorption and reduction, considering variations in structural Fe(II) content of clay minerals, redox potential and pH. Combining batch experiments and X-ray absorption fine structure (XAFS) spectroscopy, we examine how these factors influence the selenite reduction process and kinetics by identifying the reduced Se species over time. We observed that selenite adsorption and reduction are kinetically controlled. Selenite reduction is redox potential dependent and presents a slower reduction rate at pH 7 compared to pH 5. Concerning Fe(II) content in clay minerals, higher amount of structural Fe(II) results in faster selenite reduction kinetics, and only elemental Se(0) was observed as reduced species, while the absence of soluble Fe prevented precipitation of Fe selenide. Moreover, we observed a transformation of amorphous red Se(0) to metallic trigonal grey Se(0) in the reduction species as the reaction time increases. This insight into the selenite reduction mechanism provides valuable information for the development of effective approaches for selenite immobilization.