Influence of structural Fe content in clay minerals on selenite redox reactions: Kinetics and structural transformations

Abstract

Redox reactions control the environmental fate of selenium, an element of concern due to its small gap between beneficial and detrimental effects on human health and due to the longevity of the radionuclide 79Se produced in nuclear reactors. 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. Here we systematically investigate selenite (Se(IV)O32−) sorption and reduction by Fe-bearing clay minerals of the smectite group, using batch experiments and X-ray absorption fine structure (XAFS) spectroscopy to elucidate reaction mechanisms and kinetics up to 720 or 3600 h. We found a linear relationship between structural Fe(II) content and selenite reduction rate. Selenite reduction depends also on redox potential and pH, with pH 7 showing a slower reduction rate than at pH 5. Selenite first sorbs to clay edge sites by forming an inner-sphere sorption complex, before being reduced to Se(0). The first redox product was amorphous (red) Se(0), which gradually transformed into trigonal (grey) Se(0), indicative of a kinetically hindered conversion of an amorphous (or less crystalline) solid phase into the thermodynamically stable and more crystalline form of the solid. Despite an Fe2+aq concentration of up to 6·10−4 M in the clay suspension, and their thermodynamic prediction for the clays at lower Eh, we did not observe formation of iron selenides. These insights into the selenite reduction mechanisms by Fe-bearing clays provides valuable information for the development of effective approaches for selenite immobilization.