Nanocomposite Pyrite-Greigite Reactivity toward Se(IV)/Se(VI)

A nanopyrite/greigite composite was synthesized by reacting FeCl3 and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30-50 nm sized particles containing 28% of greigite (Fe2+Fe23+S4) and 72% pyrite (FeS2). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite greigite solid phase at different pH-values and with or without the addition of extra Fe2+. XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe2) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se-0, the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system.