Reduction of SeO42- Anions and Anoxic Formation of Iron(II)−Iron(III) Hydroxy-Selenate Green Rust

Abstract

Iron(II)−iron(III) hydroxy-salts known as green rusts were recently discovered as minerals present in hydromorphic soils and sediments. Due to their high reactivity, they are envisioned as potential reducing agents of a number of pollutants such as nitrate, chromate, or selenate. The interaction of selenate ions with such iron(II)-containing hydroxy compounds was studied by monitoring the oxidation processes of the iron phases with transmission Mössbauer spectroscopy measured at 14 K and by following the evolution of Se(VI) in solution by capillary electrophoresis. This interaction involved the hydroxy-selenate green rust, a compound isomorphous to the hydroxy-sulfate GR(SO42-). Its chemical composition, FeIIyFeIII2(OH)2(y+2)SeO4·8H2O, varied with time since y starts at 5.5 and ends at 4. GR(SeO42-) was obtained from Fe(OH)2 precipitates by simultaneous accumulation of SeO42- anions inside the solid phase and reduction of an equal amount of SeO42- anions to Se(IV) species. These species were found to be less mobile, partially bound to iron compounds and/or forming iron salts. Finally, the hydroxy-selenate GR2(SeO42-) can form without any other oxidizing agent than selenate itself.