Reduction of SeO42- anions and anoxic formation of iron(II)-iron(III) hydroxy selenate green rust

Iron(II)-iron(III) hydroxy-salts known as green rusts were recently discove red as minerals present in hydromorphic soils and sediments. Due to their h igh reactivity, they are envisioned as potential reducing agents of a numbe r of pollutants such as nitrate, chromate, or selenate. The interaction of selenate ions with such iron(II)-containing hydroxy compounds was studied b y monitoring the oxidation processes of the iron phases with transmission M ossbauer spectroscopy measured at 14 K and by following the evolution of Se (VI) in solution by capillary electrophoresis. This interaction involved th e hydroxy-selenate green rust, a compound isomorphous to the hydroxy-sulfat e GR(SO42-). Its chemical composition, Fe(II)yFe(2)(III)(OH)(2(y+2))SeO4. 8 H(2)O, 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, parti ally bound to iron compounds and/or forming iron salts. Finally, the hydrox y-selenate GR2(SeO42-) can form without any other oxidizing agent than sele nate itself.