pH-dependence of selenate removal from liquid phase by reductive Fe(II)-Fe(III) hydroxysulfate compound, green rust

Removal of selenate ion from the liquid phase by hydroxysulfate green rust (GR_SO4) was investigated in the pH range from 7.5 to 10.0. Batch tests showed that the total selenium concentration decreased more rapidly with increasing pH. However, the amount of selenium removed from the liquid phase increased as the pH decreased. X-ray absorption spectrometric analysis demonstrated that the Se(VI) in the selenate ion was reduced to elemental selenium Se(0) at pH 9.0, whereas the existence of small amount of intermediate Se(IV) was detected at pH 7.5. Comparing the mass balance of the amount of consumed ferrous iron in GR_SO4, [ΔFe(II)], and the amount of Se(VI) removed from the liquid phase, [ΔSe(VI)], [ΔFe(II)] is approximately six times larger than [ΔSe(VI)] in mol units. This is also indirect but convincing evidence that the oxidation of Fe(II) in GR_SO4 leads to the simultaneous reduction of Se(VI) to Se(0). Powder X-ray diffraction analysis showed that the end product of GR_SO4 depends on the pH: magnetite for pH > 9.0, goethite for pH < 8.0, and their mixture at pH 8.5. These results indicate that the solution pH has a significant effect on the reaction path of selenate removal by GR_SO4.