Role of Al(III) and Cr(III) on the formation and oxidation of the Fe(II-III) hydroxysulfate Green Rust

Abstract Aqueous GR(SO 4 2− ) suspensions were prepared by mixing Fe(II) − Fe(III) based solutions with NaOH solutions in the presence of both Cl − and SO 4 2− ions. So as to study the effects of Al(III) and Cr(III) on the formation and oxidation of GR(SO 4 2− ), 5 at.% of Fe were replaced by Cr(III), Al(III) or Cr(III) + Al(III). The presence of Al(III) and Cr(III) during the precipitation process induced a decrease of crystallinity of the obtained solid phase. One week of ageing in suspension at room temperature led in any case to a significant increase of crystallinity of GR(SO 4 2− ) but Al(III) and Cr(III) induced the formation of a small amount of magnetite. Al(III) was observed to influence significantly the oxidation process of aerated GR(SO 4 2− ) aqueous suspensions at it favors strongly the formation of goethite α-FeOOH (vs lepidocrocite γ-FeOOH).