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

Aqueous GR(SO42-) suspensions were prepared by mixing Fe(II) - Fe(III) based solutions with NaOH solutions in the presence of both Cl- and SO42- ions. So as to study the effects of Al(III) and Cr(III) on the formation and oxidation of GR(SO42-), 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 (SO42-) 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(SO42-) aqueous suspensions at it favors strongly the formation of goethite alpha-FeOOH (vs lepidocrocite gamma-FeOOH).