Stabilization of chromium(VI) by hydroxysulfate green rust in chromium(VI)-contaminated soils

Chromium (Cr)-contaminated soils pose a great environmental risk, with high solubility and persistent leaching of Cr(VI). In this study, hydroxysulfate green rust (GR_SO4), with the general formula Fe(II)₄Fe(III)₂(OH)₁₂SO₄·8H₂O, was evaluated for its efficiency in Cr(VI) stabilization via Cr(VI) reduction to Cr(III) in four representative Cr(VI)-spiked soils. The initial concentrations of phosphate buffer-extractable Cr(VI) (Cr(VI)_b) in soils 1, 2, 3, and 4 were 382.4, 575.9, 551.3, and 483.7 mg kg^-1, respectively. Reduction of Cr(VI) to Cr(III) by structural Fe(II) (Fe(II)_s) in GR_SO4 in all studied soils was fast, wherein the application of GR_SO4 markedly decreased the amount of Cr(VI)_b at the Cr(VI)_b/Fe(II)_s stoichiometric mole ratio of 0.33. The kinetics of Cr(VI) reduction by GR_SO4 could not be determined as this reaction coincided with the release of Cr(VI) from soil during the experiment. The concentration of Cr(VI)_b decreased, as the Cr(VI)_b/Fe(II)_s ratio decreased from 0.46 to 0.20, generally to below 10 mg kg^-1. Back-transformation of the generated Cr(III) was examined in the presence of manganese oxide birnessite at the birnessite/initial Cr(III) mole ratio of 4.5. The results of batch tests showed that only 5.2% of the initial Cr(III) was converted to Cr(VI) after two months, while under field capacity moisture conditions, less than 0.05% of the initial Cr(III) was oxidized to Cr(VI) after six months. The results illustrated that remediation of Cr(VI)-contaminated soils would be fast, successful, and irreversible with an appropriate quantity of fresh GR_SO4.