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)_(4) Fe(Ⅲ)_(2)(OH)_(12) SO_(4)·8 H_(2) O, 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)bat 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)bdecreased, 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_(SO_(4)).