Redox regulation of layered Fe-Cr hydroxides during Cr(VI) reduction by sulfate green rust: The overlooked electron conductivity and reactivity of Fe (III)-Cr(III) residues

In this study, unique layered Fe(III)-Cr(III) oxides were formed during the Cr(VI) removal by sulfate green rust (GRSO4). Solid state characterization confirmed that the hexagonal plate and layered structure of GRSO4 remained after fast reduction of Cr(VI). Then, sodium dithionite was further employed to reduce the Fe(III) to Fe (II) in these layered Fe(III)-Cr(III) oxides, enabling stable Fe(II)/Fe(III) cycle in the hexagonal plates. As the Fe(II) content increased in the layered Fe(III)-Cr(III) oxides, there was no significant Fe(II) and Cr(III) release into the solution and the GR-like plate remained intact, indicating that this hexagonal layer Fe-Cr oxides has stable structure during the reduction of Fe(III). Interestingly, better crystallinity was observed at higher Fe(III) reduction rate, and the layered metal oxide structure was confirmed with a basal spacing of 0.935 nm when similar to 100 % of Fe(III) was reduced to Fe(II). Multiple treatment cycles experiment demonstrated that Cr(III) loading on the layered Fe(III)-Cr(III) oxides could be increased in the following 6 runs of Fe(II)/Fe(III) redox cycle. This study provides new evidence to confirm the electron conductivity and redox reactivity of the Fe(III)-Cr(III) residues under ambient condition, which might help to formulate novel strategies for elimination of Cr(VI) or related heavy metals.