One-time removal of V(V) and Cr(VI) from aqueous solution of different pH by sulphate green rust: The overlooked adsorption and reactivity of Fe (III)-Cr(III) oxides

Efficient and economical reduction and elimination of co-pollution of vanadium (V) and chromium (Cr) from water sources is a major challenge. In this study, sulphate green rust (GR SO4 ) was successfully prepared by co-precipitation method and used for the removal of V(V) and Cr(VI). The findings from batch adsorption experiments indicated that the adsorption capacity of GR SO4 for V(V) and Cr(VI) individually was 30.66 mg⋅g −1 and 39.48 mg⋅g −1 , respectively, with equilibrium reached in 25 min and 10 min. Intriguingly, in a binary system with both V(V) and Cr(VI) present, the time to reach adsorption equilibrium for V(V) removal by GR SO4 was 30 % shorter comparison to the single system under identical conditions. By analyzing the surface characteristics, crystal structure and elemental valence changes of GR SO4 minerals before and after the reaction, it can be obtained that: i) Direct adsorption mainly drives the removal of V(V) by GR SO4 , whereas the removal of Cr(VI) is preceded by reduction and followed by adsorption. ii) In a binary coexistence system, the formation of Fe(III)-Cr(III) oxide (GR-Cr(III)) during the removal of Cr(VI) by GR SO4 formed during the removal of Cr(VI) by GR SO4 creates additional adsorption sites for V(V), which shortens the equilibrium time of the V(V) removal reaction by GR SO4 and reduces the amount of V(V) leached. These findings are of great practical significance for understanding the mechanism and practical application of GR SO4 in the synergistic removal of V(V) and Cr(VI) heavy metals and provide valuable insights into their geophysical-chemical cycling.