Citric ligand manipulated efficient spatially-separated reduction and immobilization of Cr(VI) upon electron-rich copper-iron oxides

Abstract

In-situ removal of Cr(VI) by metallic (oxides) materials is generally limited by rapid loss of surface active sites, especially in neutral water/wastewater circumstances co-existing organic ligands impurity. This study has firstly demonstrated that synthesized magnetic electron-rich copper-iron oxides composites (Cu2O-Fe3O4) could successfully achieve amazingly efficient reductive immobilization of Cr(VI) induced by common citric ligand (Cit). As Cr(VI) reduced heterogeneously on the composites, immobilization of the generated Cr(III) was almost synchronous via a “spatially-separated” way and correlated linearly with simultaneous dissolved Cu. DFT calculations indicated more appropriate adsorption energy of Cit in realizing its preferential coordination on Fe3O4component surface and Cr(VI) adsorption on Cu2Ocomponent, as compared to oxalic, tartaric and EDTA ligands. The complexed Cit (Cit≡Fe3O4component) would promote internal electron transfer from Fe3O4component to Cu2Ocomponent then adsorbed Cr(VI), causing a highly improved electron efficiency of 72.25%. Most importantly, acting as a regulator of the continuously synchronous reduction and immobilization, Cit≡Fe3O4component would tendentiously chelate newborn Cu(II) at Cu2Ocomponent surface to release bulk Cu2+-Cit complexes, refreshing the reduction sites as well as favoring the Cr(III) immobilization of great stability at the exposed sites of Fe3O4component. This study has revealed that specific impurity ligands can stimulate and tune effective reductive immobilization of heavy metal oxyanions by electron-rich materials under near-neutral conditions, providing a new method towards the efficient treatment of Cr(VI) contaminated wastewaters.