Efficient removal of natural organo-chromium(III) through self-circulating decomplex and immobilization with nanoscale zero-valent iron

Organo-chromium(III) complex is one of the chromium contaminant species, which would transform to high-toxic Cr(VI) during migrating in the environment. Such natural organo-chromium(III) (NOCr) is difficult to remove by traditional degradation or precipitation methods, due to its high stability and solubility. Herein, we demonstrated a novel NOCr removing method by transforming it to certain structures similar to Cr-Fe minerals in nature, through a self-circulating decomplex and immobilization mechanism with nano zero-valent iron (nZVI). Taking chromium glycinate (Cr-Gly) as a probe, nZVI showed a high Cr removal efficiency of 99.4% under ambient conditions. The removal process included three stages of adsorption, decomplexation, and re-immobilization. Cr-Gly was first adsorbed on the surface of nZVI by chemisorption of the oxide shell. Then, the adsorbed Cr-Gly was decomplexed and oxidized to Cr(VI) by ·OH and ¹O₂, which were generated from molecular oxygen activated by nZVI. Meanwhile, the released Cr(VI) could be in-situ adsorbed and re-reduced to Cr(III), which was further immobilized in form of Cr-O-Fe complex. As the Cr-O-Fe complexing structure was similar to that of Cr-Fe minerals (such as chromohercynite) in nature, this work explored a novel and efficient NOCr removing method that was potential to weaken chromium pollution in the environment.