Spherical Cu2O-Fe3O4@chitosan bifunctional catalyst for coupled Cr-organic complex oxidation and Cr(VI) capture-reduction

Unlike the removal of chromium (Cr) ions, the efficient treatment of Cr complexes with organic ligands is a challenging issue in wastewater treatment. The generation and accumulation of highly toxic Cr(VI) species should be minimized. In this study, well-dispersed Cu2O and Fe3O4 nanoparticles inlaid in chitosan beads were prepared. This catalyst demonstrated dual functions, i.e., catalytic oxidation of Cr(III)-organic complexes and efficient inhibition of Cr(VI) accumulation in the presence of H2O2. More than 85% of the Cr(III)-organic complexes were removed in a wide pH range and a negligible amount of Cr(VI) remained in solution in the presence of 20 mmol/L H2O2. The synergistic effect of Cu2O, Fe3O4, and the chitosan matrix was the key factor for the ligand disruption and Cr removal. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis were used to determine the underlying mechanism. The results showed that the generated Cu+ species of Cu2O transferred electrons to accelerate the Fe(III)/Fe(II) cycle during the Fenton reaction, which favored the production of center dot OH radicals. The generated Cr(VI) in the Fenton-like reaction was efficiently adsorbed by chitosan and reduced to Cr(III) by chitosan and Cu2O. The reduced Cr(III) was then incorporated into the surface structure of Fe3O4 and generated a stable complex oxides of Fe and Cr(III). Overall, this paper demonstrated a new method for the treatment of Cr(III)-organic complexes from wastewater.