Efficient one-step reductive immobilization of Cr(VI) upon citrate-modified Cu-Fe oxides in alkaline circumstances

Treatment of Cr(VI)-contaminated wastewaters by conventional reduction methods has been intensively challenged by alkaline environments. In this study, a novel copper-iron oxide composite (Cit@CuFeO) enriched with oxygen vacancies (OVs) was developed by ball milling with citric acid ligand (Cit). The material demonstrated efficient Cr(VI) removal within a broad pH range (6.0–12.0). Intriguingly, simultaneous and nearly complete in-situ immobilization of Cr(III) occurred, instead of precipitated in strong alkaline circumstances. Characterizations of the Cit@CuFeO revealed that ball milling of Cit could introduce abundant OVs into the oxide structure, serving as pivotal active sites linearly correlated with Cr(VI) capture and removal. These generated OVs could also boost electron conduction in Cit@CuFeO and cause a positive shift in the zeta potential , thereby enabling the adsorption of negative CrO 4 2- at high pH. Furthermore, DFT calculation verified that the surface-attached Cit would significantly decrease the adsorption energy of Cr(VI) at OVs sites, competitively excluding the occupancy of common inorganic anions especially CO 3 2– and thus favoring the Cr(III) immobilization under alkaline conditions. This study has provided a green functional material for the one-step remediation of alkaline Cr(VI) wastewaters without the need for acidification.