Immobilization, enrichment and recycling of Cr(VI) from wastewater using a red mud/carbon material to produce the valuable chromite (FeCr2O4)

It is a long-term goal to develop an inexpensive, eco-friendly, and effective approach for the purification of wastewater containing heavy metal elements such as Cr(VI). In this study, a red mud (RM)/carbon material with dispersed nano zero-valent iron (nZVI) is fabricated to collect, immobilize, enrich, and recycle Cr(VI) from wastewater by repeatedly using the spent sample after carbothermal regeneration. The regenerated RM/carbon sample maintains an excellent removal efficiency of Cr(VI), and the collected Cr species can be firmly immobilized and gradually accumulated through forming highly stable chromite (FeCr2O4) with the assistance of carbothermal treatment. A detailed study of structure–function relationships reveals that Cr(VI) in wastewater is first reduced to Cr(III) by nZVI, and is then collected on RM/carbon in the form of Cr-Fe hydroxide species, which is converted to a stable FeCr2O4 phase immobilized on the regenerated RM/carbon by carbothermal treatment. Moreover, the repeated use/regeneration process facilitates the gradual growth, separation, and exposure of nZVI particles on the external surface of the RM/carbon, which improves its reaction activity with H+ and Cr(VI). The elevated activity of nZVI as well as the high stability of the accumulated FeCr2O4 account for the excellent cycle efficiency of Cr(VI) removal. Nearly all of the nZVI (97.8%) in RM/carbon can be utilized to produce FeCr2O4 with more than 45 wt% Cr2O3 content in the final calcined ash, which can be recycled as a valuable substitute for natural chromite ore in the steel industry with no secondary pollution. The demonstrated superiority of the material and process offer promising technological, economic and environmental benefits for the simultaneous utilization of chromium wastewater and RM solid wastes.