Cellulose Mediated Reduction and Immobilization of Cr(VI) in Chromite Ore Processing Residue

It is a great challenge to find an effective method for the treatment of chromite ore processing residue (COPR), due to the highly toxic and mobile characteristic of Cr(VI) in the sludge. This work reported a facile strategy to thoroughly reduce and immobilize Cr(VI) that was encapsulated in COPR by biomass-assistant hydrothermal treatment. After hydrothermal treatment at 160 °C for 180 min, the leaching of Cr(VI) in COPR decreased from 138.6 mg/L to 2.31 mg/L, well below the disposal standard limit (5 mg/L). It was found that in-situ produced volatile synthesis gas (H₂, CO and CH₄) by cellulose under hydrothermal condition, was responsible for Cr(VI) reduction. The reduction kinetics were temperature-dependent and the rate constants increased from 7.8 × 10^-3 min^-1 at 120 °C to 77.9 × 10^-3 min^-1 at 180 °C. Further simulation experiments revealed that (i) Fe-hydrotalcite in COPR acted as the catalyst for the decomposition of cellulose, and (ii) cellulose can hydrothermally produce reductive gas with a high efficiency, where 0.1 g of cellulose can realize the reduction and immobilization of Cr(VI) equivalent to 14 g of COPR by 14 cycles of treatment. This study provided a promising strategy for one-step remediation of COPR by the coupled reduction-stabilization process.