Different Pathways for Cr(III) Oxidation: Implications for Cr(VI) Reoccurrence in Reduced Chromite Ore Processing Residue

Hexavalent chromium contamination is a global environmental issue and usually reoccurs in alkaline reduced chromite ore processing residues (rCOPR). The oxidation of Cr­(III) solids in rCOPR is one possible cause but as yet little studied. Herein, we investigated the oxidation of Cr­(OH)₃, a typical species of Cr­(III) in rCOPR, at alkaline pH (9–11) with δ-MnO₂ under oxic/anoxic conditions. Results revealed three pathways for Cr­(III) oxidation under oxic conditions: (1) oxidation by oxygen, (2) oxidation by δ-MnO₂, and (3) catalytic oxidation by Mn­(II). Oxidations in the latter two were efficient, and oxidation via Pathway 3 was continuous and increased dramatically with increasing pH. XANES data indicated feitknechtite (β-MnOOH) and hausmannite (Mn₃O₄) were the reduction products and catalytic substances. Additionally, a kinetic model was established to describe the relative contributions of each pathway at a specific time. The simulation outcomes showed that Cr­(VI) was mainly formed via Pathway 2 (>51%) over a short time frame (10 days), whereas in a longer-term (365 days), Pathway 3 predominated the oxidation (>78%) with an increasing proportion over time. These results suggest Cr­(III) solids can be oxidized under alkaline oxic conditions even with a small amount of manganese oxides, providing new perspectives on Cr­(VI) reoccurrence in rCOPR and emphasizing the environmental risks of Cr­(III) solids in alkaline environments.