Enhanced oxidation of Cr(III) with the mineral transformation of CrFe(OH) in the presence of Mn(II): overlooked process during the reoccurrence of Cr(VI) in reduced chromite ore processing residue.

The reoccurrence of Cr(VI) in reduced chromium ore processing residue (rCOPR) causes secondary pollution after remediation, with Mn(II)-induced catalytic oxidation identified as one of its key contributors. The mineral transformation of Cr-Fe coprecipitate (CrFe(OH) dominant phase in rCOPR) tends to form goethite along with Cr(III) in alkaline environments. However, the effect of this critical process on Cr(VI) reoccurrence remains unknown and requires further investigation. Our results showed that after the mineral transformation, approximately 10 times higher concentration of Cr(VI) (2261.67 ± 108.13 μg L) produced within 24 h than pre-transformation levels (212.92 ± 22.71 μg L). Mn (oxyhydr)oxides (MnOs) produced from Mn(II) oxidation served as the direct oxidant of Cr(III). The alteration in Cr(VI) generation kinetics after the mineral transformation indicated distinct speciation patterns and interactions of Cr(III) and MnOs in this process. Three predominant crystalline phases in MnOs, hausmannite (MnO), feitknechtite (β-MnOOH), and manganite (γ-MnOOH) were identified through XPS, XRD, SEM and TEM-EDS. Theoretical calculations revealed the hierarchical adsorption ability for Cr(III): ferrihydrite (-9.69 eV) > MnOs (-9.50, -9.30 eV) > goethite (-9.27 eV). The synergistic interplay between MnOs' oxidation capacity and Cr(III) accessibility governed the release of Cr(VI). Our findings elucidated the kinetic process and underlying mechanisms of Cr(VI) generation induced by Mn(II) oxidation with the mineral transformation of Cr-Fe coprecipitate in Cr contaminated environments. These new insights support pollution control of Cr(VI) reoccurrence in rCOPR and may guide future research on promoting the long-term stability and safety of rCOPR.