Exfoliative oxidation of CrxFe1-x(OH)3 by H2O2 increases the Cr(VI) rebound risk in alkaline Cr-bearing residues and soils.

Chromite ore processing residue (COPR) remediation reduces Cr(VI) to Cr(III) precipitates like CrxFe1-x(OH)3, but reoxidation causes rebound. We reassess Cr(VI) release from Cr(OH)3 and Cr0.5Fe0.5(OH)3 with H2O2 using batch experiments, characterizations and DFT calculations. Relative to Cr(OH)3, Cr0.5Fe0.5(OH)3 exhibits anomalously high Cr(VI) release and an abrupt pH-dependent transition between pH 10 and 11. The rate constant at pH 11 is 6.7 × that at pH 10, with no further increase to pH 12. Reactive intermediates (ROS, Fe(II), and high-valent iron species) are not kinetically controlling. Instead, Fe-Cr cluster formation accompanies accelerated Cr(VI) release, as indicated by ∼273 nm absorbance, ∼1.3 nm ultrasmall species (DLS), and an m/z 226.9 oxygen-bridged Fe-Cr dimer. These clusters are stable (log K = 6.28) and resist H2O2 oxidation in homogeneous solution. These results support an exfoliation-coupled, surface-controlled oxidation pathway: HO2- adsorption was proposed to weaken linkages between peripheral [MeO6] and central [CrO4] units within surface δ-Keggin-like structures of Cr0.5Fe0.5(OH)3, resulting in concurrent Fe-Cr cluster exfoliation and CrO42- release. Lower Cr/Fe, higher H2O2, and dissolved oxygen promote Cr(VI) release, whereas HPO42-, Mn2+, DOM, and clay minerals suppress it. This reveals an overlooked Fe-mediated pathway for Cr(VI) rebound, indicating that alkaline, Fe-rich, Mn-limited conditions require particular attention.