Cr( <scp>III</scp> ) Oxidation/Fixation by Fe‐Mn Oxides From Mn( <scp>II</scp> ) Oxidation on Al‐Substituted Ferrihydrite at Neutral/Alkaline <scp>pH</scp>

ABSTRACT The mobility of chromium (Cr) in soils is sensitive to redox conditions, and understanding the processes underlying this sensitivity is important for managing soil samples contaminated with Cr. A key but poorly understood process is the oxidation of reduced manganese (Mn) oxides on which Cr is sorbed, catalyzed by ferrihydrite (poorly crystalline hydrous ferric (Fe(III)) oxyhydroxide). We used kinetic and spectral analyses to investigate the effects of pH and ferrihydrite composition on the mineral characteristics of resulting Fe‐Mn binary oxides from Mn(II) oxidation and their subsequent oxidation and immobilization on Cr. Results showed that increased Al(III) content in ferrihydrite inhibited Mn(II) oxidation on its surfaces, leading to both decreased Cr(III) oxidation rates and reduced Cr(VI) immobilization. This result was attributed to Al(III) substitution, which inhibits the electrochemical catalytic activity of ferrihydrite, thereby suppressing Mn(II) oxidation and thus Mn(III/IV) oxide formation. Furthermore, higher pH levels (e.g., pH 8.0) promoted the more formation of Mn(III/IV) oxides, enhancing Cr(III) oxidation but decreasing Cr(VI) stabilization. This is because Mn oxides possibly block the Cr(VI) adsorption sites. Compared to Mn(IV) oxides, the Mn(III) oxides generated under neutral or alkaline conditions exhibited higher Cr(III) oxidation efficiency. These findings clarify Fe–Mn–Cr interactions in red soils, informing strategies for Cr pollution mitigation and soil remediation.