Phase-Dependent Effects of Al Substitution on Photochemical Mn(II) Oxidation by Iron Oxides

Abstract

Photocatalytic Mn2+(aq) oxidation on Fe oxide surfaces involves coupled interfacial reactions and photochemical processes and contributes to Fe–Mn redox cycling in Earth’s surface and near-surface environments. Al substitution is ubiquitous in natural Fe oxides, yet its effects on photochemical Mn2+(aq) oxidation and subsequent Mn(III/IV) oxide formation remain poorly understood. Here, we combined photochemical experiments with synchrotron-based characterization of Al-substituted goethite and hematite (0–5 mol % Al) to investigate Fe oxide-catalyzed Mn2+(aq) oxidation. Mn2+(aq) oxidation kinetics revealed contrasting effects of Al substitution between the two Fe oxide phases, with increasing Al substitution progressively suppressing Mn2+(aq) oxidation on goethite while enhancing it on hematite. At these relatively low Al substitution levels, changes in structure and band gap were minor, whereas surface charge exhibited contrasting trends between the two Fe oxide phases. These results suggest that phase-dependent mineral-ion interactions at Fe oxide surfaces may play a more important role than changes in bulk structure or intrinsic optical properties in regulating photocatalytic Mn2+(aq) oxidation at low impurity levels. Al substitution also suppressed Mn(III/IV) oxide formation on goethite, whereas Mn(III/IV) oxide products persisted on hematite but exhibited markedly altered diffraction features. Together, these observations demonstrate that Al substitution regulates both Mn2+(aq) oxidation and Mn(III/IV) oxide formation in a Fe oxide phase-dependent manner. Using Al as a representative impurity, this study provides a framework for understanding how common impurities in natural Fe oxides regulate photochemical Mn oxidation and, in turn, influence coupled Fe–Mn geochemical cycling.