Intensified Production of Hydroxyl Radical from Iron–Manganese Synergistic Coupling during Soil Oxygenation: Driven by Iron-Induced Manganese Redox Cycling and Peroxo Complex Formation

Iron (Fe)-driven biogeochemical processes are central to hydroxyl radical (•OH) production under redox fluctuations in soil. Although soil Fe and manganese (Mn) frequently coexist, their coupled effects on •OH generation remain poorly understood. This study revealed previously unrecognized synergistic interactions between Fe and Mn that enhanced •OH production during soil oxygenation, as demonstrated through soil microcosm incubation and mineral model experiments. In Fe- and Mn-enriched soils, substantial •OH (19.8-325 μM) accumulated. Pearson correlation analysis and scavenging experiments identified soil Fe(II) and Mn(II)/Mn(III) as concurrent electron donors. Mineral model systems containing both reduced Fe and Mn oxides further confirmed Fe-Mn synergism, where •OH accumulation surpassed theoretical additive values by 61.8%, and electron utilization efficiency increased nearly 3-fold. Wet chemical analysis, electron microscopy, and in situ Fourier transform infrared spectroscopy elucidated that Fe(II)-driven regeneration of Mn(II)/Mn(III) facilitated O2•- production, while atomic-scale Fe and Mn proximity induced the formation of peroxo bridging complex─both mechanisms synergistically enhancing •OH accumulation. The generated •OH further mediated oxidative transformation of soil organic carbon, copper sulfide, and organically bound copper. These findings unveil a novel pathway for •OH production under redox oscillations and underscore the overlooked role of Fe-Mn coupling in soil biogeochemical cycles.