New Pathway Mediated by Humic Acid Facilitates Long-Distance Hydroxyl Radical Production during Crystalline Iron Mineral Oxygenation

Hydroxyl radicals (^•OH) are extensively produced from crystalline iron minerals under redox oscillation, during which their oxidizing impact on pollutant dynamics is often limited by low transportation. While natural organic matter exists as a ubiquitous redox mediator, how it regulates the distance scale of ^•OH production in a heterogeneous system remains poorly understood. This study, for the first time, reports an unrecognized route mediated by humic acid (HA) in extending the spatial range of ^•OH from oxygenation of reduced hematite (rHem). The results showed that HA facilitated the spatial redistribution of ^•OH from the surface to the solution, posing inverse impacts on oxidation efficacies of pollutants with varying surface accessibilities. It was difficult to explain the enhanced free ^•OH production using electron shuttling by quinones or dissolved Fe–HA complexes. Comprehensive evidence demonstrated that adsorbed HA preferentially consumed surface-bound ^•OH to trigger a solid-to-liquid propagation of carbon-centered radicals (CCR^•), dominantly promoting formation of secondary free ^•OH far from the surface. Reactive-transport simulation and in situ fluorescence results visualized interfacial and centimeter-scale long-range production of ^•OH mediated by CCR^•. Fourier transform ion cyclotron resonance mass spectrometry verified the role of surface decarboxylation of adsorbed HA in CCR^• generation. These findings offer new insights into the environmental fate of ^•OH produced during iron redox cycling in natural and engineered systems, including those with natural organic matter.