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.