Water Vapor Condensation on Iron Minerals Spontaneously Produces Hydroxyl Radical

The hydroxyl radical (^•OH) is a potent oxidant and key reactive species in mediating element cycles and pollutant dynamics in the natural environment. The natural source of ^•OH is historically linked to photochemical processes (e.g., photoactivation of natural organic matter or iron minerals) or redox chemical processes (e.g., reaction of microbe-excreted or reduced iron/natural organic matter/sulfide-released electrons with O₂ in soils and sediments). This study revealed a ubiquitous source of ^•OH production via water vapor condensation on iron mineral surfaces. Distinct ^•OH productions (15-478 nM via water vapor condensation) were observed on all investigated iron minerals of abundant natural occurrence (i.e., goethite, hematite, and magnetite). The spontaneous ^•OH productions were triggered by contact electrification and Fenton-like activation of hydrogen peroxide (H₂O₂) at the water-iron mineral interface. Those ^•OH drove efficient transformation of organic pollutants associated on iron mineral surfaces. After 240 cycles of water vapor condensation and evaporation, bisphenol A and carbamazepine degraded by 25%-100% and 16%-51%, respectively, forming ^•OH-mediated arene/alkene hydroxylation products. Our findings largely broaden the natural source of ^•OH. Given the ubiquitous existence of iron minerals on Earth's surface, those newly discovered ^•OH could play a role in the transformation of pollutants and organic carbon associated with iron mineral surfaces.