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 O 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 (HO) 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.