Water Vapor Condensation on Iron Minerals Spontaneously Produces Hydroxyl Radical

Water vaporcondensation on iron minerals spontaneouslyproduces hydroxyl radicals, driving surface-bound pollutant degradation. The hydroxyl radical ((OH)-O-center dot) is a potentoxidantand key reactive species in mediating element cycles and pollutantdynamics in the natural environment. The natural source of (OH)-O-center dot is historically linked to photochemical processes (e.g., photoactivationof natural organic matter or iron minerals) or redox chemical processes(e.g., reaction of microbe-excreted or reduced iron/natural organicmatter/sulfide-released electrons with O-2 in soils andsediments). This study revealed a ubiquitous source of (OH)-O-center dot production via water vapor condensation on iron mineral surfaces.Distinct (OH)-O-center dot productions (15-478 nM via watervapor condensation) were observed on all investigated iron mineralsof abundant natural occurrence (i.e., goethite, hematite, and magnetite).The spontaneous (OH)-O-center dot productions were triggered bycontact electrification and Fenton-like activation of hydrogen peroxide(H2O2) at the water-iron mineral interface.Those (OH)-O-center dot drove efficient transformation of organicpollutants associated on iron mineral surfaces. After 240 cycles ofwater vapor condensation and evaporation, bisphenol A and carbamazepinedegraded by 25%-100% and 16%-51%, respectively, forming (OH)-O-center dot-mediated arene/alkene hydroxylation products. Ourfindings largely broaden the natural source of (OH)-O-center dot. Given the ubiquitous existence of iron minerals on Earth'ssurface, those newly discovered (OH)-O-center dot could play arole in the transformation of pollutants and organic carbon associatedwith iron mineral surfaces.