Mechanistic Study on Iron Solubility in Atmospheric Mineral Dust Aerosol: Roles of Titanium, Dissolved Oxygen, and Solar Flux in Solutions Containing Different Acid Anions

Atmospheric processing of mineral dust aerosols has been identified as a major contributor to bioavailable Fe in the marine environment. While numerous studies have focused on single-component Fe-bearing minerals, the impact of non-Fe-bearing minerals, emitted via natural and anthropogenic processes, on Fe dissolution remains largely unknown. The current study investigates reaction mechanisms that govern the dissolution of hematite mineral (α-Fe2O3) in the presence of a relatively common semiconductor oxide in mineral dust aerosol, that is, titania (TiO2), in three different atmospheric mineral acids, HNO3, HCl, and H2SO4. Our studies suggest that Fe dissolution in the daytime increases when mixed with TiO2 because of HO•-mediated mechanisms. These effects are further enhanced by the dissolved oxygen due to additional radical pathways arising from reactive oxygen species. The presence of oxygen increases dissolved Fe(II) under irradiated conditions for HNO3 and HCl, whereas it decreases for H2SO4, suggesting reactivity differences of the anionic radicals. In the dark, the presence of TiO2 and nitrate increases Fe dissolution due to redox coupling with nitrate under reduced conditions. The current study thus reveals vital mechanistic information on mineralogy-controlled iron dissolution in dust aerosols by anthropogenic non-Fe-bearing minerals, oxygen and solar flux with implications for global iron mobilization.