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.