Complexation strength between organic carbon and transition metal ions dominates the photochemical conversion of SO₂ to sulfates

Abstract. The photooxidation of SO₂ on organic carbon (OC) is a critical pathway for sulfate formation, yet the role of interactions between OC and transition metal ions (TMIs) in this process remains scarcely understood. We systematically investigated potential influences of TMIs (Fe³⁺, Cu²⁺ and Mn²⁺) on the conversion of SO₂ to sulfates on OC from coal combustion under irradiation. Fe³⁺ exhibited a significantly antagonistic effect, whereas Cu²⁺ and Mn²⁺ enhanced SO₂ uptake and sulfate generation on OC. Spectroscopic evidences, including absorbance decreasing and fluorescence quenching, confirmed the complexes formation of TMI with chromophores in OC. Fe³⁺ owned the strongest binding affinity with chromophores, followed by Cu²⁺ and Mn²⁺. This variation in the coordination strength dominated the generation of reactive species, such as free electrons (e⁻), superoxide radicals (•O₂⁻), H₂O₂ and hydroxyl radicals (•OH), and •OH acted as a pivotal trigger to drive the sulfate production. Extended investigations containing more metal ions demonstrated that the regulatory effects on the sulfate production were generally governed through their ability to suppress or facilitate the •OH generation.