Complexation strength between organic carbon and transition metal ions dominates the photochemical conversion of SO ₂ to sulfates

The photooxidation of SO ₂ 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 (Fe ³⁺ , Cu ²⁺ and Mn ²⁺ ) on the conversion of SO ₂ to sulfates on OC from coal combustion under irradiation. For OC and its coexistence with TMIs under experimental conditions adopted here, the steady-state uptake coefficients of SO ₂ and sulfate masses were (0.39–22.24) × 10 ⁻⁶ and (2.36–26.05) × 10 ⁻³ mg, respectively. Fe ³⁺ exhibited a significantly antagonistic effect, whereas Cu ²⁺ and Mn ²⁺ enhanced SO ₂ uptake and sulfate generation on OC. Spectroscopic evidences, including absorbance decreasing and fluorescence quenching, confirmed the complexes formation of TMI with chromophores in OC. Fe ³⁺ owned the strongest binding affinity with chromophores, followed by Cu ²⁺ and Mn ²⁺ . This variation in the coordination strength dominated the generation of reactive species, such as free electrons (e ⁻ ), superoxide radicals (⚫O2-), H ₂ O ₂ and hydroxyl radicals (⚫OH), and ⚫OH acted as a pivotal trigger to drive the sulfate production. Extended investigations confirmed a good linear relationship between ⚫OH intensity ( I ) and sulfate mass ( M _sulfates ): M _sulfates  = (1.68 ± 0.11) × 10 ⁻²  ×  I  − (0.43 ± 0.08) × 10 ⁻² , demonstrating that the regulatory effects of metal ions on the sulfate production were generally governed through their ability to suppress or facilitate the ⚫OH generation.