Synergistic Photochemistry of Dissolved Black Carbon Mediated by Iron: A Critical Pathway for Enhancing Hydroxyl Radical Generation

Dissolved black carbon (DBC) displays strong photochemical effects in surface Earth systems through reactive oxygen species production, yet the role of redox minerals during this process has been poorly characterized. While evidence emphasized significant iron-DBC coupling, the underlying mechanistic basis for their photochemical synergy requires further elucidation. This study comprehensively evaluated the coupled impacts of iron on photochemical reactive oxygen species production from a variety of DBC sources and the behind mechanisms. Results demonstrated that the synergistic photochemistry of the iron-DBC coupling system led to 1-2 orders of magnitude higher hydroxyl radical (^·OH) generation than that from DBC alone, and its ^·OH fluxes approached the previously reported global levels for natural ^·OH yields. Notably, the Fe(III)-mediated net enhancement in ^·OH generation was 2 times higher for DBC than for dissolved organic matter, yet with a minor contribution of iron redox cycling (<6%). Experimental and modeling results revealed that the photo-generation of organic peroxides (ROO^·), which were initiated by the oxygenation of carbon-centered radicals produced upon photolysis of Fe(III)-DBC complexes, served as a critical pathway driving >75% of additional ^·OH production. Especially, the photogenerated ROO^· from DBC associated with Fe(III) showed higher reduction potentials (E_red⁰) beneficial for ^·OH generation via intermolecular propagation reactions. Ultrahigh-resolution mass spectrometry and model compound analyses suggested the dominant roles of unsaturated carbon structures and Fe(III)-promoted decarboxylation in facilitating the significant photogeneration of ROO^· from DBC. The findings explored the unrecognized photochemical pathway of iron-DBC coupling, providing new insights into the biogeochemical role of DBC and the natural origins of ^·OH.