Photoinduced thiol formation in dissolved organic matter: The role of sulfur-containing precursor and source of electron

Abstract

Photochemical transformation of dissolved organic matter (DOM) drives the evolution of its thiol moieties, potentially increasing their abundance in DOM. However, the sulfur-containing precursors and electron sources within DOM responsible for thiol photogeneration remain ambiguous, limiting our understanding of the biogeochemical cycles of thiol and thiophilic toxic metals in aquatic ecosystems. Here, we demonstrate the process and mechanism of photoinduced thiol formation in DOM using various sulfur-containing models with varying oxidation states. The disulfide and thioether moieties in DOM are identified as the primary precursors for thiol photogeneration. These disulfide/thioether moieties are directly or indirectly reduced by photogenerated reductive species (PRS, i.e., hydrated electrons (e_aq^-), triplet state molecules (³R*), and superoxide (·O₂^-)) derived from the phenolic hydroxyl (ArOH) and aromatic carboxyl/carbonyl. The e_aq^- is the main reductive species for thiol photogeneration. ArOH and aromatic carboxyl can directly eject e_aq^- from the singlet excited state (¹R*), while aromatic carbonyl produces e_aq^- via ¹R* and ³R*. Notably, e_aq^- can also indirectly reduce disulfide and thioether by producing ·O₂^- under oxic conditions. These findings emphasize the comprehensive effects of PRS from DOM on the evolution of thiol in aquatic ecosystems, providing important insights into the cycles of thiol and metals.