Photosensitized formation of sulfate and volatile sulfur gases from dissolved organic sulfur: Roles of pH, dissolved oxygen, and salinity.

The photodegradation of dissolved organic sulfur (DOS) is a potential source of aqueous sulfate and its chemical precursors in surface water. However, the photochemical fate of DOS and factors that control its fate still remain unclear. Herein, we employed a DOS model featuring a photosensitizer (humic acids, HA) to investigate the photochemical degradation pathways of DOS in various natural water sources, from which we observed the substantial photosensitized formation of sulfate, methanesulfonic acid (MSA), carbonyl sulfide (COS), and carbon disulfide (CS). However, the photochemical production of sulfate and MSA tends to be more efficient than COS and CS. The formation of sulfur-containing photodegradation products was also strongly affected by the identity of the organic sulfur precursor, the oxygen concentration, and the pH, while the salinity did not significantly influence the production ratios. Our results revealed that the photosensitization of DOS contributed significantly to the overall production of sulfate and MSA production, especially in acidic and oxygen-enriched environments, which was attributed to the photochemical production of reactive intermediates, such as excited CDOM (CDOM*) and reactive oxygen species (ROS). Considering the coexistence of DOS and photosensitizers in aquatic environments, photochemistry may play an essential role in the fate of aquatic DOS.