Selective Photochemical Oxidation of Reduced Dissolved Organic Sulfur to Inorganic Sulfate

The chemical nature and stability of reduced dissolved organic sulfur (DOSRed) have implications on the biogeochemical cycling of trace and major elements across fresh and marine aquatic environments, but the underlying processes governing DOSRed stability remain obscure. Here, dissolved organic matter (DOM) was isolated from a sulfidic wetland, and laboratory experiments quantified dark and photochemical oxidation of DOSRed using atomic-level measurement of sulfur X-ray absorption near-edge structure (XANES) spectroscopy. DOSRed was completely resistant to oxidation by molecular oxygen in the dark and underwent rapid and quantitative oxidation to inorganic sulfate (SO42–) in the presence of sunlight. The rate of DOSRed oxidation to SO42– greatly exceeded that of DOM photomineralization, resulting in a 50% loss of total DOS and 78% loss of DOSRed over 192 h of irradiance. Sulfonates (DOSSO3) and other minor oxidized DOS functionalities were not susceptible to photochemical oxidation. The observed susceptibility of DOSRed to photodesulfurization, which has implications on carbon, sulfur, and mercury cycling, should be comprehensively evaluated across diverse aquatic environments of differing DOM composition.