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.