Electron Transfer Budgets and Kinetics of Abiotic Oxidation and Incorporation of Aqueous Sulfide by Dissolved Organic Matter

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227–0.325 h–1. The main transformation products were elemental S (S0) and thiosulfate (S2O3 2–), yielding electron accepting capacities of 2.82–1.75 μmol e– (mg C)−1. Native iron contents in the HA could account for only 6–9% of this electron transfer. About 22–37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.