Determination of Hydroxyl Radical Production from Sulfide Oxidation Relevant to Sulfidic Porewaters

Hydroxyl radical (·OH) production from the reaction between aqueous total sulfide ([H₂S]_T = [H₂S] + [HS^–] + [S^2–]) and dissolved oxygen is potentially an environmentally important reaction when anoxic, sulfidic water is exposed to oxygen. Using hydroxyterephthalate (hTPA) formation from the reaction of terephthalic acid (TPA) with ·OH as a probe for ·OH production, hydrogen peroxide was verified as an essential intermediate, and the production of free ·OH, versus lower energy hydroxylating agents, was established. The optimal conditions for the quantification of ·OH production kinetics and yield were determined by varying TPA and total sulfide concentrations. An initial total sulfide concentration of 10 μM and TPA concentration of 2 mM was used to find a yield of 15 mmol of ·OH per mole [H₂S]_T. Additionally, a pseudo-first-order model elucidated a maximum rate of production of 1.04 (±0.05) × 10^–4 moles of ·OH per hour per mole of [H₂S]_T. Experiments with sulfidic wetland porewaters containing up to 294 mg/L of dissolved organic carbon (DOC) revealed that [H₂S]_T, and not reduced DOC, was the dominant source of ·OH. A simple model considering a water containing [H₂S]_T, DOC, and methane exposed to a constant concentration of oxygen (∼50 μM) gave steady state values of [·OH] ranging from 5.7 × 10^–19 to 3.2 × 10^–18 M. The results indicate that [H₂S]_T should be considered a source of dark formation of ·OH in addition to ferrous iron and reduced DOC.