Hydroxyl radical (·OH) production from the reaction between aqueous total sulfide ([H2S]T = [H2S] + [HS-] + [S2-]) 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.
Bibliographical noteFunding Information:
Thanks to Grant Wallace for performing sample collection and Zi-Yao Ngai for performing preliminary experiments. Dr. Dave N. Mushet and Matthew Solensky at the U.S. Geological Survey Northern Prairie Wildlife Research Center provided expertise, field support, and assistance with sample collection. This work was funded by the National Science Foundation (EAR-1245135 with ancillary support from CBET-1434148), the University of Minnesota Undergraduate Research Opportunities Program (SML), and the Joseph T. and Rose S. Ling Professorship. This NMR instrument was supported by Grant 1S10OD021536 (G. Veglia) from National Institute of General Medical Sciences. We also appreciate the helpful suggestions of Patrick Brezonik.
- hydrogen peroxide
- hydroxyl radical