The reactive oxygen species (ROS) superoxide (O2-) and hydrogen peroxide (H2O2) are oxidants and reductants of metals which affect metal speciation and bioavailability in natural waters.1 Known sources of ROS include the photo-oxidation of dissolved organic matter (DOM) and dark, particle-associated production attributed to biological sources.2 Recent studies have shown that hydroxyl radical (OH) is produced during oxidation of reduced DOM by O2, with H2O2 implicated as an intermediate.3-4 This work suggests that reduced DOM may be another source of O2- and H2O2, but direct measurements of production rates and yields have not yet been performed. This reaction pathway is environmentally significant, since DOM can be reversibly reduced under anoxic conditions and then produce ROS as it oxidizes when it enters oxic surface waters. Determining the contribution of DOM to ROS production in turn improves the current understanding of factors influencing metal speciation and cycling. In this work, dark production rates of O2- and H2O2 from DOM were quantified. Preliminary results show that even a freeze-dried humic isolate generates dark ROS upon redissolution. Additional experiments explore the dependence of production rates on temperature, oxygen concentration, and DOM concentration and redox state (manipulated using direct electrochemical reduction3). We also present field measurements showing significant dark production of O2- and H2O2 in filtered water samples. For example, filtered water from Crown Hill Lake (Wheat Ridge, CO) had dark H2O2 production rates range from 20(±5) to 44(±2) nM h-1 while Mirror Lake (Lyons, CO) had rates of 17(±3)-18(±3) nM h-1. Our results suggest that DOM may play a complementary role in the dark production of ROS. References: 1. Voelker, B. M.; Sedlak, D. L. Iron reduction by photoproduced superoxide in seawater. Mar. Chem. 1995,50,93 2. Rose, A. L.; Webb, E. A.; Waite, T. D.; Moffett, J.W., Measurement and implications of nonphotochemically generated superoxide in the equatorial Pacific Ocean. Environ. Sci. Technol. 2008, 42, 2387. 3. Page, S. E.; Sander, M.; Arnold, W. A.; McNeill, K. Hydroxyl radical formation upon oxidation of reduced humic acids by oxygen in the dark. Environ. Sci. Technol. 2012, 46, 1590. 4. Page, S. E.; Kling, G. W.; Sander, M.; Harrold, K. M.; Logan, J. R.; McNeill, K.; Cory, R. M. Dark formation of hydroxyl radical in arctic soil and surface waters. Environ. Sci. Technol. 2013, 47, 12860.

Dark production of superoxide and hydrogen peroxide during oxidation of reduced dissolved organic matter in natural waters

249th National Meeting (2015)

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