Bachelor of Science
Major or Concentration
Chemistry (ACS Certified)
Dissolved natural organic matter (DOM) is an important natural photosensitizer, which produces a variety of reactive oxygen species including singlet oxygen, hydroxyl radical, superoxide (O2-) and hydrogen peroxide (H2O2). H2O2 is formed primarily through the dismutation of O2-. Previous work has suggested that neither singlet oxygen nor excited triplet states of DOM are responsible for H2O2 production. Rather, reducing DOM intermediates likely form O2-, which in turn produce H2O2. One such intermediate may be a charge-separated DOM species (DOM+/-) formed by charge transfer (CT) relaxation of excited singlet states. In the CT model, the electron transfer is believed to occur between donors (e.g., phenols and methoxy aromatics) and acceptors (e.g., ketones, aldehydes, and quinones). Using a polychromatic approach (where DOM samples were irradiated with multiple long pass filters), the effect of pH on the wavelength dependence of H2O2 quantum yields for two DOM isolates was studied. These isolates included Suwannee River Natural Organic Matter because it is primarily terrestrial sourced material and Pony Lake Fulvic Acid because it is primarily microbial sourced material. Experiments were conducted over a pH range 4 to 10 for both environmental significance and mechanistic significance. As pH increases, O2- apparent quantum yields increase, as predicted by the charge transfer model, regardless of source type. H2O2 quantum yields further will increase depending on pH. Chloroethanol was used as a scavenger of aqueous electrons to determine the role of photoionization of DOM in O2- production at different wavelengths. No appreciable decrease in H2O2 quantum yields was observed at any wavelength, suggesting no appreciable amount of aqueous electron is contributing to O2- production and subsequent H2O2 production.
Laszakovits, Juliana R., "pH Dependence of the Spectral Distribution of Hydrogen Peroxide Quantum Yields" (2016). Student Research Submissions. 60.