Temperature Dependence of the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter

The temperature dependence of the photochemical production of the hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by measuring the apparent temperature dependence of the quantum yield (Φ_a) for this process. Temperature dependent Φ_a values were analyzed using the Arrhenius equation. Apparent activation energies obtained for DOM isolates purchased from the International Humic Substances Society ranged from 16 to 34 kJ mol^–1. Addition of 40 units mL^–1 catalase, used to hinder the hydrogen peroxide (H₂O₂)-dependent pathway to •OH, did not impact the observed activation energy. However, an increase in activation energy was observed in lower molecular weight DOM obtained by size fractionation. We also measured the temperature dependence of <i>p</i>-benzoquionone photolysis as a model compound for DOM and observed no temperature dependence (slope <i>p</i> = 0.41) for the formation of phenol from oxidation of benzene (the •OH probe used), but a value of about 10 kJ mol^–1 for <i>p</i>-benzoquinone loss, which is consistent with formation of a quinone-water exciplex. These data provide insight into DOM photochemistry as well as provide parameters useful for modeling steady state •OH concentrations in natural systems