Relationships Between the Photochemical and Photophysical Properties of Dissolved Organic Matter

摘要

Naturally occurring dissolved organic matter (DOM) is a major constituent in aquatic water bodies, including lakes, rivers, estuaries and bays, and the open ocean. DOM is often the primary absorber of sunlight in these systems. This absorption of light leads to excited electronic states of DOM molecules that subsequently undergo photophysical and photochemical processes to form transient reactive intermediates (RI), such as the hydroxyl radical ( •OH), singlet oxygen, and triplet state DOM. Because of DOM's complexity, it is difficult to obtain a precise description of the photophysical and photochemical processes that occur in DOM. The goal of this thesis is to better understand the origin of both DOM's optical properties and the mechanisms of •OH formation from DOM photolysis.;After an introduction of the topic of DOM optical properties and photochemistry, the relationship between DOM molecular weight and photophysics/photochemistry is evaluated. Although this initial study seemed to provide evidence for the charge-transfer (CT) model for DOM photophysics, later studies examining the effect of temperature and solvent polarity on DOM absorbance and fluorescence indicated that CT interactions are likely not playing a dominant role in DOM. Specifically, absorption and emission position were insensitive to changing temperature and solvent polarity, contrary to hypotheses based donor-acceptor complexes of organic molecules.;The photochemical formation of •OH from DOM was also investigated. The quantum yield for •OH formation from DOM was shown to be temperature-dependent and described by the Arrhenius equation. The activation energies of different isolates for •OH formation correlated to DOM physicochemical properties, such as molecular weight. In addition, the temperature-independence for photohydroxylation of a •OH probe compound by p-benzoquinone, a model photosensitizer in DOM, was consistent with this species forming a quinone-water exciplex and not free •OH. Finally, a long-term monitoring study of the Florida Everglades was conducted to assess the ability to model •OH formation using DOM physicochemical parameters. •OH quantum yields were poorly modeled by all factors investigated, indicating that •OH precursors comprise a small and variable portion of the overall DOM pool.