Phototransformation of Organic Contaminants in Aquatic Systems: Characterization of Long-Lived Photooxidants Derived from Dissolved Organic Matter

The increasing interest in the fate of organic contaminants in the aquatic environment requires the elucidation of relevant abiotic transformation processes. These processes often involve the ubiquitous dissolved organic matter (DOM), a complex mixture of organic compounds present in the aquatic environment. Upon light absorption of DOM a range of photochemically produced reactive intermediates (PPRI) form, which may induce the transformation of organic contaminants. PPRI reacting as oxidants have been proposed to consist of two pools, short-lived photooxidants, such as singlet oxygen (1O2), the hydroxyl radical (.OH) and triplet states of chromophoric DOM (3CDOM*), and long-lived photooxidants (LLPO), whose nature is still unclear. LLPO-induced transformations were hypothesized to cause a concentration-dependent enhancement effect observed for 3CDOM*-induced transformations. Thereby, pseudo-first-order phototransformation rate constants for electron-rich phenols increased when their initial concentrations were lowered from 5.0 to 0.1 mikroM. Phenolic moieties within DOM have been proposed as LLPO precursors. This thesis tests the hypothesis that LLPO are relevant PPRI, formed, concomitantly with 3CDOM* upon DOM light absorption, from phenolic DOM precursors. Experimental methods used in this thesis include steady-state irradiations, laser flash photolysis and studies with DOM surrogates and pre-oxidized DOM. The occurrence of LLPO-induced phototransformations was tested by analyzing target compound transformation kinetics for two initial concentrations, namely 5.0 and 0.1 mikroM in steady-state irradiation experiments. The aforementioned enhancement effect for the lower initial concentration was taken as an indicator for the involvement of LLPO in the phototransformation. Firstly, the relevance of LLPO in the phototransformation of various aquatic contaminants was evaluated. Electron-rich phenols, anilines and phenylureas were identified to be susceptible to LLPO-induced transformations. The LLPO reduction potential could be estimated to lie between 1.0 to 1.3 V vs. SHE. Secondly, LLPO-induced target compound transformation was mimicked by photochemically produced electron-poor phenoxyl radicals. Enhanced target compound transformation was observed for 0.1 mikroM initial concentration with any of the tested phenoxyl radicals, which were generated by the photosensitized oxidation of the parent phenols in aqueous solution. Reactivity increased proportionally with the one-electron reduction potential of the electron-poor phenoxyl radical. Finally, characterization of LLPO precursors was attempted by DOM pre-oxidation. A DOM isolate, Suwannee River fulvic acid, was pre-oxidized, by chlorine and ozone (with and without t-BuOH), characterized by measurements of UV absorbance measurements and electron donating capacity and subsequently tested for its photoreactivity. The chemical pre-oxidation lead to changes of DOM, such as decrease in aromaticity and phenol content, in accordance to previous studies. LLPO precursors were concluded to be composed of oxidant susceptible sites and to likely include phenolic DOM moieties. The results presented in this thesis corroborate the hypothesis of LLPO formed from phenolic DOM precursors upon DOM light absorption. One consequence drawn from this thesis is the recommendation to include target compound concentrations < 1 mikroM in laboratory-scale phototransformation experiments.