Stephanie Christa Remke

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Impact of chlorination and ozonation of dissolved organic matter on its photo-induced production of long-lived photooxidants and excited triplet states

Urs von Gunten, Joanna Maria Houska, Silvio Canonica, Stephanie Christa Remke

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (3CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA254), the ratio of absorbance at lambda = 254 nm and lambda = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre -oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial con-centrations ([DMOP]0 = 0.1 and 5.0 mu M). Linear inter-correlations were observed for the relative changes in SUVA254, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., kobs0.1/rabsCDOM and kobs 5.0 /rabs CDOM, for 0.1 and 5.0 mu M, respec-tively) exhibited the following distinct trends: The LLPO-dominated kobs 0.1/rabs CDOM ratio decreased with increasing oxidant dose and with decreasing SUVA254 and EDC, while the 3CDOM*-dominated kobs 5.0 /rabs CDOM ratio positively correlated with E2:E3. Finally, it was concluded that precursors of 3CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

PERGAMON-ELSEVIER SCIENCE LTD2023

Photochemical oxidation of phenols and anilines mediated by phenoxyl radicals in aqueous solution

Urs von Gunten, Silvio Canonica, Stephanie Christa Remke

Reactive intermediates formed upon irradiation of chromophoric dissolved organic matter (CDOM) contribute to the degradation of various organic contaminants in surface waters. Besides well-studied "short-lived" photo oxidants, such as triplet state CDOM (3CDOM*) or singlet oxygen, CDOM-derived "long-lived" photooxidants (LLPO) have been suggested as key players in the transformation of electron-rich contaminants. LLPO were hypothesized to mainly consist of phenoxyl radicals derived from phenolic moieties in the CDOM. To test this hypothesis and to better characterize LLPO, the transformation kinetics of selected target compounds (phenols and anilines) induced by a suite of electron-poor model phenoxyl radicals was studied in aerated aqueous solution at pH 8. The phenoxyl radicals were generated by photosensitized oxidation of the parent phenols using aromatic ketones as photosensitizers. Under steady-state irradiation, the presence of any of the electron-poor phenols lead to an enhanced abatement of the phenolic target compounds (at an initial concentration of 1.0 x 10(-7) M) compared to solutions containing the photosensitizer but no electron-poor phenol. A trend of increasing reactivity with increasing one-electron reduction potential of the electron-poor phenoxyl radical (range: 0.85-1.12 V vs. standard hydrogen electrode) was observed. Using the excited triplet state of 2-acetonaphthone as a selective oxidant for phenols, it was observed that the reactivity correlated with the concentration of electron-poor phenoxide present in solution. The rates of transformation of anilines induced by the 4-cyanophenoxyl radical were an order of magnitude smaller than for the phenolic target compounds. This was interpreted as a reduction of the radical intermediates back to the parent compound by the superoxide radical anion. Laser flash photolysis measurements confirmed the formation of the 4-cyanophenoxyl radical in solutions containing 2-acetonaphthone and 4-cyanophenol, and yielded values of (2.6 5.3) x 10(8) M-1 s(-1) for the second order rate constant for the reaction of this radical with 2,4,6-trimethylphenol. These and further results indicate that electron-poor model phenoxyl radicals generated through photosensitized oxidation are useful models to understand the photoreactivity of LLPO as part of the CDOM.

PERGAMON-ELSEVIER SCIENCE LTD2022

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.

EPFL2021