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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.
Urs von Gunten, Silvio Canonica, Stephanie Christa Remke
Urs von Gunten, Frank Jonathan Leresche