Transformation Kinetics of Phenols in Water: Photosensitization by Dissolved Natural Organic Material and Aromatic Ketones

Abstract

Different dissolved natural organic materials photosensitize the transformation of a series of methyl and methoxy phenols at pH 8 with a very similar high selectivity (reactivity range ≈50). This selectivity falls in the range of that achieved using the aromatic ketones benzophenone (BP), 3′-methoxyacetophenone (3′-MAP), and 2-acetonaphthone (2-AN) as model photosensitizers. For both natural and model sensitizers, the photooxidation at pH 8 is not controlled by singlet oxygen. Deuterium isotope effects (<I>k</I>_H/<I>k</I>_D) of different phenols are 0.7-1.3 for humic and fulvic acids and 1.1-1.7 for BP, suggesting an electron transfer mechanism. In contrast, the isotope effect for 2-AN of ≈4 indicates an H-atom abstraction reaction. The results show that reactive excited triplet states are important photooxidants in natural waters and should be considered for assessing the abiotic degradation of chemicals. The reactive triplet state concentration is estimated to be ≈10^-14 M in the top meter of Lake Greifensee under summer noon sunlight, and this leads to a half-life of ≈7 h for 2,4,6-trimethylphenol. Further still uncharacterized photooxidants derived from the dissolved organic material are also involved in the phototransformation of the phenols.