Constraining the yields and pathways of long-lived photo-oxidants: Suggestion of a formation process not involving organic matter triplet states

Abstract Long-lived photo-oxidants (LLPOs) are reactive transients photoproduced by chromophoric dissolved organic matter in sunlit surface waters, and play important roles in the photodegradation of contaminants such as phenols and anilines. Previous works identified LLPOs with phenoxy radicals, tentatively assuming they are formed upon oxidation of organic-matter phenols by the excited triplet states of chromophoric dissolved organic matter (³CDOM*). In this work we generated reactive (electron-poor) phenoxy radicals by direct UVA photolysis of 2-nitrophenol and 4-nitrophenol, and determined their reactivity with 2,4,6-trimethylphenol in water. Our main findings are: (i) LLPOs are unlikely to derive only from ³CDOM* because, to explain 2,4,6-trimethylphenol degradation, one should assume quite long LLPO lifetimes (equal to the proposed upper limits), very elevated reactivity (comparable to the most unstable/reactive phenoxy radicals), and the involvement in LLPO generation of most ³CDOM* that escape quenching by O₂; (ii) results are more easily explained by assuming that LLPOs are also formed by direct photolysis of organic-matter phenols that absorb environmental UV radiation.