Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters?

Here we show that fluorescent compounds that could be classified as "M-like" (marine-like) fulvic acids are formed upon phototransformation of phenol by a triplet sensitiser (anthraquinone-2-sulphonate, AQ2S). The relevant process most likely involves phenol oxidation to phenoxyl radical by triplet AQ2S, followed by dimerisation of phenoxyl radicals into phenoxyphenols and dihydroxybiphenyls. It might be the first step of an oligomerization process that bears resemblance with the expected formation pathways of humic-like substances (HULIS) in the atmosphere. Such a process could account for the formation in surface waters of compounds having similar fluorescence properties as "M-like" fulvic acids. Presently it is thought that such species are formed upon photo-fragmentation of larger humic and fulvic acids ("top-down" pathway), and we propose that an opposite, "bottom-up" pathway could also be operational.