Photochemical transformation of benzotriazole, relevant to sunlit surface waters: Assessing the possible role of triplet-sensitised processes

The corrosion inhibitor 1H-benzotriazole (pK_a = 8.4) can exist in two different forms in natural waters, and photochemical transformation is a potentially significant attenuation pathway for both of them. Depending on conditions, the modelled half-life times range from some days/weeks to several months. In sunlit water bodies, the acidic (neutral) form would undergo direct photolysis (accounting for up to 7% of total phototransformation) and, most notably, reaction with the hydroxyl radicals (^·OH) and the triplet states of chromophoric dissolved organic matter (³CDOM*). The basic (anionic) form would undergo significant transformation with ^·OH and ³CDOM*. The ^·OH reactions would be more important at low dissolved organic carbon (DOC) and the ³CDOM* processes at high DOC. In the presence of highly reactive triplet-state model compounds, the two benzotriazole forms react with similar rate constants. In this case, they would show comparable half-life times in surface-water environments. With less reactive triplet states, the rate constant of the anionic form can be a couple of orders of magnitude higher than that of the neutral one. Under these circumstances, the neutral form could be considerably more photostable than the anionic one at high DOC. Therefore, depending on ³CDOM* reactivity, the solution pH may or may not play an important role in the photoattenuation kinetics of 1H-benzotriazole in sunlit natural waters, especially at high DOC. Both forms of benzotriazole yield hydroxyderivatives as their main transformation intermediates under all the relevant photochemical reaction pathways. These intermediates could be formed via ^·OH-induced hydroxylation, or upon electron abstraction followed by reaction with water. Differently from UVC irradiation data reported in previous studies, the concentration of aniline upon excitation of 1H-benzotriazole under environmentally significant UV wavelengths was always below the detection limit of the analytical method used in this work (5 μmol L⁻¹). - Highlights: • Triplet sensitisation can be important for the photodegradation of benzotriazole. • Anionic benzotriazole reacts fast with excited triplet states. • Neutral benzotriazole needs very reactive triplet states to undergo fast reaction. • Lifetimes in sunlit surface waters might be highly affected by triplet reactivity. • Hydroxyderivatives are formed as intermediates in all tested conditions.