Phototransformation of monuron induced by nitrate and nitrite ions in water: Contribution of photonitration

The nitrate and nitrite-induced phototransformation of monuron in aqueous solution has been investigated using 300–450 nm light irradiation. The degradation kinetics was estimated under various conditions of concentrations of substrate and inducer, oxygen content and pH. From the numerous photoproducts identified using LC–ESI-MS–MS and their evolution curves a degradation scheme was proposed. Besides oxidation of the N-terminus group (major pathway in the presence of NO3−) and a minor contribution of direct photolysis, the other photoproducts originated from Cl/OH substitution, regiospecific hydroxylation and/or nitration of the phenyl ring. The two last processes occurred competitively under the NO3− conditions even at “environmental” concentrations of the inducer yielding significant levels of nitro-derivatives. Their production was found to be oxygen-dependent in contrast to the NO2− conditions.