Photodegradation of metoprolol in the presence of aquatic fulvic acids. Kinetic studies, degradation pathways and role of singlet oxygen, OH radicals and fulvic acids triplet states

[Display omitted] •Metoprolol indirect photodegradation occurs in presence of aquatic fulvic acids, FA.•The photosensitizer effect of aquatic FA is mainly attributed to OH.•Involvement of other reactive species, 1O2 and/or some 3FA*was assessed.•Formation mechanisms were proposed for the several metoprolol photoproducts.•Relation between reactive species and photoproducts was established. Metoprolol is a pharmaceutical used for the treatment of cardiovascular diseases and disorders, whose frequent detection in surface waters raises concern. Indirect photodegradation is an important degradation pathway in waters and dissolved organic matter has a major role as photosensitizer. In this study, metoprolol photodegradation, in the absence and in the presence of fulvic acids extracted from the Vouga River (Portugal) (VRFA), was assessed under simulated sunlight. While metoprolol direct photodegradation was deniable, indirect photolysis occurred under the presence of VRFA. It followed a pseudo-first order kinetics and after 72 h of irradiation there was a decrease of metoprolol concentration of ∼80 %. The OH radical (OH) was verified to be the main reactive species (RS) responsible for the photosensitized degradation of metoprolol, but other RS are also involved, probably triplet excited states of FA (3FA*) and singlet oxygen (1O2), as demonstrated by the higher inhibition of the photodegradation in presence of sodium azide than in presence of 2-propanol. Based on a previous identification of photoproducts, tentative degradation mechanisms were here proposed. Photoproducts analysis after 24 h irradiation in the absence and presence of scavengers, shown that different RS are involved in the formation of different products/intermediates.