Mechanism of degradation of ketoprofen by heterogeneous photocatalysis in aqueous solution

•A mechanism is proposed for photolysis and heterogeneous photocatalysis of ketoprofen with TiO2.•The efficacy of composites MWCNT–TiO2 is discussed and compared with that of synthesized TiO2.•ΔG, pKa, and E° for ketoprofen and related species, are calculated using computational chemistry.•The variables affecting the process are discussed, and energetic efficiency factors analyzed. The photocatalytic degradation of ketoprofen (KP), 2-(3-benzoylphenyl)-propionic acid, was studied under UV–vis irradiation using synthesized TiO2 (anatase), composites of multi-walled carbon nanotubes with TiO2 anatase (20-MWCNT–TiO2), and commercial anatase. Different factors that affect this process are compared: irradiation source, load of catalyst, initial concentration of KP, pH, concentration of dissolved oxygen, addition of co-oxidants, and the presence of tert-butanol as HO scavenger. The fastest degradation of KP, with a rate constant (42±3)×10−4s−1, was obtained under UV irradiation in air atmosphere, using 1.2gL−1 of 20-MWCNT–TiO2. Composites of 20-MWCNT–TiO2 also afforded the fastest degradation with UVA–vis irradiation, in the absence of oxygen, with a rate constant (13.6±0.7)×10−4s−1. In this case, 60% mineralization of KP was observed after 30min. Gibbs free energies, pKa values, and the standard reduction potential of KP and related species, calculated by DFT (6-311G(d,p)), are in agreement with the observed behavior. A feasible degradation mechanism is proposed, both for the photolysis and photocatalyzed processes. Similar transient species are involved, both routes leading to similar photoproducts, the main is (3-ethylphenyl)(phenyl)methanone (m/z=211g/mol), resulting from decarboxylation of KP.