Mechanism of degradation of ketoprofen by heterogeneous photocatalysis in aqueous solution

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 center dot scavenger. The fastest degradation of KP, with a rate constant (42 +/- 3) x 10(-4) s(-1), was obtained under UV irradiation in air atmosphere, using 1.2 g L-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) x 10(-4) s(-1). In this case, 60% mineralization of KP was observed after 30 min. Gibbs free energies, pK(a) 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=211 g/mol), resulting from decarboxylation of KP. (c) 2013 Elsevier B.V. All rights reserved.