Kinetics and the mechanism of the photocatalytic degradation of mesotrione in aqueous suspension and toxicity of its degradation mixtures

The photocatalytic degradation of the herbicide mesotrione (0.05 mM) in aqueous suspensions of TiO2 Degussa P25 and ZnO was examined as a function of the different operational parameters. The optimum of the catalyst loading was found to be 2.00 mg mL−1 under UV light at the pH 4.0 (TiO2 Degussa P25) and pH 9.2 (ZnO). In the first stage of the reaction, the photocatalytic degradation of mesotrione followed the pseudo-first order kinetics, in which the heterogeneous catalysis proceeds mainly via •OH radicals, and partly also via holes. The results showed that the disappearance of mesotrione led to the formation of a number of organic intermediates and ionic byproducts, whereas its complete mineralization occurred after about 4 h. Tentative photodegradation pathways were proposed and discussed as a function of the type of photocatalyst. A comparison of the evolution of toxicity that was evaluated in vitro in the rat hepatoma (H-4-II-E) and human fetal lung (MRC-5) cell lines with the degradation kinetics indicates that the cell growth of both cell lines was only mildly affected by the mixture of mesotrione and its photocatalytic degradation intermediates obtained using TiO2 Degussa P25, and moderately affected by the mixtures obtained using ZnO, whereby at 120 min toxicity decreased.