Photocatalytic degradation of the herbicide clomazone in natural water using TiO2: kinetics, mechanism, and toxicity of degradation products.

The photocatalytic degradation of the herbicide clomazone (0.05mM) in aqueous suspensions of TiO2 Degussa P25 was examined as a function of the different operational parameters. The optimum concentration of the catalyst was found to be 0.50mgmL(-1) under UV light at the pH 10.3. In the first stage of the reaction, the photocatalytic degradation of clomazone followed the pseudo-first order kinetics, with and the heterogeneous catalysis proceeding via OH radicals. The results also showed that the disappearance of clomazone led to the formation of a number of organic intermediates and ionic byproducts, whereas its complete mineralization occurred after about 55min. Tentative photodegradation pathways were proposed and discussed. A comparison of the evolution of toxicity that was evaluated in vitro in rat hepatoma (H-4-II-E) and human fetal lung (MRC-5) cell lines with the degradation kinetics indicates that the irradiation contributed to the decrease of the toxicity of the mixture that is no longer dominated by the parent compound. The study also encompassed the effect of the quality of natural water on the rate of removal of clomazone.