Photocatalytic ozonation of clopyralid, picloram and triclopyr. Kinetics, toxicity and influence of operational parameters

BACKGROUND: Photolysis, photocatalysis, ozonation and photocatalytic ozonation in the presence of titania have been applied to the elimination of three pyridine carboxylic acids herbicides (clopyralid, picloram and triclopyr) in water. RESULTS: Photocatalytic ozonation led to the best results in terms of herbicide elimination rate and mineralization. The herbicides were relatively recalcitrant to direct ozonation (rate constants: 20 M⁻¹ min⁻¹ for clopyralid and triclopyr, and 105 M⁻¹ min⁻¹ in the case of picloram). Herbicides degradation proceeds by hydroxyl radicals attack (rate constants: 0.73, 3.80 and 1.73 × 10⁹ M⁻¹ s⁻¹ for clopyralid, picloram and triclopyr, respectively). Monitoring of chloride and nitrates suggests dechlorination as the primary stage in the process. Further ring opening led to the accumulation of nitrates and the appearance of some low weight carboxylic acids. Toxicity decreased at the end of the process. CONCLUSION: Tested herbicides slowly react with ozone. Photocatalytic ozonation shows different degrees of synergism, leading to 80% mineralization in less than 1 h. Small amounts of TiO₂ (i.e. 0.005 g L⁻¹) maintains the efficiency of the photocatalytic ozonation. Quantitative free chloride and nitrates rapidly appear in the O₃/UVA/TiO₂ system. First intermediates in the photocatalytic ozonation of clopyralid show higher toxicity than the parent compound. © 2014 Society of Chemical Industry