Heterogeneous photocatalytic removal of the herbicide clopyralid and its comparison with UV/H2O2 and ozone oxidation techniques Original Research Article

Clopyralid is a herbicide that has recently been reported to occur in drinking water at concentrations above the Permitted Concentration Value (PCV) of 0.1 μg/L for an individual pesticide (EU directive 98/83/EC). An extensive laboratory study on clopyralid removal with UV/TiO2, was carried out and was compared to UV/H2O2 and O3 removal efficiencies. The effectiveness of three TiO2 photocatalysts (Degussa P25, VP Aeroperl, Hombifine N) was studied and Degussa P25 was selected since it outperformed the other two. Complete removal of clopyralid was achieved with UV/TiO2 in about 90 min at an optimum catalyst concentration of 1 g/L. Pseudo-zero-order kinetics were suitable to describe the first stage of the photocatalytic reaction in the concentration range 0.078–0.521 mM. pH was found to significantly affect the removal rates of clopyralid due to changes in TiO2 surface charges and clopyralid ionisation degree. The rate constant was maximum at pH 5 and its value was 2.1 × 10−6 ± 4.3 × 10−7 M min−1. Pure oxygen bubbled in solution was found to slightly affect unfavourably the photocatalytic removal of clopyralid as compared to air. With UV/H2O2 and O3 systems, the initial removal rates were high but these systems were not effective in achieving high removal percentages overall.