Heterogeneous and homogeneous photocatalytic degradation of the insecticide imidacloprid in aqueous solutions

The heterogeneous and homogeneous photocatalytic degradation of imidacloprid, a systemic chloronicotinoid insecticide. has been investigated in aqueous solutions using artificial UV-A or visible illumination. Three processes under various experimental conditions were evaluated namely, TiO2/UV-A, photo-Fenton/UV-A and photo-Fenton/vis with respect to their activity for substrate degradation and mineralization. The initial apparent photonic efficiency decreased in the order photo-Fenton/UV-A > TiO2/UV-A > photo-Fenton/vis. For the TiO2/UV-A process, the efficiency increased considerably when TiO2 is combined with Fe3+ and H2O2 presumably due to the synergistic effect of homogeneous and heterogeneous photocatalytic reaction. On the other hand, the homogeneous photocatalytic reactions were enhanced in the presence of the oxalate ions. Imidacloprid degradation was accompanied by the formation of several reaction by-products as confirmed by GC/MS analysis. while ammonium, nitrate and chloride ions have been detected in the liquid phase as mineralization products. By-products were less ecotoxic to marine bacteria than the insecticide itself. (C) 2008 Published by Elsevier B.V.