Comparison of UV/H2O2 and UV/TiO2 for the degradation of metaldehyde: Kinetics and the impact of background organics.

The kinetics of photodegradation of the pesticide metaldehyde by UV/H(2)O(2) and UV/TiO(2) in laboratory grade water and a natural surface water were studied. Experiments were carried out in a bench scale collimated beam device using UVC radiation. Metaldehyde was efficiently degraded by both processes in laboratory grade water at identical rates of degradation (0.0070 and 0.0067 cm(2) mJ(-1) for UV/TiO(2) and UV/H(2)O(2) respectively) when optimised doses were used. The ratio between oxidant and metaldehyde was significantly higher for H(2)O(2) due to its low photon absorption efficiency at 254 nm. However, the presence of background organic compounds in natural water severely affected the rate of degradation, and whilst the pseudo first-order rate constant of degradation by UV/H(2)O(2) was slowed down (0.0020 cm(2) mJ(-1)), the degradation was completely inhibited for the UV/TiO(2) process (k' = 0.00007 cm(2) mJ(-1)) due to the blockage of active sites on TiO(2) surface by the background organic material.