The influence of polychromatic light on methomyl degradation in TiO2 and ZnO aqueous suspension

The photocatalytic degradation of the insecticide methomyl in water using TiO2 Merck Eusolex (R) T (anatase) under Osram ultra-vitalux (R) lamp light (315-400 nm, 300W) was studied. The effect of the operational parameters, such as initial concentration of catalyst, initial methomyl concentration, initial salt concentration (NaCl), and pH, was studied. The optimal concentration of catalyst was found to be 2.0 g/L. Using the Langmuir-Hinshelwood mechanism as a base, a pseudo-first-order kinetic model was illustrated and the adsorption equilibrium constant and the rate constant of the surface reaction were calculated (K-MT = 0.079 L/mg and k(C) = 0.134mg/L min, respectively). The photodegradation rate was higher in acidic than in alkaline conditions. The presence of Cl- ions significantly affected the photodegradation of the pollutant. The rate of photodecomposition of methomyl was measured using UV spectroscopy and high performance liquid chromatography, while its mineralization was followed using ion chromatography and total organic carbon analysis. In addition, several photocatalysts were used: ZnO (Merck), TiO2 P-25 (Degussa), TiO2 Merck (anatase), and TiO2 Merck R-706 (rutile), and were compared with TiO2 Merck Eusolex (R) T (anatase).