Photocatalytic degradation of a typical agricultural chemical: metalaxyl in water using TiO2 under solar irradiation

The photodegradation and mineralization of the metalaxyl [methyl N-(2,6-dimethyl-phenyl)-N-(methoxyacetyl)-alaninate], which is a popular benzenoid fungicide, was conducted in the presence of TiO2 photocatalyst under solar irradiation. An initial metalaxyl concentration of 50 ppm was completely degraded in presence of TiO2 after 30 min irradiation, while no degradation was observed in absence of TiO2 under solar irradiation. The effect of different parameters, such as amount of TiO2, initial pH, light intensity, reaction temperature and irradiation time, on the photocatalytic degradation of metalaxyl was evaluated. The drop of total organic carbon as a consequence of mineralization of metalaxyl was detected during the photocatalytic process. The kinetics of photocatalytic degradation followed a pseudo-first order law according to Langmuir–Hinshelwood model, and the rate constant was 0.105 min−1. Ammonium ion and CO2 were speculated as the end-products after completing degradation of metalaxyl. The five types of intermediate products were identified by GC–MS during the decomposition of metalaxyl. In order to investigate the degradation pathway of metalaxyl, the point charge and frontier electron density at each atom on the molecule were determined by using MOPAC stimulation. The degradation mechanism was proposed from the identified intermediates. The solar photocatalytic degradation method can become an effective technique for the treatment of metalaxyl-polluted water.