Photocatalyzed Degradation of Agrochemicals in Water

1. The phtocatalyzed degradation of agrochemicals (simazine, benthiocarb, thiram, diazinon, fenitrothion, isoprothiolane, propyzamide, dichlorvos, fenobucarb, and iprobenfos) has been investigated in an aqueous suspension of TiO_2 using a ultra high-pressure mercury lamp (lambda>290nm and 365nm) at 35ﾟC.2. The degradation is much faster than direct photolysis without TiO_2 and depends on the first order kinetics to the concentrations of the agrochemicals. 3. The degradation depends on the structures of the agrochemicals. The half-lives of the agrochemicals ranged from 4.11-11.9min (lambda>290nm) and 5.0-24.3min (lambda=365nm). Quantum yields were estimated to be 0.0086-0.032min (lambda=365nm). The agrpchemicals were ultimately mineralized to form Cl^-, SO_4^<2->, NO_3^-.4.Several factors affecting the degradation were examined in details with iprobenfos. 5. Optimum concentration of TiO_2 was found to be 0.5-1.0mg/ml. The initial reaction rate (r_0) is proportional to the photons absobed by TiO_2 (I_a) and quantum yield (PHI). r_0 follows the langmuir-llinshelwood relationship with the initial concentration (C_0) of iprobenfos and the partial pressure of O_2 in the bubbling gas (pO_2). The action of adsorbed O_2 (O_2(ads)) is discussed. We propose the empirical rate of the photocatayzed degradation of iprobenfos as follows : r_0=I_aPHIpO_2, _<ads>(iprobenfos)_<ads>. Fair agreement of r_0 was found between experiment and calculation. 6. Based on the GC/MS analysis of the degradation products, degradation pathways were examined. One pathway is OH attack on alpha, beta, and gamma bond of iprobenfos, another pathways is OH attack on the benzene ring of iprobenfos followed by the alpha cleavage. It was found that the photocatalyzed degradation could be applicable to the degradation of water contaminants.