Chemical Oxidation And UV Photolysis On Degradation Of Micro Organic Pesticides In Water

Pesticide is one of the most important organic micro-pollutants in water environments. Pesticides in water are highly toxic and difficult to be removed by biodegradation, it thus imposed a serious threat to the water environments and human health. Therefore, it is important to find out an efficient way for degradation of the pesticide residues in water for the management of water environments. In this study, degradation efficiency of four kinds of commonly used pesticides（malathion, diazinon, S-metolachlor and carbaryl） by oxidation of potassium permanganate, sodium hypochlorite and hydrogen peroxide as well as UV irradiation at 254 nm was investigated. In addition, the effect of photocatalytic oxidation degradation of atrazine was studied.（1） Degradation efficiency of four kinds of pesticides（diazinon, malathion, S-metolachlor and carbaryl） was examined at solution pH values of 4.0, 7.0 and 10.0 by oxidation of potassium permanganate, sodium hypochlorite and hydrogen peroxide as well as UV irradiation at 254 nm. The results indicate that the degradation efficiency of the four pesticides was significantly affected by p H and the oxidation methods. Certain degrees of degradation for the four pesticides by UV irradiation could be achieved under all the experimental conditions. The degradation efficiencies of diazinon on alkaline condition by sodium hypochlorite oxidation treatment were higher than that in acid or neutral conditions, more than 90% of diazinon was degraded after 180-minute reaction. The oxidation efficiencies of malathion and carbaryl by the four oxidation methods were relatively higher in alkaline condition than that in acid or neutral conditions. However, a substantial degradation efficiency of S-metolachlor was observed only with UV irradiation. More than 90% of S-metolachlor was degraded within 90 minute, and it could be removed completely after 180 minute reaction. The oxidation of the four pesticides by UV irradiation was found to follow the pseudo-first-order reaction kinetics.（2） Degradation of atrazine by irradiation of 500 W medium pressure mercury lamp（λ=365nm UVA） was investigated. The results show that the initial concentration of atrazine has a great impact on the degradation efficiency by UVA, the degradation rate was decreased as the initial concentration of atrazine increased, and that the influence of atrazine degradation rate by pH was not significant. All the degradation reactions could complet within a relatively short reaction time under each pH condition. The effects of H₂O₂ on the degradation efficiency of atrazine when the solution in neutral condition（pH=7.0） were examined. The degradation rate of atrazine showed a trend of increase first and then decreased with the increase in the dosage of H₂O₂. In this study, the degradation rate of atrazine reached the maximum when the H₂O₂ dosage was 5mg/L, and under this condition the degradation rate of atrazine was better at high p H than in acidic or neutral pH values. Under all the above experimental conditions, more than 90% of atrazine was degraded within 20 minute, and it could be removed completely after 40-minute reaction. In addition, they all followed the pseudo-first-order reaction kinetics. Finally, the effects of TiO₂ on the degradation efficiency of atrazine were examined in neutral solution condition（pH=7.0）. The results show that the degradation efficiency of atrazine decreased with the increase in the dosage of TiO₂.