Study On Photodegradation Of Profenofos In Aqueous Solution

The photodegradation of pesticides is an important pathway for their decomposition and transformation in natural environment. Pesticide molecules can absorb the light energy and be photodegraded themselves (direct degradation). The degradation may also be sensitized by another compound (or other compounds), which is refered to as indirect degradation. Through the study of the photodegradation of pesticides, we can get clues on the products and pathways for their photodegradation in natural environment. Thus, realize the safty of pesticides,in order to instruct the use of pesticides and synthesize of new products. It is therefore of great significance to study the photodegradation of pesticides.Profenofos is a novel organophosphorus pesticide; it's effective and less toxic. In this thesis, the photodegradation kinetics of profenofos in aqueous solution irradiated by UV lamp and sunlight was investigated. The effects of solution media, pH, concentrations of nitrate and nitrite, metallic ions(Fe3+,Cu2+,Zn2+,Hg2+), acetone, H2O2, pyrethroids and humic acid (HA), on photodegradation of profenofos under UV irradiation were also studied.The main results and conclusions were summarized as follows:1. Under sunlight, the direct photolysis rate of profenofos in aqueous solution was very slow, which is a direct combined result of the absorption characteristics of profenofos and the terrestrial solar spectrum. Profenofos can only absorb the light under 294nm, while the shortest wavelength of the terrestrial solar spectrum is 286.3nm. No degradation of profenofos was observed in the dark, which is an indication that the hydrolysis of profenofos under our experimental conditions is neglectable. Profenofos degraded remarkably under ultraviolet radiation and showed a first-order kinetics.2. The pH value has significant effect on the photodegradation of prefenofos; the photolysis rate constant decreased in the order of pH11> pH9> pH7> pH5> pH3. The pH of solution tends to decline in the process of photodegradation, apparently owing to some acid materials produced.3. The photolytic rates of profenofos in different solution media decreased in the order of: deionized water> synthetic seawater> tap water> natural seawater. The half-life of profenofos was 25.39h in deionized water, while it was 32.69h in seawater. The effect might be related to the physicochemical properties of the water samples.4. The photodegradation rate of profenofos decreased with the increasing initial concentration. It may be caused by the competitive absorption of the overmuch profenofos molecules to photons. In any case, the photodegradation processes obeyed a first-order kinetics.5. Both nitrate and nitrite showed photoquenching effects on degradation of profenofos in the dosage range studied. The photoquenching was enhanced with the increasing concentrations of nitrate and nitrite.6. Fe3+ exhibited distinct photosensitizing effect on the degradation of profenofos, while Cu2+ quenched profenofos degradation and the quenching effect showed a direct proportion to the dosage ratio. Zn2+ and Hg2+ had no observable effect on the degradation.7. The photodegradation of profenofos was greatly promoted by H2O2, owing to the effect of·OH produced by H2O2 upon UV irradiation. H2O2 with a concentration of 5mmol/L reduced the half-life of profenofos from 25.69h to 12.72h in deionized water, from 32.69h to 16.94h in natural seawater.8. HA had photoquenching effect on profenofos photodegradation. The quenching effect was increased with increasing HA concentration. It was suggested that HA baffle the absorption of light energy by profenofos.9. Acetone accelerated the photodegradation of profenofos. Acetone with a concentration of 0.5% (V/V) reduced the half-life of profenofos from 25.69h to14.17h in deionized water, from 32.69h to 17.31h in natural seawater. Studies also showed that the photolysis rate decreased when the concentration of acetone was increased to 1.0%. 10. Both fenvalerate and fenpropanate exhibited photosensitizing effects on the degradation of profenofos, and the sensitizing effects of fenpropanate was more remarkable than that of fenvalerate. The photosensitizing effects showed direct proportion to the dosage ratio.