PHOTODEGRADATION AND HYDROLYSIS OF CHLORPYRIFOS IN AQUEOUS SYSTEMS
DOI:
https://doi.org/10.4314/njt.334.856Keywords:
Chlorpyrifos, photodegradation, Hydrolysis, organophosphorus pesticide, nitrate ionAbstract
Hydrolysis and photodegradation of chlorpyrifos a widely used organophosphorus pesticide was studied. Hydrolysis is a dominant process for pesticide loss in the environment. Chlorpyrifos by virtue of its wavelength of maximum absorption is susceptible to direct photolysis. Degradation via hydrolysis was studied at pHs 4, 7, 9 and 11 while photodegradation was carried out in the presence and absence of nitrate solution at ~30oC for a period of seven days. The rate of degradation was determined for both conditions as a function of the phosphate formation within the reaction solution. Results obtained indicated that both processes were found to follow a first order rate kinetics with rate constants for the hydrolysis experiments to be kpH4 as 1.54× 10-3hr-1, kpH7 4.63× 10-3hr-1, kpH9 9.21×10-3hr-1 and kpH11 2.56 × 10-2hr-1 with corresponding half – lives of 449.52 hrs, 149.76 hrs, 75.36 hrs and 27.12 hrs respectively. Chlorpyrifos was found to be most stable in acidic medium with stability decreasing with increase in pH. The photochemical degradation was also observed to follow first order rate kinetics with degradation rates of 7.08 x 10-4 hr-1 and half-life of 978.35 hrs for solutions without nitrate ion and 1.25 x 10-2 hr-1 with corresponding half-life of 55.44 hrs for solutions with nitrate ions. Photodegradation was observed to be faster in the presence of nitrate compared with those without nitrate (kNO3- > k). It can be concluded that the presence of nitrate ions acts as a photo-sensitizer towards the degradation rate of organophosphorus pesticide studied thus accelerating the process. Comparison of both processes revealed that pH 11 was most effective for the degradation of chlorpyrifos in aqueous systems. Implication for this study is relevant in understanding the fate of chlorpyrifos once released into the environment with possible application in water/wastewater treatment.
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