Environmental Fate of Temephos: Photodegradation Versus Hydrolysis

Abstract

The environmental fate of temephos a current-used pesticide was studied. It was subjected to photochemical degradation, acid and base driven hydrolysis to determine which process plays a more crucial role on its fate once released into the environment. Temephos is susceptible to direct photolysis as a result of its wavelength of maximum absorption (λ ≈ 390nm). Hydrolysis was carried at pH 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 1.22× 10-3hr-1, kpH7 1.17× 10-3hr-1, kpH9  5.11×10-3hr-1 and kpH11 5.53 × 10-3hr-1  with corresponding half – lives of 568.03 hrs, 592.31 hrs, 135.62hrs and 125.3 hrs respectively. Temephos was found to be most stable in acidic medium with stability decreasing with increase in pH. There is a strong relationship between rate of degradation and the pH of the aqueous solution for acid and base driven hydrolysis of temephos. The photochemical degradation was also observed to follow first order rate kinetics with degradation rates of 1.25 x 10-3 hr-1 and half-life of 544.80 hrs for solutions without nitrate ion and 1.59 x 10-2 hr-1 with corresponding half-life of 43.54 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 thus 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 temephos in aqueous systems. Implication for this study is relevant in understanding the fate of temephos once released into the environment with possible application in water/wastewater treatment.