KINETICS OF AZADIRACHTIN HYDROLYSIS IN MODEL AQUATIC SYSTEMS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The hydrolysis of azadirachtin-A isomer (AZ-A) was studied at 20 degre es C in the dark in buffered distilled water at pH 4, 7 and 10, and in unbuffered sterilized and unsterilized pond water. Individual solutio ns were fortified in triplicate with pure AZ-A and formulated AZ-A sep arately. Hydrolysis of AZ-A in pond water was studied by using pure AZ -A only. At pH 10, AZ-A fortified either in pure form or as formulatio n, was hydrolysed rapidly and the DT50 was only about 2 h. At pH 4, th e DT50 values for the pure and formulated AZ-A were 19.2 and 38.3 d, r espectively, indicating that the chemical is relatively stable in acid ic medium. The stability was diminished at pH 7 and the corresponding DT50 values were 12.9 and 30.5 d. The data show that the hydrolysis of AZ-A is greatly influenced by pH in the order pH 10 >>> pH 7 > pH 4. The differences in DT50 values between pure AZ-A and formulated AZ-A, at pH 4 and 7, suggest that hydrolysis is considerably retarded by the surfactants in the formulation. The average pH and DT50 values for th e sterilized and unsterilized pond water were 8.08 +/- 0.49 and 7.36 /- 0.28, and 6.91 d and 11.94 d, respectively. The faster degradation of pure AZ-A in sterilized water, compared to the unsterilized water, was likely due to chemical hydrolysis. Microbial action in the degrada tion of AZ-A in the unsterilized pond water appeared to be minimal.