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South African Journal of Chemistry

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Speciation and Persistence of Dimethoate in the Aquatic Environment: Characterization in Terms of a Rate Model that Takes Into Account Hydrolysis, Photolysis, Microbial Degradation and Adsorption of the Pesticide by Colloidal and Sediment Particles

MF Zaranyika, J Mlilo

Abstract


The kinetics of the degradation of dimethoate in an aquatic microcosm ecosystem, and a distilled water control, charged with the pesticide was studied over a period of 90 days. The concentration of dimethoate was monitored in the distilled water control, as well as the water phase and sediment phase of the  experiment. The loss of the pesticide after a given time period was calculated and plotted as a function of time. Biphasic linear rates of dimethoate degradation were observed for both the water phase (3.12 and 0.358 μg mL –1 day–1, respectively) and the sediment phase (108 and 1.03 μg g–1 day–1, respectively) in the experimental microcosm, as well as the distilled water control (0.694 and 0.0388 μg mL –1 day–1, respectively). The linear rates of degradation in distilled water are attributed to hydrolysis and  photochemical degradation. The biphasic linear rates of degradation in the experimental microcosm are attributed to microbial degradation of pesticide adsorbed by sediment or colloidal particles, and an  enzymatic kinetics model is presented to  account for the observed kinetics. The factors that affect the rates of degradation, possible dimethoate  pollution remediation strategies, and characterization of the different speciation forms in terms of rates of degradation and apparent adsorption/desorption thermodynamic properties, are discussed.

KEYWORDS: Dimethoate, degradation rates, pesticide, adsorption/desorption kinetics, chemisorption.




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