Degradation and Adsorption of Lambda-cyhalothrin on Athi River and Coastal Soils and Sediments in Kenya

Abstract

Lambda-Cyhalothrin is extensively used in agriculture, horticulture and public health management in Kenya. The pesticide is effective against many vegetable pests, rice and other related disease vectors. This study investigated adsorption of L-Cyhalothrin onto sediments of Athi River and degradation on Kwale and Athi River soils. Sample analysis was conducted using UV-Visible spectrophotometry and the results were analyzed by fitting data into isotherm models including Langmuir, Freundlich, Temkin, Dubinin-Radushvich and Scathard plot to understand the environmental impacts of this pesticide. The models presented different numerical values but the results demonstrated similar characteristics. It stands out that Dubinin-Radushvich (D-R) with average R2: 93.48, 93.75 and 89.8 fitted best in this experiment followed by Freudlich R2: 78.28, 89.95 and 81.18 and Quasi-Langmuir R2: 81.6, 87.45 and 70.53 for all samples from the upstream, midstream, and downstream of Athi River, respectively. The spontaneity of the adsorption process was also realized in ΔG values as predicted by Langmuir and Freundlich. In both models, the ΔG values for the midstream were negatives -9.781 KJ/Kmol and -2.720 KJ/Kmol for Langmuir and Freudlich, respectively. This showed complete spontaneous characteristics, whereas the ΔG value for upstream was positive (9.286 KJ/Kmol) for Langmuir and negative (-4.462.12 KJ/Kmol) for Freundlich. The downstream recorded a positive ΔG value (3.0968 KJ/Kmol) for Freudlich while Langmur was negative (-5.4415KJ/Kmol). Other important parameters in Freudlich, such as the number of adsorbed molecules, n and the apparent equilibrium constant K were determined. The average values were 1.04, 0.73, and 0.39 for n and 6.36, 2.89, and 0.54 for K, for upstream, midstream and downstream, respectively. Degradation of L-Cyhalothrin was faster in Kwale soil than Athi River soil with half-life between 4 to 5 weeks for Kwale soil was faster than Athi River soil. Adsorption modeling and degradation studies of pesticide residues are vital to environmentalist and policy makers in determining the application and disposal mechanisms for these pollutants.