Adsorption characteristics of captafol pesticide by sediment and soil samples: apparent thermodynamic properties using spectroscopic methods

This study involved adsorption of captafol on red soil obtained from Kwale county and the sediments collected from Ngong river, by varying the initial concentration of the adsorbate, shaking time and weight of adsorbent. The sediment collected from Ngong river and the red soil obtained from Kwale county used were analysed for pH, texture, cation exchange capacity and organic carbon content. The adsorption was determined by measuring concentration of the pesticide using UV-Vis-NIR spectrophotometer before and after the attainment of equilibrium..Freundlich and Langmuir adsorption isotherms were used to study adsorption behaviour of the pesticide at equilibrium conditions. The relationship between sediments and soil characteristics and thermodynamic properties was explored following Gibbs free energy expressions. Captafol was found to absorb at 442 nm. A calibration for captafol showed a linear relationship for concentration range from 0.2 to 40 ppm, and slight deviation as the concentration increased to 100 ppm. This was in accordance with the Beer’s law. Freundlich isotherm fitted well for most of the data. Adsorption rate for captafol for red soil was found to be 0.035 g mg/min and 0.0245 m mg/min for sediment. Thermodynamics parameters showed that adsorption process was exothermic and spontaneous. Gibbs energy (ΔG), apparent equilibrium constant (K’) and number of adsorption sites (n) were some thermodynamic properties investigated. The calculated values for K’ were 57.34±4.6 and 58.16±4.7, ΔG: -9.98±0.19 (kj/mol) and -10.05±0.21 (kj/mol), n: 1.08±0.03 and 1.10±0.01 for the sediment and red soil, respectively.