Adsorption of Dursban (Chlorpyrifos) Pesticide by Loam Soil from Limuru, Kenya: Apparent Thermodynamic Properties

Abstract

The adsorption phenomenon of chlorpyrifos by suspended loam soil particles from Limuru, Kenya in an aqueous solution was studied using a model which assumes the adsorption/ desorption relations: nSX S nX ⇔ + , ] [ ] /[] [ S X SX K n n = and ln[x]ads = ln(nK’) + nln[x]e + [sxn]w, where X is the chemical species of interest; S is the substrate; K is the adsorption/desorption equilibrium constant; nSX is the particle-pesticide complex. The apparent adsorption equilibrium constant; [X ]ads is concentration of X in adsorbed state in suspension. [X ]e is the concentration of X in solution at equilibrium and [ nSX ]w is the pesticide adsorption site complex in the suspension at equilibrium. The amount of chlorpyrifos adsorbed was determined against variation of mass of sediment, concentration of chlorpyrifos and contact time using UV-Visible spectrophotometer at 254nm. The data obtained in this study best fitted the quasi-langmuir adsorption isotherm with regression values of up to 0.992. The calculated values of the apparent K, n and 'G∆ were found to be 118.665, 0.244 and -11.7946kj/mol respectively. The negative value for 'G∆ confirmed the fact that adsorption reaction occurs spontaneously as expected. Moreover, adsorption of chlorpyrifos onto suspended/dissolved sediment particles decreased with increase in mass of the substrate. Key words: Adsorption, pesticide, chlorpyrifos, adsorption equilibrium constant, UV-Visible spectrophotometer