Kinetics and equilibrium studies of the herbicide 2,4-dichlorophenoxyacetic acid adsorption on bituminous shale

Abstract

The adsorptive removal of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by bituminous shale (BS) has been studied by means of batch technique. Kinetic data well fit to McKay equation at the lowest initial concentration of 6 x 10(-5) M, which assumes a two-resistance diffusion model. The effective film-diffusion coefficients, which correspond to initial fast stage of the adsorption, are in the magnitude similar to 10(-11) trills whereas particle-diffusion coefficients corresponding to the latter slow stage are 10(-14) m(2)/s in the studied concentration range of (0.6-4.0) x 10(-4) M. Both film- and particle-diffusion coefficients decrease as the initial concentration increases while the effective diffusion coefficient of 7.73 x 10(-13) m(2)/s calculated from Paterson's equation based on a one-resistance diffusion model changes neither initial concentration nor temperature. Average activation energies calculated by applying Arrhenius equation to film-diffusion coefficients are found to be positive whereas they change around zero for particle-diffusion process. Thermodynamic parameters estimated according to Eyring equation based on the transition state theory indicate the existence of both energy and entropy barrier in the system.