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

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 × 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 ∼10−11 m2/s whereas particle-diffusion coefficients corresponding to the latter slow stage are ∼10−14 m2/s in the studied concentration range of (0.6–4.0) × 10−4 M. Both film- and particle-diffusion coefficients decrease as the initial concentration increases while the effective diffusion coefficient of 7.73 × 10−13 m2/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.Experimental equilibrium data points are found in accordance with the curves calculated according to Freundlich, Dubinin–Radushkevich (D–R) and Langmuir isotherm equations. Adsorption capacities of the BS for 2,4-D calculated from isotherm parameters increase with increasing temperature and decreasing pH.