Investigation of boundary layer effect of intra-particle diffusion on methylene blue adsorption on activated carbon

Methylene blue (MB) is a model dye used in many adsorption studies, and it is chemisorbed on activated carbon (AC) adsorbent. The adsorption of an adsorbate by a natural adsorbent is a phenomenon where kinetics is often complex despite the necessity of having reliable information on such reactions which is much needed to predict the efficiency of industrial processes of which rate of adsorption plays an integral part. Such information is, at times, not available under different experimental conditions, limiting desired applications. The research reported herein was thus performed to investigate rate of adsorption of MB onto coconut shell activated carbon by fitting experimental data to kinetics and diffusion models. According to the regression analysis of linearized pseudo-order models applied for various experimental conditions, the pseudo first order (PFO) model is found to be best suited to describe the kinetics of the MB-AC system having rate constants in the range 0.0799 – 0.2437 min⁻¹ under different experimental conditions at the ambient temperature. The rate constants determined through the PFO model at different solution temperatures are indicative of chemisorption of MB on AC surface. Application of the Webber and Morris intra-particle diffusion (IPD) model, which accounts for the boundary layer effect on mass transfer, indicates that adsorption kinetics may be controlled by both film diffusion and intra-particle diffusion sequentially, and that the thickness of the boundary layer on the AC surface, which is a measure of the intercept of the linearized Webber and Morris IPD model, is sensitive to experimental conditions. Consequently, experimental parameters would be able to control the adsorption behaviour of the MB-AC system, which can be investigated by monitoring the magnitude of the initial adsorption factor.