Kinetics, equilibrium and thermodynamic studies of l-tryptophan adsorption using a cation exchange resin

► We develop an empirical model to reproduce the equilibrium data at different pHs. ► We develop a theoretical model based on ion exchange and proton transfer reaction. ► The particle diffusion is the rate-limiting step of the exchange process. ► The diffusion in the reaction layer is more important than that in the solid phase. In this study, the adsorption equilibrium of l-tryptophan by a strong acid styrene cation exchange resin (0 0 1 × 7) was investigated. The equilibrium data were reproduced by both empirical and theoretical treatments. A simple empirical model was able to reproduce the measured total uptake of l-tryptophan. A theoretical model based on ion exchange and the proton transfer reaction was developed to fit the experimental data and to obtain the parameters related to both phenomena. The theoretical model developed herein enables the relation between l-tryptophan uptake, pH and the amount of resin to be determined. Kinetic experiments under different initial l-tryptophan concentrations and initial pHs were performed. The homogeneous particle diffusion model and the shell progressive model were used to define the controlling mechanism of the overall ion exchange process. The results show that the process is always controlled by rate diffusion of l-tryptophan through the polymeric matrix of the resin. The l-tryptophan diffusion coefficient predicted by the homogeneous particle diffusion model in the order of 10 −9 m 2/s is lower than that calculated by the shell progressive model, which has an order of magnitude of 10 −7 m 2/s.