Cooperative adsorption of L-tryptophan and sodium ion on a hyper-cross-linked resin: Experimental studies and mathematical modeling.

The adsorption separation of L-tryptophan (L-Trp) by the weakly polar hyper-cross-linked resin XDA-200 was studied. First, the adsorption equilibria of different species of L-Trp on the resin were compared. Then, the adsorption isotherms and adsorption kinetics of L-Trp were studied at different pH values. Finally, the dynamic adsorption and separation processes of L-Trp in a packed bed of the resin were studied. The distribution coefficient of L-Trp± between the resin and an aqueous solution of L-Trp (55.69) was found to be markedly larger than that of L-Trp+ (27.53) and L-Trp- (10.42). An adsorption isotherm model depending on pH was established to simulate the adsorption equilibrium data of L-Trp. The cooperative adsorption of sodium ion (Na+) with L-Trp- cannot be ignored when the solution pH is higher than 8.0. Thus, a modified surface diffusion model considering cooperative adsorption of Na+ with L-Trp- was established. The model fitted the kinetic curves for L-Trp adsorption under different pH values satisfactorily. The surface diffusion coefficient of L-Trp first decreased and then increased as pH increased from 3 to 12. In this study, a modified film-surface diffusion model considering cooperative adsorption of Na+ with L-Trp- is proposed. Further, we show that our proposed model can predict the chromatographic peaks of L-Trp, L-glutamic acid (L-Glu), and Na+ satisfactorily.