L-Tryptophan (l-Trp) was separated from its aqueous solution by hyper-cross-linked resins. The adsorption and desorption performances of l-Trp on different resins were compared. The weakly polar resin XDA-200 was selected as an excellent adsorbent with high adsorption amount and easy elution. The resin has a high adsorption selectivity and strong salt resistance. The adsorption mechanism of l-Trp on resin XDA-200 was elucidated based on adsorption thermodynamics experiments, molecular dynamics simulations, and adsorption kinetics experiments. The dynamic separation process of l-Trp was finally studied. The adsorption of l-Trp on resin XDA-200 is a spontaneous process driven by adsorption enthalpy. l-Trp^± is the most favorable form for l-Trp adsorption on resin XDA-200 because of the strongest affinity of l-Trp^± to the resin and relatively low water solubility. The adsorption of l-Trp is mainly based on π–π and hydrophobic interactions. Surface diffusion is the sole rate-limiting step of l-Trp mass transfer on resin XDA-200. l-Trp was separated satisfactorily from l-glutamic acid (l-Glu) and NaCl with both the recovery rate and purity of l-Trp higher than 99% in the fixed bed packed with resin XDA-200.

The mass-transfer process of l-tryptophan (l-Trp) in the hydrophobic interaction/ion-exchange mixed-mode resin HD-1 particles and fixed bed was studied experimentally and theoretically. The adsorption kinetics of l-Trp in single-component and multicomponent adsorption systems was investigated under different pH conditions. The co-adsorption of sodium ions (Na⁺) and l-Trp anions was found to be negligible. A modified liquid-film linear driving force model considering the physical adsorption of l-Trp zwitterions and anions as well as ion exchange of l-Trp cations was proposed. The dissociation equilibria of l-Trp molecules and functional groups on the resin were introduced in the model. The model could well fit the kinetic adsorption curves of l-Trp at different pH values. The presence of Na⁺ and the impurity amino acid l-glutamic acid (l-Glu) did not significantly affect the mass-transfer rate of l-Trp. The dynamic adsorption processes of l-Trp under different pH and concentration conditions were studied. A modified transport-dispersive model considering axial diffusion, liquid-film mass transfer, and a combined physical adsorption and ion-exchange equilibrium was established, which could predict the adsorption breakthrough curves of l-Trp well. During the dynamic adsorption process, the pH of mobile phase in the fixed bed changed with changing the l-Trp concentration in the mobile phase. l-Trp was well separated from Na⁺ and l-Glu with the purity of l-Trp higher than 99%, the recovery rate higher than 95%, and a concentration of 4.69 × 10^-3 mol/L. The elution chromatographic peaks of l-Trp, l-Glu, and Na⁺ and the pH of the outlet solution were predicted satisfactorily.

The adsorption amount and selectivity of L-tryptophan (l-Trp) on the hydrophobic interaction and ion exchange mixed-mode chromatography medium HD-1 were studied as well as the salt resistance of the resin via adsorption equilibrium experiments. The adsorption mechanisms of l-Trp were illuminated by combining adsorption equilibria and a kinetics analysis. The separation effect was studied by dynamic separation experiments in a fixed-bed. The results indicate that an increase of the concentration proportion of l-Trp zwitterion benefits the adsorption of l-Trp. The resin shows a high adsorption selectivity for l-Trp at different pH values. The adsorption amount of l-Trp is not affected significantly by NaCl. Various groups play a role in the adsorption of l-Trp. An adsorption energy lower than 8 kJ/mol indicates that the adsorption of l-Trp is mainly based on non-electrostatic interactions, with an electrostatic interaction as a supplement. The adsorption equilibrium model considering the dissociation equilibrium of the resin and l-Trp proposed in this work can simulate the adsorption equilibrium data of l-Trp at different pH values as well. The mass transfer rate of l-Trp is controlled by intraparticle and liquid film diffusion simultaneously. The fixed-bed packed with resin HD-1 can separate l-Trp with the purity of l-Trp higher than 99%, recovery rate higher than 95%, and concentration of 4.69 × 10^–3 mol/L.