Electro-Driven in Situ Construction of Functional Layer Using Amphoteric Molecule: The Role of Tryptophan in Ion Sieving

The permselective separation of monovalent ions from the solution with the coexistence of multivalent ions was required in the industry of the wastewater treatment and resource recycling. Here, in this work, a novel electro-driven in situ modification method was utilized in the positively charged tryptophan solution to prepare highly permselective cation exchange membrane (CEM). We have optimized the process conditions and discussed the modification mechanism by drawing upon the fouling phenomenon in the membrane separation processes. A series of conventional characterization methods such as scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and electrochemical impedance spectroscopy (EIS) were used to investigate the structure and performance changes after the modification. The SEM-energy-dispersive X-ray (SEM-EDX) was introduced for analyzing the transfer of Na+ and Mg2+ in the functional modification layer and the membrane matrix, thus illustrating the sieving mechanism for the modified membranes. The resultant membranes were observed to have the changed interstructure and the multivalent-ion-repulsive modification layer. Due to the channel filling and electrostatic repulsion effect, the electro-driven tryptophan endowed the pristine CMX with superior monovalent cation permselectivity (P-Mg2+(Na+): 6-35), which was higher than that of pristine CMX and the commercial Neosepta CIMS. It confirms that the proposed electro-driven tryptophan in situ modification method could effectively help improve the membrane permselectivity by structurally forming stable crystallization on and within the membrane, which provides a feasible choice for the cation exchange membrane modification.