High flux nanofiltration membrane via surface modification using spirocyclic quaternary ammonium diamine for efficient antibiotics/salt separation

Efficiently separating antibiotics from monovalent salt is a practical challenge in the pharmaceutical industry. In this study, a novel spirocyclic quaternary ammonium diamine monomer (SBI) was synthesized and used as a surface modifier of polyamide (PA)-based nanofiltration membrane to enhance antibiotic separation. The SBI monomer was reacted with the residual acyl chloride groups and successfully grafted into the PA film. The rigid spirocyclic structure of SBI increased the molecular weight cut-off (MWCO) of the membrane from 348 Da to 382 Da, which loosened the selective layer and reduced the water transport resistance. The SBI-modified membrane had an excellent pure water permeance of 33.4 L·m−2·h−1·bar−1, 2.5 times as high as the pristine one. Moreover, the introduction of quaternary ammonium moieties partially neutralized the surface negative charge of the membrane, reducing the Donnan exclusion. The resulting modified membrane demonstrated high antibiotic rejection and low NaCl rejection. The erythromycin ERY/NaCl selectivity was 20.7 in 10 g·L−1 NaCl solution, and ERY was concentrated from 100 to 419 ppm in 7 h. This study indicates that the surface modification using the SBI monomer is a potential method to improve the antibiotics/salt separation.