A layer-by-layer-assembly modified semi-aromatic polyamide membrane with enhanced fouling resistance

Membrane fouling is a persistent challenge that hinders the widespread application of membrane technology for water treatment. This study introduced an innovative layer-by-layer-assembly approach (LBL), i.e., chitosan (Cs), β-cyclodextrin (β-CD), glycidyl trimethyl ammonium chloride (GTAC), to modify the semi-aromatic polyamide nanofiltration membrane to enhance the separation performance and fouling resistance. The results show that the LBL surface coating, i.e., amino group in Cs and/or hydroxyl group in β-CD reacted with the unhydrolyzed acyl chloride group in TMC and/or epoxy group in GTAC, partially neutralized the negatively charged membrane surface with enhanced hydrophilicity. The modified membrane (named as PTCBG) was on par with or even better than the leading commercial NF270 membrane in terms of permeability and selectivity (A value: 13.1–13.7 vs. 14.2–14.3 L·m⁻²·h⁻¹·bar⁻¹; A/B value: 9.2–46.9 vs. 3.1–8.8 bar^-1, for rejection tests of typical divalent salts including CaCl₂, MgCl₂ and MgSO₄). The flux decline rate (FDR) decreased significantly from 89% for virgin membrane to 29% for PTCBG membrane using dodecyl trimethyl ammonium bromide (DTAB) as a model foulant. The corresponding flux recovery rate (FRR) was 102%. It indicates the enhanced reversible anti-organic fouling property, which is attributed to the weakened hydrophobic and electrostatic attraction. The antibacterial rate of PTCBG relative to virgin membrane was 75% and 68% for Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The FDR of PTCBG membrane exposed to E. coli and S. aureus was 20% and 51%, much lower than 35% and 65% for virgin one. It consistently reflected the enhanced biofouling resistance for PTCBG membrane, as a result of combined effects of weak adsorption and strong sterilization on bacteria, attributed to quaternary ammonium introduced by GTAC. This study provides an innovative and straight-forward strategy to fabricate polyamide membranes with good permeability/selectivity and excellent fouling resistance.