Surface engineering of loose nanofiltration membrane with acid-resistant through thermal crosslinking of zwitterionic cyanuric chloride with PEI on PES substrate for efficient dye/salt selective separation

The treatment of silk and wool printing industry acidic wastewater by thin-film polyamide nanofiltration (NF) membrane faces significant challenges due to the stability of acid-resistance. In this study, zwitterionic cyanuric chloride (ZCC) synthesized by cyanuric chloride (CC) as a raw material was employed to fabricate a zwitterionic loose NF membrane (ZLNF-PEI-ZCC) by thermal crosslinking with polyethyleneimine (PEI) on a polyethersulfone (PES) membrane as a supporting layer. In contrast to the M ZCC membrane, the M CC membrane was fabricated using PEI and CC. The results showed that ZCC was successfully introduced onto the surface of the PES membrane. As the ZCC content increased, the thickness of the functional layer increased from 169 nm (0.1 wt% ZCC) to 237 nm (0.6 wt% ZCC), whereas the surface average pore size decreased from 1.74 nm to 1.16 nm. Meanwhile, the molecular weight cut-off (MWCO) of ZLNF decreased from 1619 Da to 763 Da and the maximum water contact angle decreased from 65.81° to 48.60°. Specifically, after being exposed to a H 2 SO 4 solution (pH = 1) for 720 h, the M ZCC-0.4 membrane (fabricated with 2 wt% PEI and 0.4 wt% ZCC) maintained stable separation performance with a high rejection of AB-234 94.31% and a low rejection of Na 2 SO 4 15.80% and the permeance of 17.92 L m -2 h -1 bar -1 . The density functional theory (DFT) calculations showed that the PEI-CC exhibits a significantly stronger binding energy with H + (ΔE = -97.19 kcal mol -1 ) than the zwitterionic modified PEI-ZCC (ΔE = -56.51 kcal mol -1 ). This confirms that the zwitterionic modification substantially attenuates the membrane’s affinity for protons. In summary, this work presents a promising strategy for developing highly acid-resistant NF membranes for efficient dye/salt separation in strongly acidic wastewater.