Improved permeability of tight acid resistant nanofiltration membrane via citric acid post-treatment

Development of acid resistant nanofiltration membranes (ARNFMs) with high metal rejection, high flux and low molecule weight cut-off (MWCO) remains a big challenge. The separation performance of polyamine ARNFM is partially limited by the dense structure caused by the high concentration of organic phase solution. Herein, in this work, a novel effective post-treatment of citric acid (CA) was utilized to further improve the separation property of dually charged polyamine ARNFMs with low MWCO. An appropriate CA post-treatment (1.5 mol/L, 30 min, 65 °C) could gently remove off the oligomers and CA molecules would react with the redundant protonated/non-protonated amino, imine and hydroxyl groups of the polyamine NFM. Therefore，after the post-treatment, the membrane flux was enhanced 7.1 times with a high MgCl2 rejection (97.6%) and a low MWCO around 220 Da. Furthermore, CA post-treatment not only had no significant influence on the acid stability of this membrane, but would also protect the membrane from the drastic effect of HCl (hydrolyze -Cl into –OH) in turn. All the results indicated that CA post-treatment has positive effects on polyamine ARNFMs, and the resulted dually charged polyamine ARNFMs have a good application potential for acidic wastewater treatment.