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 degrees 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.

Automated summary

The development of acid resistant nanofiltration membranes (ARNFMs) with high metal rejection, high flux, and low molecule weight cut-off (MWCO) is a challenge. In this study, a novel post-treatment method using citric acid (CA) was used to enhance the separation performance of dually charged polyamine ARNFMs. The CA treatment effectively removed oligomers and reacted with redundant amino, imine, and hydroxyl groups. The treated membranes showed significantly improved flux, high MgCl2 rejection, and low MWCO. The CA treatment did not affect the acid stability of the membranes, and even protected them from the harmful effects of hydrochloric acid. This study demonstrates the potential application of CA-treated polyamine ARNFMs in acidic wastewater treatment.