Super Li+/Mg2+ sieving and regenerable potentials of a nanofiltration membrane with intermediate layers of positively-charged hyperbranched polyamide

Abstract

Constructing an intermediate layer has attracted extensive research interests in improving the separation efficiency of nanofiltration (NF) membrane. However, present NF membranes with intermediate layers face the problem of limited Li⁺/Mg²⁺ sieving efficiency and costly regeneration. Here we reported the grafting of intermediate layer of positively-charged hyperbranched polyamides (HBPA/4-APL) on polysulfone ultrafiltration membrane (PSF-UFM) as substrate by diazotization coupling reaction, and then to form a NF membrane via the interfacial polymerization process. This study presented a novel approach to fabricate NF membrane with intermediate layers of HBPA/4-APL between negatively-charged polyamide skin layers. This study clearly elucidated that HBPA/4-APL intermediate layer could strictly regulate the structure of polyamide (PA) nanomembranes formed on PSF-UFM, resulting in enhanced ion sieving selectivity. Our membrane exhibited uniform nanostripes, fine-tuned aperture, and thinner thickness. Our membrane had a maximum water flux of 2.49 times that of PA nanomembranes formed on pristine PSF-UFM without HBPA/4-APL intermediate layer. The separation factor of Li⁺/Mg²⁺ was 72.19, 63.75 and 60.34 when the Mg²⁺/Li⁺ mass ratio in the feed solution was 7.65:1, 15.3:1 and 30.6:1, respectively. Our membrane had regenerable capability after in-situ HCl washing. So our NF membrane showed great promise for Li⁺/Mg²⁺ separation. This work also offered a new strategy of robust regenerable membranes.