Branched polymer grafted loose nanofiltration membrane with high surface potential for dye desalination

Herein, a thin and high-potential loose nanofiltration (LNF) membrane was fabricated by one-step cross-linking with branched polyethyleneimine (PEI) using the reactive sites on the styrene maleic anhydride/chlorinated polyvinyl chloride membrane surface to for alkylation and nucleophilic addition reactions, respectively. As the molecular weight of branched PEI increased, the thickness of LNF membranes decreased and their pore sizes and surface potential increased (i. e., potential value of LNF membrane above 59 mV at pH = 7). Density functional theory calculations were used to understand the separation mechanism of LNF membranes. The rejection mechanisms of these membranes for brilliant blue R250, methyl blue, Congo red, and acid fuchsin were based on size-sieving, with rejection for all >95 %. However, the rejection mechanisms of bromocresol green and bromothymol blue were affected by the size-electrostatic synergy. Pleasingly, the NaCl rejection was <10 %. Furthermore, the 22 k-PEI-modified LNF membrane could not only effectively separate dye/NaCl under high pressure and high salinity but also maintain long-term stable desalination performance. Notably, the assessment of dye rejection in LNF membranes revealed that it is not solely based on the molecular weight of dye but is also related to the geometric shape of the organic substance.