A tight nanofiltration membrane with multi-charged nanofilms for high rejection to concentrated salts

The fractionation or rejection of concentrated salts by nanofiltration membrane remains the biggest challenge owning to the charge neutrality and concentration polarization. A tight nanofiltration membrane with ultrathin dually charged nanofilm down to 17 +/- 5 nm was synthesized to improve the rejection of highly concentrated salts up to 20 g/L. The ultrathin positive polyethylenimine (PEI) and negative poly(acrylic acid) (PAA) polyelectrolyte nanofilm was assembled on the loose nanofiltration membrane via the aid of bio-glue dopamine. The physicochemical property of dependent nanofilm was characterized by the surface morphology (SEM, AFM), element variation (XPS), surface charge (Zeta potential), molecular weight cut off and stokes radius respectively. The influence of dopamine deposition time and PEI concentration on separation performance was investigated. The optimum membrane exhibited outstanding rejection to diluted multivalent salts (1 g/L, 98.5% MgSO4, 98.3% Na2SO4, 97.2% MgCl2), high removal of heavy metal ions (1 g/L, 93.5% Cd2+, 95.2% Cu2+, 92.7% Pb2+) as well as high removal of dyes (100% Congo red, 99.93% Victoria blue B, 99.91% Brilliant green, 99.82% Basic red 2, 99.03% Neutral red). Moreover, the resultant membrane showed exceptional rejection to highly concentrated salts (20 g/L, 93.4% Na2SO4, 92.6% MgCl2, 93.5% MgSO4), exceeding the previously reported membranes. Both the multi-charged nanofilms and nano-scaled thickness contributed to the outstanding nanofiltration performance.