Construction of an electroneutral zinc incorporated polymer network nanocomposite membrane with enhanced selectivity for salt/dye separation

Electroneutral nanofiltration (NF) membranes are crucial to the treatment of textile wastewaters with high salinity, but it is challenging to generate electroneutral thin-film composite (TFC) membranes in a broad pH range. In this paper, we demonstrate the fabrication of an electroneutral NF composite membrane with the selective layer comprising of a zinc cation-integrated cross-linked polyvinyl alcohol (PVA)-polyethyleneimine (PEI) polymer network, which was deposited on an interlayer membrane of polydopamine-wrapped carbon nanotubes. The highly ionizable coordinative bonds formed between the Zn(II) ionic species and the amino groups in PEI facilitated ionization to generate hydrophilic and nearly electroneutral membranes with tenable electroneutrality over a broad pH range of 5.0–8.6. Subsequently, the membrane afforded excellent rejection (>99.7%) of bromothymol blue, congo red and direct yellow dye molecules while the low salt retentions of 3.35% (NaCl) and 11.57% (Na 2 SO 4 ) were realized with moderately high water flux of up to 84 L m −2  h −1 at 5 bar. Further prolonged filtration of a salt/dye mixed feed system demonstrated that the salt presence did not affect the dye separation efficiency leading to the attainment of an unprecedented high NaCl/Congo red selectivity of 932. This highlighted the potentiality of the presented approach for designing membranes with tunable surface charges for the purification of saline textile wastewaters.