A robust dually charged membrane prepared via catechol-amine chemistry for highly efficient dye/salt separation

Novel nanoporous membranes with superior permeability and exceptional molecular selectivity are highly desirable for wastewater treatment and reclamation. Herein, a 3D dual-charged multilayer membrane for efficient separation of dyes/salts is fabricated via catechol-amine chemistry surface engineering. Polyethyleneimine (PEI) is firstly coated on the hydrolyzed polyacrylonitrile substrate to construct a positively charged intermediate layer, followed by codeposition of tannic acid (TA) and poly-γ-glutamic acid (γ-PGA) to engineer a negatively charged top layer. During the polyphenol-induced competitive reaction processes, covalent interactions, hydrogen bonding and electrostatic adsorption among TA, γ-PGA and PEI hinder the rapid and uneven self-polymerization of TA, thus loosening the separation layer structure. In addition, the pre-reaction between TA and γ-PGA can weaken those competitive reactions, further tuning the pore size and dual charge layer thus improving the separation performance. Benefited from the novel loose dual-charged structure, the resultant membrane exhibits outstanding water permeability (36.9 Lm−2h−1bar−1) with low salt rejections (11.1% for Na2SO4 and 14.6% for NaCl) and high rejection to both positively and negatively charged dyes. It works well even for rejecting small-molecule dyes (100% congo red, 98% methyl green, 96% methyl orange and 86% methylene blue), far superior to the state-of-the-art membranes. Despite the loose structure, the membrane still possesses excellent acid resistance, and its hydrophilic surface contributes to excellent anti-fouling performance. Such thin-film composite membranes with tunable dual-charged multilayer prepared via this new strategy provides a promising candidate for separation of small charged molecules and inorganic salts, especially in textile industry.