Novel composite nanofiltration membranes containing zwitterions with high permeate flux and improved anti-fouling performance

Abstract

This article provides a novel method for introducing zwitterionic polymers into nanofiltration membranes, with the aim of achieving high water flux and good anti-fouling performance. Terpolymers P(DMC-HEA-DMAPS) (PDHD) composed of 2-methacryloyloxy ethyl trimethylammonium chloride (DMC), 2-hydroxyethyl acrylate (HEA) and 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) were synthesized via aqueous phase free-radical copolymerization. Composite nanofiltration membranes (CNFMs) were prepared with PDHD and glutaraldehyde (GA) via the combination of surface coating and chemical cross-linking methods. Chemical structures and compositions of PDHDs and CNFMs were characterized by (attenuated total reflectance) Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and atomic force microscopy. The water permeability of CNFMs was examined with nanofiltration test and dynamic water contact angle measurement. Optimum nanofiltration performance was obtained for CNFM3 with 4.35 mol% DMAPS, e.g. $R_{{\text{MgCl}}_2}=96\text{.5}\%$, $J_{{\text{MgCl}}_2}=47{\text{.8\hspace{0.17em}L\hspace{0.17em}m}}^{-2}{\text{\hspace{0.17em}h}}^{-1}$ (testing with 1 g L⁻¹ aqueous MgCl₂ solution at 25 °C and 0.6 MPa). Moreover, the extent of fouling for CNFM3 was significantly reduced and most of the fouling was reversible during the MgCl₂ and protein filtration test. Therefore, the water permeability and anti-fouling property of CNFMs were significantly improved with introducing zwitterionic groups into the membranes.

Introduction

The global water crisis and the growing demand for environmental protection spurs people to find new energy-efficient technologies for water treatment [1]. Nanofiltration (NF) as an environment-friendly separation technology has attracted more and more research attention. Nanofiltration is a pressure-driven process intermediate between reverse osmosis and ultrafiltration, and combines the advantages of high-salt rejection and low-energy consumption [2]. Nanofiltration membranes with a pore size in the range of 0.5–2.0 nm are usually negatively/positively charged. They separate substances based mainly on the electrostatic repulsive effect and the steric hindrance effect [3]. Recently, they have been widely used in the desalination of brackish water and seawater [4], wastewater treatment [5], and industrial separation [6], etc. Therefore, nanofiltration membranes with good separation performance and anti-fouling property are required for the more complicated applications.

To address these requirements, some hydrophilic and low-fouling polymers have been employed for preparing nanofiltration membranes [7], [8]. For example, Qiu et al. [9] prepared nanofiltration membranes by grafting polyacrylic acid onto the surface of cardo polyetherketone (PEK-C) ultrafiltration membranes. Asatekin et al. [10] prepared nanofiltration membranes by coating the comb copolymer polyacrylonitrile-graft-poly (ethylene oxide) (PAN-g-PEO) selective layer on a polyacrylonitrile (PAN) ultrafiltration membrane support. In addition, polyvinyl alcohol (PVA) as a common hydrophilic polymer, has been introduced into polyamide composite nanofiltration membranes via physical adsorption or in situ interfacial polymerization [11], [12], [13]. According to previous reports, the water flux and anti-fouling performance of these nanofiltration membranes have been improved. However, there are still some disadvantages, such as the uncontrollability of surface graft polymerization, the rapid autoxidization of polyether, and the instability of physical adsorption, and so on. Therefore, it is necessary to develop new membrane materials for conveniently preparing nanofiltration membranes with both good separation and anti-fouling performance.

In recent years, zwitterionic polymers, containing both anionic and cationic groups in the same monomer unit, such as polyphosphobetaine, polysulfonbetaine and polycarboxybetaine, have been attractive as a new type of anti-fouling materials [14]. It is found that zwitterionic surfaces can form a “free water” hydration layer because of the intra- and inter-ether proximity between the oppositely charged groups, resulting in excellent biocompatibility and anti-fouling property [15], [16]. Until now, most studies of zwitterionic polymers have focused on the improvement of materials’ biocompatibility [17], [18]. Only a few research groups have used zwitterionic polymers to modify microfiltration and ultrafiltration membranes. Initially, Hawes and co-workers grafted zwitterionic 2-methacryloyloxy-ethyl phosphorylcholine onto surfaces of poly(vinylidene fluoride) (PVDF) and cellulose acetate (CA) microfiltration membranes with the plasma etching technique [19]. After that, Zhao et al. tethered the zwitterionic poly(sulfobetaine methacrylate) onto polypropylene (PP) membrane surface through a combined method consisting of UV-induced surface graft polymerization and surface-initiated atom transfer radical polymerization (ATRP) [20]. Because these surface graft polymerization methods usually need special equipment, complicated preparation process and extensive use of organic solvents and monomers, they are not suitable for up-scale production. Jiang and coworkers [21], prepared poly(acrylonitrile-([3-(methacryloylamino)propyl]-dimethyl(3-sulfopropyl) ammonium hydroxide)) random copolymers, and blended them with PAN to prepare asymmetric ultrafiltration membranes via a phase-inversion method. Zhang et al. also prepared ultrafiltration membranes with carboxybetaine and sulfobetaine poly(arylene ether sulfone) through a similar method [22]. And all of the above studies have shown that the permeability and anti-fouling property of these separation membranes were improved via introducing zwitterions into the membranes. However, as far as we know, there have been no reports about employing zwitterionic polymers for preparing nanofiltration membranes.

In this study, a new type of zwitterionic terpolymers P(DMC-HEA-DMAPS) (PDHDs) was prepared with 2-methacryloyloxy ethyl trimethylammonium chloride (DMC), 2-hydroxyethyl acrylate (HEA) and 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) by a aqueous phase free-radical copolymerization method. Composite nanofiltration membranes (CNFMs) were successfully prepared with terpolymers PDHDs and glutaraldehyde (GA) through surface coating and chemical cross-linking methods. Chemical structures and compositions of PDHDs and CNFMs were characterized with (ATR)-FTIR, XPS, SEM and AFM. The permeability and anti-fouling property of CNFMs were evaluated systematically, and both of which were improved a lot with introducing zwitterions into the membranes. Therefore, PDHDs as a new type of zwitterionic terpolymers are promising materials for preparing composite membranes with good nanofiltration performance.