Novel solvent-resistant nanofiltration membranes using MPD co-crosslinked polyimide for efficient desalination

Highlights

• A novel solvent-resistant NF membrane was prepared for efficient desalination.

• Changes in properties of PI substrate affect the performance of NF membrane.

• The membrane showed highly increased rejection rate for univalent salts.

• The membrane also displayed excellent solvent resistance.

• The membrane has promising application in treatment of high-salinity wastewater.

Abstract

Solvent-resistant nanofiltration (NF) membranes have numerous application prospects. In this study, a novel solvent-resistant nanofiltration membrane with a double layered polyamide structure was prepared for efficient desalination. The substrate was polyimide crosslinked with m-phenylenediamine (MPD), and the surface layer was polyamide prepared by typical interfacial polymerization. The crosslinking was confirmed by Fourier transform infrared spectroscopy. Investigation of the effects of different reaction conditions on membrane performance showed that the optimal reaction conditions were found to be: 5% aqueous MPD solution with crosslinking time of 10 min and piperazine concentration of 0.1% (m/v). The rejection rate for sodium sulfate and sodium chloride were 99.13% and 97.45%, respectively. The membrane morphologies were studied by scanning electron microscopy and atomic force microscopy. The membrane showed electronegativity in the pH range of 3.0–10.0 and its contact angle was 54.28°, indicating excellent hydrophilicity. After soaking in several solvents for a week, the membrane still had a high rejection rate of more than 90% for Na₂SO₄, demonstrating its excellent solvent resistance.

Introduction

Typical NF membranes are normally prepared by interfacial polymerization (IP) method at the interface between two immiscible phases. Diamine and acyl chloride monomers are used to obtain a dense and thin film composite (TFC) structure on the porous substrate. They are widely applied in seawater desalination, sewage reuse, and drinking water treatment. Presently, the development of solvent-resistant nanofiltration membrane is still a hot topic of research [1,2]. For example, in the chemical, petroleum, and pharmaceutical industries, treatment of high-salt wastewater is very difficult, due to the complex nature of its components. It not only contains a large amount of organic solvents, but also high amounts of salts, such as sodium chloride and sodium sulfate [3].

However, designing of solvent-resistant membranes still remains a challenge due to the chemical instability of conventional polymers, such as polysulfone, polyacrylonitrile, and polyethersulfone in organic solvents [[4], [5], [6], [7], [8], [9]]. There are currently three methods for preparing solvent-resistant membranes. The simplest method is to prepare the membrane using a chemically resistant polymer, such as polyether ether ketone [10], polyphenylsulfone [11], or cellulose [12]. However, polymers used in membrane preparation are generally difficult to dissolve and almost no polymer can withstand all solvents, which limits their applications. The most common method of preparing solvent-resistant membranes involves chemical crosslinking with ultrafiltration (UF) membrane or NF membrane [[13], [14], [15], [16]]. Crosslinking not only increases the chemical stability, but also reduces the pore size. However, this approach reduces the free volume available, resulting in low flux.

The third option is to design membranes with two layers. The surface layer is usually prepared by conventional methods such as interfacial polymerization, coating, etc. The substrate is a cross-linked UF membrane or others, which is resistant to organic solvents [5,17]. The advantages of solvent-resistant membrane are that high flux and high selectivity can be easily achieved and the two layers can be operated separately [[18], [19], [20], [21], [22]]. For example, Zhou et al. brought about cross-linking of vanillic alcohol and 1,3,5-benzenetricarbonyl trichloride (TMC) monomers in ethylenediamine (EDA). A composite NF membrane was synthesized by interfacial polymerization (IP) on an ether imide substrate. The prepared membrane showed DMSO permeability of 21 L m⁻² h⁻¹ MPa⁻¹ [23]. Hai et al. used poly (ethyleneimine) with a mixture of EDA and a branched polymer to crosslink poly (ether imide), which was further used to synthesize NF membranes by IP method, using 1,2,4,5-benzene tetracarbonyl chloride (BTC) monomer. The membrane showed permeation of 30.4 L m⁻² h⁻¹ MPa⁻¹ and 90.4% rejection of Rhodamine B in aqueous solution [24].

Compared to traditional polymeric membranes, polyimide (PI) membranes have higher solvent resistance, particularly to organic solvents, and are most commonly used for the development of solvent-resistant membranes [[25], [26], [27], [28], [29]]. A well-established method for increasing the stability of polyimides in corrosive solvents is to crosslink them with diamines [30]. Different diamines are used for cross-linking to improve their chemical resistance in harsh environments [[31], [32], [33], [34], [35], [36], [37]]. Lee et al. prepared solvent-resistant membranes through IP reaction of m-phenylenediamine (MPD) with 1,2,4,5-benzene tetracarboxylic acyl chloride (BTAC), followed by imidization reaction to convert the polyamide acid to PI. Then the substrate and the surface layer crosslinked integrally, which further enhanced the solvent resistance. Their study demonstrated that PI could react with MPD through covalent interactions, wherein a strong bond could be formed between the substrate and epidermal layer [38]. This provided some inspiration for our research work.

Hence, in this study, the novel PI developed before and MPD were co-crosslinked. Subsequently, a polyamide layer was built on the surface of crosslinked PI membrane, resulting in the formation of a double layered polyamide membrane. The effects of different reaction conditions on the membrane performance were also investigated. The membranes prepared under optimized reaction conditions were characterized for morphology and surface properties. Filtration experiments showed that the solvent-resistant membrane had excellent stability against a variety of organic solvents. In addition, the membrane not only showed a very high rejection rate for divalent salts, but also a good increase in rejection rate for monovalent salts, which greatly expands its scope of possible applications.