Polyvinyl alcohol hydrogel-supported forward osmosis membranes with high performance and excellent pH stability
Graphical abstract
Introduction
Forward osmosis (FO) is a membrane-based separation process that is operated by a transmembrane osmotic pressure difference between high- and low-concentration solutions. FO is currently regarded as a promising separation technology for renewable energy generation, wastewater treatment and desalination because of its high energy efficiency [1]. However, the lack of high-performance and durable FO membranes has hampered the commercial implementation of the FO process. FO membranes are typically fabricated by creating a polyamide (PA) selective layer on a porous support through interfacial polymerization (IP) [1], [2], [3], [4]. The FO performance of the resulting thin film composite (TFC) membrane can be optimized by separately tuning its selective and support layer structures.
As for the support, reducing internal concentration polarization (ICP) is critically important to enhance FO flux. A thinner, more hydrophilic and more porous support with highly interconnected pores can minimize ICP by promoting internal salt transport [5], [6]. Various polymers, including polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyethersulfone (PES) and polysulfone (PSF), have been fabricated into porous films via phase inversion or electrospinning for use as the supports of TFC FO membranes [7], [8], [9], [10], [11]. However, many of these supports have insufficient hydrophilicity, reducing their water wettability and consequently aggravating ICP [5]. Hence, significant efforts have been made to reduce ICP by incorporating hydrophilic porous particles (e.g., silica, modified carbon nanotube, zeolite and metal organic framework) into the support or by modifying the support with hydrophilic materials [8], [12], [13], [14]. Although these strategies effectively mitigated ICP, they suffer from certain technical bottlenecks, including high production costs, complicated manufacturing procedures and difficulty in scaling-up the fabrication process [11], [15]. In addition to high separation performance, FO membranes require sufficient pH stability for practical applications [16], [17]. However, commercial FO membranes have been reported to exhibit poor durability under harsh pH conditions [3], [18], [19], [20], while the pH stability of FO membranes currently in development has not been extensively assessed. Hence, a new class of hydrophilic and pH-resistant polymeric materials that can be formed into a highly porous structure is required to develop FO membranes with high performance and satisfactory pH stability.
Polyvinyl alcohol (PVA), often referred to as hydrogel, has been widely used for various applications, including tissue engineering, drug delivery and food packaging, owing to its high hydrophilicity, excellent pH stability, low cost and easy processability [21], [22], [23]. They can be readily fabricated into porous or dense films in either a neat or crosslinked form. In fact, PVA has been employed to functionalize and modify reverse osmosis (RO) and FO water treatment membranes to improve their separation performance, fouling resistance and chlorine stability [24], [25], [26]. Furthermore, PVA has been fabricated into dense selective layers for pervaporation, nanofiltration and FO applications [27], [28], [29] and porous layers for ultrafiltration [30]. PVA has also been fabricated into a porous nanofiber mat via electrospinning for use as the support of the FO membrane [31]. However, the lack of the scalability and mechanical robustness of the electrospun PVA support restrict its practical application. Furthermore, the beneficial feature of the PVA support in terms of pH stability has not been demonstrated.
In this work, we demonstrate that a porous PVA hydrogel support prepared via a commercially viable non-solvent-induced phase separation (NIPS) process can be used to fabricate a high-performance FO membrane with excellent pH stability. An intrinsically hydrophilic PVA support with high porosity and great pore interconnectivity was formed via NIPS and subsequent chemical crosslinking. We also employed toluene-assisted interfacial polymerization (TIP), which utilizes toluene as an organic phase for IP [32], [33], [34], to synthesize a PA layer on the fabricated hydrophilic PVA support. This is because TIP has proven to be effective for fabricating a highly-performing PA layer on a hydrophilic support, something that is difficult to achieve with conventional IP, which utilizes aliphatic hydrocarbons as the organic phase [32], [33], [34]. The FO performance of the PVA-supported TFC (PVA-TFC) membrane fabricated via TIP using optimal protocols was assessed with varying the draw solution (DS) concentration and compared with that of commercial and previously reported FO membranes. The pH stability of the proposed membrane under harsh pH conditions (pH 2 and 12) was also compared with that of commercial FO membranes. Furthermore, the structures and properties of the PVA-TFC membrane were comprehensively characterized to identify the relationship between membrane properties and resulting performance.
Section snippets
Materials
PVA (>99% hydrolyzed, Mw = 85–124 kg mol−1), polyvinyl pyrrolidone (PVP, Mw = 40 kg mol−1), sodium dodecyl sulfate (SDS, 99%) and glutaraldehyde (GA, 25%) aqueous solution were obtained from Sigma-Aldrich. Trimesoyl chloride (TMC, 98%) and m-phenylenediamine (MPD, 98%) were obtained from TCI. Toluene, ethanol (EtOH), sodium sulfate (Na2SO4, anhydrous), sodium hydroxide (NaOH), sodium chloride (NaCl, 99%), hydrochloric acid (HCl, 35%) and sulfuric acid (H2SO4, 95%) were received from Daejung Chemical.
Optimization of the PVA support
The structures of the PVA supports fabricated using dope solutions with different PVA concentrations are displayed in Fig. 2. All of the fabricated PVA supports exhibited an asymmetric structure; the top surface had larger pores with higher porosity than the bottom surface, which was caused by the polymer concentration gradient across the cast film depth created during solvent/non-solvent exchange [41]. When the PVA concentration was raised from 8 to 12 wt%, the surface pore size and porosity of
Conclusions
A highly-performing and pH-resistant FO membrane was made using a porous PVA hydrogel support via TIP. The thinness, strong hydrophilicity, high porosity and excellent pore interconnectivity of the PVA support prepared via NIPS and subsequent chemical crosslinking led to a considerably low S. The TIP process created a highly selective and permeable PA layer on the PVA support. Consequently, the fabricated PVA-TFC membrane produced higher FO water flux and salt selectivity than commercial and
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2019R1A2C1002333 and 2019M3E6A1064103) and the Technology Innovation Program (20010914) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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