Effect of sulphonated polyethersulfone substrate for thin film composite forward osmosis membrane
Graphical abstract
Introduction
The forward osmosis (FO) process has been emerging as an alternative technology for desalination, water treatment and power generation in recent years [1], [2]. FO utilizes osmotic pressure for spontaneous water transport from the feed to the draw solution (DS) across a semi-permeable membrane. Compared to the pressure driven membrane technology such as reverse osmosis (RO), the FO process displays several merits that can potentially surpass the RO technology for certain applications especially where DS recovery and regeneration are not essential [3]. Practically, FO can be used as a pre-treatment or in hybrid units for desalination. Applying FO as a pre-treatment could minimize RO fouling and scaling while at the same time provides easier cleaning process for desalination or water treatment plant [4]. FO-nanofiltration or FO-pressure assisted osmosis hybrid systems have been proposed for fertigation by using fertilizer as DS [5], [6], [7], [8]. Other potential and more practical applications of the FO process include juice and food concentration [9], protein and pharmaceutical enrichment [10], and power generation [11], [12], [13], [14].
Lack of high performing membranes and DS that can be recycled effectively still remains as a major obstacle for the commercialisation of the FO technology [2], [5], [15]. The flat-sheet cellulose triacetate (CTA) FO membrane produced by Hydration Technologies Inc. (HTI, Albany, OR) is an FO membrane which is commercially available in the market produced at a large scale, although few more companies such as Oasys (Boston, MA) have started to make FO membranes but still in limited quantities. Regardless of its broad applications for the FO process, CTA FO membrane has relatively lower water flux than the few reported polyamide-based thin film composite (TFC) FO membranes [16]. Recently, attention has been drawn to the development of asymmetric flat sheet or hollow fibre FO membranes via phase inversion technique followed by interfacial polymerization [17], [18]. This TFC FO membrane has been inspired from the membrane fabrication approach that was originally developed for the RO process. It contains a very thin polyamide (PA) rejection layer and a polysulfone (Psf) support layer casted on a fabric support (usually made of non-woven polyethylene terephthalate or PET fabric) that gives additional mechanical strength to the membrane structure [19]. Chemically-modified TFC-RO membrane has also been investigated for the FO process and it has been found that the existing RO membrane can be used for all engineered osmosis applications [20]. However, the thick nonwoven support fabric and the hydrophobic Psf support layer result in high internal concentration polarization (ICP) effects that significantly reduce the water flux during the FO process [20]. Tiraferri et al. [21] and Yip et al. [17] suggested that an optimal FO membrane should consist of a very thin PA rejection layer on top of a highly porous finger-like structure substrate to decrease the ICP. Later studies have also demonstrated that membrane hydrophilicity plays a major role in inducing water flux across semi-permeable membranes [22]. A report by Wang et al. confirmed that FO performance can be further enhanced in TFC membranes by increasing membrane hydrophilicity [23]. Hence, in addition to the membrane support layer morphological structure, the other properties such as hydrophilicity has been manipulated to enhance the water flux in the FO process. However, hydrophilic polymers pose several disadvantages as membrane substrates, which include: (1) swelling that can negatively affect the membrane substrate strength as well as the stability of the PA rejection layer due to overstretched substrate, and; (2) interfacial polymerization formation inside the pores instead of just at the membrane top surface [24]. But on a positive note, increased substrate hydrophilicity can improve binding between the PA rejection layer and the substrate top surface compared to that of a substrate with a relatively hydrophobic nature and rough surface [23], [25]. Furthermore, interfacial polymerization has a significant effect on permeability regardless of substrate hydrophilicity [24]. Based on polymer solution and casting condition, each substrate can produce a different top skin layer and structural morphology. The physical and chemical properties of the substrate can therefore affect the PA layer formation resulting in different membrane performances [24].
Recently, sulphonated polyethersulfone (SPES) has been studied as a promising material in the proton exchange membrane fuel cell (PEMFC) applications and also for both RO and FO membranes to increase hydrophilicity and fouling resistance [26], [27]. It is also assumed that the sulphonation of the polymer materials can introduce not only the hydrophilic nature to the membrane substrate which enhances the water flux of the resultant FO membranes, but also may help change its membrane substrate morphology. Therefore, the objectives of this study are: (1) to investigate the influence of sulphonation of PES on the formation of substrate morphology and its mechanical strength, and (2) to investigate the effect of blended sulphonated materials as membrane substrates on FO performance. The synthesized SPES was directly blended with PES at specific ratios to prepare modified membranes.
Section snippets
Chemicals
Polyethersulfone (PES) granules (Mn: 55,000 — Good fellow, UK) and N-methyl-2-pyrrolidone (NMP) (Sigma-Aldrich Pty. Ltd., Australia) were used for the fabrication of membrane substrates. M-phenylenediamine (MPD) with > 99% purity and trimesoyl chloride (TMC) with 98% purity (Sigma-Aldrich Pty. Ltd., Australia) were used as received in this study for the interfacial polymerization process. N-hexane from Sigma-Aldrich with > 99.0% purity was utilized as the solvent for TMC. Sodium chloride (NaCl)
Effect of membrane substrates
Sulphonated PES membrane substrate was synthesized by blending PES with the SPES and was compared with the pure PES substrate. The effect of sulphonation on the PES substrate formation by phase inversion was investigated in terms of its membrane morphology and changes in membrane hydrophilicity and mechanical strength. Fig. 2 shows the SEM images of membrane substrates casted with different concentrations of sulphonated polymer as explained in Table 1. The height of the casting knife was
Conclusions
The effect of sulphonation on the PES substrate to synthesize TFC-FO membranes was studied through substrate characterization such as morphology, wettability, tensile strength and TFC membrane performances. The following summary is drawn from this work:
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Sulphonation of PES substrate changes the substrate morphology from finger-like to a sponge-like substrate at higher degree of sulphonation, increases substrate hydrophilicity, improves membrane permeability coefficient (A value), and decreases
Acknowledgments
This study was supported by the Australian Postgraduate Award (APA), the Australian Research Council (ARC) Discovery Projects (DP140100835), and the National Centre of Excellence in Desalination Australia (NCEDA), which is funded by the Australian Government through the Water for the Future initiative.
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