A novel thin film composite forward osmosis membrane using bio-inspired polydopamine coated polyvinyl chloride substrate: Experimental and computational fluid dynamics modelling
Introduction
Water scarcity and increasing safe freshwater demand due to the rapid population growth, climate change and industrialization, has motivated researchers for development of novel technologies for desalination and water reclamation (Adnan et al., 2019). Among the numerous technologies for supplying potable water, seawater desalination by the membrane-based processes such as reverse osmosis (RO) is known as one of the most effective methods due to its high separation properties (Shabani et al., 2020). However, it is not energy efficient and has high-cost of operation and maintenance (Blandin et al., 2016; Park et al., 2015). Forward osmosis (FO) is an emerging and unique membrane process with high potential for seawater desalination and water reuse (Ghanbari et al., 2016). Unlike RO that uses external high hydraulic pressure for water permeation, osmotic pressure generated by the osmotic agent is the driving force of FO process to extract water molecules from the FS (feed solution: low concentration aqueous solution) to the DS (draw solution: high concentration aqueous solution) through a semipermeable membrane (Sahebi et al., 2020, 2016; Shabani and Rahimpour, 2016). The main advantages of FO process are low carbon footprint and energy consumption, low fouling propensity and high water recovery (Alturki et al., 2013; Chiao et al., 2019; Kwon et al., 2019; Majeed et al., 2016). These have led to widespread applications, not only in wastewater treatment and desalination, but also in power generation, food processing, pharmaceutical industries and fertigation in agriculture sector (Lin et al., 2020; Sahebi et al., 2020; Zhao et al., 2012).
Although the advantages of FO are interesting, the development of robust and high-quality membrane and suitable DS still remains the main concern. An ideal FO membrane should possess high WF and solute rejection, low RSF, internal concentration polarization (ICP) and fouling propensity and high chemical and mechanical stability, simultaneously (Kwon et al., 2015; Qasim et al., 2015). Amongst FO membranes, TFC FO membranes have been employed widely due to their brilliant permeability and selectivity (Kuang et al., 2016). A TFC asymmetric membrane is made of two different layers: A thin active layer of polyamide (PA) is placed onto a support layer. Both active and support layers can be optimized separately for modification of TFC membranes. High solute rejection and low RSF are achieved by controlling the PA top layer, while the support layer plays a crucial role in determining high WF (Morales-Torres et al., 2016). Hence, many researchers have focused on optimization of the TFC membrane support layer to decrease ICP and improve FO performance. The ICP problem arises due to difficult solute diffusion in the membrane support layer resulting in reduced effective driving force and thus WF (Tavakol et al., 2020; Zhao et al., 2012). It has been found that minimization of structural parameter (S) of the substrate can alleviate the ICP effect. Reduction of thickness and tortuosity and also increment of support layer porosity can reduce the S parameter (Ma et al., 2020; Nakagawa et al., 2020; Pan et al., 2017a). Moreover, hydrophilicity of the support layer has significant effect on the reduction of ICP (Ghanbari et al., 2016; Pan et al., 2017b).
Polysulfone (PS), polyethersulfone (PES), cellulose acetate (CA) and cellulose triacetate (CTA) are widely used as the main polymer for preparation of FO membranes (Baniasadi et al., 2021; Pardeshi et al., 2017). Although CA and CTA are hydrophilic polymers, they have some limitations such as hydrolysis and biological attack. On the other hand, apart from advantages of PS and PES polymers, high cost and hydrophobicity make them undesirable for FO membranes (Akther et al., 2015; Pardeshi et al., 2017; Zuo et al., 2017). Recently, among all polymers, polyvinyl chloride (PVC) has been accepted as an outstanding choice for membrane fabrication because of its excellent mechanical and thermal characteristics, solubility in various solvents, easy to modify and chemical resistance to acids, bases, chlorine and solvents. Another prominent property of PVC is its very lower price compared with the other polymers (Farjami et al., 2020; Pardeshi et al., 2017; Zheng et al., 2018a). These attributes of PVC have made it as a suitable candidate for membrane preparation. However, hydrophobic nature of PVC is its important drawback. It is necessary to increase the membrane hydrophilicity by hydrophilic modification to improve the membrane performance. A large number of previous studies have focused on designing proper FO membranes by incorporation of hydrophilic inorganic particles into polymeric solutions such as PS/titanium dioxide (TiO2) (Emadzadeh et al., 2014), PS/silica (Liu and Ng, 2015), PES/carbon nanotube (CNT) (Wang et al., 2013), PS/graphene oxide (GO) (Park et al., 2015) and PES/metal-organic framework (MOF) (Arjmandi et al., 2019). However, due to poor dispersion of nanoparticles in dope solutions, achieving homogeneous distribution of these nanoparticles into the membranes’ substrate is difficult (Shah et al., 2019). Apart from the nano-enabled support membranes, different substrates’ chemical modification techniques such as polymer blending (Sahebi et al., 2016), dip coating (Park et al., 2018), vacuum filtration (Zhao et al., 2017) have been used to improve FO performance.
PDA, a mussel inspired polymer, has been widely used in membrane technology to modify the membranes. Dopamine (DA, 3,4-dihydroxyphenethylamine), as a precursor for PDA formation, forms PDA in an alkaline condition via oxidative self-polymerization (Zarghami et al., 2019b). Due to catechol and amine functional groups of DA, adherent PDA thin layer can be created on any substrate (Zarghami et al., 2019a). PDA modification has been used in various applications such as ultrafiltration (UF), nanofiltration (NF), FO, and RO membranes (Arena et al., 2011; Han et al., 2012; Shah et al., 2019; Wang et al., 2019; Zarghami et al., 2019a; Zhao et al., 2014). Studies in the literature have shown that PDA has positive effect on membrane performance by increasing membrane hydrophilicity and it may actively interact with trimesoyl chloride (TMC) monomer during interfacial polymerization (IP), which leads to formation of a higher quality PA layer (Han et al., 2012).
There are only a few studies which have used PVC (Pardeshi et al., 2017; Zheng et al., 2018a, 2018b) as support membranes in FO applications. All of them utilized a hydrophilic additive for preparation of the polymeric dope solution including poly(vinyl butyral) (PVB) [44], sulfonated polysulfone (SPSU) (Zheng et al., 2018a) or layered double hydroxide LDH nanoparticles (Pardeshi et al., 2017) for enhancement of the support membrane performance. However, there is the lack of study focused on the PVC-based TFC membrane and its substrate’s surface coating in FO process.
Thus, the main objective of this study was to develop the low-cost hydrophilic PVC substrate using a fast and facile bio-inspired surface coating technique for preparation of a TFC membrane. Initially, due to the lack of studies on optimization of PVC membrane in the FO process, the effect of PVC concentration on morphology of the substrate was investigated to achieve a TFC membrane with high FO performance. Subsequently, to improve hydrophilicity and separation performance of the membrane, the optimum PVC membrane selected in the previous step was surface functionalized by the PDA interlayer at different conditions. Then, CFD simulation was employed to confirm the obtained experimental results. In the developed CFD model, all resistivity values, ICP, ECP (external concentration polarization), RSF, and sorption coefficients were applied to ameliorate the precision of its prediction. In the next step, 1000 W F and RSF values were calculated and their average was reported. As far as the authors know, this is the first report about application of the bio-inspired PVC membrane in concentration-driven processes. Also, this research is the first evaluation in terms of optimizing the PVC substrate in FO process.
Section snippets
Materials
PVC powder (MW = 90,000) was purchased from Arvand Petrochemical Co., Iran. N-methyl-2-pyrrolidone (NMP) (≥ 99.5 %) and n-hexane (>95.0 %) were supplied by Merck. Trimesoyl chloride (TMC) (98 %), m-phenylenediamine (MPD) (99.5 %), and dopamine hydrochloride (DA) (99 %) were provided by Sigma-Aldrich, Exir and Acros Organics, respectively. Tris (hydroxymethyl) aminomethane (Tris) (≥ 99.0 %) was obtained from Bio Basic. Sodium chloride (NaCl) (99 %) was supplied by Dr. Mojallali Co., Iran.
PVC membranes preparation
Phase
Model definition
A CFD model, finite element method, for the FO membrane process was developed by COMSOL Multiphysics® (Version 5.4, COMSOL Inc.). The diagram of FO and PRO modes and physical properties (calculated by OLI Stream Analyzer 3.2) of the solution are presented in Fig. 3 and Table 2, respectively.
Governing equations
To simplify the CFD model, the below assumptions were considered:
- 1
Steady-state conditions
- 2
Thermodynamic equilibrium at the interface of active layer
- 3
Incompressible and laminar flow through FS and DS sides
- 4
Morphology and characteristics of the membrane substrate
The TFC FO membranes compose of two layers (substrate and PA rejection layer) which can be developed, separately. Studies have shown that optimization of the substrate has great effects on the active layer formation and FO performance (Zheng et al., 2018b). Hence, first, PVC substrates with different polymer concentrations were prepared to study their influence on the resultant TFC-FO membrane. The values of thickness, porosity, mean pore size, and pure water permeability (PWP) are presented in
Conclusions
In this study, a new bio-inspired PVC TFC-FO membrane was developed. This purpose was pursued by 3 steps. At first, the PVC membranes with different concentrations were fabricated and evaluated in both FO and PRO modes using DI water as FS and 1 M NaCl as DS. Then, the selected PVC membrane was modified with PDA. The following results were summarized:
- 1
Lower PVC polymer concentration results in the thinner and more porous substrate which has lower S parameter and higher FO performance. The
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
This work was supported by Iran National Science Foundation (INSF) (Grant numbers: 98010526 and 96008182).
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2023, ChemosphereCitation Excerpt :Overall, the presence of MPs may aggregate membrane fouling; hence, more systematic studies are warranted to investigate the fouling impact of MPs and possible mitigation methods. Within membrane technologies, forward osmosis (FO) membrane systems have attracted much attention as an emerging technology for advanced wastewater treatment over the past decade, owing to their spontaneous process driven by osmotic pressure instead of hydraulic pressure (Hartanto et al., 2016; Kim et al., 2020b; Li et al., 2018; Li et al., 2019; Ly et al., 2019; Shabani et al., 2021; Zargar et al., 2015; Xie et al., 2015; Zargar et al., 2020). In the FO process, water permeates through a semipermeable membrane from the feed solution (FS) to the draw solution (DS), due to the natural osmotic pressure of the DS, while other solutes/pollutants are rejected (Hartanto et al., 2019; Mi and Elimelech, 2010).