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Separation performance of poly(vinyl alcohol) based nanofiltration membranes crosslinked by malic acid for salt solutions

In this study, poly(vinyl alcohol) (PVA) based nanofiltration (NF) membranes were prepared by coating a thin PVA film on polysulfone ultrafiltration support substrates. The PVA film was cross-linked using malic acid in the presence of HCl as a catalyst. The impacts of crosslinker content and PVA molecular weight on physicochemical properties and separation performance of the prepared membranes were investigated. The obtained membranes were characterized using FTIR spectra, swelling degree, and sessile drop contact angles, respectively. Then, the separation performance of the NF membrane was systematically evaluated for pure water; magnesium sulfate (MgSO4) as well as sodium chloride (NaCl) solutions using a custom fabricated 4-cell crossflow desalination system. On increasing the malic acid content, the extent of crosslinking degree increased and disrupted the crystallinity of the PVA film. The salt rejection of the prepared membranes was found to increase and then decrease through the maximum point of malic acid content for 20 wt%, while the water permeability showed the opposite trend. Moreover, the results revealed that the prepared membrane with higher molecular weight exhibited lower water permeability but better salt rejection.

Moreover, the results revealed that the prepared membrane with higher molecular weight exhibited lower water permeability but better salt rejection. and maintenance cost as compared with RO process [2].

INTRODUCTION
Polyamide (PA) based NF membranes have been successful commercialized for brackish water desalination. PA membranes are fabricated through the interfacial polymerization using multifunctional amine and acyl chloride monomers. They show high water flux, good rejection multivalent ions, but low anti-fouling property, low chemical stability, and weak chlorine tolerance [1][2][3][4][5][6]. In developing countries, the high fabrication cost also is one of the obstacles restricting the application of PA based NF membranes [2]. Recently, poly(vinyl alcohol) (PVA) has been intensively used for preparing NF membranes owing to its good physical and chemical stability, low cost, commercial availability and excellent filmforming property [2][3][4][5][6] [1,[3][4][5][6]. Previous studies demonstrated that malic acid, a dicarboxylic acid with an additional hydroxyl group in its molecule, was a good crosslinking agent for making PVA membranes [1,[4][5][6]. The PVA membranes crosslinked by malic acid exhibited not only good chemical stability and separation performance but also high anti-fouling property [1,6].
This work focuses on the preparation of PVA based NF membranes by coating a crosslinked PVA thin film on the surface of polysulfone ultrafiltration (UF) substrates. The PVA thin film was crosslinked by malic acid in the presence of HCl as a catalyst. The effects of malic acid content and PVA molecular weight on the physicochemical properties water permeability and salt rejection of the prepared membranes were systematically investigated and thoroughly discussed.

Chemicals and materials
PVA powders (Mw 31kDa and Mw 61kDa) were purchased from Sigma-Aldrich. Malic acid (C4H6O5) with the purity of 99% received from Merck was used as crosslinking agent. The commercial UF membrane (PS20-Dow-filmtec) was utilized as the supporting substrate where the crosslinked PVA film was coated. HCl (35%) was received from Merck.

Membrane preparation
PVA solutions with a concentration of 0.1 wt/v% were prepared by dissolving PVA in deionized (DI) water at 90 o C under constant stirring for 2h. Next, PVA solutions were cooled to room temperature and then, crosslinking agent malic acid was added along with 2M HCl as catalyst under continuous stirring to produce the coating solution. The content of malic acid was varied according to the crosslinker per PVA weight ratio of 5 wt% to 60 wt%. The supporting substrate was taped onto the glass plate, and only the membrane surface side was contacted with PVA solution in dip coating process. PVA solution was coated onto supporting membrane for 10 min. The PVA coated membrane was dried at the ambient temperature for 24h. The obtained membrane was immersed into the same PVA solution again for 10s and dried in air for 24h. Finally, the obtained membrane was cured at 100 o C for 1h to accelerate the crosslinking reaction in PVA film [3].

Membrane characterization and separation performance
The derived membranes were characterized by using a Bruker FTIR spectrometer. Three replicate FTIR spectra were obtained for each membrane type, with each spectrum averaged from 100 scans collected from 400 to 4000 cm -1 at 4 cm -1 increments. Pure water contact angles were determined from measured sessile drop contact angles on membranes using the contact angle goniometer. Six equilibrium contact angles were measured for each sample.
For swelling experiments, the pieces of dried membranes with the dimension of 3×3 cm were immersed in pure water at 30 o C for 48h to reach equilibrium swelling. The swollen membranes were wiped carefully using tissue paper for removing residual solution on the membrane surfaces. Then, the swollen membranes were weighted by a mass balance (accuracy ± 0.0001 g). The degree of swelling was defined as Wherein, WS (g) and WD (g) were the mass of the swollen membrane and the mass of the dried membrane, respectively.
Where Qp was the permeate water flow rate and Am was the effective membrane area. The water permeability of the prepared membranes was determined as Wherein, A was the water permeability, P was the operational pressure and  was the osmotic pressure of the salt solution. Feed and permeate conductivities were used to calculate the observed salt rejection using the following equation

Effect of malic acid content on separation performance of prepared NF membrane
The FTIR spectra of the prepared membrane was presented in Fig. 1   membranes [5,6]. It was obviously acknowledged that semi-crystalline PVA had the impermeable crystalline region and the permeable amorphous matrix [1,[3][4][5][6]. The crystal structure, forming from hydrogen linkage, depleted both the sorption sites and the mobility of the polymer chains, which allowed the high transport of solvent and solute molecules through the membrane [1,[3][4][5][6]. Meanwhile, the polymer chains in the amorphous structure were much more mobile and thus, the mass transfer of water and solute molecules throughout the membrane was promoted. The XRD spectra described the crystallinity of the prepared membranes were shown in Fig. 2. It revealed that the crystalline peak (at 2 = 26.6 o ) was reduced with the increase in malic acid content. Moreover, at higher malic acid content (>20 wt%) the crystalline peak was observed to destroy mostly and separate into two small peaks. The decrease in the crystalline peak implied that crosslinking reaction disrupted the crystallinity and induced the increase in the amorphous fraction of the PVA film. The decrease in the crystallinity of the PVA film might be due to the incomplete crosslinking reaction, resulting in the addtion of large carboxylic acid moieties in the PVA film. Moreover, the large molecular network with high degree of crosslinking was also contributed to inhibited the chain segment motion for crystallization in the PVA film. Fig. 3 showed the effects of the malic acid content on the water contact angle and swelling degree of the prepared membranes. The swelling degree was observed to reduce significantly when raising the malic acid content from 5 wt% to 20 wt%, and go up as increasing the malic acid content above 20 wt%. The water contact angle indicated the hydrophilic property of the membrane surface. The higher water contact angle signified the lower hydrophilicity of the membrane surface. The water contact angle was found to rise as increasing malic acid content. On increasing the crosslinker content, much more hydroxyl groups in the PVA matrix reacted with carboxylic groups on the malic acid to produce the ester crosslinking linkages [1,[3][4][5][6][7]. Thus, the increase of the crosslinking density resulted in the decrease of hydrophilicity of the membrane surface.  NaCl for 2 g/L (Fig. 4). The water permeability of the prepared membranes decreased with malic acid content increasing from 5 wt% to 20 wt%, and then increased when increasing malic acid content above 20 wt%. Meanwhile, solute rejection presented the opposite trend. Salt rejection of the prepared membranes increased and then decreased with the malic acid content of 20 wt%. From the results, it was suggested that both the hydrophilic and crystallinity affected the separation performance of the prepared membranes. At lower crosslinker content (<20 wt%), the crystallinity of the resulting membrane was not completely demolished, while the hydrophilic property was declined. The reduction in hydrophilicity dominantly diminished the affinity between the water molecules and membrane surface. Therefore, the permeability was reduced, but the salt rejection of the prepared membranes was improved.
However, at higher crosslinking agent content (>20 wt%), the disruption of the crystalline regions in the PVA membrane was superior. The more amorphous fractions were formed in the PVA membrane, promoting the higher permeability and transport of the water and salt molecules through the resulting membranes.
Accordingly, the crosslinked PVA membranes had higher water permeability but lower salt rejection as increasing the malic acid content above 20 wt%.  membrane prepared by PVA molecular weight of 61 kDa had lower hydrophilic property, swelling degree and water permeability as compared with that made by PVA molecular weight of 31 kDa.

Effect of PVA molecular weight on the separation performance of NF membranes
Meanwhile, the MgSO4 and NaCl rejection of 61 kDa PVA membrane was double as compared to 31 kDa PVA membrane (Fig. 7). It could be explained by XRD results that the NF membrane formed by PVA molecular of 31 kDa possessed higher amorphous fractions than that formed by 61 kDa. In the amorphous regions, the PVA chain network was more flexible resulting in the easy penetration and diffusion of the species, like water and solute molecules in the PVA membrane. As a result, the membrane made from PVA molecular of 31 kDa exhibited higher permeability but lower salt rejection than that made from PVA molecular of 61 kDa.
The crosslinked PVA-based NF membrane synthesized with 61 kDa PVA concentration of 0.1 wt%, the malic acid content of 20 wt% was utilized to evaluate the separation performance for salt solutions with different concentration from 0.5 to 15 g/L. The results of water permeability, MgSO4 rejection and NaCl rejection of the prepared membrane were shown in Fig. 8. It described that the water permeability was slightly reduced from 5.0 to 4.2 x 10 -12 mPa -1 s -1 and the salt rejection was also decreased with the increase of salt concentration in the range of 0.5-15 g/L. In particular, the retention of MgSO4 was decreased approximately from 73% to 62% and the rejection of NaCl was reduced from 39% to 16%, respectively.