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Fabrication of high-performance PVA/PAN composite pervaporation membranes crosslinked by PMDA for wastewater desalination

Fabrication of high-performance PVA/PAN composite pervaporation membranes crosslinked by PMDA for... The pyromellitic dianhydride (PMDA) crosslinked poly(vinyl alcohol) (PVA) was coated on top of the PAN ultrafiltration membrane to form a PVA/PAN composite PV membranes for wastewater desalination. The composite membranes have high application value in industrial wastewater treatment. By varying the membrane fabrication parameters including the weight percent (wt%) of the PMDA, the crosslink temperature and duration, membrane with the best desalination per- formance was obtained. The composite membrane with a 2-lm-thick PVA selective layer containing 20 wt% of PMDA and being crosslinked at 100 C for 2 h showed the highest NaCl rejection of 99.98% with a water flux of 32.26 L/(m h) at 70 C using the 35,000 ppm NaCl aqueous solution as feed. FTIR spectroscopy, wide-angle X-ray diffraction, ther- mogravimetric analysis and scanning electron microscope have been used to characterize the structures and properties of both the crosslinked PVA dense films and PVA/PAN composite membranes. The effects of the concentrations of PMDA, the crosslinking time and temperature to the membrane water contact angle, swelling degree, salt rejection and water flux were systematically studied. Keywords Pervaporation  Desalination  Crosslinked PVA  Composite membranes  Wastewater treatment 1 Introduction lot of RO saline water desalination plants have been built, environmental problem caused by the disposal of high Water shortage has become a global issue that threats our concentrated brine water from the RO process has drew daily life (Newman 1995; Wang et al. 2015). Wastewater much attention to the public (Hou et al. 2017; Mitra et al. desalination is considered as one of the most practical 2009; Yang et al. 2010; Yu et al. 2012). In our opinion, methods to solve this problem. Currently, reverse osmosis pervaporation (PV) technology provides a practicable (RO) is the dominating technology due to its intrinsic solution to treat the brine water. The driving force of a PV advantages including: high salt rejection, good efficiency, process is the vapor pressure difference between the feed easy to be operated, and energy-saving (Abufayed et al. and permeate sides of the membrane. Theoretically, PV 2003; Duong and Chung 2014; Sheikh et al. 2003). After a technology has the ability to treat high concentrated brine water as long as a low water vapor pressure in the permeate side can be maintained by applying vacuum in the mem- brane permeate side. Similar to membrane distillation Handling editor: Fanxing Li. (MD), phase transforming of the permeate is involved. Edited by Xiu-Qin Zhu However, the separation process of PV follows the solu- tion-diffusion mechanism (Huang et al. 2001; Shao and & Bing Cao Huang 2007). That is, the feed mixture first dissolves in the Bcao@mail.buct.edu.cn membrane feed side. Then, the permeates diffuse through & Pei Li the membrane and detach to the downstream side in a lipei@mail.buct.edu.cn vapor state (Cheng et al. 2017; Das et al. 2011; Hong et al. College of Materials Science and Engineering, Beijing 2011; Rachipudi et al. 2011; Xu et al. 2017; Zhang et al. University of Chemical Technology, Chaoyang District North 2009). Therefore, a PV membrane has a dense selective Third Ring Road 15, Beijing 100029, China 123 Petroleum Science (2018) 15:146–156 147 layer which can mitigate the fouling problem encountered 2 Experimental by a typical MD process where the pores of the MD membrane are easily fouled during the separation process. 2.1 Materials In the last decade, researchers start to study the feasibility of PV membranes for salty water desalination (Chaudhri PVA-124 with a molecular weight (Mw) of 105,000 was et al. 2015; Cho et al. 2011b; Drobek et al. 2012; Liang purchased from Guoyao Chemical Reagent Co., Ltd (Bei- et al. 2014, 2015; Wang et al. 2016b; Xie et al. 2011; jing, China). The PAN ultrafiltration membranes with a Zwijnenberg et al. 2005). molecular weight cutoff of 400 K were got from Ande Developing membrane materials with high permeability Membrane Separation Technology and Engineering Co., and selectivity is the key to fabricate high-performance PV Ltd (Beijing, China). PMDA with a purity [96% was membranes. Researchers mainly focus on synthesizing bought from Aladdin Industrial Corporation (Shanghai, inorganic materials (Cho et al. 2011b; Drobek et al. 2012) China). Sulfuric acid, sodium hydroxide and sodium or polymeric materials for the application of pervaporation chloride were obtained from Beijing Chemical Works. All desalination. The reported data demonstrate that both chemicals were used without further purification. inorganic and polymeric PV membranes have high salt rejections but the water fluxes are relatively low 2.2 Membrane preparation (0.2–22.87 L/(m h)) when separating a 1 wt% NaCl solution at 63 C (Wang et al. 2016a; Xie et al. 2011; The crosslinked PVA dense film and PVA/PAN composite Zwijnenberg et al. 2005). Poly(vinyl alcohol) (PVA), with membrane were prepared in the following procedures. a super hydrophilicity and good film-forming ability, is an First, prepare the PVA dope solutions. Specifically, 3 g of excellent polymeric material for PV. Since PVA is soluble PVA was added in 100 mL deionized water. The mixture in water, modification methods such as crosslink have to be was stirred for at least 5 h at 90 C to obtain a homoge- used to increase its stability. Crosslinking PVA by small neous solution. After the solution was cooled down to molecules is a straight forward way to increase PVA’s 70 C, varied amount of PMDA was added and 2 drops of stability. How to mitigate the decrease in water flux after sulfuric acid were put in subsequently as a hydrolysis crosslink reaction is the key of the selection of crosslinking agent. Figure 1a shows the hydrolysis mechanism of the method. PMDA molecule in the presence of sulfuric acid. Then, the In this paper, the hydrolyzed PMDA was chosen as the PVA solution was further stirred to ensure that all chemi- crosslinked in the consideration of its stiff aromatic cals were totally dissolved. At last, the mixture was filtered molecular structure and hydrophilic –COOH groups. After and settled overnight to remove all non-dissolved particles carrying out the esterification crosslink reaction between and air bubbles. the –COOH group of the hydrolyzed PMDA and –OH The crosslinked PVA dense film was prepared by a group of PVA, the left unreacted –COOH group may act as solution-casting method. A 5-mL PVA dope solution was the facilitate transport agent to water molecule, and the placed onto a PTFE plate. After being dried in air for 48 h phenyl group in the PMDA molecule may increase the at room temperature, a PVA film was formed and peeled PVA fractional free volume (FFV) to benefit the water off. The PVA dense film was heated at 100 C for 2 h to molecule diffusion. Therefore, the PMDA crosslinked PVA carry out the crosslink reaction as shown in Fig. 1b and polymer may have high water flux as well as good stability then used for characterizations including FTIR, TGA, in water. In order to improve the mechanical strength of the DSC, WAXD, swelling degree and PV test. crosslinked PVA membrane, a PAN ultrafiltration mem- To prepare the PVA/PAN composite membrane, a PAN brane was adopted as the membrane substrate. Therefore, ultrafiltration membrane was cut into a 696 cm square we fabricated a series of PVA/PAN composite pervapor- which was immersed in a 1.0 mol/L NaOH solution at tation membranes. FTIR, TGA, DSC, WAXD, SEM and 60 C for 0.5 h to hydrolyze the PAN membrane surface. the swelling degree measurements were carried out to After that, the PAN membrane was soaked in deionized characterize the physiochemical properties of the com- water several times to guarantee no alkali residue left in the posite membranes. And PV desalination tests were per- membrane. Afterward, the PAN membrane was stick on a formed at different temperatures and NaCl concentrations glass plate and the PVA/PAN composite membrane was to evaluate the separation performances of the prepared obtained by the dip-coating method. Specifically, the PAN membranes. membrane was dipped into the 3 wt% PVA dope solution for 30 s. Then, the composite membrane was taken out and dried in air for 48 h at room temperature followed by heated at a predetermined temperature for certain period to 123 148 Petroleum Science (2018) 15:146–156 HOOC (a) COOH (b) O O HOOC COOH C C Sulfuric acid (PVA) OO (PMDA) 70 °C C C O O C COOH Crosslinking region C COOH Crystalline region Fig. 1 a The hydrolysis mechanism of PMDA and b the PMDA crosslinked PVA diagram 2.3.4 Scanning electron microscopy (SEM) perform the crosslink reaction. Using this method, the composite membrane with a 2-lm-thick PMDA cross- The surface and cross-sectional morphologies of PVA/ linked PVA coating layer were fabricated. The PVA layer with different amounts of PMDA crosslinkeds were deno- PAN composite membranes were monitored by scanning electron microscope (S 4700, Hitachi). The samples were ted as: M-a, M-b, M-c, M-d, and M-e, respectively, cor- responding to 0%, 5%, 10%, 20% and 30% of the mole fractured in liquid nitrogen and coated with gold before concentrations of –COOH group of the hydrolyzed PMDA test. molecule to the –OH group of the PVA molecule. 2.3.5 Water uptake and contact angle 2.3 Membrane characterization The static water contact angles of all PVA dense films were measured by a contact angle meter (CAM200) at the room 2.3.1 Fourier transform infrared spectroscopy (FTIR) temperature. A 2 lL water drop was introduced onto the film surface for the contact angle measurement. At least An ATR-FTIR spectrometer (Spectrum RX I) purchased from PerkinElmer (USA) was used to confirmed the three different locations of each sample were measured to get the average value of the water contact angle. The occurrence of the chemical reaction between the hydro- lyzed PDMA and the PVA molecules. The dry PMDA swelling degree of the films was measured based on the crosslinked PVA films were placed on the sample holder, following procedure. The film samples were first vacuum- and the sample surface was scanned by the ATR-FTIR dried at room temperature overnight to obtain their dry -1 instrument. The frequency was set from 600 to 4000 cm weights W (g). Then, the dried samples were immersed in -1 and the spectra were recorded with a resolution of cm . deionized water at room temperature till the swelling equilibriums were reached. After wipe off the spare water on the film surfaces, their weights W (g) were recorded. 2.3.2 Wide-angle X-ray diffraction (WAXD) The swelling degree (S) of the films was calculated using Eq. (1): The crystallinities of both the pure PVA and the PDMA crosslinked PVA films were investigated using a wide- W  W s d S ¼  100% ð1Þ angle X-ray diffractometer (D-8 advanced, Brucker’s). The sample was first dried in vacuum for 24 h and then being Three repeated experiments were taken for each sample scanned with a scanning rate of 0.3 /min from 5 to 60. and the average result was recorded. 2.3.3 Thermal property analysis 2.3.6 Pervaporation test Thermogravimetric analysis (TGA) of pure PVA and Pervaporation tests were carried out using a lab-made crosslinked PVA film samples were performed with a TA pervaporation unit presented in Fig. 2. The membrane with instruments Q500 at a heating rate of 10 C/min from 30 to an effective surface area of 12.56 cm was put in the 800 C under nitrogen. Differential scanning calorimetry membrane cell, and the pressure of the permeate side (DSC) was performed on a TA Instruments Q20 (downstream) was maintained at 100 Pa. The feed solu- calorimeter at a heating rate of 10 C/min from 30 to tions with different NaCl concentrations range from 1 to 250 C under nitrogen. 7 wt% were circulated using a peristaltic pump in the membrane feed side and their temperatures were main- tained using a water bath. The permeate was collected by a 123 Petroleum Science (2018) 15:146–156 149 Pressure meter Cold trap Feed solution Membrane cell Liquid nitrogen Insulation pail Peristaltic pump Isothermal water bath Vacuum pump Fig. 2 The schematic diagram of the pervaporation unit cold trap immersed in liquid nitrogen. The pervaporation 3 Results and discussion tests were conducted at temperatures range from 30 to 70 C, and at each condition the experiment was repeated 3.1 Membrane characterization for three times and the average value was recorded. Water flux (L/(m h)) is determined using Eq. (2): 3.1.1 FTIR spectroscopy J ¼ ð2Þ A  t Figure 3 shows the FTIR spectra of the pure PVA and crosslinked PVA films (M-a to M-e). All the films have where m is the mass (or volume) of permeate, kg (or L); -1 2 characteristic peaks at 2800–3000 and 3320 cm , repre- A is the effective membrane area, m ; and t is the testing senting the stretching vibrations of the C–H band and the time, h. The salt concentrations of the feed and the per- O–H band in the PVA molecules (Rachipudi et al. 2011; meate are determined by a conductivity meter (Oakton Xie et al. 2011). As the PMDA concentration increases, Con 110) which is calibrated using standard NaCl solutions more –OH groups of the PVA molecules react with the with different concentrations ranging from 0 to –COOH group of the hydrolyzed PMDA crosslinked, and 100,000 ppm. A calibration curve is then constructed. The -1 the peak intensity of the O–H band (3320 cm ) gradually salt rejection R is calculated using Eq. (3): -1 decreases. In addition, peaks at 1750 and 1275 cm , c  c f p which represent the C=O band and –C–O–C– band of R ¼  100% ð3Þ aromatic ester, respectively, increase with the PMDA concentration. All the above-mentioned characteristic where c is the concentration of feed solution; and c is the f p concentration of the permeate. Membrane permeability P (Barrer) is calculated using Eq. (4) (Baker et al. 2010): 3320 1750 1275 P ¼ D K ¼ j ð4Þ i i i i P  P M-e i0 il where D is the diffusion coefficient of water, cm /s; K is i i 3 3 the sorption coefficient of water, cm (STP)/cm cmHg; P M-d i0 and P are the water partial pressures on feed side and il permeate side of the membrane; j is the molar flux, cm i M-c (STP)/(cm s). M-b M-a 3600 3000 2400 1800 1200 600 -1 Wavenumber, cm Fig. 3 FTIR spectroscopy of PVA dense films with different dosages of PMDA Intensity, a.u 150 Petroleum Science (2018) 15:146–156 peaks prove the occurrence of the crosslink reaction and the crosslinking degree increases with the concentration of PMDA. 3.1.2 TGA analysis M-a The effects of the PDMA crosslinked to the thermal sta- M-b bilities of the PVA polymer are investigated using TGA. M-c Figure 4 shows that all polymers show similar thermal degraded pattern. The first weight loss below 120 Cis M-d attributed to the vaporization of the absorbed water in all M-e polymers. In the temperature region of 120–250 C, a 49.46% weight loss is observed for the pure PVA film 10 20 30 40 50 60 which is identified as the decomposition of side chain and 2 , degree θ some amorphous carbons decomposition (Lai et al. 2015). Fig. 5 Wide-angle X-ray diffraction patterns of pristine PVA and For the crosslinked PVA, the weight loss in this region PMDA crosslinked membranes gradually decreases with the increase in the PMDA con- centration. At the highest PMDA concentration (M-e), the weaker and broader as the crosslinking degree increases. weight loss reduces to 32.43%. This phenomenon indicates For the pure PVA, the presence of large amounts of –OH that the crosslink reaction significantly increases the ther- groups and the flexible PVA polymer chains result in high mal stability of PVA. The last weight loss region between crystalline degree. As the crosslinking degree increases, 300 and 600 C is probably relative to decomposition of more and more –OH groups react with the –COOH groups the polymer backbones. Compared with the 27.2% weight of the PMDA crosslinked and the crosslinked PVA poly- loss of the pure PVA, the weight loss of M-e is only mer chains are less flexible. Hence, it is difficult for the 11.26%. Again, this result indicates a better thermal sta- crosslinked PVA polymer chains to pack and form crystal bility of the PMDA crosslinked PVA polymer. regions. And the less crystal domains of the crosslinked PVA polymer shall benefit the water transport, since the 3.1.3 WXRD crystal regions are impermeable to any penetrant. Figure 5 shows the WXRD patterns for the pure PVA and 3.1.4 DSC crosslinked PVA films. All polymers have one diffraction peak with a 2h value of 20, which presents the crystalline According to Fig. 6, the pure PVA polymer has a glass domains of PVA (Liang et al. 2014). For pure PVA, the transition temperature (T ) around 75 C. As the peak is very sharp and strong indicating its high crys- crosslinking degree increases, the glass transition region tallinity. But for the crosslinked PVA, the peak becomes becomes less prominent and disappears for M-d and M-e. This indicates that the nature of the PVA polymers grad- ually turn from thermal plastic to thermal set material with the increase in crosslinking degree. The DSC curves also show that the melting temperature (T ) of all polymers decreases from 223 C of M-a to 175 C of M-e and the melting peaks decrease as the crosslinking degree increa- ses. This is in accordance with the WAXD results. Recall that, the crystallinity of highly crosslinked PVA polymer is M-d low. Therefore, the less and small PVA crystals are easily M-e melt and it leads to a lower melting temperature and M-c smaller melting peak of the crosslinked PVA (Rachipudi M-b et al. 2011). M-a 100 200 300 400 500 600 700 800 900 Temperature, °C Fig. 4 Thermogravimetric analysis of pure PVA and crosslinked PVA films Weight, % Intensity, a.u Petroleum Science (2018) 15:146–156 151 170.5% to 110.9%. This is due to that the –OH groups of the PVA polymer is consumed by the –COOH groups of M-e the hydrolyzed PMDA molecule. Hence, the crosslinked PVA polymer is less hydrophilic and absorbs less water. With the concentration of PMDA increases further, more unreacted –COOH groups are left in the PVA film which M-d leads to a slightly higher hydrophilicity. However, due to the increased crosslinking degree, the swelling degree continuously decreases to 90% for M-d (20% PMDA). As the concentration of PMDA increase to 30%, more and M-c more unreacted –COOH groups increase the water contact angle of M-e to 52.5. The higher hydrophilicity of the M-e film overplays the increased crosslinking degree. There- fore, the swelling degree slightly increases to 90%. M-b 3.2 Morphology of the PVA/PAN composite membranes M-a The surface and cross-sectional morphologies of PAN ultrafiltration membrane and PVA/PAN composite mem- brane with 20% PMDA crosslinked are shown in Fig. 8. The cross section of the PAN ultrafiltration membrane has 50 100 150 200 250 large finger-like micro-voids underneath the skin layer Temperature, °C which can efficiently cut down the mass-transfer resistance Fig. 6 The DSC curves of the pure PVA and PMDA crosslinked films of water vapor during the pervaporation desalination pro- (dash lines are to guide eyes) cess. Besides, the top surface of the PAN membrane (Fig. 8b) has many pores in sizes of 10–20 nm which are 60 180 formed after the hydrolysis treatment. These pores will allow the water vapor to pass through the dense layer region of the PAN membrane and reduce the resistance of the PAN substrate. Figure 8c shows that the dense PVA layer has a thickness of 1.8–2 lm, and Fig. 8d indicates that the PVA layer is defect free which can ensure a high salt rejection of the composite membrane. 3.3 Pervaporation performance 3.3.1 The effect of the content of PMDA to the separation 510 15 20 25 30 performance of the dense PVA films Concentration of PMDA, wt% The pervaporation desalination performances of the Fig. 7 The water contact angle and swelling properties of PVA films crosslinked PVA films with different PMDA contents are with different contents of PMDA studied. As shown in Table 1, all films show similar high salt rejections (99.9%). As the PMDA content increases 3.1.5 Effects of PMDA content on water uptake and water from 5 to 20 wt%, the water flux increases from 9.88 to contact angle of the PVA films 16.47 L/(m h). However, as the PMDA content further increases to 30%, the water flux reduces to 16.02 L/(m h). Figure 7 shows the water contact angles and swelling The increase in the water flux might be due to that more degrees of all crosslinked PVA films. All films show a unreacted –COOH groups left in the PVA film as the water contact angle less than 57 indicating their hydro- PMDA content increases to 20%. The –COOH groups act philic natures. When the concentration of PMDA increases as the facilitate transport agent to water molecules and from 5 to 10 wt% the water contact angle increases from result in an increase in the water flux. However, as the 42.1 to 57.4 and the swelling degree decreases from PMDA concentration increases to 30%, the higher Water contact angle, degree Heat flow Endo up, mW Swelling degree, % 152 Petroleum Science (2018) 15:146–156 Fig. 8 SEM images of cross section (a) and top surface (b) of the PAN ultrafiltration membrane, and of the cross section (c) and surface (d)of the PVA/PAN composite membrane (M-d) –COOH and –OH groups. The reaction rate of the esteri- Table 1 Pervaporation performance of PMDA crosslinked PVA dense membranes at 50 C using 35,000 ppm NaCl as the feed fication shall be boosted at higher temperatures. Insuffi- solution cient crosslinked PVA membrane may result in low salt -2 -1 5 rejection since the un-crosslinked PVA is soluble in water. Flux, L m h Permeability, 10 barrer Rejection, % But over crosslinked PVA membrane may lead to low M-b 9.88 1.50 99.96 water flux due to that the highly inflexible PVA chain M-c 12.32 1.87 99.98 structure hinders the diffusion of water molecules. There- M-d 16.47 2.51 99.98 fore, it is important to figure out the most suitable crosslink M-e 16.02 2.47 99.98 temperature to obtain the composite membrane with suf- ficiently high salt rejection and good water flux. Figure 9a shows the water contact angles and the swelling degrees of the M-d dense films crosslinked at different temperatures crosslinking degree may reduce the chain flexibility of the from 70 to 140 C for 2 h. Figure 9b shows the desalina- PVA polymer and hence the water flux decreases. Since tion performance of the corresponding PVA/PAN com- M-d has the highest water flux and salt rejection, the best posite membranes crosslinked at the same temperatures. As concentration of PMDA crosslinker is selected to be the crosslink temperature increases from 70 to 100 C, the 20 wt% for fabricating PVA/PAN composite membranes. water contact angle slightly increases from 33.2 to 39.4 but the swelling degree significantly decreases from 3.3.2 The effects of the crosslinking temperature 106.4% to 87.3%. This indicates that the crosslink reaction and reaction time to the separation performance is more completed at 100 C than at 70 C, and that the of composite membranes water flux reduces from 35 to 32 L/(m h) indicates that highly crosslinked PVA structure hinders the diffusion of In this study, PVA is crosslinked by the hydrolyzed PMDA water molecules. Note that, the salt rejection of the PVA/ crosslinked via the esterification reaction between the PAN composite membrane crosslinked at 70 C is below 123 Petroleum Science (2018) 15:146–156 153 (a) (b) 20 20 90 60 80 100 120 140 60 80 100 120 140 Temperature, °C Temperature, °C Fig. 9 a The water contact angles and swelling properties and b the water flux and salt rejections (using the 35,000 ppm NaCl solution as the feed at 70 C) of M-d composite membranes crosslinked at different temperatures for 2 h 99%. When the crosslinked temperature increases to crosslink time needs to be at least 2 h to ensure a salt 100 C, the salt rejection is higher than 99.95%. Therefore, rejection higher than 99.95%. And further increase in the the crosslink temperature needs to reach 100 C to ensure crosslink time only reduces the water flux. Therefore, the an efficient salt rejection. With the further increases in the best crosslink time for M-d composite membrane is crosslink temperature, both the membrane swelling degrees selected at 2 h. and the water fluxes significantly decrease. Since increases the crosslink temperature from 100 to 140 C just reduces 3.3.3 The effects of the NaCl concentration the water flux from 32 to 22 L/(m h). The best crosslink and temperature of the feed solution temperature is selected at 100 C. to the separation performance Similar to the crosslink temperature, the crosslink time also determine the degree of the crosslinking of the PVA Figure 11a shows that the water flux increases with tem- membrane which influences the desalination property. perature and decreases with the increase in the NaCl con- Figure 10a shows that the water contact angles and swel- centration of the feed solution. The increase in the water ling degrees change from 35.5 to 89.8, and 118.6% to flux can be explained by the increases in the saturate water 51.3%, respectively, as the crosslink time gradually vapor pressure in the membrane feed side and the increase increases to 24 h. The phenomena indicate the esterifica- in the diffusivity of the water molecule in the membrane. tion reaction continues with time and the resulting PVA Note that the driving force of PV process is the vapor films become less hydrophilic. Figure 10b shows that the pressure difference between the membrane feed and 100 110 45 (a) (b) 80 90 60 70 30 40 50 20 30 15 90 0 5 10 15 20 25 0 5 10 15 20 25 Time, h Time, h Fig. 10 The effect of crosslink time on a water contact angles and swelling degrees of M-d film and b water flux and salt rejections of M-d composite membranes Water contact angle, degree Water contact angle, degree Swelling degree, % Swelling degree, % 2 Flux, L/(m h) Flux, L/(m h) Rejection, % Rejection, % 154 Petroleum Science (2018) 15:146–156 40 4.0 (a) (b) 0 0 3.6 35000 ppm 35000 ppm 50000 ppm 50000 ppm 3.2 70000 ppm 70000 ppm 2.8 2.4 2.0 1.6 1.2 30 40 50 60 70 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 Temperature, °C 1/T, 1/K Fig. 11 a The relations of water flux to the NaCl concentration and temperature of the feed of the M-d composite membrane crosslinked for 2 h at 100 C; b the Arrhenius plots of the water flux and the feed temperature at different NaCl concentrations permeate side and the saturate water vapor pressure is in becomes more severe. This will further decrease the water exponentially proportional to the temperature in the unit of concentration in the membrane feed side. Therefore, these kelvin of the feed solution. Therefore, when the tempera- two phenomena cause the water flux to decrease from 2 2 ture of the feed solution increases from 30 to 70 C, the 35.12 L/(m h) for pure water to 18.02 L/(m h) for the driving force of the water transport shall dramatically 70,000 ppm NaCl feed solution at 70 C. increase. In addition, transport in PV process follows the Figure 11b shows that the logarithmic membrane water solution-diffusion mechanism where the diffusivity is fluxes are linearly correlated with the reciprocal tempera- governed by the size of the penetrant, polymer free volume tures of the feed solutions. This is well fit to the Arrhenius and polymer chain flexibility. As the temperature increases, correlation as described in Eq. (5). the PVA polymer chain becomes more flexible that favors p;i J ¼ A exp  ð5Þ the diffusion of the water molecules. Therefore, the i i RT increases in both the driving force and the diffusivity of the where A is pre-exponential factor; R is the gas constant; water molecules lead to the significantly improved water T is the absolute temperature and E is the apparent flux. On the contrary to the effect of the temperature, the p,i activation energy for permeation and reflects the amount of increase in the NaCl concentration shall decrease the water the energy barrier when the component passes though the flux. The high NaCl content dilutes the water concentration membrane. Table 2 shows the apparent activation energies so that the saturate water vapor pressure in the membrane feed side decreases. It reduces the driving force of the calculated from the slop of Fig. 11b and compares it to the literature data. The water activation energies of feeds with water molecule. Furthermore, at a high NaCl concentration, concentration polarization in the membrane feed side different salt concentrations are very similar Table 2 Pervaporation performance of different membranes Material NaCl, T, Membrane Flux, Rejection, Positive activation energy, -2 -1 -1 ppm C thickness, lm Lm h % kJ mol Poly(vinyl alcohol)/maleic anhydride/silica 2000 65 10 11.7 99.9 23.8 (Xie et al. 2011) Graphene oxide/polyacrylonitrile (Liang et al. 35,000 90 0.1 65.1 99.8 22.19 2015) Polyether ester (Quin˜ones-Bolan˜os et al. 3200 22 160 0.16 – 44.3 2005) Fluoroalkylsilane-ceramic (Kujawski et al. 30,000 40 23 5 – 51 2007) NaA zeolite (Cho et al. 2011a) 35,000 69 – 1.9 99.9 – PVA/PMDA (this study) 35,000 70 2 32.26 99.8 23.6 Flux, L/(m h) lnJ Petroleum Science (2018) 15:146–156 155 -1 (23.6–24.3 kJ mol ) for the PVA/PAN composite mem- link to the Creative Commons license, and indicate if changes were made. branes. The positive value of E implies that water flux p,i shall increase with the temperature of the feed solution (Wang et al. 2016a). According to Table 2, our PVA/PAN References composite PV membrane exhibits the second highest water flux. The good performance can be attributed to two rea- Abufayed AA, Elghuel M, Rashed M (eds). Desalination: a viable sons. First, the activation energy of the PVA/PAN com- supplemental source of water for the arid states of North Africa. In: Conference on desalination strategies in South Mediterranean posite membrane is very low, which means that water Countries. 2003. https://doi.org/10.1016/S0011-9164(02)01050- molecules need less energy to diffuse through the mem- brane. Second, the dense layer thickness of the PVA/PAN Baker RW, Wijmans JG, Yu H. Permeability, permeance and composite membrane is only 2 lm which reduces the selectivity: a preferred way of reporting pervaporation perfor- mance data. 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Fabrication of high-performance PVA/PAN composite pervaporation membranes crosslinked by PMDA for wastewater desalination

Zhang, Rui; Xu, Xiaoying; Cao, Bing; Li, Pei
Petroleum Science , Volume 15 (1) – Jan 5, 2018
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Springer Journals
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Copyright © 2017 by The Author(s)
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
eISSN
1995-8226
DOI
10.1007/s12182-017-0204-z
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Abstract

The pyromellitic dianhydride (PMDA) crosslinked poly(vinyl alcohol) (PVA) was coated on top of the PAN ultrafiltration membrane to form a PVA/PAN composite PV membranes for wastewater desalination. The composite membranes have high application value in industrial wastewater treatment. By varying the membrane fabrication parameters including the weight percent (wt%) of the PMDA, the crosslink temperature and duration, membrane with the best desalination per- formance was obtained. The composite membrane with a 2-lm-thick PVA selective layer containing 20 wt% of PMDA and being crosslinked at 100 C for 2 h showed the highest NaCl rejection of 99.98% with a water flux of 32.26 L/(m h) at 70 C using the 35,000 ppm NaCl aqueous solution as feed. FTIR spectroscopy, wide-angle X-ray diffraction, ther- mogravimetric analysis and scanning electron microscope have been used to characterize the structures and properties of both the crosslinked PVA dense films and PVA/PAN composite membranes. The effects of the concentrations of PMDA, the crosslinking time and temperature to the membrane water contact angle, swelling degree, salt rejection and water flux were systematically studied. Keywords Pervaporation  Desalination  Crosslinked PVA  Composite membranes  Wastewater treatment 1 Introduction lot of RO saline water desalination plants have been built, environmental problem caused by the disposal of high Water shortage has become a global issue that threats our concentrated brine water from the RO process has drew daily life (Newman 1995; Wang et al. 2015). Wastewater much attention to the public (Hou et al. 2017; Mitra et al. desalination is considered as one of the most practical 2009; Yang et al. 2010; Yu et al. 2012). In our opinion, methods to solve this problem. Currently, reverse osmosis pervaporation (PV) technology provides a practicable (RO) is the dominating technology due to its intrinsic solution to treat the brine water. The driving force of a PV advantages including: high salt rejection, good efficiency, process is the vapor pressure difference between the feed easy to be operated, and energy-saving (Abufayed et al. and permeate sides of the membrane. Theoretically, PV 2003; Duong and Chung 2014; Sheikh et al. 2003). After a technology has the ability to treat high concentrated brine water as long as a low water vapor pressure in the permeate side can be maintained by applying vacuum in the mem- brane permeate side. Similar to membrane distillation Handling editor: Fanxing Li. (MD), phase transforming of the permeate is involved. Edited by Xiu-Qin Zhu However, the separation process of PV follows the solu- tion-diffusion mechanism (Huang et al. 2001; Shao and & Bing Cao Huang 2007). That is, the feed mixture first dissolves in the Bcao@mail.buct.edu.cn membrane feed side. Then, the permeates diffuse through & Pei Li the membrane and detach to the downstream side in a lipei@mail.buct.edu.cn vapor state (Cheng et al. 2017; Das et al. 2011; Hong et al. College of Materials Science and Engineering, Beijing 2011; Rachipudi et al. 2011; Xu et al. 2017; Zhang et al. University of Chemical Technology, Chaoyang District North 2009). Therefore, a PV membrane has a dense selective Third Ring Road 15, Beijing 100029, China 123 Petroleum Science (2018) 15:146–156 147 layer which can mitigate the fouling problem encountered 2 Experimental by a typical MD process where the pores of the MD membrane are easily fouled during the separation process. 2.1 Materials In the last decade, researchers start to study the feasibility of PV membranes for salty water desalination (Chaudhri PVA-124 with a molecular weight (Mw) of 105,000 was et al. 2015; Cho et al. 2011b; Drobek et al. 2012; Liang purchased from Guoyao Chemical Reagent Co., Ltd (Bei- et al. 2014, 2015; Wang et al. 2016b; Xie et al. 2011; jing, China). The PAN ultrafiltration membranes with a Zwijnenberg et al. 2005). molecular weight cutoff of 400 K were got from Ande Developing membrane materials with high permeability Membrane Separation Technology and Engineering Co., and selectivity is the key to fabricate high-performance PV Ltd (Beijing, China). PMDA with a purity [96% was membranes. Researchers mainly focus on synthesizing bought from Aladdin Industrial Corporation (Shanghai, inorganic materials (Cho et al. 2011b; Drobek et al. 2012) China). Sulfuric acid, sodium hydroxide and sodium or polymeric materials for the application of pervaporation chloride were obtained from Beijing Chemical Works. All desalination. The reported data demonstrate that both chemicals were used without further purification. inorganic and polymeric PV membranes have high salt rejections but the water fluxes are relatively low 2.2 Membrane preparation (0.2–22.87 L/(m h)) when separating a 1 wt% NaCl solution at 63 C (Wang et al. 2016a; Xie et al. 2011; The crosslinked PVA dense film and PVA/PAN composite Zwijnenberg et al. 2005). Poly(vinyl alcohol) (PVA), with membrane were prepared in the following procedures. a super hydrophilicity and good film-forming ability, is an First, prepare the PVA dope solutions. Specifically, 3 g of excellent polymeric material for PV. Since PVA is soluble PVA was added in 100 mL deionized water. The mixture in water, modification methods such as crosslink have to be was stirred for at least 5 h at 90 C to obtain a homoge- used to increase its stability. Crosslinking PVA by small neous solution. After the solution was cooled down to molecules is a straight forward way to increase PVA’s 70 C, varied amount of PMDA was added and 2 drops of stability. How to mitigate the decrease in water flux after sulfuric acid were put in subsequently as a hydrolysis crosslink reaction is the key of the selection of crosslinking agent. Figure 1a shows the hydrolysis mechanism of the method. PMDA molecule in the presence of sulfuric acid. Then, the In this paper, the hydrolyzed PMDA was chosen as the PVA solution was further stirred to ensure that all chemi- crosslinked in the consideration of its stiff aromatic cals were totally dissolved. At last, the mixture was filtered molecular structure and hydrophilic –COOH groups. After and settled overnight to remove all non-dissolved particles carrying out the esterification crosslink reaction between and air bubbles. the –COOH group of the hydrolyzed PMDA and –OH The crosslinked PVA dense film was prepared by a group of PVA, the left unreacted –COOH group may act as solution-casting method. A 5-mL PVA dope solution was the facilitate transport agent to water molecule, and the placed onto a PTFE plate. After being dried in air for 48 h phenyl group in the PMDA molecule may increase the at room temperature, a PVA film was formed and peeled PVA fractional free volume (FFV) to benefit the water off. The PVA dense film was heated at 100 C for 2 h to molecule diffusion. Therefore, the PMDA crosslinked PVA carry out the crosslink reaction as shown in Fig. 1b and polymer may have high water flux as well as good stability then used for characterizations including FTIR, TGA, in water. In order to improve the mechanical strength of the DSC, WAXD, swelling degree and PV test. crosslinked PVA membrane, a PAN ultrafiltration mem- To prepare the PVA/PAN composite membrane, a PAN brane was adopted as the membrane substrate. Therefore, ultrafiltration membrane was cut into a 696 cm square we fabricated a series of PVA/PAN composite pervapor- which was immersed in a 1.0 mol/L NaOH solution at tation membranes. FTIR, TGA, DSC, WAXD, SEM and 60 C for 0.5 h to hydrolyze the PAN membrane surface. the swelling degree measurements were carried out to After that, the PAN membrane was soaked in deionized characterize the physiochemical properties of the com- water several times to guarantee no alkali residue left in the posite membranes. And PV desalination tests were per- membrane. Afterward, the PAN membrane was stick on a formed at different temperatures and NaCl concentrations glass plate and the PVA/PAN composite membrane was to evaluate the separation performances of the prepared obtained by the dip-coating method. Specifically, the PAN membranes. membrane was dipped into the 3 wt% PVA dope solution for 30 s. Then, the composite membrane was taken out and dried in air for 48 h at room temperature followed by heated at a predetermined temperature for certain period to 123 148 Petroleum Science (2018) 15:146–156 HOOC (a) COOH (b) O O HOOC COOH C C Sulfuric acid (PVA) OO (PMDA) 70 °C C C O O C COOH Crosslinking region C COOH Crystalline region Fig. 1 a The hydrolysis mechanism of PMDA and b the PMDA crosslinked PVA diagram 2.3.4 Scanning electron microscopy (SEM) perform the crosslink reaction. Using this method, the composite membrane with a 2-lm-thick PMDA cross- The surface and cross-sectional morphologies of PVA/ linked PVA coating layer were fabricated. The PVA layer with different amounts of PMDA crosslinkeds were deno- PAN composite membranes were monitored by scanning electron microscope (S 4700, Hitachi). The samples were ted as: M-a, M-b, M-c, M-d, and M-e, respectively, cor- responding to 0%, 5%, 10%, 20% and 30% of the mole fractured in liquid nitrogen and coated with gold before concentrations of –COOH group of the hydrolyzed PMDA test. molecule to the –OH group of the PVA molecule. 2.3.5 Water uptake and contact angle 2.3 Membrane characterization The static water contact angles of all PVA dense films were measured by a contact angle meter (CAM200) at the room 2.3.1 Fourier transform infrared spectroscopy (FTIR) temperature. A 2 lL water drop was introduced onto the film surface for the contact angle measurement. At least An ATR-FTIR spectrometer (Spectrum RX I) purchased from PerkinElmer (USA) was used to confirmed the three different locations of each sample were measured to get the average value of the water contact angle. The occurrence of the chemical reaction between the hydro- lyzed PDMA and the PVA molecules. The dry PMDA swelling degree of the films was measured based on the crosslinked PVA films were placed on the sample holder, following procedure. The film samples were first vacuum- and the sample surface was scanned by the ATR-FTIR dried at room temperature overnight to obtain their dry -1 instrument. The frequency was set from 600 to 4000 cm weights W (g). Then, the dried samples were immersed in -1 and the spectra were recorded with a resolution of cm . deionized water at room temperature till the swelling equilibriums were reached. After wipe off the spare water on the film surfaces, their weights W (g) were recorded. 2.3.2 Wide-angle X-ray diffraction (WAXD) The swelling degree (S) of the films was calculated using Eq. (1): The crystallinities of both the pure PVA and the PDMA crosslinked PVA films were investigated using a wide- W  W s d S ¼  100% ð1Þ angle X-ray diffractometer (D-8 advanced, Brucker’s). The sample was first dried in vacuum for 24 h and then being Three repeated experiments were taken for each sample scanned with a scanning rate of 0.3 /min from 5 to 60. and the average result was recorded. 2.3.3 Thermal property analysis 2.3.6 Pervaporation test Thermogravimetric analysis (TGA) of pure PVA and Pervaporation tests were carried out using a lab-made crosslinked PVA film samples were performed with a TA pervaporation unit presented in Fig. 2. The membrane with instruments Q500 at a heating rate of 10 C/min from 30 to an effective surface area of 12.56 cm was put in the 800 C under nitrogen. Differential scanning calorimetry membrane cell, and the pressure of the permeate side (DSC) was performed on a TA Instruments Q20 (downstream) was maintained at 100 Pa. The feed solu- calorimeter at a heating rate of 10 C/min from 30 to tions with different NaCl concentrations range from 1 to 250 C under nitrogen. 7 wt% were circulated using a peristaltic pump in the membrane feed side and their temperatures were main- tained using a water bath. The permeate was collected by a 123 Petroleum Science (2018) 15:146–156 149 Pressure meter Cold trap Feed solution Membrane cell Liquid nitrogen Insulation pail Peristaltic pump Isothermal water bath Vacuum pump Fig. 2 The schematic diagram of the pervaporation unit cold trap immersed in liquid nitrogen. The pervaporation 3 Results and discussion tests were conducted at temperatures range from 30 to 70 C, and at each condition the experiment was repeated 3.1 Membrane characterization for three times and the average value was recorded. Water flux (L/(m h)) is determined using Eq. (2): 3.1.1 FTIR spectroscopy J ¼ ð2Þ A  t Figure 3 shows the FTIR spectra of the pure PVA and crosslinked PVA films (M-a to M-e). All the films have where m is the mass (or volume) of permeate, kg (or L); -1 2 characteristic peaks at 2800–3000 and 3320 cm , repre- A is the effective membrane area, m ; and t is the testing senting the stretching vibrations of the C–H band and the time, h. The salt concentrations of the feed and the per- O–H band in the PVA molecules (Rachipudi et al. 2011; meate are determined by a conductivity meter (Oakton Xie et al. 2011). As the PMDA concentration increases, Con 110) which is calibrated using standard NaCl solutions more –OH groups of the PVA molecules react with the with different concentrations ranging from 0 to –COOH group of the hydrolyzed PMDA crosslinked, and 100,000 ppm. A calibration curve is then constructed. The -1 the peak intensity of the O–H band (3320 cm ) gradually salt rejection R is calculated using Eq. (3): -1 decreases. In addition, peaks at 1750 and 1275 cm , c  c f p which represent the C=O band and –C–O–C– band of R ¼  100% ð3Þ aromatic ester, respectively, increase with the PMDA concentration. All the above-mentioned characteristic where c is the concentration of feed solution; and c is the f p concentration of the permeate. Membrane permeability P (Barrer) is calculated using Eq. (4) (Baker et al. 2010): 3320 1750 1275 P ¼ D K ¼ j ð4Þ i i i i P  P M-e i0 il where D is the diffusion coefficient of water, cm /s; K is i i 3 3 the sorption coefficient of water, cm (STP)/cm cmHg; P M-d i0 and P are the water partial pressures on feed side and il permeate side of the membrane; j is the molar flux, cm i M-c (STP)/(cm s). M-b M-a 3600 3000 2400 1800 1200 600 -1 Wavenumber, cm Fig. 3 FTIR spectroscopy of PVA dense films with different dosages of PMDA Intensity, a.u 150 Petroleum Science (2018) 15:146–156 peaks prove the occurrence of the crosslink reaction and the crosslinking degree increases with the concentration of PMDA. 3.1.2 TGA analysis M-a The effects of the PDMA crosslinked to the thermal sta- M-b bilities of the PVA polymer are investigated using TGA. M-c Figure 4 shows that all polymers show similar thermal degraded pattern. The first weight loss below 120 Cis M-d attributed to the vaporization of the absorbed water in all M-e polymers. In the temperature region of 120–250 C, a 49.46% weight loss is observed for the pure PVA film 10 20 30 40 50 60 which is identified as the decomposition of side chain and 2 , degree θ some amorphous carbons decomposition (Lai et al. 2015). Fig. 5 Wide-angle X-ray diffraction patterns of pristine PVA and For the crosslinked PVA, the weight loss in this region PMDA crosslinked membranes gradually decreases with the increase in the PMDA con- centration. At the highest PMDA concentration (M-e), the weaker and broader as the crosslinking degree increases. weight loss reduces to 32.43%. This phenomenon indicates For the pure PVA, the presence of large amounts of –OH that the crosslink reaction significantly increases the ther- groups and the flexible PVA polymer chains result in high mal stability of PVA. The last weight loss region between crystalline degree. As the crosslinking degree increases, 300 and 600 C is probably relative to decomposition of more and more –OH groups react with the –COOH groups the polymer backbones. Compared with the 27.2% weight of the PMDA crosslinked and the crosslinked PVA poly- loss of the pure PVA, the weight loss of M-e is only mer chains are less flexible. Hence, it is difficult for the 11.26%. Again, this result indicates a better thermal sta- crosslinked PVA polymer chains to pack and form crystal bility of the PMDA crosslinked PVA polymer. regions. And the less crystal domains of the crosslinked PVA polymer shall benefit the water transport, since the 3.1.3 WXRD crystal regions are impermeable to any penetrant. Figure 5 shows the WXRD patterns for the pure PVA and 3.1.4 DSC crosslinked PVA films. All polymers have one diffraction peak with a 2h value of 20, which presents the crystalline According to Fig. 6, the pure PVA polymer has a glass domains of PVA (Liang et al. 2014). For pure PVA, the transition temperature (T ) around 75 C. As the peak is very sharp and strong indicating its high crys- crosslinking degree increases, the glass transition region tallinity. But for the crosslinked PVA, the peak becomes becomes less prominent and disappears for M-d and M-e. This indicates that the nature of the PVA polymers grad- ually turn from thermal plastic to thermal set material with the increase in crosslinking degree. The DSC curves also show that the melting temperature (T ) of all polymers decreases from 223 C of M-a to 175 C of M-e and the melting peaks decrease as the crosslinking degree increa- ses. This is in accordance with the WAXD results. Recall that, the crystallinity of highly crosslinked PVA polymer is M-d low. Therefore, the less and small PVA crystals are easily M-e melt and it leads to a lower melting temperature and M-c smaller melting peak of the crosslinked PVA (Rachipudi M-b et al. 2011). M-a 100 200 300 400 500 600 700 800 900 Temperature, °C Fig. 4 Thermogravimetric analysis of pure PVA and crosslinked PVA films Weight, % Intensity, a.u Petroleum Science (2018) 15:146–156 151 170.5% to 110.9%. This is due to that the –OH groups of the PVA polymer is consumed by the –COOH groups of M-e the hydrolyzed PMDA molecule. Hence, the crosslinked PVA polymer is less hydrophilic and absorbs less water. With the concentration of PMDA increases further, more unreacted –COOH groups are left in the PVA film which M-d leads to a slightly higher hydrophilicity. However, due to the increased crosslinking degree, the swelling degree continuously decreases to 90% for M-d (20% PMDA). As the concentration of PMDA increase to 30%, more and M-c more unreacted –COOH groups increase the water contact angle of M-e to 52.5. The higher hydrophilicity of the M-e film overplays the increased crosslinking degree. There- fore, the swelling degree slightly increases to 90%. M-b 3.2 Morphology of the PVA/PAN composite membranes M-a The surface and cross-sectional morphologies of PAN ultrafiltration membrane and PVA/PAN composite mem- brane with 20% PMDA crosslinked are shown in Fig. 8. The cross section of the PAN ultrafiltration membrane has 50 100 150 200 250 large finger-like micro-voids underneath the skin layer Temperature, °C which can efficiently cut down the mass-transfer resistance Fig. 6 The DSC curves of the pure PVA and PMDA crosslinked films of water vapor during the pervaporation desalination pro- (dash lines are to guide eyes) cess. Besides, the top surface of the PAN membrane (Fig. 8b) has many pores in sizes of 10–20 nm which are 60 180 formed after the hydrolysis treatment. These pores will allow the water vapor to pass through the dense layer region of the PAN membrane and reduce the resistance of the PAN substrate. Figure 8c shows that the dense PVA layer has a thickness of 1.8–2 lm, and Fig. 8d indicates that the PVA layer is defect free which can ensure a high salt rejection of the composite membrane. 3.3 Pervaporation performance 3.3.1 The effect of the content of PMDA to the separation 510 15 20 25 30 performance of the dense PVA films Concentration of PMDA, wt% The pervaporation desalination performances of the Fig. 7 The water contact angle and swelling properties of PVA films crosslinked PVA films with different PMDA contents are with different contents of PMDA studied. As shown in Table 1, all films show similar high salt rejections (99.9%). As the PMDA content increases 3.1.5 Effects of PMDA content on water uptake and water from 5 to 20 wt%, the water flux increases from 9.88 to contact angle of the PVA films 16.47 L/(m h). However, as the PMDA content further increases to 30%, the water flux reduces to 16.02 L/(m h). Figure 7 shows the water contact angles and swelling The increase in the water flux might be due to that more degrees of all crosslinked PVA films. All films show a unreacted –COOH groups left in the PVA film as the water contact angle less than 57 indicating their hydro- PMDA content increases to 20%. The –COOH groups act philic natures. When the concentration of PMDA increases as the facilitate transport agent to water molecules and from 5 to 10 wt% the water contact angle increases from result in an increase in the water flux. However, as the 42.1 to 57.4 and the swelling degree decreases from PMDA concentration increases to 30%, the higher Water contact angle, degree Heat flow Endo up, mW Swelling degree, % 152 Petroleum Science (2018) 15:146–156 Fig. 8 SEM images of cross section (a) and top surface (b) of the PAN ultrafiltration membrane, and of the cross section (c) and surface (d)of the PVA/PAN composite membrane (M-d) –COOH and –OH groups. The reaction rate of the esteri- Table 1 Pervaporation performance of PMDA crosslinked PVA dense membranes at 50 C using 35,000 ppm NaCl as the feed fication shall be boosted at higher temperatures. Insuffi- solution cient crosslinked PVA membrane may result in low salt -2 -1 5 rejection since the un-crosslinked PVA is soluble in water. Flux, L m h Permeability, 10 barrer Rejection, % But over crosslinked PVA membrane may lead to low M-b 9.88 1.50 99.96 water flux due to that the highly inflexible PVA chain M-c 12.32 1.87 99.98 structure hinders the diffusion of water molecules. There- M-d 16.47 2.51 99.98 fore, it is important to figure out the most suitable crosslink M-e 16.02 2.47 99.98 temperature to obtain the composite membrane with suf- ficiently high salt rejection and good water flux. Figure 9a shows the water contact angles and the swelling degrees of the M-d dense films crosslinked at different temperatures crosslinking degree may reduce the chain flexibility of the from 70 to 140 C for 2 h. Figure 9b shows the desalina- PVA polymer and hence the water flux decreases. Since tion performance of the corresponding PVA/PAN com- M-d has the highest water flux and salt rejection, the best posite membranes crosslinked at the same temperatures. As concentration of PMDA crosslinker is selected to be the crosslink temperature increases from 70 to 100 C, the 20 wt% for fabricating PVA/PAN composite membranes. water contact angle slightly increases from 33.2 to 39.4 but the swelling degree significantly decreases from 3.3.2 The effects of the crosslinking temperature 106.4% to 87.3%. This indicates that the crosslink reaction and reaction time to the separation performance is more completed at 100 C than at 70 C, and that the of composite membranes water flux reduces from 35 to 32 L/(m h) indicates that highly crosslinked PVA structure hinders the diffusion of In this study, PVA is crosslinked by the hydrolyzed PMDA water molecules. Note that, the salt rejection of the PVA/ crosslinked via the esterification reaction between the PAN composite membrane crosslinked at 70 C is below 123 Petroleum Science (2018) 15:146–156 153 (a) (b) 20 20 90 60 80 100 120 140 60 80 100 120 140 Temperature, °C Temperature, °C Fig. 9 a The water contact angles and swelling properties and b the water flux and salt rejections (using the 35,000 ppm NaCl solution as the feed at 70 C) of M-d composite membranes crosslinked at different temperatures for 2 h 99%. When the crosslinked temperature increases to crosslink time needs to be at least 2 h to ensure a salt 100 C, the salt rejection is higher than 99.95%. Therefore, rejection higher than 99.95%. And further increase in the the crosslink temperature needs to reach 100 C to ensure crosslink time only reduces the water flux. Therefore, the an efficient salt rejection. With the further increases in the best crosslink time for M-d composite membrane is crosslink temperature, both the membrane swelling degrees selected at 2 h. and the water fluxes significantly decrease. Since increases the crosslink temperature from 100 to 140 C just reduces 3.3.3 The effects of the NaCl concentration the water flux from 32 to 22 L/(m h). The best crosslink and temperature of the feed solution temperature is selected at 100 C. to the separation performance Similar to the crosslink temperature, the crosslink time also determine the degree of the crosslinking of the PVA Figure 11a shows that the water flux increases with tem- membrane which influences the desalination property. perature and decreases with the increase in the NaCl con- Figure 10a shows that the water contact angles and swel- centration of the feed solution. The increase in the water ling degrees change from 35.5 to 89.8, and 118.6% to flux can be explained by the increases in the saturate water 51.3%, respectively, as the crosslink time gradually vapor pressure in the membrane feed side and the increase increases to 24 h. The phenomena indicate the esterifica- in the diffusivity of the water molecule in the membrane. tion reaction continues with time and the resulting PVA Note that the driving force of PV process is the vapor films become less hydrophilic. Figure 10b shows that the pressure difference between the membrane feed and 100 110 45 (a) (b) 80 90 60 70 30 40 50 20 30 15 90 0 5 10 15 20 25 0 5 10 15 20 25 Time, h Time, h Fig. 10 The effect of crosslink time on a water contact angles and swelling degrees of M-d film and b water flux and salt rejections of M-d composite membranes Water contact angle, degree Water contact angle, degree Swelling degree, % Swelling degree, % 2 Flux, L/(m h) Flux, L/(m h) Rejection, % Rejection, % 154 Petroleum Science (2018) 15:146–156 40 4.0 (a) (b) 0 0 3.6 35000 ppm 35000 ppm 50000 ppm 50000 ppm 3.2 70000 ppm 70000 ppm 2.8 2.4 2.0 1.6 1.2 30 40 50 60 70 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 Temperature, °C 1/T, 1/K Fig. 11 a The relations of water flux to the NaCl concentration and temperature of the feed of the M-d composite membrane crosslinked for 2 h at 100 C; b the Arrhenius plots of the water flux and the feed temperature at different NaCl concentrations permeate side and the saturate water vapor pressure is in becomes more severe. This will further decrease the water exponentially proportional to the temperature in the unit of concentration in the membrane feed side. Therefore, these kelvin of the feed solution. Therefore, when the tempera- two phenomena cause the water flux to decrease from 2 2 ture of the feed solution increases from 30 to 70 C, the 35.12 L/(m h) for pure water to 18.02 L/(m h) for the driving force of the water transport shall dramatically 70,000 ppm NaCl feed solution at 70 C. increase. In addition, transport in PV process follows the Figure 11b shows that the logarithmic membrane water solution-diffusion mechanism where the diffusivity is fluxes are linearly correlated with the reciprocal tempera- governed by the size of the penetrant, polymer free volume tures of the feed solutions. This is well fit to the Arrhenius and polymer chain flexibility. As the temperature increases, correlation as described in Eq. (5). the PVA polymer chain becomes more flexible that favors p;i J ¼ A exp  ð5Þ the diffusion of the water molecules. Therefore, the i i RT increases in both the driving force and the diffusivity of the where A is pre-exponential factor; R is the gas constant; water molecules lead to the significantly improved water T is the absolute temperature and E is the apparent flux. On the contrary to the effect of the temperature, the p,i activation energy for permeation and reflects the amount of increase in the NaCl concentration shall decrease the water the energy barrier when the component passes though the flux. The high NaCl content dilutes the water concentration membrane. Table 2 shows the apparent activation energies so that the saturate water vapor pressure in the membrane feed side decreases. It reduces the driving force of the calculated from the slop of Fig. 11b and compares it to the literature data. The water activation energies of feeds with water molecule. Furthermore, at a high NaCl concentration, concentration polarization in the membrane feed side different salt concentrations are very similar Table 2 Pervaporation performance of different membranes Material NaCl, T, Membrane Flux, Rejection, Positive activation energy, -2 -1 -1 ppm C thickness, lm Lm h % kJ mol Poly(vinyl alcohol)/maleic anhydride/silica 2000 65 10 11.7 99.9 23.8 (Xie et al. 2011) Graphene oxide/polyacrylonitrile (Liang et al. 35,000 90 0.1 65.1 99.8 22.19 2015) Polyether ester (Quin˜ones-Bolan˜os et al. 3200 22 160 0.16 – 44.3 2005) Fluoroalkylsilane-ceramic (Kujawski et al. 30,000 40 23 5 – 51 2007) NaA zeolite (Cho et al. 2011a) 35,000 69 – 1.9 99.9 – PVA/PMDA (this study) 35,000 70 2 32.26 99.8 23.6 Flux, L/(m h) lnJ Petroleum Science (2018) 15:146–156 155 -1 (23.6–24.3 kJ mol ) for the PVA/PAN composite mem- link to the Creative Commons license, and indicate if changes were made. branes. The positive value of E implies that water flux p,i shall increase with the temperature of the feed solution (Wang et al. 2016a). According to Table 2, our PVA/PAN References composite PV membrane exhibits the second highest water flux. The good performance can be attributed to two rea- Abufayed AA, Elghuel M, Rashed M (eds). Desalination: a viable sons. First, the activation energy of the PVA/PAN com- supplemental source of water for the arid states of North Africa. In: Conference on desalination strategies in South Mediterranean posite membrane is very low, which means that water Countries. 2003. https://doi.org/10.1016/S0011-9164(02)01050- molecules need less energy to diffuse through the mem- brane. Second, the dense layer thickness of the PVA/PAN Baker RW, Wijmans JG, Yu H. Permeability, permeance and composite membrane is only 2 lm which reduces the selectivity: a preferred way of reporting pervaporation perfor- mance data. 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Petroleum Science – Springer Journals

Published: Jan 5, 2018

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