In this paper, a novel positively charged membrane was prepared through interfacial polymerization technique between polyethyleneimine in aqueous phase and trimesoyl chloride in organic phase. Next, cross-linking of polyamide (PA) layer using ρ-xylylene dichloride (XDC) and glutaraldehyde (GA) was studied. The influences of cross-linking concentrations on the separation and permeation performance of membrane were also investigated. Membranes were characterized in terms of their chemical structure, the cross-sectional and surface morphologies, contact angles, molecular weight cut-off (MWCO) and effect of pH feed solution. The salt rejection sequence of CaCl2 >NaCl > Na2SO4 showed a positive charge at the membrane surface after cross-linking reaction. The MWCO of primary PA membrane decreased from 1,135 to 775 and 885 Da for XDC and GA, respectively. XDC membrane shows highest CaCl2 divalent cationic rejection (95.5%) and lowest water flux (21.1 L/m2.h). This study illustrates a promising method for fabrication of positively charged membrane in cation separation.
Nanofiltration (NF) is a novel and powerful separation technology that displays separation characteristics in the intermediate range between ultrafiltration (UF) and reverse osmosis (RO) (Zhanga et al. 2011). Pore size of NF membranes changes between UF and RO membranes and the nominal molecular weight cut-off (MWCO) of a NF membrane ranges from 200 to 1,000 Da (Cheng et al. 2012). Transfer mechanism of NF membranes involves both pore sieving as in UF and diffusion as in RO. Separation mechanisms involve steric hindrance and Donnan exclusion effects (electrostatic interaction of charges between solutes and membrane surface). These characteristics lead to a relatively high permeate flux at low operating pressure and high selectivity. These advantages have led to a growing list of NF membrane applications, such as water softening (Mulder 1996), dyes substances separation and purification (Koyuncu 2002), wastewater treatment (Van der Bruggen et al. 2003), natural organic matter separation (Schafer et al. 1998), recovery of valuable pharmaceutical products (Tessier et al. 2005), and removal of heavy metal ions (e.g. Ni2+, Cd2+, Cu2+, Zn2+, Co2+, Pb2+) from industrial wastewater before discharging (Bougen et al. 2001; Mohamm et al. 2004; Bouranene et al. 2008).
Heavy metals are harmful to public health, plants and animals because of their higher toxicity, non-biodegradable and persistent nature (Gonzalez-Munoz et al. 2006). Separation of heavy metals from wastewater has received considerable attention during recent years caused by the stringent environmental laws. Heavy metals are found naturally in the earth but become concentrated with human industrial activities, such as electroplating, mining, battery manufacturing, metal finishing, paints and treated woods, textile industries, and automotive industries (Li et al. 2008). Several methods have been developed for the treatment of heavy metals in wastewater, such as chemical precipitation, coagulation/coprecipitation, adsorption, ion exchange, and floatation and end-up standard discharges (Ku et al. 2001; Kurniawan et al. 2006; Fu & Wang 2011). However, low selectivity, high costs, long time of operation, and the large sludge containing toxic compounds produced during these processes are the main disadvantage of these methods. Different types of membrane separation operations show the greatest promise in the commercial treatment of heavy metal wastewater. Among membrane processes RO operation was limited because of high energy consumption and low water permeability. Microfiltration and UF processes have a low rejection for heavy metal treatment. But NF technology with acceptable rejection, low energy consumption (moderate pressure), and high water permeability is a promising technology for removal of heavy metal from wastewater and other applications (Muthukrishnan & Guha 2006). However, according to the NF separation mechanism (Donnan exclusion effects) positively charged NF membranes are more efficient for the removal or recovery of heavy metal and multivalent cations. Several methods, such as coating (Ba et al. 2010; Wang et al. 2013), chemical cross-linking (Huang et al. 2006; Sun et al. 2011), UV photo-grafting (Deng et al. 2011; Liu et al. 2015), and plasma grafting (Buonomenna et al. 2009) were developed for prepared positively charge NF membranes.
So, positively charged NF membranes have gained significant attention in recent years. An et al. (2011) prepared positively charged copolymers of poly (2-methacryloyloxy ethyl trimethylammonium chloride-co-2-hydroxyethyl acrylate) (PDMCHEA) through free radical copolymerization. The water flux and salt rejection for MgCl2 and NaCl were 19.1 and 20.6 L/m2.h and 94.3 and 60.7% at 25 °C under 0.6 MPa, respectively. Li et al. (2011) prepared hydrophilic and positively charged NF onto polysulfone UF substrate by UV-induced graft polymerization of N,N-dimethylaminoethyl methacrylate and followed by quaternization by chemical cross-linking. The rejections of the composite membranes to different salts followed the order Na2SO4 < NaCl< MgSO4< MgCl2< CaCl2. Zhang et al. (2014) prepared NF membranes using the phase inversion method using cardopoly(arylene ether sulfone) with pendant tertiary amine groups (PES-TA). Zeta potential measurements indicated that the PES-TA membranes were positively charged below pH 10.7. Zhang et al. (2014) synthesized novel positively charged composite nanofiltration membranes were facilely prepared by polydopamine (PDA) deposition followed by poly(ethyleneimine) (PEI) grafting on polyethersulfone (PES) substrates. The rejection of salts was increased but the pure water flux was decreased with the increase of PDA deposition time, PEI concentration, PEI reaction temperature and time. Yu et al. (2015) prepared high flux positively charged nanofiltration membrane via ‘blending-phase inversion’ method using silica spheres modified by poly (ionic liquid) brushes. The results revealed that the hybrid membranes have enhanced surface hydrophilicity, water permeability, thermal stability, and mechanical properties.
Most of the commercial NF membranes have a negatively charge and are often made from polyamide using interfacial polymerization into the microporous substrate (An et al. 2011). In the interfacial polymerization technique, two reactive monomers usually amine and acyl chloride are dissolved in two insoluble phases, such as water and n-hexane, and the polymerization reaction occurs onto the interface of phases on the surface of the microporous support (Akbari & Mojallali Rostami 2014). As a result, a dense ultra-thin active layer formed separately onto a microporous support and it has a major role in determination of membrane characters, such as water flux, selectivity, morphology, roughness, hydrophilicity and type of membrane charge.
The main aim of the current study is preparation of a novel positively charged membrane based on polyamide membrane using interfacial polymerization method. Positively charged PA NF membrane was prepared by interfacial polymerization of PEI and triethylamine (TEA) in aqueous phase and trimesoyl chloride (TMC) in organic phase. The following PA active top layer was cross-linked using -xylylene dichloride (XDC) and glutaraldehyde (GA) agents. Performance of PA NF membranes was determined by filtration of aqueous feed solutions containing 1,000 ppm of NaCl, Na2SO4, and CaCl2 salts. The morphological, topological and chemical characteristics of the resulting membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and FT-IR, respectively. MWCO of PEI/TMC, XDC and GA membranes were calculated using filtration different molecular weight of polyethylene glycol (PEG). The relationship between salt rejection, water flux, and cross-linking agent concentrations was discussed.
Polyacrylonitrile (PAN) fibers were purchased from textile wastes of SepehrManufacturing Company and were used as polymer for support membranes preparation. PEI (Mw ∼ 2,000) and GA purchased from Sigma-Aldrich Company. N,N-dimethylformamide (DMF), TMC, TEA, n-hexane, sodium hydroxide (NaOH) and hydrochloride acid (HCl) were obtained from Merck Chemicals (Germany). Also, PEG with different molecular weight were purchased from Merck Company. p-XDC purchased from Across. All chemical agents were used without further purification.
Preparation of microporous polyacrylonitrile support membrane
Microporous PAN membranes were used as a supporting substrate for the PA membrane, which were prepared by traditional phase-inversion method (Schafer et al. 2005). Homogeneous casting solution was prepared by dissolving of PAN (16% wt) in DMF (84% wt) at 70 °C for 12 hours. Polymer solutions were cast on flat glass plates with a uniform thickness of 250 μm by a casting knife. Immediately after casting, the glass plates were immersed in a coagulation bath containing distilled water at 25 °C acting as the non-solvent. Microporous PAN substrate membrane stored in deionized water at ambient temperature for after modification and experiments.
Fabrication of PA membrane
The selective PA active layer of NF membrane was prepared by conventional interfacial polymerization method onto the microporous PAN membrane. First, PAN supporting membrane was dipped into the PEI/TEA aqueous solution for 60 min and then the excess solution was removed using pieces of filter paper until the solution remained on the membrane surface. Then, the support membrane with aqueous phase was installed in a frame and the top surface of membrane was exposed to the TMC organic solution for 60 seconds. The effect of PEI (0.5, 1, 1.5, 2 and 2.5% w/w) and TMC (0.1, 0.2, 0.3 and 0.4% w/v) concentrations was investigated. Finally the membrane was thoroughly rinsed with de-ionized water and stored in deionized water at 4 °C before carrying out evaluation studies.
Cross-linking PA membrane by XDC and GA
Separation and permeation properties
Characterization of membranes
Fourier transform infrared spectroscopy (FT-IR) (Nicolet Magna-IR 550) was employed to investigate the surface chemical composition changes of PAN membrane before and after modification and cross-linking in the region of 400–4,000 cm−1. The membrane surface hydrophilicity was characterized by water contact angle measurement using a contact angle goniometer (DSA 100 KRUSS GMBH, Germany) equipped at room temperature. A total of 4 μL of ultrapure water was lowed onto a dry membrane surface with a microsyringe and after 3 seconds, the value of water contact angle was recorded. At least five measurements in different locations of the membrane surface sample were carried out and then the curves of contact angle as a function of the drop age were plotted to compare the hydrophilicity of different membranes.
Membranes surface and cross-section morphologies were obtained by SEM (SEM KYKY-EM 3,200, China). The membrane samples were fractured in liquid nitrogen to avoid destroying the cross-section structure and then sputtered with gold before observation. AFM (model Park Scientific Instrument, CP Auto Probe) was used to image the membranes. The images were obtained in the same way from different places of each membrane surface sample at room temperature in air. Dry membranes were cut and glued on glass substrate and then membrane surfaces were imaged in a scan size of 5 μm × 5 μm.
The MWCO of the membranes was evaluated with Filtration of different molecular weight PEG (200, 400, 1,000, and 1,500 g/mol). PEG concentration was measured using a total organic carbon analyzer (TOC-VCPHSHIMADZU).
Optimization of PEI and TMC concentration
A key advantage of the PA-TFC membrane is that the materials for microporous support and PA active top layer can be chosen and optimized separately to reach the best separation performance and membrane stability (Petersen 1993). The microporous support membrane was prepared for suitable strength against operating conditions and PA active layer being responsible for membrane water flux and selectivity. The selective PA active layer of TFC membrane was prepared by conventional interfacial polymerization method on the microporous PAN membrane. Concentrations of PEI in aqueous phase and TMC in organic phase were separately investigated to gain the best performance of PA-TFC membrane.
The variation of PA-TFC membrane performance with the concentration of TMC in organic phase showed the same trend and is shown in Figure 3. At first, water flux declined rapidly as the concentration of TMC in organic phase increased between 0.1 (394.8 L/m2.h) and 0.2 (43.2 L/m2.h), because at the 0.1% concentration a fault-free PA active top layer on the PAN support surface was not formed. At the higher concentration water flux declined with a slight slope from 43.2 at 0.2% to 24.8 L/m2.h at 0.4%. More TMC concentration namely more TMC monomers at interface of the two phases, so the PA active top layer became thicker, leading to more resistant to cross-flow water molecules transfer and resulting lower flux (Berezkin & Khokhlov 2006). Salt rejections show that the rejection increased as the concentration of TMC increased from 0.1 to 0.3% and then rejection approximately become constant. As a result, the modified PA-TFC membrane with 0.3% of TMC concentration seems to be the best result, because at the higher concentration of TMC water flux declined from 32 to 24.8 L/m2.h without considerably change in CaCl2 salt rejection.
The effect of XDC and GA concentration on the PA-TFC membrane performance
In general, the change tendencies of the salt rejection and the water flux of membrane are opposite. Also, the rejection behaviors of NaCl, Na2SO4 and CaCl2 of the cross-linked membranes are depicted in Figure 4. The salt rejection order follows: CaCl2 > NaCl > Na2SO4, which was in agreement with the Schaep &Vandecasteele (2001) and Huang et al. (2008) sequences. It was found that CaCl2 and NaCl rejection of the cross-linked membrane increased with raising the XDC (from 0.2 to 0.6% w/w) or GA (from 0.2 to 1% w/w) concentration as water flux declined. With increasing cross-linking concentration the degree of chemical cross-linking reaction between PEI and XDC or GA agents is improved, leading to a denser barrier PA active top layer with many positive charges on the membrane surface. As a result divalent cationic CaCl2 salt rejection increased from 85.2 to 95.5% for XDC and 83 to 88.7% for GA cross-linked membranes. Also, divalent anionic Na2SO4 salt rejection decreased due to aggregation of positive charges on the membrane surface. In both membranes, the salt rejection for NaCl first increased and then decreased because it contained equal monovalent anions and cations. However, the CaCl2 rejection decreased and the pure water flux increased when the XDC or GA concentration was higher than 0.6 and 1% w/w, respectively. Also, comparison of two diagrams indicated that more variation of water flux and salt rejection occurred when XDC applied as a cross-linking agent. This phenomenon probably contributed to higher XDC reactivity rather than GA cross-linking agent and quarternized amine groups with independent positively charge.
Effect of pH feed solution on the membrane performance
The effect of pH on rejection salt ions was investigated by addition of NaOH (pH = 11) or HCl (pH = 3) to salt feed solution and varying its pH accordingly. In general, NF separation mechanisms frequently involve both distribution of membrane pore size and Donnan exclusion effects (Yu et al. 2010). Ions rejection by NF membranes is dependent on the electrical interaction between ions and membrane surfaces charge. So, pH of feed solution can be dependent on ion rejection because membrane surface charges varied with the pH. The functional groups on the PA active top layer (carboxylic acid, amine, and amide) and pH feed solution are responsible for charge density value of the membrane surface (Teixeira et al. 2005). Therefore, charged properties of cross-linking membranes were evaluated; pH adjusted at 3, 7, and 11 using HCl and soda (NaOH).
Molecular weight cut-off
|MWCO (Da, ±10)||–||1138||775||885|
|Pore size (nm)||–||1.33||1.09||1.17|
|Contact angle (°, ±1 °)||71.4||62.4||43.5||55.7|
|MWCO (Da, ±10)||–||1138||775||885|
|Pore size (nm)||–||1.33||1.09||1.17|
|Contact angle (°, ±1 °)||71.4||62.4||43.5||55.7|
Water contact angle of membranes
The contact angle of water is usually used to investigate the changes in the molecular structure and hydrophilicity of the membrane surfaces. The water contact angles of PAN support substrate, polyamide and cross-linked with XDC and GA membranes are shown in Table 1. As expected, all the polyamide membranes have a lower water contact angle than those of the PAN support membrane. Also, the contact angle decreases with the cross-linking of polyamide top layer. The water contact angle of polyamide, XDC and GA membranes were 62.4 °, 43.5 ° and 55.7 °, respectively. As a result, the enhancement of the hydrophilicity of the cross-linking membranes was due to the quaternization reaction of polyamide and formation independent cation on the membrane surface. XDC membrane has a smallest water contact angle because XDC cross-linked has a higher activity compared with GA agent.
Figure 7(b) shows spectrum of polyamide TFC NF membrane synthesis using PEI monomer. The traces of PAN support membrane and polyamide active layer can be found in the spectra of TFC membrane. Two peaks at 1,627 and 1,729 cm−1 are related to stretch vibration of amide and acid carbonyl groups in polyamide chain structures, respectively (Zoua et al. 2010). Two peaks at 3,429 cm−1 and 1,370 correspond to stretch vibration N—H and C—N groups of PEI polymer chains. The FT-IR spectra confirm that the interfacial polymerization of polyamide active layer was successfully done on the surface of PAN support membrane.
Figures 7(c) and 7(d) show FT-IR spectra of polyamide membrane after cross-linking reaction using XDC and GA, respectively. The strong peak at 677 cm−1 is attributed to stretching vibration of C—Cl bond of XDC (Li et al. 2011). Also, the bending vibration of CH2–Cl of XDC compound is shown at 1,261 cm−1 that proves cross-linking reaction of PEI chains at polyamide active layer. In Figure 7(d) GA peaks such as vibration aldehyde carbonyl and vibration aldehyde C—H overlapping with polyamide peaks are seen. In this case we have reverted to other tests such as SEM, AFM, MWCO and contact angle analysis and performance tests.
Scanning electron microscopy
The PAN support membrane became rougher and had a higher roughness and membrane surface shows many small fine grain-like and belt-like structures. Surface morphology variation demonstrates that thin and dense polyamide active layer was successfully formed on the PAN support membrane. After cross-linking reaction, the grain-like structure gradually decreased and the belt-like structures increased and became larger. This phenomenon is probably attributed to the quaternization cross-linking reaction between XDC and GA and active amine groups in PA backbone. As seen in Figures 8(d), 8(f) and 8(h), XDC membrane has fewer grains compared with PA and GA membrane, respectively. This may be due to higher reactivity of XDC agent.
Atomic force microscopy
|Roughness (±3) (Sa)||Root mean square (Sq)||Peak maximum (Sz)|
|Roughness (±3) (Sa)||Root mean square (Sq)||Peak maximum (Sz)|
In the present study novel positively charged PA-TFC membranes were successfully prepared by cross-linking PA layer with XDC and GA agents. It was observed that the quaternization of PA layer affected nanofiltration properties and led to higher rejection of divalent cationic salt (CaCl2). This demonstrates that the new positively nanofiltration membrane is appropriate for separation of cationic material solutions, which is essential for some industries. For both types of cross-linking agents, it was observed that the rejection of cationic salt increased with an increase in agent concentration. The XDC membrane indicated positive charge in the pH range from 3 to 11 due to the quaternization of the tertiary amine groups in the PA layer.