MgCl2 Rejection Research Articles

Preparation of positively charged composite nanofiltration membranes by quaternization crosslinking for precise molecular and ionic separations

Developing nanofiltration (NF) membranes with highly efficient and precise separation ability is of great significance for the molecular and ionic separations. In this work, positively charged composite NF membranes were engineered via soaking of polysulfone (PSF) ultrafiltration membranes in poly(N-vinyl imidazole) (PVI) solutions followed by a quaternization crosslinking step. The PVI was firmly attached to the PSF membrane by this method and acted as an active separation layer of the composite NF membrane. The obtained composite NF membrane featured a high rejection (83%) to vitamin B12 (molecular radius: 0.74 nm) but a low rejection (24.6%) to vitamin B2 (molecular radius: 0.47 nm), exhibiting a great potential in precisely molecular separation. Furthermore, the ionic separation ability of the composite NF membrane was confirmed with a rejection order of Na2SO4 < MgSO4 < NaCl < MgCl2 and the MgCl2 rejection reached up to 90.1%. Compared to conventional polyamide NF membranes, the developed PVI/PSF composite NF membranes were characterized with high separation precision to organic molecules, higher rejection over cationic ions than over anionic ones, better chlorine resistance and stability in long-term operation. In addition, the membrane fabrication process is convenient and easily scaled up in industry. This work offers a novel alternative of NF membranes for high precision in molecular and ionic separations.

Positively charged capillary nanofiltration membrane with high rejection for Mg2 + and Ca2 + and good separation for Mg2 + and Li +

High water permeability and good separation property are greatly desired in water production due to energy concerns. To explore nanofiltration (NF) membrane with high permeability for cations separation, a positively charged NF membrane was fabricated via interfacial polymerization using polyethersulfone (PES) three-channel capillary ultrafiltration (UF) membrane as substrate, polyethyleneimine (PEI) as the aqueous precursor. The NF membrane preparation conditions were optimized. To enhance the permeability of the prepared NF membrane, modified hydroxyl contained multi-walled carbon nanotubes (MWCNTs-OH), grafting with piperazine (PIP), were utilized. The resultant NF membrane showed increased water flux from 20.8L·m−2·h−1 to 56.1L·m−2·h−1 at 4bar after adding 0.01wt% modified MWCNTs-OH in aqueous solution. Interestingly, MgCl2 rejection of the membrane also increased from 94.2% to 96.9%. The positively charged NF membrane exhibited above 97% rejection for divalent cations (Mg2+ and Ca2+) and low rejection (<70%) for monovalent cations (Na+ and Li+), and it also showed long durability and good separation for Mg2+ and Li+ when the membrane was used to separate mixed salts solution simulated the composition of salt lake brine. The fabricated membrane would have potential for effective water softening and for reclamation of lithium from brine or seawater with high Mg2+/Li+ ratio.

Codeposition of catechol–polyethyleneimine followed by interfacial polymerization for nanofiltration membranes with enhanced stability

ABSTRACTThin‐film composite (TFC) nanofiltration (NF) membranes were fabricated via the codeposition of catechol (CCh) and polyethyleneimine (PEI) followed by subsequent interfacial polymerization with trimesoyl chloride (TMC) on the surface of polysulfone ultrafiltration substrates. The detailed structures and surface properties were characterized by X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, ζ potential analysis, and water contact angle measurement. The surface properties, including the roughness, hydrophilicity, surface potential, and NF performances, were facilely tuned through variation of the codeposition time of CCh–PEI for the prepared TFC membranes. The optimized membrane achieved a high rejection (ca. 93%) of MgCl2 with a flux of around 31 L m−2 h−1 under 0.7 MPa. The results also reveal that the codeposition process endowed the final membranes with much better structural stability in alcohol and improved chlorine resistance compared to commonly interfacial polymerized ones with PEI and TMC. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45422.

Constructing zwitterionic surface of nanofiltration membrane for high flux and antifouling performance

Antifouling performance of nanofiltration membrane is critical in applications like water treatment. In this study, surface zwitterionization was conducted by 1, 3-propanesultone ring-opening directly on the nanofiltration membrane prepared by interfacial polymerization of 3,3′-diamino-N-methyldipropylamine and 1, 3, 5-benzenetricarboxylic chloride. The zwitterionic structures improve the hydrophilicity of nanofiltration membrane, thus increase the water permeability and antifouling performance. The zwitterionic membrane have a 72Lm−2h−1 water flux at the optimize condition, which was about 2.4 times that of the pristine membrane without zwitterionization, and with a 89% rejection of MgCl2. Meanwhile, the zwitterionic membrane will have better antifouling performance to protein and polysaccharose. The flux recovery ratio of bovine serum albumin, sodium alginate and lysozyme were all above 95%. The prepared zwitterionic membrane presents prominent high water flux and fouling-resistance performance, indicating it's promising application potential in organic wastewater treatment.

Alkyl amine functional dextran macromonomer‐based thin film composite loose nanofiltration membranes for separation of charged and neutral solutes

ABSTRACTThin film composite (TFC) nanofiltration membranes with defined porous structure of the separation layer are desirable for the concentration of neutral solute and separation of salts from a mixture. Herein, we report the formation of TFC membranes composed of polyamide (PA) separation layer by the interfacial polymerization between new dextran‐butyl amine (Dex‐NH2) macromonomer and trimesoyl chloride on polysulfone support membrane. The membranes prepared with 1%–1.5% (wt/vol) of Dex‐NH2 exhibited water permeance of 110–116 L m−2 h−1 MPa−1 with 62%–71% rejection of Na2SO4 and 12%–14% rejection of MgCl2. The membranes also showed about 91% rejection of poly(ethylene glycol) of molecular weight 2000 g/mol and about 11% rejection of NaCl. A decrease in permeance and ions selectivity was observed with increasing concentration of Dex‐NH2. The dextran chains attached to the PA network restrict the diffusion of Dex‐NH2 toward the interfacial zone and thereby assist the formation of porous and thin PA layer compared to that when free amine (alkyl diamine) was used. These membranes are applicable for the separation of small molecular weight neutral solutes from mixture containing monovalent salts. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45301.

Fabrication of composite nanofiltration membranes by dopamine-assisted poly(ethylene imine) deposition and cross-linking

Positively charged composite nanofiltration (NF) membranes with good stability were prepared by dopamine (DA) assisted poly(ethylene imine) (PEI) deposition on a polysulfone ultrafiltration (UF) substrate followed by a cross-linking step. Attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electronic microscopy, and atom force microscopy were employed to characterize the surface chemistry and morphology of the obtained composite membranes. The DA and PEI co-deposition conditions were optimized based on knowledge of the co-deposition mechanism. The effects of the cross-linker concentration, cross-linking time, and reaction temperature on the permeation and separation properties of the prepared composite membranes were investigated in detail. Under optimized conditions, the MgCl2 rejection and permeation flux of the composite membrane reached 80.4% and 19.6 L/(m2·h), respectively (the feed was 0.01 mol/L of MgCl2 solution under a test pressure of 0.4 MPa). The rejection of various salts followed the order MgCl2≈CaCl2>MgSO4>NaCl>Na2SO4, suggesting the membranes were positively charged. The composite membranes showed good durability under alkaline aqueous conditions. This study provided new insights into the fabrication of mussel-inspired thin-film composite nanofiltration membranes.

Nanofiltration Membranes with Narrow Pore Size Distribution via Contra-Diffusion-Induced Mussel-Inspired Chemistry.

Nanofiltration membranes (NFMs) are widely used in saline water desalination, wastewater treatment, and chemical product purification. However, conventional NFMs suffer from broad pore size distribution, which limits their applications for fine separation, especially in complete separation of molecules with slight differences in molecular size. Herein, defect-free composite NFMs with narrow pore size distribution are fabricated using a contra-diffusion method, with dopamine/polyethylenimine solution on the skin side and ammonium persulfate solution on the other side of the ultrafiltration substrate. Persulfate ions can diffuse through the ultrafiltration substrate into the other side and in situ trigger dopamine to form a codeposited coating with polyethylenimine. The codeposition is hindered on those sites completely covered by the polydopamine/polyethylenimine coating, although it is promoted at the defects or highly permeable regions because it is induced by the diffused persulfate ions. Such a "self-completion" process results in NFMs with highly uniform structures and narrow pore size distribution, as determined by their rejection of neutral solutes. These near electrically neutral NFMs show a high rejection of divalent ions with a low rejection of monovalent ions (MgCl2 rejection = 96%, NaCl rejection = 23%), majorly based on a steric hindrance effect. The as-prepared NFMs can be applied in molecular separation such as isolating cellulose hydrogenation products.

Novel antifouling nano-enhanced thin-film composite membrane containing cross-linkable acrylate-alumoxane nanoparticles for water softening

A novel thin-film composite (TFC) nanofiltration membrane was prepared using polymerization of pyrrole monomers on the PES ultrafiltration membrane. To improve the characteristics of hydrophobic polypyrrole (PPy) thin-film layer, cross-linkable acrylate-functionalized alumoxane nanoparticles with different concentrations were embedded into the thin-film during polymerization process, and thin-film nanocomposite (TFNC) membranes were prepared. The characteristics and performance of TFC and TFNC membranes were assessed through the morphological analyses (SEM, AFM), measurement of hydrophilicity and solid–liquid interfacial free energy, water permeability and Mg2+ removal tests. Addition of proper amount of nanoparticles into the polymerization mixture led to the preparation of membranes with more hydrophilic, thinner and smoother active layer as well as higher water permeability compared to TFC control membrane. TFNC membrane prepared with 0.025g of nanoparticles was the most efficient membrane since it exhibited the highest rejection of MgCl2 and MgSO4 salts. Antifouling capability of membranes, in terms of flux recovery and fouling parameters, demonstrated the high tolerance of TFNC against fouling.

Dual-Charged Hollow Fiber Membranes for Low-Pressure Nanofiltration Based on Polyelectrolyte Complexes: One-Step Fabrication with Tailored Functionalities.

A new nanofiltration (NF) hollow fiber membrane is developed by using two oppositely charged polyelectrolytes coagulating into a polyelectrolyte complex (PEC) onto polyether sulfone base polymer. The particular membrane architecture emerges during a single-step procedure, allowing setting both the porous negatively charged support of the hollow fiber and the separation layer containing also the positive polyelectrolyte (PEI/PDADMAC) through a single layer dry-jet wet spinning process. The novelty is two-pronged: the composition of the hollow fiber membrane itself and its fabrication procedure (one-step fabrication of membranes employing polyelectrolytes). These result in highly permeable hollow fiber membranes with a stable separation layer and performance at par with the membranes reported in literature obtained by multistep processes. More importantly, the membranes are obtained through a simple, very fast (one-step), and less expensive procedure. The best performance among these newly obtained hollow-fiber membranes is achieved by PD5% hollow fiber (MWCO of 300 Da), which showed 7.6 L/m(2)·h·bar permeability and ∼90% rejection of MgCl2, MgSO4, and Na2SO4 at 2 bar pressure. Thus, the resulting membranes not only have the advantages of the hollow-fiber configuration, but perform very well at extremely low pressures (the lowest reported in the literature). The broad impact of the results presented in this Article lies in the potential to dramatically reduce both the fabrication (duration and complexity) and the price and desalination costs of highly performing NF hollow fiber membranes. These might result in interesting potential applications and open new directions toward designing efficient functional NF hollow fibers for water desalination.

Chemically crosslinked rGO laminate film as an ion selective barrier of composite membrane

In this research, a novel approach of sulfonation followed by mild reduction was employed first time to prepare well separated rGO laminate composite membranes. The combined multistep approach offers hydrophilic regions as well as suitable interlayer spacing of 0.63nm and 0.74nm for ion selectivity and water permeation. In-situ Crosslinking reaction has been conducted on the assembled rGO film to successfully improve the stability of the rGO film in water. The promising water permeability of 61.7LMH/kPa was achieved from optimized rGO interlayer distance by carefully controlled sulfonation and partial reduction conditions (sample Low-R). It was found that the net surface charges Donnan exclusion mechanism played a significant role in the salt rejection efficiency, which was subject to the ratio of cationic and anionic species valency present in the solution. The prepared rGO membrane High-R achieved 80.5% rejection of Na2SO4, followed by 52.0% rejection of NaCl, and 13.7% rejection of MgCl2. It can be seen that there was an interplay between water flux and salt rejection performance, the balance can be achieved by adjusting the reduction conditions. Stronger reduction condition resulted in higher salt rejection rate, while the water flux would decrease to some extent, it was due to the less remaining surface functional groups and resulted in smaller interlayer distance. This research has tested and confirmed of using a novel approach, a relatively small quantity of well prepared and assembled two-dimensional rGO laminates could form an integrated barrier for effective ion selectivity.

Thin film composite nanofiltration membranes fabricated from quaternized poly(ether ether ketone) with crosslinkable moiety using a benign solvent

Thin film composite nanofiltration membranes were fabricated through dip-coating and in situ cross-linking of quaternized poly(ether ether ketone) containing a certain amount of tertiary amine groups (QAPEEKs) on polyacrylonitrile (PAN) support. The effects of the variables in membrane formation such as the coating polymer concentration, the curing temperature, and the cross-linking agent types on resultant membrane were studied and the membrane properties such as the barrier layer chemical structure, the surface element composition and morphology were investigated. The obtained performance of uncross-linked and cross-linked QAPEEK-70 thin film composites in nanofiltration test was compared. The results indicated that the cross-linking improved the composite membranes’ performance. For instance, the membrane cross-linked by bisphenol A diglycidyl ether (BPADGE) named M-C-BPADGE exhibited a MgCl2 rejection of 97.8%, a water flux of 11.8Lm−2h−1, a MWCO of 800Da and corresponding pore size of 0.69nm, while for its uncross-linked membrane named M-U, a MgCl2 rejection of 91.2%, a water flux of 13.5Lm−2h−1, a MWCO with 960Da and a pore size of 0.77nm were found. Furthermore, the M-C-BPADGE membrane exhibited selectivities of 16.0 for separation of mixed Mg2+ and Na+ cations, much larger than selectivity of 5.2 obtained for M-U, suggesting that the cross-linked membranes are promising in cation separation.

Stability and permeation behavior of a porous membrane modified by polyelectrolyte networks enabled by electro-deposition and cross-linking for water purification

A novel nanofiltration (NF) membrane is developed via polyethyleneimine (PEI) deposition on a hydrolyzed polyacrylonitrile (PAN) substrate enhanced by an electric field and chemical cross-linking to achieve improved desalting properties. Compared to the PEI static-deposition process, dramatic increase in salt rejection and substantial reductions of deposition time and PEI concentration are achieved for the membranes prepared by electro-deposition. What's more, the resulting membrane exhibited both stable flux and salt rejection in filtrating seawater for a relatively long term due to a significant membrane stability attributed to the electro-deposition. Desalting properties in terms of salt rejection and flux were found to be strongly affected by both hydrolysis and electro-deposition conditions. Under the optimal fabricating conditions, the resulting NF membrane exhibited a high flux of 24L/m2h, excellent MgCl2 rejection of ∼92% and low NaCl rejection of ∼39% when filtrating 2000mg/L salt solution at 0.5MPa. Under filtration of raw seawater at 1.8MPa, the resulting membrane exhibited a high flux of 30.2L/m2h and a significant improvement in seawater softening efficiency compared to various commercial NF membranes, which is attributed to its excellent separation efficiency between divalent and univalent ions. The current investigation demonstrated that polyelectrolyte NF membranes fabricated by electro-deposition and cross-linking led to excellent separation performance and satisfactory stability and durability that can be used in seawater softening for a relatively long time.

A novel type of polyelectrolyte complex/MWCNT hybrid nanofiltration membranes for water softening

A novel type of polyelectrolyte complex (PEC)/multi-walled carbon nanotube (MWCNT) hybrid nanofiltration (NF) membranes were prepared via in-situ ionic cross-linking between sodium alginate (SA), polyethyleneimine (PEI), and MWCNTs. Chemical structures and compositions of PECs and MWCNT hybrid PECs (HPECs) were characterized by Fourier transform infrared spectroscopy (FTIR). Morphologies of PEC NF membranes (PEC-NFMs) and HPEC NF membranes (HPEC-NFMs) were characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM); and their hydrophilicity was characterized by water contact angle (CA). Effects of PEI and MWCNT content on the surface zeta potential and NF performance were investigated thoroughly. It was found that the surface charge character of PEC-NFMs changed from negative to positive with increasing the PEI content. The water flux of HPEC-NFMs dramatically increased from 13.4 to 27.0Lm−2h−1 with increasing the mass ratio of MWCNTs to SA from 0 to 0.050, and the MgCl2 rejection maintained at around 93.5% (testing with 1gL−1 aqueous MgCl2 solution at 25°C and 0.6MPa). Moreover, the HPEC-NFM-3 exhibited a good Na+/Mg2+ selectivity. This study provided an effective approach for fabricating positively charged NF membranes with enhanced NF performance, which suits well for water softening.

High-flux composite hollow fiber nanofiltration membranes fabricated through layer-by-layer deposition of oppositely charged crosslinked polyelectrolytes for dye removal

Composite hollow fiber nanofiltration membranes with an active skin layer comprising only one polyelectrolyte bilayer were prepared through layer-by-layer deposition of oppositely charged polyelectrolytes of sodium carboxymethyl cellulose (CMCNa) and polyethylenimine (PEI) on polypropylene hollow fiber substrate, with glutaraldehyde crosslinking between each deposition. Membrane performance was optimized by varying the contents of CMCNa and PEI of the deposition solutions and the obtained membranes were characterized in terms of physico-chemical characteristics and permeation properties. Experiments revealed that the obtained membrane was positively charged and exhibited a salt rejection order of MgCl2>CaCl2>KCl>NaCl>MgSO4>Na2SO4 at neutral pH. The optimized membrane had a molecular weight cut-off of about 730g/mol and possessed a pure water permeability of 14.2l/m2hbar, MgCl2 rejection of 93.2% and NaCl rejection of 36.2% in filtration of 500mg/l salt aqueous solution at 3.0bar. The desired membrane could also effectively remove organic dyes from aqueous solution, showing retentions of 99.8%, 99.6% and 99.4% to Brilliant green, Victoria blue B and Congo red, respectively, under the trans-membrane pressure of 3.0bar. These promising results demonstrate that the technique developed in this study is potentially applicable for preparing high-flux nanofiltration membrane for dye removal and partial desalination.

Layer-by-layer assembly of aquaporin Z-incorporated biomimetic membranes for water purification.

We fabricated a biomimetic nanofiltration (NF) membrane by immobilizing an Aquaporin Z (AqpZ)-incorporated supported lipid bilayer (SLB) on a layer-by-layer (LbL) complex polyelectrolyte membrane to achieve excellent permeability and salt rejection with a high stability. The polyelectrolyte membranes were prepared by LbL assembly of poly(ethylenimine) (PEI) with positive charges and poly(sodium 4-styrenesulfonate) (PSS) with negative charges alternately on a porous hydrolyzed polyacrylonitrile (H-PAN) substrate. AqpZ-incorporated 1,2-dioleloyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-3-trimethylammo-nium-propane (chloride salt) (DOTAP) vesicles with positive charges were deposited on the H-PAN/PEI/PSS polyelectrolytes membrane surface. The resulting biomimetic membrane exhibited a high flux of 22 L·m(-2)·h(-1) (LMH), excellent MgCl2 rejection of ∼97% and NaCl rejection of ∼75% under an operation pressure of 0.4 MPa. Due to the attractive electrostatic interaction between SLB and the polyelectrolyte membrane, the biomimetic membrane showed satisfactory stability and durability as well as stable NF flux and rejection for at least 36 h. In addition, the AqpZ-containing biomimetic membrane was immersed in a 0.24 mM (critical micellar concentration, CMC) Triton X-100 solution for 5 min. The flux and rejection were slightly influenced by the Triton X-100 treatment. The current investigation demonstrated that the AqpZ-incorporated biomimetic membranes fabricated by the LbL method led to excellent separation performances and robust structures that withstand a high operation pressure for a relatively long time.

Effect of LSCF content on the performance of LSCF/PES mixed matrix membranes

Mixed matrix membranes (MMM) possess very promising properties that have remarkably surpassed that of polymeric and inorganic membranes. LSCF which is often used in preparation of cathode materials and solid electrolyte materials has been demonstrated to have good conductivity in a water solution. So it is introduced in the preparation of La0.6Sr0.4Co0.2Fe0.8O3/polyethersulfone (LSCF/PES) mixed matrix membrane to improve the rejection of inorganic salts and the conducting MMMs were prepared via the phase inversion method in this work. The LSCF nanoparticles were introduced in the membrane pores of the PES microporous membrane. The content of PES and LSCF were changed to improve the membrane performance. When the PES concentration is 16wt.% and the LSCF weight ratio of the polymer is 20%, the membrane flux is 90.4L·h−1·m−2, and the rejection of MgCl2 is 25.8% at 0.3MPa and 20°C. The membrane performance is greatly affected by the concentration of LSCF and aggregation of the nanoparticles.

Thin film composite nanofiltration membranes fabricated from polymeric amine polyethylenimine imbedded with monomeric amine piperazine for enhanced salt separations

Thin film composite (TFC) nanofiltration membranes were fabricated by interfacial polymerization using polymeric amine polyethylenimine (PEI) and monomeric amine piperazine (PIP) as the amine reactant. Membranes with a single-ply polyamide layer were produced by reacting trimesoyl chloride (TMC) with mixed amines of PEI and PIP, and incorporation of a small amount of PIP in PEI was found to increase the permeation flux effectively while still maintaining a good solute rejection. For instance, adding 10wt% PIP in the amine reactant solution resulted in a 6-fold increase in permeation flux, while a 91.6% MgCl2 rejection was maintained. In addition, 2-ply polyamide membranes were also prepared by two cycles of PEI–TMC and PIP–TMC interfacial reactions separately, and they showed a higher rejection than the single-ply polyamide membrane. At a low PIP/PEI concentration ratio, the single-ply polyamide membranes formed with mixed amines of PIP and PEI tended to be more permeable than the 2-ply polyamide membranes. However, it was demonstrated that by properly controlling the PIP/PEI concentration ratio, the 2-ply polyamide membranes with both a higher permeation flux and salt rejection than conventional single-ply polyamide membranes could be produced. The resulting membranes were characterized for chemical composition, surface hydrophilicity, surface charge and morphology of the polyamide skin layer.

A novel positively charged composite nanofiltration membrane prepared by bio-inspired adhesion of polydopamine and surface grafting of poly(ethylene imine)

Novel positively charged composite nanofiltration membranes were facilely prepared by polydopamine (PDA) deposition followed by poly(ethylene imine) (PEI) grafting on polyethersulfone (PES) substrates. Scanning election microscope (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), zeta potential measurement and water contact angle measurement were employed to characterize the surface chemical composition and morphology of the resultant membranes. 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. The salts rejection followed the sequence: MgCl2>CaCl2>MgSO4>Na2SO4, confirming that the membranes were positively charged. The rejection of MgCl2 could reach 73.7%, whereas the rejection of CaCl2 was 57.1%. Moreover, the membranes exhibited a superior rejection of up to 96.5% for cationic dyes.

Poly(styrene sulfonic acid) sodium modified nanofiltration membranes with improved permeability for the softening of highly concentrated seawater

Poly(styrene sulfonic acid) sodium salt (PSSS)/polypiperazine-amide nanofiltration (NF) membrane and PSSS/SiO2/polypiperazine-amide NF membrane were prepared by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on polyethersulfone (PES) supporting membrane. Effect of PSSS and silica concentrations were discussed. The active surface of the membrane was characterized by using SEM and AFM. The results showed that PSSS/polypiperazine-amide NF membrane prepared under the optimum conditions exhibited Na2SO4 rejection of 96.1% and water flux with 55.7Lm−2h−1 at operation pressure of 0.6MPa. The rejection of Na2SO4, MgSO4, MgCl2 and NaCl follows a decreasing in order of 96.1%, 85.2% 45.4% and 21.1% respectively. After addition of 0.2% (w/v) silica in the aqueous phase, the water flux increased 42.7% compared with only polypiperazine-amide NF membrane. The PSSS/polypiperazine-amide NF membrane shows a promising capability for softening highly concentrated seawater from 8–20wt.%.

Preparation and characterization of novel positively charged copolymer composite membranes for nanofiltration

A novel copolymer poly(ether ether ketone) bearing pendant tertiary amine groups (TAPEEK-x) was synthesized by 3,3′-dimethylaminemethylene-4,4′-biphenol (DABP), 4,4′-bisfluorodiphenylketone (BFDPA) and bisphenol A (BPA). After reaction with iodomethane, the copolymer turned to poly(ether ether ketone) with pendant quaternary ammonium groups (QAPEEK-x). Afterward, QAPEEK-x copolymer was dissolved in formic acid and coated on PAN ultrafiltration membrane as a composite membrane. The NF membrane preparation conditions, such as drying temperature, formic acid concentration, copolymer concentration, feed concentration, and tolerance in chlorine solution were investigated. The experiments showed that the membrane whose coating polymer concentration was 0.5 wt% in 88% formic acid had the best flux of 12.6 L m−2 h−1 at 0.4 MPa and best rejection of 99.4% for 500 ppm MgCl2. In addition, the membrane showed excellent tolerance ability of chlorine with a little decrease of MgCl2 rejection. To some extent, this study provides a new insight to improve membrane separation capability and stability in the desalination process.