Active Transport of Sodium Ions through Interpolymer Carboxylic Membranes

Abstract

The active transport of sodium ions in the system NaOH/membrane/NaCl-HCl has been investigated. The membranes used were carboxylic membranes of the interpolymer type polyethylene/poly(methacrylic acid-co-divinylbenzene), PE/poly(MA-co-DVB), containing ～32 wt% of poly(MA-co-DVB) and 1.8–9.6 wt% of DVB. The best transport characteristics were achieved for membranes with 1.8–3.0 DVB wt% and 0.1 M HCl. However, the observed Na⁺ fluxes and selectivities were rather small. To explain the observed effects, a three-layer laminate model of the carboxylic membrane has been postulated where the working membrane can be viewed as consisting of three parallel layers. The first layer, contacting alkaline solution, is regarded as an ideal carboxylic ion-exchange membrane. The second layer, contacting acidic solution, is regarded as a nonionic gel. The third layer is the intermediate part localized between the above-defined layers. It has the properties of an ion-exchange membrane to some extent but it is relatively nonselective and contains coions. It has been stated that for the most effective action of the interpolymer membrane, the thickness of the second membrane layer should be as small as possible during the whole process. This can be achieved at low HCl concentration. On the other hand, low HCl concentration cannot secure the required driving force. Moreover, the net effect of a DVB content increase in the membrane was a flux decline. Therefore, the carboxylic membranes are not effective for the active transport of Na⁺ ions in the system studied.     

Use of Micellar Solutions as Draw Agents in Forward Osmosis

Recent advances in membrane technologies have enhanced the viability of water treatment strategies that employ semipermeable barriers. Forward osmosis (FO), which exploits the natural osmotic pressure gradient between a “draw” solution and a “feed” solution to produce potable water, offers a low‐energy, low‐cost alternative to more conventional treatment methods. Surfactants, because of their tendencies to aggregate into micelles and to adsorb at interfaces, provide intriguing osmotic pressures and offer exploitable properties by which draw solutions can be regenerated. The effectiveness of surfactant‐based FO using cellulose triacetate membranes has been assessed in terms of water flux and reverse surfactant diffusion using cetylpyridinium chloride, sodium dodecylsulfate, and Triton X‐100. The ratios of water flux to surfactant flux exceeded 600 L mol^?1 for all surfactants studied. Surfactant recoveries of over 99 % were achieved by ultrafiltration using regenerated cellulose membranes.     

State of the water in crosslinked sulfonated poly(ether ether ketone)

We investigated the material properties of different crosslinked sulfonated poly(aryl ether ketone) membranes, focusing on the effect of the degree of sulfonation and crosslinking density on the water uptake, the physical state of the water, and the pore size distribution within the membranes. We observed that the degree of sulfonation and, in particular, the ion‐exchange capacity (IEC) had less effect on the control of the extent of water absorbed than the crosslinking density of the membranes. Crosslinking also enabled the membranes to reach a higher water contents without losing mechanical integrity. Moreover, increasing the crosslinking density resulted in the presence of more bound water, without dissolution of the membrane. The crosslinked membranes had lower methanol permeability and electroosmotic drag values. Only at low IEC values and low water uptake in partially crystalline sulphonated poly(ether ether ketone), SPEEK could the presence of nanometer pores in the water‐equilibrated crosslinked membranes be confirmed by thermoporometry and the pore size distributions were then comparable to those reported for Nafion membranes. At higher IEC values, the water uptake was extremely high, up to 300%, and then the structure of the swollen membranes was more analogous to that of a dilute aqueous solution of the sulfonated polymer, and no nanopores were present. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation and characterization of semi‐interpenetrating network polystyrene/PVDF cation exchange alloy membranes

The polystyrene‐DVB/PVDF alloy particles were prepared by pulverizing the polymerization product of styrene/DVB/PVDF in DMF, and then sulfonated with concentrated sulfuric acid to gain the cation exchange alloy powder, which was directly thermoformed by a hot‐press machine to form the titled cation exchange alloy membranes with the structure of semi‐interpenetrating polymer network. The effects of the polystyrene‐DVB to PVDF mass ratio and the DVB content in the monomers on the physical and electrochemical properties of the prepared alloy membranes were investigated. While the Fourier transform infrared spectroscopy (FTIR) confirms the components of membranes, the scanning electron microscopy (SEM) reveals that the alloy membranes possess a uniform distribution of functional groups, and a more dense structure with the increases of DVB content and PVDF content. The optimal prepared membranes have the area electrical resistance values within 3.0–6.6 Ω·cm², obviously superior to the commercial heterogeneous cation exchange membrane, as well as the moderate water contents of 35–40% and the desirable permselectivity with a transport number more than 0.95. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1220‐1227, 2013     

Permeabilities to salts and water of macrocyclic polyether–polyamide membranes

The polyether–polyamide (PC-6) strongly absorbs sodium salts from their aqueous solutions. Membranes based on PC-6 and on its polymeric alloys with poly(vinylpyrrolidone) (PVP) are, however, much more permeable to water than to salts. The membrane permselectivity is due to the low mobility of the absorbed salts. Desorption experiments were conducted to determine the values of the diffusion coefficients of the sodium ions in the investigated membranes. They were found to vary from 5 × 10^-12 cm²/sec in loose PC-6 membranes to 1.7 × 10^-9 cm²/sec in the polymeric alloy containing 30% PVP. Water permeation experiments with the alloy membranes yielded values of the diffusion coefficients in the range of 2–5 × 10^-7 cm²/sec. The apparent “energy of activation of the diffusion” of sodium ions in such membranes was found to be essentially indentical (～12 kcal) with the energy of activation of the decomplexation of the sodium–“crown” complex. The ramifications of the proposed “site to site jump” diffusion mechanism were discussed. The permeability characteristics of PC-6 membranes were found to be strongly affected by their “history.”. The observed phenomenon was explained in terms of reversible changes in the structure of the polymeric network, in the presence and in the absence of the absorbed salts. It has been found that PVP has a stabilizing effect on the permeability characteristics of the membranes. Reverse osmosis experiments indicated that their intrinsic osmotic characteristics seem to be superior to those of the commercially used materials. Their salt rejections are in the range of 95–99.5%, and their permeabilities to water are at least one order of magnitude higher than those of the unmodified aromatic polyamides.     

Electrodialytic properties of aromatic and aliphatic type hydrocarbon-based anion-exchange membranes with various anion-exchange groups

Anion-exchange membranes (AEMs) with various membrane structures were prepared by introducing various amines: trimethylamine (TMA), triethylamine (TEA), tri-n-propylamine (TPA) and tri-n-butylamine (TBA) into precursor membranes prepared from chloromethylstyrene (CMS)–divinylbenzene (DVB) and glycidyl methacrylate (GMA) – DVB. Their properties for ionic transport and anti-organic fouling were examined. Almost all of the prepared AEMs have excellent ionic transport properties: transport number of anions >0.94 and membrane resistance <4 Ωcm². The voltage change through the AEMs during electrodialysis operation using solutions containing sodium dodecylbenzene-sulfonate as a foulant indicated that there are three cases of fouling mechanism being related to membrane structure: (a) aliphatic AEMs show lower fouling than aromatic ones; (b) the lower the water content of an AEM, the more remarkable the fouling; (c) the longer the chain length of the alkyl groups of the anion-exchange groups of an AEM, the more remarkable the fouling.     

纳滤和反渗透膜形貌结构与膜性能关系探索

纳滤和反渗透膜表面形貌结构、亲疏水性的性质与膜脱盐率、水通量等性能存在一定关系。对几款商用纳滤、反渗透膜进行表面形貌结构、表面粗糙度、亲水性表征。结果表明,纳滤膜表面平整粗糙度低、亲水性强、脱盐率较低,但水通量高。反渗透膜表面存在大量疏松的峰谷结构,比纳滤膜粗糙度更大、亲水性强。对比两款海水反渗透膜,推测调整反渗透膜"叶片"大小和数量可调节反渗透膜的脱盐率和水通量性能。     

Facile modification of aliphatic polyketone-based thin-film composite membrane for three-dimensional and comprehensive antifouling in active-layer-facing-draw-solution mode

The application of “active-layer-facing-draw-solution” (AL-DS) mode, which allows a considerably high water flux in forward osmosis (FO) processes, is hindered by severe fouling occurring within the porous support of the FO membranes. We designed a series of “three-dimensionally” antifouling FO membranes by an extremely convenient and scalable approach, by using in situ reduced aliphatic polyketone (PK) membranes (rPK) and the silver-nanoparticles-immobilized rPK-Ag membranes as the substrates for thin-film composite (TFC) FO membrane preparation. This modification imparted enhanced hydrophilicity compared with the original PK-TFC membrane, without affecting the morphology and transport properties. Benefiting from the three-dimensional antifouling structure, the modified TFC membranes (i.e., rPK-TFC and rPK-Ag-TFC membranes) demonstrated excellent and comprehensive fouling resistance towards a variety of organic foulants, as well as biofouling resistance towards Escherichia coli. These results provide useful insights into the fabrication of antifouling FO membranes for water purification purposes and pressure retarded osmosis (PRO) process.     

Electrochemical synthesis of N2O5 by oxidation of N2O4 in nitric acid with PTFE membrane

Electrochemical synthesis of dinitrogen pentoxide (N₂O₅) by oxidation of dinitrogen tetroxide (N₂O₄) in a plate-and-frame electrolyzer was investigated. As the separator, different porous polytetrafluoroethylene (PTFE) membranes were tested in this process and the effects of hydrophilicity and of hydrophobicity on the electrolysis were discussed. The transport of N₂O₄ and water from catholyte to anolyte through membrane occurred in the electrolysis, especially at the end of the electrolysis. The water transport had a much more effect on the electrolysis than that of the N₂O₄ diffusion. The hydrophobic PTFE membranes had better performance on control of water transport from catholyte to anolyte than that of the hydrophilic ones. Hydrophobicity can increase the chemical yield of N₂O₅. The membranes with a low hydrophobic surface were preferred. All the hydrophobic PTFE membranes with low resistance have the specific energy of 1.1-1.5 kWh kg⁻¹ N₂O₅. The current efficiency of 67.3-80.2% and chemical yield of 58.9-60.9% were achieved in production of N₂O₅. The technique of replacing the catholyte with fresh nitric acid can minimize the transport of N₂O₄ and water to a great extent, it can further improve the chemical yield and reduce the specific energy.     

Transport within ion-exchange membranes

For the purpose of investigating electromigration in ion-exchange membranes, d.c. conductivity measurements have been carried out in (a) a strong-acid membrane in hydrogen and sodium forms and (b) a strong-base membrane in hydroxyl and chloride forms. These measurements have been conducted in the absence of external electrolyte from virtually dry to fully water-saturated membrane conditions. From our specific conductance data, electrical ion mobilities have been calculated. Ionic mobilities as functions of membrane water content have been correlated for both the low and high-hydration ranges by means of our semi-theoretical exponential equation that relates ion mobility to membrane water content. Hydrogen and hydroxyl ions have been shown to be transferred by an additional fast transport mechanism above certain levels of membrane water absorptions. At low membrane water contents, they electromigrate like sodium and chloride ions under an applied electric field.     

The effect of corrugated membranes on salt splitting

An investigation of the electrohydrolysis of sodium sulfate using a corrugated Nafion^® 117 membrane is reported. A comparison of the performance of a flat and corrugated Nafion^® 117 in a two-compartment membrane electrolysis cell is made. Corrugating the membrane increased the active membrane area by 57% compared to the projected area. The effect of flow rate, current density and salt concentration on current efficiencies, transport properties and achievable product concentrations are presented. The results show a large improvement on transport properties, current efficiencies and product formation using corrugated membranes. Corrugated membranes gave an improvement of up to 77% on achievable base concentration and an increase of approximately 22% in current efficiency.     

Triblock SEBS/DVB crosslinked and sulfonated membranes: Fuel cell performance and conductivity

A set of styrene-ethylene-butylene-styrene triblock copolymer (SEBS) membranes with 10 or 25 wt% divinyl-benzene (DVB) as a crosslinking agent were prepared and validated. Physicochemical characterization revealed suitable hydrolytic and thermal stability of photo-crosslinked membranes containing 25 wt% DVB and post-sulfonated. These compositions were evaluated in H₂/O₂ single cells, and electrical and proton conductivities were furtherly assessed. The membranes with the milder post-sulfonation showed greater proton conductivity than those with excessive sulfonation. In terms of electrical conductivity, a universal power law was applied, and the values obtained were low enough for being used as polyelectrolytes. At the analyzed temperatures, the charge transport process follows a long-range pathway or vehicular model. Finally, fuel cell performance revealed the best behavior for the membrane with 25 wt% DVB, photo-crosslinked during 30 min and mild sulfonated, with a promising power density of 526 mW·cm⁻². Overall, the results obtained highlight the promising fuel cell performance of these cost-effective triblock copolymer-based membranes and indicate that higher sulfonation does not necessarily imply better power density.     

The formation mechanism of asymmetric membranes

Aromatic polyamide asymmetric “skin type” membranes have been prepared by the Loeb-Sourirajan technique. Two different structures were obtained, depending on the rate of precipitation. Low precipitation rates produced membranes with sponge-like structures. These membranes usually had high salt rejections and low water fluxes. High precipitation rates produced membranes with large finger-like pores. These membranes had low salt rejections and high water fluxes. Possible mechanisms producing these two structures are discussed. It is proposed that high concentrations of polymer in the casting solution, and hence high viscosities at the point of precipitation, or a thick viscous sublayer in advance of the precipitating polymer front both favour sponge-structured membrane formation. The suggested mechanisms are supported by data showing the effect of various preparation parameters such as polymer concentration and casting solution and precipitation bath composition on membrane structure.     

Highly permeable reverse osmosis membranes incorporated with hydrophilic polymers of intrinsic microporosity via interfacial polymerization

Enhancing the water permeation while maintaining high salt rejection of existing reverse osmosis (RO) membranes remains a considerable challenge. Herein, we proposed to introduce polymer of intrinsic microporosity, PIM-1, into the selective layer of reverse osmosis membranes to break the trade-off effect between permeability and selectivity. A water-soluble a-LPIM-1 of low-molecular-weight and hydroxyl terminals was synthesized. These designed characteristics endowed it with high solubility and reactivity. Then it was mixed with m-phenylenediamine and together served as aqueous monomer to react with organic monomer of trimesoyl chloride via interfacial polymerization. The characterization results exhibited that more “nodule” rather than “leaf” structure formed on RO membrane surface, which indicated that the introduction of the high free-volume of a-LPIM-1 with three dimensional twisted and folded structure into the selective layer effectively caused the frustrated packing between polymer chains. In virtue of this effect, even with reduced surface roughness and unchanged layer thickness, the water permeability of prepared reverse osmosis membranes increased 2.1 times to 62.8 L·m^-2·h^-1 with acceptable NaCl rejection of 97.6%. This attempt developed a new strategy to break the trade-off effect faced by traditional polyamide reverse osmosis membranes.     

Characterization of sulfonated poly(styrene-co-pyrrolidone) pore-filling membranes for fuel cell applications

Pore-filling membranes using three monomers, i.e., styrene, N-vinyl pyrrolidone (VP), and divinylbenzene (DVB), are prepared for polymer electrolyte fuel cell (PEFC) applications. A porous polyethylene (PE) film substrate is used to enhance the dimensional stability of the prepared membranes. The proton conductivity and the water uptake of the styrene/VP/DVB membranes are similar to those of the styrene/DVB membranes, even though their ion exchange capacity is slightly lesser than that of the styrene/DVB membranes. Furthermore, the thermal stability of the styrene/VP/DVB membranes is higher than that of the styrene/DVB membranes, and, for the same DVB content, the membranes containing VP exhibit better oxidative stability. VP increases the membrane’s water-absorbing ability due to its intrinsic hydrophilic property and decreases weak α-hydrogen derived from the sulfonated styrene. Finally, the membrane-electrode assembly (MEA) using the 80/10/10 (Styrene/VP/DVB in weight percentage) membrane shows better performance than that using the 90/0/10 membrane.     

An interpolymer anionic composite reverse osmosis membrane derived from poly(vinyl alcohol) and poly(styrene sulfonic acid)

An interpolymer anionic composite membrane for reverse osmosis was prepared from poly(vinyl alcohol) and poly(styrene sulfonic acid). The effects of composition of a casting solution, heat-curing periods, and casting thickness on the reverse osmosis performance of resulted membranes have been examined. A mixture of water and ethyl alcohol (12/7, wt %) was found to be a proper solvent for casting an interpolymer membrane on the supporter. The composite membrane was formed by casting the polymer solution in ultrathin film on a microporous polypropylene supporter, evaporating the solvent, and heat-curing at 120°C for a proper period. the optimum composition of a casting solution was as follows: wt % of poly(vinyl alcohol)/poly(styrene sulfonic acid)/solvent was 3/2/95. The membrane heat-cured at 120°C for 2 h has a good performance for reverse osmosis, viz., water flux of 9.1–28.4 L/m².h at salt rejection level of 88.1–93.4% under applied pressure of 80 kg/cm² with 0.5% NaCl aqueous solution. The formation mechanism of a water-insoluble membrane was discussed.     

Effect of crosslinking on the physicochemical properties of proton conducting PVDF-g-PSSA membranes

Poly(vinylidene fluoride) (PVDF) membranes, radiation-grafted with styrene and sulfonated, were studied as a candidate material for polymer electrolyte fuel cell (PEFC). In particular the effect of the use of crosslinkers in the polymer structure was investigated using bis(vinyl phenyl)ethane (BVPE) and divinylbenzene (DVB) as reagents. Water uptake in the H+ form, proton conductivity and ion exchange capacity of the PVDF-g-PSSA membranes, as well as transport properties of oxygen and hydrogen were determined at room temperature. Crosslinking with DVB resulted in a more pronounced decrease in the properties; the use of BVPE had no significant influence. Even on the permeation of oxygen and hydrogen the BVPE had little effect: the diffusion coefficient and solubility remained at the same level as for the non-crosslinked membranes. Increasing the membrane thickness was found to be at least as effective in reducing the oxygen permeation rate as using crosslinkers.     

Salt splitting with radiation grafted PVDF membranes

Sulphonated PVDF cation-exchange membranes have been formulated for the splitting of sodium sulphate by electrohydrolysis. Three membranes with different degree of grafting were tested in a two-compartment membrane cell. The effect of flow rate, current density and salt concentration on the performance of each membrane is described. The different flow conditions in front of the membranes did not significantly affect the current efficiency. Productivity was greater at higher current densities, although a slight decrease in the current efficiency was observed. The SPVDF with a 22.7% degree of grafting performed slightly better than the other cation-exchange membranes. The new materials gave acceptable selectivity; low electrical resistance; and excellent chemical, thermal, and mechanical stability. They resulted in superior performance to the commercially available Nafion® 117, enabling an increase of approximately 20% in current efficiencies and sodium transport rates.     

Water softening by diffusion using cation exchange membranes

Water softening by an Interionic diffusion process across a cation- exchange membrane is studied. A twenty compartment laboratory scale unit with an effective crosssectional area of 63 cm² is assembled and studied for water softening using an interpolymer cation exchange membrane developed in this institute. Alternate compartments formed by thin gaskets are fed with synthetic hard waters and sodium chloride as regenerant solution in a counter current series flow arrangement. Results on softening of hard water of 500, 1000, 1500 ppm as CaCO₃, containing sodium chloride in the ratios s 1:0, 1:1 and 1:2 are given. The regenerate concentrations of 0.5, 1.0 and 1.5 N are studied for softening efficiency. Diffusion of salt and water through the membranes and the effect of regeneration levels to achieve 80% softening is also investigated. The commercial prospects for a continuous softening system by this method are discussed.     

Transport and conductivity properties of an advanced cation exchange membrane in concentrated sodium hydroxide

An electrochemical concentrator for application to the chlorine-caustic industry is currently under development. In it 30 to 35 wt % NaOH enters the anolyte and catholyte chambers and exits at 20 and 50 wt %, respectively. Consequently, in support of the electrochemical concentrator development, the conductance and transport properties of advanced cation exchange membranes in concentrated sodium hydroxide, are being investigated. The membrane voltage drop, sodium ion transport and water flux of these membranes in 20 to 35 wt % sodium hydroxide anolyte and 30 to 50 wt % sodium hydroxide catholyte at 75°C are presented. To better understand the behaviour of these membranes, electrolyte sorption measurements were conducted in the anolyte/catholyte environment appropriate for the electrochemical concentrator. The water uptake data appear to correlate well with the conductance data and the combined NaOH and water sorption data are consistent with the sodium ion transport data.