Performance Of Ion Exchange Membranes Research Articles

Exponential Water Uptake in Ionomer Membranes Results from Polymer Plasticization

Water content is the most influential factor in the performance of ion-exchange membranes (IEMs), controlling their mechanical rigidity, ionic conductivity, and degradation rates. Membranes absorb water exponentially at high water activity (aw), making that region of the sorption isotherm the most influential on membrane properties. Plasticization of the polymer by water has been proposed to cause this exponential uptake at high aw. However, an integrated microscopic picture of the structure, thermodynamic, and mechanical properties of IEMs as a function of aw has remained elusive. Here, we use large-scale molecular simulations validated with experimental measurements to compute the sorption isotherms, Young’s modulus, polymer dynamics, and structure of IEMs. The simulations unveil that the exponential increase in water uptake (WU) coincides in all cases with the glass to rubber transition of the membrane, as measured through its Young’s modulus and segmental polymer dynamics. Functionalization of the polymer with alkyl groups further contributes to its plasticization, increasing the WU at a given aw and ion-exchange capacity (IEC). The alkyl chains act synergistically with water to plasticize the polymer matrix and allow water penetration in the membrane. The simulations reveal that the width of the water channels depends on the ratio λ of water to ions in the polymer but is independent of its IEC. We conclude that differences in the polymer matrix─not the water channels─are responsible for the distinct uptake response of IEMs to the thermodynamic driving force of water activity.

Integration of hydroxyapatite from fish scales and polyethersulfone membrane for protease separation from Bacillus subtilis

ABSTRACT In the present study, an ion exchange membrane for thermostable alkaline protease separation from Bacillus subtilis was prepared via integration of hydroxyapatite from fish scales (FSHAp) and polyether sulfone membrane. The prepared membranes were characterized in terms of morphology, surface chemistry, ion exchange capacity, water uptake and fouling analysis. The performance of ion exchange membranes were evaluated by protease separation at different pH (3, 5, 7, 9, and 11) and ionic strength (0.05, 0.1, 0.15, 0.2, and 0.25 M). Characterization study revealed that PES/FSHAp-1000 membrane promoted the highest performance with a high permeability coefficient of about 64.0 L/m2.h.bar and membrane porosity around 90.1%. It also had a high ion exchange capacity of around 7.09 meq/g and water uptake for about 97.08%. This membrane’s hydrophilicity was also improved by 53%, and its flux recovery ratio (FRR) was about 97.79%. Fourier transform infra-red Spectroscopy (FTIR) and X-ray- diffractometer (XRD) ascertained the successful integration of FSHAp onto the membrane’s surface. Optimum recovery of protease (92.30%) obtained when the elution process performed using a glycine-NaOH buffer at pH 11 and 2.0 M ionic strength.

Anion exchange membranes with twisted poly(terphenylene) backbone: Effect of the N-cyclic cations

In order to investigate the relationship towards the cationic structures and ion exchange membrane performance, three kinds of twisted poly(terphenylene)-based anion exchange membranes (AEMs) with N-cyclic cations were prepared via facile Friedel-Crafts type polycondensation and quaternization. The steric hindrance of the N-cyclic cations is gradually increased from the small piperidinium to the sterically protected N-spirocyclic quaternary ammonium (QA). The twisted poly(terphenylene)s backbone promotes the self-assembly of the polymer chain and forms a microphase separated morphology, resulting in a highest conductivity of 68.7 mS cm−1 (80 °C) for the polymer tethered with piperidinium groups (m-TPNPiQA). The relative conductivity (conductivity/swelling ratio) of m-TPNPiQA is even higher than that of the commercial Fumapem FAA-3-50 membrane. Increasing the size of the QA is helpful to constrain water absorption and related swelling but has a negative effect on the chemical stability. β-Hofmann elimination degradation is observed for all of the AEMs during a stability test by 1H NMR analysis. The m-TPNPiQA demonstrates less than 6% ionic exchange capacity loss after 240 h in 5 M NaOH solution at 80 °C. The results demonstrated that the membrane performance is associated well with the features of the cationic groups. A high performance AEM can be achieved by grafting appropriate cations onto aryl ether-free backbone.

Effect of Surface Alignment on Connectivity in Phosphonium-Containing Diblock Copolymer Anion-Exchange Membranes

The performance of ion-exchange membranes in fuel cells is critically linked to their ion-conducting morphology. In this report, we analyzed the connectivity of anion-exchange membranes (AEMs) by electrostatic force microscopy (EFM). As a tapping-mode AFM-based technique, EFM probes height, phase, and electrostatic force gradient through two-pass interleave scans, which reveals the direct relationship between membrane surface morphology and connectivity of the ionic domain. The AEMs are diblock copolymers of a polyisoprene (PIp) block and a polystyrenic block containing the phosphonium ion (P(R3P+)MS). Different alignments of cylinders were observed in the bulk and on the surface for AEMs with different ion-exchange capacities (IECs) and preparation methods. The impacts of the cylinder alignment on the connectivity of the ionic domain were then investigated by EFM. For AEM 13 with IEC = 0.44 mmol/g and a hexagonal morphology, cylinders aligned parallel to the surface, which led to many disconnected regions observed throughout the membrane. In contrast, the perpendicularly aligned channels of AEM 16 with IEC = 0.87 mmol/g consisted only of a well-connected ionic phase throughout the membrane.

Effect of HIx solution concentration on ion-exchange membrane performance in electro-electrodialysis

Electro-electrodialysis (EED) has been applied to the thermochemical water-splitting iodine-sulfur process for hydrogen production to enhance the hydrogen iodide (HI) component in the HI–I2–H2O mixture (HIx solution). In this work, the HIx solution concentration dependence of conductivity and transport number as the membrane-performance indexes of Nafion 212 and a styrene-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) membrane was investigated experimentally. From a theoretical analysis of measurements based on the new EED model, the effects of the H+ and I− contents and the diffusion coefficients of H+ and I− in the evaluated membranes on conductivity and transport number were as follows: HIx solution concentration dependence of the H+ and I− contents exhibited a significantly small or different variation in comparison with that of conductivity and transport number, indicating that the dominant factors for conductivity and transport number would not be the H+ and I− contents; thereby, the effect of the diffusion coefficients of H+ and I− should not be ruled out. In terms of the conductivity of Nafion 212, the diffusion coefficient of H+ is the dominant factor because the existence of I− was negligible, whereas in the case of the ETFE-St membrane, the effect of the diffusion coefficient of I− should be considered. The diffusion coefficient of I− could more strongly affect transport number for the evaluated membranes rather than the diffusion coefficient of H+, indicating that the H+ selectivity of the membranes could be determined relatively based on variations of I− behavior in the membranes.

The effect of heterogeneity in ion-exchange membrane structure on Donnan Exclusion

Ion-exchange membrane (IEM) can be classified into homogeneous and heterogeneous membrane according to the heterogeneity of membrane structure. Results of various studies indicated that the degree of heterogeneity significantly affects the properties and performance of IEMs, such as conductivity, permselectivity, mechanical and chemical stability, energy consumption, and limiting current density. In the case of transport properties, it may be associated with efficacy of Donnan exclusion influenced by the presence of interstitial solution and inert phase in the membrane matrix. Therefore, understanding the effect of heterogeneity on Donnan exclusion is important to provide preliminary information about IEM transport properties. In this paper, the effect of IEC/Wu on Donnan exclusion is discussed. The effect of heterogeneity on Donnan exclusion is expressed as a correlation of IEC/Wu ratio with Donnan equilibrium constant, KD, and transport number of counter-ion, ti.

Effects of hypochlorite exposure on the structure and electrochemical performance of ion exchange membranes in reverse electrodialysis

We performed chlorination experiments to understand the impact of hypochlorite exposure on both cation exchange membranes (CEMs) and anion exchange membranes (AEMs) regarding their properties and performance in a reverse electrodialysis (RED) stack. Changes in membrane morphology, surface elemental composition and chemical bounding suggested the chlorine incorporation in the form of C-Cl bonds and side-chain cleavage of –SO3- or –NR3+ containing molecular fractions. These observations were further supported by observed increases in hydrophobicity and decreases in fixed charged groups of the membranes, respectively. Compared to CEMs, AEMs were less chlorine resistant such that the development of more extensive cracks in the membrane structure further increased water content and dramatically decreased membrane conductivity. The performance of both chlorinated CEMs and AEMs were tested in an RED stack and the results showed the reduced RED power density was largely attributed to the deteriorated electrical properties of AEMs.

The Role of Ion Exchange Membranes in Membrane Capacitive Deionisation.

Ion-exchange membranes (IEMs) are unique in combining the electrochemical properties of ion exchange resins and the permeability of a membrane. They are being used widely to treat industrial effluents, and in seawater and brackish water desalination. Membrane Capacitive Deionisation (MCDI) is an emerging, energy efficient technology for brackish water desalination in which these ion-exchange membranes act as selective gates allowing the transport of counter-ions toward carbon electrodes. This article provides a summary of recent developments in the preparation, characterization, and performance of ion exchange membranes in the MCDI field. In some parts of this review, the most relevant literature in the area of electrodialysis (ED) is also discussed to better elucidate the role of the ion exchange membranes. We conclude that more work is required to better define the desalination performance of the proposed novel materials and cell designs for MCDI in treating a wide range of feed waters. The extent of fouling, the development of cleaning strategies, and further techno-economic studies, will add value to this emerging technique.

Impacts of HPAM molecular weights on desalination performance of ion exchange membranes and fouling mechanism

This study aimed at investigating the desalination performance of ion exchange membranes (IEMs) treating polymer flooding wastewater and influence of partially hydrolyzed polyacrylamide (HPAM) molecular weights (MWs) on membrane fouling. Adsorption of HPAM on IEMs and molecular weight distributions (MWDs) of HPAM in chemical cleaning solutions were analyzed to study IEM fouling characteristics. Experimental results indicated that HPAM affected the desalination performance most significantly compared to oil. Moreover, cation exchange membranes (CEMs) were easier to be polluted by HPAM with lower MWs while anion exchange membranes (AEMs) were easier to be polluted by HPAM with higher MWs. It was also found that the heterogeneous IEM fouling caused by HPAM was more severe than the homogeneous IEM. Furthermore, CLSM, FTIR, electrostatic interaction and interface thermodynamics analysis indicated that HPAM caused membrane fouling and the main mechanism was electrostatic interaction.

Ion selection of charge-modified large nanopores in a graphene sheet

Water desalination becomes an increasingly important approach for clean water supply to meet the rapidly growing demand of population boost, industrialization, and urbanization. The main challenge in current desalination technologies lies in the reduction of energy consumption and economic costs. Here, we propose to use charged nanopores drilled in a graphene sheet as ion exchange membranes to promote the efficiency and capacity of desalination systems. Using molecular dynamics simulations, we investigate the selective ion transport behavior of electric-field-driven KCl electrolyte solution through charge modified graphene nanopores. Our results reveal that the presence of negative charges at the edge of graphene nanopore can remarkably impede the passage of Cl(-) while enhance the transport of K(+), which is an indication of ion selectivity for electrolytes. We further demonstrate that this selectivity is dependent on the pore size and total charge number assigned at the nanopore edge. By adjusting the nanopore diameter and electric charge on the graphene nanopore, a nearly complete rejection of Cl(-) can be realized. The electrical resistance of nanoporous graphene, which is a key parameter to evaluate the performance of ion exchange membranes, is found two orders of magnitude lower than commercially used membranes. Our results thus suggest that graphene nanopores are promising candidates to be used in electrodialysis technology for water desalinations with a high permselectivity.

Role of conditioning on water uptake and hydraulic permeability of Nafion ® membranes

The performance of ion-exchange membranes under pressure differential conditions is important for a number of separations and for fuel cell applications. Fluid transport properties under pressure are also key to understanding electrochemically-driven pumping and pressurization systems. The physicochemical properties and performance of perfluorosulfonic acid membranes varies considerably with conditioning treatment. In this study, hydraulic permeability and water uptake are measured as a function of membrane treatment for Nafion ® membranes that are contacted with aqueous electrolyte solutions. Several conditioning/contacting solution treatments are evaluated including exchanged cation, manner of cation exchange, anion effect, and heat treatment. For Nafion ® 117 membranes, these treatments resulted in water content ranging from 3.8 to 20.2 water molecules/fixed sulfonate site and hydraulic permeability ranging from 3.78 × 10 −16 to 1.79 × 10 −13 m/s Pa. Similar to water uptake, a systematic decrease in hydraulic permeability is observed with the alkali series Li + > Na + > K +. Interestingly, the as-received H + form and the Li + form have nearly identical water content but the permeability of the H + form is a factor of 6–7 higher than the Li + form, regardless of heat treatment. Evaluation based on an equivalent pore model and Poiseuille flow shows good agreement for pore radii ranging from 1.0 to 2.0 nm.

Comparative performance of ion exchange membranes for salt splitting

Comparative performance of ion exchange membranes for salt splitting

Comparative performance of ion exchange membranes for salt splitting

Comparative performance of ion exchange membranes for salt splitting

Comparative performance of ion exchange membranes for electrodialysis of nickel and cobalt

The extraction of nickel and cobalt from their sulfate solutions by electrodialysis in a modified three compartment cell is described. Two cation exchange membranes, the perfluorosulfonic Nafion® 117 and a new sulfonated PVDF membrane, are compared under similar operating conditions. The membranes are used as either flat structures or as corrugated structures. The effect of flow rate, current density, salt concentration and temperature on the performance of each membrane is described. The performance is characterised in terms of transport properties, current efficiencies and concentrations of metal ions transported through each membrane. The performance of the PVDF membrane was as good as; if not slightly better, than that of the commercial Nafion 117. A significant improvement with the use of corrugated membranes on the amounts of metal extracted is observed. The corrugated Nafion 117 membrane gave superior current efficiencies compared to the flat one with the same amount of charged passed. Separation of cobalt from nickel by electrodialysis in mixed solution has also been investigated.

Pervaporation of water—ethanol through ion exchange membranes

The performance of ion exchange membranes in sorption and pervaporation of water—ethanol has been studied as function of membrane type (ion loading) and feed composition, and has been compared with the performance of cellulose triacetate. A relatively uniform pattern of preferential water sorption for all membranes studied, reasonably excepting the H + form and CTA, contrasts sharply with distinct differences in selectivity and flux between cation exchange and anion exchange membranes. The key to an understanding of the interactions involved appears to be the activity of the sorbed ethanol, which is increased in case of cation exchange membranes, and lowered in case of anion exchange membranes. Increased ethanol activity is traced to a salting-out effect ultimately leading to phase separation within the membrane fluid, as evidenced by a disproportionately high ethanol flux and the adverse effect of selective diffusion reducing overall selectivity. Lowering the ethanol activity enhances selectivity, thus favoring anion exchange membranes for the separation under consideration. When projected to comparable thickness, anion exchange membranes are superior to cellulose triacetate, the OH − form being the most promising of the membranes studied

Present status of electrodialysis in Japan

In Japan, the research and development of ion-exchange membrane and electrodialysis was mainly directed to sea water concentration. After about 20 years efforts, the performance of ion-exchange membrane becomes superior, and electrodialysis becomes one of the most stable technology.Japan Tobacco and Salt Public Corporation (previous name, Japan Monopoly Corporation) and the three soda manufacturing companies made an effort to produce foodgrade salt commercially with sea water concentration by electrodialysis. The seven salt manufacturing companies now produce foodgrade salt (about 1 million ton a year) under the protection of monopolization.Based on such circumstances, desalination by electrodialysis was also considered a promising process, and desalination plants of saline water by electrodialysis were gradually enlarged. In 1976, fresh water production reached to total amount, 19,200 m3/day; municipal water 2,600 m3/day, industrial water 15,600 m3/day.