Surface Of Cation-exchange Membrane Research Articles

Influence of Concentration Gradients on Electroconvection at a Cation-Exchange Membrane Surface.

Membrane-based systems, such as electrodialysis, play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrated and diluate streams from a feed solution. It is known that under overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion depletion generating instability and vortices termed electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) that is both aligned with and opposed to the applied electric field. Experiments were conducted by coflowing NaCl solutions of different concentrations (0.1-100 mM) on each side of the membrane, and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show that electroconvection is enhanced when the electric field is opposite to the direction of the concentration gradient.

Transport Properties and Structure of Anisotropic Composites Based on Cation Exchange Membranes and Polyaniline

A comprehensive characterization of basic heterogeneous cation-exchange MK-40 and Ralex CMH membranes and composite membranes with polyaniline based on them was performed, including the determination of conductivity and diffusion permeability, measurement of current–voltage curves in solutions of sodium, calcium, magnesium chlorides and hydrochloric acid, distribution curves of water binding energy and effective pore radii, as well as an assessment of the transport-structural parameters of microheterogeneous model. The time of polyaniline synthesis on the surface of cation-exchange membranes by successive diffusion of an oxidizing agent and monomer solution through the membrane into water was determined. Samples with an anisotropic structure and asymmetric electrotransport properties were obtained. Based on the analysis of the electrotransport properties, structure characteristics and model transport-structural parameters of membranes in solutions of singly and doubly charged ions, the obtained materials were defined as promising for use in the processes of electrodialysis desalination of multicomponent solutions.

Impact of heterogeneous cation-exchange membrane surface modification on chronopotentiometric and current–voltage characteristics in NaCl, CaCl2 and MgCl2 solutions

Chronopotentiograms and current-voltage curves were measured to characterize the transport of sodium, calcium, and magnesium ions through cation-exchange membranes MK-40 and MK-40MOD. A microfluidic electrodialysis cell was used to visualize electroconvective vortices. The MK-40MOD membrane was obtained by covering the heterogeneous surface of a commercial MK-40 membrane with a thin (about 15 μm) homogeneous film of the MF-4SK sulfonated fluoropolymer. It has been shown that the combination of relatively high hydrophobicity of the MK-40MOD membrane surface with a certain electrical and geometrical (surface undulation) heterogeneity facilitate the generation of electroconvective vortices. The effect of vortices is manifested in reduction of the potential drop under a fixed current density and potential oscillations on the chronopotentiogram. It was found that in all studied systems potential drop in stationary state decreases in the order Na+>Ca2+>Mg2+. This correlates with the amplitude of potential drop oscillations (which increases in the range Na+<Ca2+<Mg2+) and with the size of electroconvection vortices observed in the microfluidic cell: its size is higher in the case of MgCl2 compared to the CaCl2 solution. Oscillations of potential drop on chronopotentiogram were registered not only in stationary state, but also at times of the order of several seconds, lower than the transition time. These oscillations occurred in the case of MgCl2 for both membranes, in the case of CaCl2 for the MK-40MOD membrane. No earlier oscillations were observed in the case of NaCl. The appearance of earlier potential drop oscillations is accompanied by an increase in the transition time and a reduction in the potential drop. The earlier oscillations are explained by the onset of unstable equilibrium electroconvection recently described by Rubinstein and Zaltzman. Stronger electroconvection in the case of MgCl2 in comparison to CaCl2 and NaCl is attributed to the impact of the residual space charge of bulk solution, which is the highest for the MgCl2 among the studied electrolytes. The contribution of bulk electroconvection is discussed on the basis of the estimation of the thicknesses of the diffusion layer and its constituting zones as functions of the current density.

Effect of homogenization and hydrophobization of a cation-exchange membrane surface on its scaling in the presence of calcium and magnesium chlorides during electrodialysis

In this paper, two cation-exchange membranes, i.e. MK-40 and MK-40MOD, were studied by chronopotentiometry and voltammetry, MK-40MOD being obtained by covering the heterogeneous surface of a commercial MK-40 membrane with a homogeneous 20µm thick Nafion® film. Electrodialysis process was realized in an electrodialysis flow-through laboratory cell, in which the cation-exchange membrane under study formed a desalination chamber with an auxiliary anion-exchange (Neosepta AMX-SB) membrane. 0.02M and 0.04M solutions of CaCl2, MgCl2 and NaCl were used. The current densities were changed in the range from 0.25 ilimth to 2.5 ilimth, where the theoretical limiting current density, ilimth, was calculated using the Leveque equation. The potential drop over the modified MK-40MOD membrane and the water splitting at this membrane turned out to be lower in all studied cases. Formation of scaling was observed only in the case of the MK-40/0.04M MgCl2 system at current densities in the range from 1.1 ilimth to 1.4 ilimth. For these current densities, the (quasi)steady state value of potential drop slowly increased with time and the crystals of Mg(OH)2 were found on the ion-exchange particles embedded onto the MK-40 membrane surface facing the desalination chamber. At higher currents, stronger electroconvection at this membrane and higher water splitting at the AMX-SB membrane (the latter providing lower pH in the desalination chamber) prevented scaling. No scaling was found on the modified membrane at any current. It is due to the Nafion® film, which is relatively more hydrophobic than pristine MK-40 and which provides a “better” distribution of current lines near the surface, thus enhancing electroconvection and decreasing water splitting.

Electrochemical impedance spectroscopy fingerprints the ion selectivity of microgel functionalized ion-exchange membranes

Surface modification methods are applied to alter interfacial phenomena and improve ion transport through membranes. In this work we present a novel method for tailoring the surface of cation-exchange membranes based on the deposition of thin microgel monolayers. The charge of such layers exerts a strong influence on the monovalent-ion-selectivity, and this is reflected in the electrochemical impedance responses. Membranes coated with uncharged microgels show similar behavior to that of unmodified ones, with impedance spectra dominated by low-frequency diffusional arcs. However, membranes modified with positively charged microgels exhibit an increased resistance due to the hindered transport of cations through the modification. An additional high-frequency capacitive arc is obtained with the monovalent-ion-selective membranes, which is attributed to concentration polarization effects at the membrane/modification interface. The characteristic frequency of this arc decreases with the valency of the ion, thus proving that multivalent ions pass through the modification layer at rates much slower than monovalent ones. Accordingly, electrochemical impedance spectroscopy has been used to feature monovalent-ion-selective properties of layered membranes.

Simultaneous regeneration of inorganic acid and base from a metal washing step wastewater by bipolar membrane electrodialysis after pretreatment by crystallization in a fluidized pellet reactor

This study aims to investigate the feasibility of an integrated system consisting of a fluidized pellet reactor and bipolar membrane electrodialysis (EDBM) for regeneration of acid and base from wastewater with a high concentration of calcium and nickel ions. The fluidized pellet reactor was used to reduce the potential of scaling on the membranes by crystallization. EDBM was applied to regenerate acid/base from this pretreated wastewater. The results show that calcium and nickel can be removed by the fluidized pellet reactor with sufficient efficiency (74% and 73% without filtration, 74% and 94.4% with filtration, respectively). After pretreatment, it was feasible to regenerate mixed acid and base using EDBM. The current efficiency was 69% for the acid, 80% for the base; the energy consumption was 5.5kWh/kg acid and 4.8kWh/kg base at a current density of 60mA/cm2 and an initial concentration of 0.2N. In a long-term experiment, the pretreatment increased cation exchange membrane performance and a high concentration of acid and base up to 1.76N and 2.41N was produced with only slight scaling on the cation exchange membrane surface. This integrated process is not only technically but also environmentally feasible, reducing water pollution and yielding valuable products to achieve sustainable development.

Intensification of demineralization process and decrease in scaling by application of pulsed electric field with short pulse/pause conditions

Pulsed electric field (PEF) can affect positively the membrane scaling mitigation and the intensification of electrodialytic (ED) process. Recent studies considered the use of PEF with relatively long pulse/pause conditions (5–40s). The present work was focused on the application of specific PEF conditions with short pulse/pause duration as a following perspective step in improving demineralization. Therefore, in this study six conditions were tested (1s/0.33s, 1s/1s, 2s/0.5s, 2s/0.67s, 3s/0.33s, 3s/1s) in order to investigate the influence of short PEF lapses and to find the optimum among all PEF conditions. Results indicate that PEF conditions of 2s/0.5s and 2s/0.67s allowed the highest demineralization rates. A decrease of scaling on the cation-exchange membrane surface in comparison with previous data was observed in all PEF conditions. In addition, absence of scaling on the anion-exchange membrane was observed for the first time in such extreme conditions of demineralization and solution composition. These results highlighted the successful use of short pulse/pause lapses during electrodialysis.

Competitive migration behaviors of multiple ions and their impacts on ion-exchange resin packed microbial desalination cell

Mixed ion-exchange resins packed microbial desalination cell (R-MDC) could stabilize the internal resistance, however, the impacts of multiple ions on R-MDC performance was unclear. This study investigated the desalination performance, multiple ions migration behaviors and their impacts on R-MDCs fed with salt solution containing multiple anions and cations. Results showed that R-MDC removed multiple anions better than multiple cations with desalination efficiency of 99% (effluent conductivity <0.05 ms/cm) at hydraulic retention time of 50 h. Competitive migration order was SO4(2-)>NO3(-)>Cl(-) for anions and Ca(2+)≈Mg(2+)>NH4(+)>Na(+) for cations, jointly affected by both their molar conductivity and exchange selectivity on resins. After long-term operation, the existence of higher concentration Ca(2+) and Mg(2+) caused the electric conductivity of mixed resins decrease and scaling on the surface of cation-exchange membrane adjoined with cathode chamber, suggesting that R-MDC would be more suitable for desalination of water with lower hardness.

Long-term performance and characterization of microbial desalination cells in treating domestic wastewater

Microbial desalination cell represents a new technology for simultaneous wastewater treatment, water desalination, and energy production. This study characterized the long-term performance of MDC during wastewater treatment and identified the key factors that caused performance decline. The 8-month operation shows that MDC performance decreased over time, as indicated by a 47% decline in current density, a 46% drop in Columbic efficiency, and a 27% decrease in desalination efficiency. Advanced electrochemical, microscopy, and spectroscopy analyses all confirmed biofouling on the anion exchange membrane, which increased system resistance and reduced ionic transfer and energy conversion efficiency. Minor chemical scaling was found on the cation exchange membrane surface. Microbial communities became less diverse at the end of operation, and Proteobacteria spp. was dominant on both anode and AEM fouling layer surface. These results provide insights into the viability of long-term MDC operation on reactor performance and direct system development through membrane optimization.

Evolution with Time of Hydrophobicity and Microrelief of a Cation-Exchange Membrane Surface and Its Impact on Overlimiting Mass Transfer

Surface properties were measured together with electrochemical characteristics of a CMX (Neosepta, Tokuyama Corp.) cation-exchange membrane. Relative hydrophobicity was controlled by the contact angle; XPS and SEM were used for characterizing chemical composition and microrelief of the surface, respectively. Voltammetry, chronopotentiometry, and mass transfer rate measurements were made as well. A "fresh" membrane and samples after 10, 25, 100, and 150 h of operation in an electrodialysis cell at an overlimiting current equal to 3 theoretical limiting currents, in a 0.02 M NaCl solution, were characterized. Some electrochemical properties were also measured for a Neosepta cation-exchange membrane, aged 2 years, in an industrial food process. It was found that the hydrophobicity of the CMX membrane has increased after the first 10 h of operation; more and more cavities of the dimension of the order of 1 μm have appeared with time testifying electrochemical erosion of the surface. The limiting current density (i(lim)) and the overlimiting transfer rate through the CMX membrane increased with time of its operation under overlimiting current. In the case of new CMX, i(lim) was very close to the theoretical value i(lim)(theor) calculated by the Lévêque equation. After 10 h of operation, i(lim) increased by 5%, and after 25, 100, and 150 h, the increase was by 30%, 70%, and 100%, respectively. Similarly, the mass transfer rate was found to increase up to 5 times (when desalting 0.005 M NaCl under 3 V) in comparison with the theoretical value. The ensemble of data was explained by the hypothesis that the passage of intensive current produces erosion of the ion-exchange polymer forming a continuous phase in CMX. This erosion results in exposure at the surface of the other constituent of CMX: small (about 100 nm) particles of relatively hydrophobic polyvinylchloride. Increasing surface hydrophobicity facilitates the slip of electroconvective vortexes along the surface. Besides, the geometry of the cavities gives rise to appearing tangential electric force applied to the extended space charge density at cavity's walls. As the local limiting current density within a cavity is lower than at the flat surface, electroconvective vortices arise at current densities lower than i(lim)(theor). With time, the number and the size of cavities increase (apparently, due to paired electroconvective vortices occurring inside them) that seems the main reason for overlimiting transfer increase.

Enhancing Ion Transfer in Overlimiting Electrodialysis of Dilute Solutions by Modifying the Surface of Heterogeneous Ion-Exchange Membranes

The desalination of dilute NaCl solutions with heterogeneous Russian commercial and modified ion-exchange membranes was studied in a laboratory cell imitating desalination channels of large-scale electrodialysers. The modification was made by casting a thin film of a Nafion-type material on the surface of cation-exchange membrane, and by processing with a strong polyelectrolyte the surface of anion-exchange membrane. It was shown that the modifications resulted in an increase of mass transfer coefficient and in a decrease in water splitting rate, both by up to 2 times. The effect of mass transfer growth is explained by higher surface hydrophobicity of the modified membrane that enhances electroconvection. The decrease in water splitting rate in the case of cation-exchange membrane is due to homogenization of its surface layer. In the case of anion-exchange membrane the effect is due to grafting of quaternary ammonium bases onto the original membrane surface layer. The suppression of water splitting favors development of electroconvection. In turn, intensive electroconvection contributes to deliver salt ions to membrane surface and thus reduces water splitting.

Characterization of cation‐exchange membrane fouling during bipolar membrane electrodialysis of monosodium glutamate isoelectric supernatant

AbstractBACKGROUND: A novel procedure that involved regeneration and recycling of ammonia and sulfuric acid from monosodium glutamate isoelectric supernatant with bipolar membrane electrodialysis (BMED) was proposed. As the performance of the membranes deteriorated during the batch runs, fouling of the cation‐exchange membrane (CEM) in contact with the base cell was studied.RESULTS: During ten consecutive batches of BMED, some operating parameters deteriorated gradually. Using scanning electron microscopy observations, fouling deposits were found on the CEM surface on the base cell side. Using Fourier transform infrared spectroscopy and reversed‐phase high‐performance liquid chromatography (RP‐HPLC), the organic fouling fraction of the CEM foulants was found to contain eight amino acids. Using X‐ray energy‐dispersive analysis, the mineral fouling fraction was shown to be mainly O and Ca elements, and a little Mg. Using X‐ray diffraction, the inorganic foulant was identified as CaCO3, mainly in the form of calcite and a little aragonite.CONCLUSION: The CEM was subject to membrane fouling consisting of an organic fouling fraction and a mineral fraction. The organic fraction occurred as ions with some positive charges from the isoelectric supernatant and probably existed in the form of amino acids or their peptides. The mineral fraction was mainly CaCO3 calcite and aragonite, and probably a little amorphous Ca and Mg hydroxides. Copyright © 2011 Society of Chemical Industry

Membrane fouling caused by amino acid and calcium during bipolar membrane electrodialysis

AbstractBACKGROUND: Bipolar membrane electrodialysis (BMED) has been widely applied in the recovery/production of organic acids and in the treatment of wastewater containing ammonium sulfate, sodium nitrate, sodium acetate and ammonium nitrate. However, membrane fouling is still one of the major problems in the electrodialysis process. Since calcium and amino acid are present naturally in fermentation wastewater, this study was carried out to determine the effects of calcium and amino acid on membrane fouling when simulated fermentation wastewater containing ammonium sulfate was treated by BMED.RESULTS: Calcium formed a scale on the cation exchange membrane (CEM) surface in contact with the base cell, but this had no significant adverse effect on the BMED performance. Amino acid, however, caused CEM fouling of the inner membrane, which hampered the BMED process. The coexistence of calcium and amino acid aggravated the membrane fouling, as observed morphologically on the CEM surface on the base cell side. Elemental mapping analysis showed that the membrane foulant was composed of calcium hydroxide and amino acid.CONCLUSION: The CEM fouling caused by calcium and that due to amino acid, which were distributed differently on the membrane, had different effects on the BMED performance. The coexistence of amino acid and calcium deteriorated the CEM fouling during BMED. Copyright © 2008 Society of Chemical Industry

Removal of hardness ions from tap water using electromembrane processes

Performances of electromembrane systems were investigated for removal of the hardness materials in a tap water. Six cells of ED, EDR, and EDIR systems with an electrode area of 12.5cm x 8.0 cm were operated. The results showed scaling formation due to water dissociation on the surface of cation exchange membrane, but EDR and EDIR enabled to avoid the scaling problem. EDIR operation lowered the resistance and power consumption compared to EDR due to the conductance of ion exchange resins. This study demonstrated the feasibility of EDR and EDIR processes for water softening.

Effects of charge density on water splitting at cation-exchange membrane surface in the over-limiting current region

To determine the correlation between surface properties and concentration polarization (CP) behaviors, cation exchange membranes with varying fixed charge densities were prepared and characterized by using several electrochemical analyses such as chronopotentiometry, zeta potential, and current-voltage measurements. Results showed that CP behavior depended mainly on surface charge density. With higher surface charge density, stronger electroconvection was observed, suggesting that an increase in the surface charge density increased the concentration of the counter ions at the membrane surface. As such, the electric field around the membrane surface was strengthened at a current over the limiting current density. Water splitting was also proportional to the surface charge density. This result was consistent with the classical electric field-enhanced water splitting theory, indicating that water splitting increased due to increases in the electric field and prepolarization of water molecules at the membrane-solution interface of the cation-exchange membrane.

Modification of transport properties of ion exchange membranes: XIV. Effect of molecular weight of polyethyleneimine bonded to the surface of cation exchange membranes by acid–amide bonding on electrochemical properties of the membranes

In order to prepare a cation exchange membrane having excellent monovalent cation permselectivity, a thin polyethyleneimine (PEI) layer was formed on one surface of the membrane by sulfonyl–amide bonding. The electrochemical properties of the cation exchange membranes, which had been prepared by the reaction of an aqueous PEI solution with chlorosulfonylated membranes, were examined in accordance with the number of the successive PEI reaction batches. The monovalent cation permselectivity decreased with the increase in the number of reaction times of the reused PEI solution. The smaller molecular weight of PEI was found to be consumed during the reaction, which means that PEI with higher molecular weight has difficulty reacting with the chlorosulfonylated membrane.

低温プラズマプロセスによるイオン導電性薄膜の作製

Ultra-thin, uniform, pinhole-free ionically conductive polymer films having fixed ion-exchange groups were prepared by plasma polymerization. Monovalent cation perm-selective membranes were prepared by depositing an ultra-thin anionic exchange layer containing fixed pyridine rings on cation exchange membrane surface. The thin solid polymer electrolyte films with single mobile species having fixed sulfonic acid groups or carboxylic acid groups were also prepared.

Modification of properties of ion exchange membranes V. Structure of cationic polyelectrolyte on the surface of cation exchange membranes

Electrodialytic transport properties of cation exchange membrane are changed by adhesion of polyethylene imine on the membrane surface. The effect of conformation of polyethylene imine molecules on the transport properties of the membrane, thickness of adherent layer and mechanism of adhesion were discussed. Observed were relative transport numbers of calcium ions to sodium ions,P Na Ca , current efficiency, electric resistance of the membrane during electrodialysis and pH-shift of anolyte after electrodialysis.