AMX Membrane Research Articles

Can the electrochemical performance of heterogeneous ion-exchange membranes be better than that of homogeneous membranes?

Competition between homogeneous and heterogeneous ion-exchange membranes (IEMs) lasts for decades. Low fraction of conductive surface area, Θ, of IEMs causes lower limiting current density, higher voltage and water splitting rate at a same average current density. On the other hand, heterogeneous IEMs are less costly. Additionally, as it was found recently, electrically heterogeneous surface enhances electroconvection. In this paper, we consider a heterogeneous anion-exchange MA-41 membrane (Shchekinoazot) and two its modifications. The first one (MA-41P) is prepared in the same way as the MA-41 membrane and contains the same resin particles, but of a larger size; these larger particles are rearranged on the surface to form agglomerates separated by non-conductive regions. The value of Θ for the MA-41P membrane is 1.5 times greater than that for the MA-41 and the height of “hills” formed by the resin particles on the surface is 3 times higher. The second membrane (MA-41 PM) is obtained from the MA-41P by treatment of its surface with a bifunctional polymer solution allowing transforming the functional tertiary and secondary amino groups into the quaternary ones, Θ remains the same. We compare the main physico-chemical (ion-exchange capacity, water content), surface (SEM-EDS analysis, optical microscopy, contact angle) and electrochemical (pH-metry, voltammetry, chronopotentiometry, impedancemetry, water splitting and mass transfer rate) properties of the three mentioned above membranes with those of a homogeneous Neosepta AMX membrane (Astom), in a 0.02 M NaCl solution. The experiments show that the water splitting rate decreases in the sequence MA-41 >MA-41P>AMX≥MA-41 PM For the membranes in this sequence above the experimental limiting current densities normalized at the theoretical limiting current density are 0.6, 0.8, 1.3 and 1.25, respectively. However, the voltage at a same overlimiting current density is still greater across the MA-41 PM than across the AMX membrane.

Effect of ampholyte nature on current-voltage characteristic of anion-exchange membrane

Current-voltage characteristics (CVCs) of a Neosepta AMX membrane are studied in NaH2PO4 (pH = 4.7, C = 0.02 mol/L) and KC4H5O6 (pH = 3.6, C = 0.02 mol/L) solutions. It is shown that in the case of NaH2PO4 there are two plateaus in the CVC, which correspond to achievement of the first, ilimI, and second, ilimII, limiting currents. ilimI occurs when the NaH2PO4 salt diffusion to the membrane surface is saturated, ilimII refers to the saturation of the proton flux when the membrane is almost completely converted into the HPO42− form. In the case of potassium hydrotartrate, there is no state corresponding to ilimI. The difference in CVC for two ampholytes is due to the difference in the ratio between the dissociation constants related to the first (Ka1) and second (Ka2) steps of ampholyte dissociation. In the case of NaH2PO4, where pKa1 and pKa2 differ greatly, a solution of this salt contains nearly exclusively the singly charged phosphate form. However, a KC4H5O6 solution contains, together with the C4H5O6− anion, about 15% of the doubly charged tartrates, which take part in charge transfer. Approximate analytical expressions are obtained for ilimI and ilimII in the case of monosodium hydrophospate solution. Their application gives the ilimI and ilimII values, which are in good agreement with the experiment and with numerical calculations using a mathematical model based on the Nernst-Planck equation, the electroneutrality condition, and the local chemical equilibrium condition in the membrane and two adjacent diffusion boundary layers.

An investigation on polymer ion exchange membranes used as separators in low-energy microbial fuel cells

An ion exchange membrane is a polymer matrix of cross-linked polyvinyl chloride macromolecular chains, on which are grafted ionic functional sites. The transport numbers of two metal ions and proton through these membranes were determined using the well-known electrochemical Hittorf's method. The cation exchange membrane Nafion, having sulfonic reactional site (-SO3-), yielded the transport number of copper relatively less than that of sodium, leaving way to proton to be transported. In contrast, the anion membrane of alkyl ammonium reactional site (-NR4+) prevented the transfer of cation metals and allowed the proton to displace by defect of permselectivity. These two membranes were used as separators in microbial fuel cells (MFC). In effect, the MFC with the anion membrane AMX, gave relatively higher power density (1.10mW/m(2)) compared to the cation exchange membrane Nafion (0.45mW/m(2)). Really, with the AMX separator, the metal ions present in the wastewater anolyte compartment were blocked by the membrane giving way to the protons to displace freely and be reduced efficiently at the cathode. In the mono-compartment cell, the ions moved sideways between anode and cathode, yielding the highest power density (11.90mW/m(2)) so far obtained. The pH adjustment of anolyte and cathode compartments support, therefore, strongly the cell voltage evolutions. In addition, the cyclic voltammetry results showed that the electroactive biofilm of the bioanode using the AMX membrane described a diffusion-limited process, while that of the Nafion membrane made in evidence the adsorption monolayer of redox species. Besides, the electrochemical impedance spectroscopy revealed that the charge transfer resistance at the interface bioanode/biofilm decreased drastically from 728 to 18cm(2) for Nafion MFC during 11 days of functioning.

Internal cross-linked anion exchange membranes with improved dimensional stability for electrodialysis

Anion exchange membranes (AEMs) with a high ion exchange capacity, striking water uptake and excellent dimensional stability were prepared via an internal crosslinking networks strategy. Internal crosslinking networks were formed by reacting 4,4′-bipyridine with brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO). 4,4′-bipyridine not only provides a functional group but also comprises a cross-linking agent without requirements of post-functionalization. The variation of the 4,4′-bipyridine amount into the casting polymer solution was explored to regulate the performance of the anion exchange membranes, and the membrane properties were evaluated by AFM, ion exchange capacity (IEC), water uptake, the linear expansion ratio, tensile strength, thermal stability, membrane area resistance and electrodialysis experiments, etc. The results showed that the cross-linked membrane with the IEC of 1.98mmol/g has much more outstanding dimensional stability (water uptake: 11.68%; swelling ratio: 3.8%) than non-cross-linked BPPO-Tri membrane (water uptake: 53.26%; swelling ratio: 7.71%) and commercial Neosepta AMX membrane (water uptake: 60.29%; swelling ratio: 5.08%), at the high temperature (50°C). When being applied in ED application, the cross-linked BPPO-20 membrane (NaCl remove: 59.7%; energy consumption: 5.97kWh/kg NaCl) exhibits slightly higher desalination efficiency and lower energy consumption than commercial Neosepta AMX membrane (NaCl remove: 58.3%; energy consumption: 6.51kWh/kg NaCl), suggesting its promising application in ED.

Current-voltage characteristic of anion-exchange membrane in monosodium phosphate solution. Modelling and experiment

A current-voltage characteristic of a Neosepta AMX membrane in a monosodium phosphate solution was investigated experimentally and theoretically. It is shown that the experimental curve has two sections of the overlimiting "plateau" and, accordingly, two limiting current densities. This feature is not observed in the presence of electrolytes, such as sodium chloride. For a better understanding of the phenomenon, a one-dimensional three-layer steady-state transfer model taking into account the possibility of ampholyte species transformation depending on the local pH is developed. Comparison of the calculations with the experimental data and the interpretation of the current-voltage characteristic are made. The first limiting current is due to the vanishing of single-charged phosphate anions in the solution at the membrane surface. The following growth of current density is caused by the protons generated during the transformation of the single-charged phosphate anions (H2PO4−) into the double-charged ones (HPO42−) when entering the membrane. This transformation is due to the increase in pH of the AMX internal solution with increasing current density. The second limiting current is observed when the membrane is almost completely converted into the form of double-charged phosphate anions. In this state the proton flux generated in the H2PO4−–HPO42− transformation at the depleted solution/membrane interface approaches its maximum.

Modification of the AMX membrane surface by polyethyleneimine: Effect of ionic strength on the membrane ion exchange selectivity

AbstractIn order to improve the selectivity of the AMX membrane, a polyethyleneimine (PEI) layer was adsorbed on its surface by the immersion of the considered membrane in a solution of PEI. The adsorption process of PEI was studied and the results were well correlated with the Langmuir model and with Lagergren's pseudo‐first‐order kinetic model.Equilibrium ion exchange isotherms of , , and systems, using modified AMX membrane, were established at various ionic strengths from 0.1 to 0.5 mol/L and at constant temperature of 298 K.The results obtained with the modified membrane, in this range of ionic strengths, showed that the affinity order was: at I = 0.1 mol/L and I = 0.2 mol/L and it was for I = 0.3 mol/L and I = 0.5 mol/L. The ion exchange selectivity coefficients , thermodynamic ion exchange equilibrium constants , and the standard free enthalpies of the ion exchange reactions were determined for the three binary systems. These results were compared with those obtained with the unmodified membrane. It was observed that for the modified membrane the ion exchange selectivity towards sulphate ions decreased and increased towards monovalent anions.

Production of lactobionic acid by BMED process using porous P84 co-polyimide anion exchange membranes

Abstract Porous P84 co-polyimide anion exchange membranes are prepared by phase inversion process using isopropanol (IPA) or water as the non-solvent. Membrane area resistances are in the range of 0.7–2.2 Ω cm2, lower than those of commercial anion exchange membrane AMX (2.0–3.5 Ω cm2) and dense P84 membrane (>80 Ω cm2). The ion exchange capacities (IECs) are in the range of 0.6–0.9 mmol/g and the water uptake (WR) values are 100–160%, both of which are higher than those of dense P84 membrane. The porous P84 co-polyimide membranes are utilized in bipolar membrane electrodialysis (BMED) process to produce lactobionic acid (LBA) and NaOH from sodium lactobionic. The pore morphology and IECs influence the output of LBA significantly. Membrane with big tear-like pores shows higher WR value and can produce 0.0555–0.0565 mol/L LBA. Sponge-like pores with IEC of 0.9 mmol/g can yield 0.0724 mol/L LBA after 180 min running under 20 V, while sponge-like pores with IEC of 0.7 mmol/g yield reduced LBA concentration of 0.0593 mol/L, illustrating that higher IEC is in favour of the passage of LB−. The porous P84 membranes have improved efficiency as compared with commercial membrane AMX (0.0451 mol/L) and the dense P84 membrane (merely 0.0162 mol/L) in the production of LBA. Meanwhile, the Na+ leakage through the porous membrane is similar to that of the commercial membrane. Hence, the porous membranes, due to their low resistances, can have high potential for producing organic acid with high molecular weight.

Method for determining ion exchange membrane resistance for electrodialysis systems

Ion transport resistance is an important characteristic of ion exchange membranes. It is not a constant but is influenced by among other things the salt concentration in the external solution. A vast number of publications investigated the membrane resistance; however, the use of different methods leads to different results. Different approaches and possible flaws are discussed and a method for measuring membrane resistance is developed, evaluated, and described. It is shown that this method can be used in a wide concentration range of 0.001–5M NaCl. The method is based on electrical impedance spectroscopy within an electrochemical cell containing four electrodes. An AC is applied on two work electrodes, while two thin platinum iridium wires are used as reference electrodes to evaluate the sample impedance. Membrane resistance is obtained from a differential impedance measurement of a solution without and with a membrane. Resistance of AMX and CMX membranes is determined.

Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents

Surface modification of ion-exchange membranes is already proved to be an effective and low-cost way to improve membrane properties. Usually, the object of modification are cation-exchange membranes. We study commercial anion-exchange Neosepta AMX membrane and two its modifications obtained by electrospray deposition method. Hydrophobic non-conducting spots of a fluoropolymer are deposited with different density; they screen 8 or 12% of the membrane surface. When a constant overlimiting current density is applied to an electrodialysis cell with a membrane sample, in all studied cases there were voltage oscillations superimposed on the general upward trend in voltage–time dependence. For the first time, we observed on the chronopotentiograms of ion-exchange membranes the oscillations arisen essentially earlier than the transition time, where the potential drop (PD) across the membrane (reduced by the initial ohmic drop) was 20–40mV. The modified membranes showed higher amplitude of oscillations, lower voltage growth and lower water splitting rate. The limiting and overlimiting steady-state current was up to 1.5 times higher over modified membranes. The early oscillations are explained by equilibrium electroconvective instability recently theoretically studied by Rubinstein and Zaltzman [24]. Thе occurrence of this mode of electroconvection at low PD is explained by undulated surface (commercial AMX) and hydrophobic spots (modified membranes) favouring higher tangential electric force applied to equilibrium space charge region. At higher voltages occurring in quasi-steady state, there are stronger and more frequent oscillations governed by non-equilibrium electroconvection. It is shown that electroconvection can not only enhance mass transfer, but essentially reduce water splitting.

In-situ crosslinking of anion exchange membrane bearing unsaturated moieties for electrodialysis

Abstract This study reports the preparation of the in-situ crosslinked anion exchange membrane that does not require the use of crosslinkers or catalysts. A polyelectrolyte bearing flexible unsaturated side chains was synthesized via the Menshutkin reaction with poly(2,6-dimethyl-1,4-phenylene oxide) and N,N-Dimethylvinylbenzylamine. The crosslinked derivatives were then prepared by the thermal crosslinking of the unsaturated side chains during the membrane formation process. This approach incorporates crosslinks, bearing quaternary ammonium cations, between the polymer chains in order to mitigate against excessive water swelling, and to enable the high ion contents to provide favorable low resistance of ion transport. Additionally, the resultant dense crosslinked network has the additional advantage of improving anion selective permeability of membrane. When being applied in ED application, the crosslinked membranes exhibit much higher desalination efficiency than commercial Neosepta AMX membrane, suggesting its potential application in ED.

Effect of ionic strength on the ion exchange equilibrium between AMX membrane and electrolyte solutions

The effect of ionic strength variation on the ion exchange equilibrium between AMX anion exchange membrane and electrolyte solutions containing the most dominant anions on natural waters (Cl−, NO3−, and SO42−) was studied. All experiments were carried out at a constant temperature of 25 °C. Ion exchange isotherms were established, at different ionic strengths from 0.1 to 0.5 M, for the systems (Cl−/NO3−), (Cl−/SO42−) and (NO3−/SO42−). Obtained results showed that for I = 0.1 M the affinity order is SO42 −>NO3−>Cl−. For I = 0.2 M this order is NO3−>SO42−>Cl−. For 0.3 and 0.5 M the AMX membrane becomes more selective for chloride than for nitrate or sulfate. Selectivity coefficients KNO3−Cl−, K2Cl−SO42− and K2NO3−SO42−, thermodynamic constants, and separation factors were calculated and decreased with the increase of ionic strength.

Synthesis of an aminated poly(vinylidene fluride-g-4-vinyl benzyl chloride) anion exchange membrane for membrane capacitive deionization(MCDI)

In this study, PVDF was dehydrofluorinated using an alkaline treatment and modified using 4-vinyl benzyl chloride to prepare a PVDF-g-VBC copolymer. A solution of the PVDF-g-VBC copolymer was anion exchanged to introduce a quaternary ammonium group using a quaternary ammonium reaction. In addition, an ion exchange membrane was prepared using a casting method, and a PVDF-g-VBC ion exchange membrane was made by heat treatment. Raman and Fourier-transform infrared (FT-IR) spectroscopy were performed to elucidate the molecular structure. The water uptake (W.U), ion exchange capacity (IEC) and electrical resistance (E.R) of the membrane were measured using a weight method, titration, and LCR meter, respectively. The water uptake of the PVDF-g-VBC ion exchange membrane ranged from 12.5% to 56.2%, and the results were lower than that of AMX (Tokuyama) (30%). The ion exchange capacity and electrical resistance range were 0.39–1.31meq/g and from 15.50 to 2.00Ωcm2, respectively. The electrical resistance was observed to decrease as the ion exchange capacity increased. The lowest electrical resistance measured, 2.00Ωcm2, is lower than that of an AMX membrane (2.40Ωcm2). When the anion exchange membrane was applied to the membrane capacitive deionization (MCDI) process, an excellent salt removal rate of 79.4% was achieved, which was 51.6% higher than when the membrane was applied in the capacitive deionization (CDI) process, 27.8%.

Water desalination by neutralization dialysis with ion-exchange membranes: Flow rate and acid/alkali concentration effects

Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a three-compartment stack with ion-exchange membranes, and the process feasibility was evaluated. The pH and the ionic conductivity of a model saline solution (0.02M NaCl) were measured during different ND operations in batch mode. The influence of the flow rate, the solution concentration in the alkali and acid compartments was investigated. It was found that, at the beginning of the operation, the ion-exchange kinetics was mainly controlled by the membrane diffusion. The maximal fluxes of exchanging ions (Jmax) (Na+ and H+ across CMX membrane, and Cl− and OH− across AMX membrane) were theoretically evaluated. The calculated and the experimentally measured fluxes at the beginning of batch operation were close. However, after some minutes of solution desalination, the salt concentration decreased, which involved diffusion boundary layer controlled kinetics. Hence, with time, the fluxes decreased and were considerably smaller than Jmax. The increase/decrease in the acid and alkali solution concentration resulted in corresponding increase/decrease in the ion-exchange rate. Recommendations have been provided to reduce the desalination duration.

Enhancement of counter-ion transport through ion-exchange membranes in electrodialytic processes

Ion-exchange membranes (IEMs) are ionic conducting materials. They have various applications such as: fuel cell (PEMFC), electrochemical synthesis (Cl2/NaOH), desalination, purification, separation, and environment. Despite these applications, several aspects are still unknown, such as: the membrane structure, the conduction mechanisms, and concentration polarization. The main obstacle in electro-membrane processes such as electrodialysis is the concentration polarization phenomenon, which remains one of the incomprehensible phenomena in IEM transport. This phenomenon is common to all systems operating a selective ionic transfer through an interface; it arises from the difference in ions mobility in the solution and in the membrane. A better understanding of concentration polarization can help to improve the membrane performance, the process efficiency, and in the reduction the process operation cost. In this research, we studied the effect of the ammonia buffer (NH3/NH4+) on the counter-ion transfer through the anion- and the cation-exchange membranes AMX and CMX, respectively. The results show that the ammonia addition facilitates the counter-ion transfer in both cases and gives a total elimination of the system polarization, but with different behaviors of CMX and AMX membranes. The classical concentration polarization theory remains insufficient to explain the obtained results.

Electrodialytic separation of boric and hydrochloric acids

Regeneration of boron-selective sorbents produces the acidic effluents of high boron content. Electrodialysis was proposed as a method for hydrochloric acid regeneration from the effluents composed of 0.1kmol/m3 hydrochloric acid and boric acid concentration ranging from 0.1 to 2.5kgB/m3. Boric acid mass transfer across anion-exchange membranes was investigated using a bench-scale electrodialyzer of an effective membrane area of 16cm2. An extended Nernst–Planck model was used for transport parameters estimation. Comparison of Neosepta AMX and ACM membranes showed that boric acid transport across the latter is much slower, allowing more effective separation of hydrochloric and boric acids. The boric acid transport in the investigated case is mainly due to diffusion of boric acid across the membrane.

Anion-conducting Pore-filling Membranes with Optimization of Transport Number and Resistance for Reverse Electrodialysis

Abstract Pore-filling membranes filled with quaternized poly(styrene-co-ethylene glycol dimethacrylate) were prepared with the degree of crosslinking (DC) for reverse electrodialysis (RED). It was shown in power density performance that the pore-filling membranes with low DC had lower open circuit voltage (Voc) and resistance, but those with high DC had higher Voc and resistance than the commercial AMX membrane. Thus, the compromise between their transport number and resistance is necessary for the application of pore-filling membranes in RED.

Application of Adsorption Models for Fluoride, Nitrate and Sulfate Removal by AMX Membrane

Application of Adsorption Models for Fluoride, Nitrate and Sulfate Removal by AMX Membrane

Mass transfer mechanism and chemical stability of strongly basic anion-exchange membranes under overlimiting current conditions

The dynamics of changes in overall and partial voltammetric characteristics with respect to chloride and hydroxide ions is studied by the method of rotating membrane disk (RMD) under the conditions of stabilized diffusion layer thickness for the original strongly basic MA-41P and homogeneous AMX membranes and also for the modified heterogeneous MA-41P-M membrane at high current densities. For unmodified anion-exchange membranes at currents exceeding the limiting value, the hydrolysis of fixed ammonium bases produces secondary and ternary amino groups which are catalytically active in the reaction of water molecule dissociation. The hydrolysis of amino groups in the membrane surface layer is the mechanism of degradation of electrochemical characteristics of strongly basic membranes. This results in the increase of transport numbers with respect to hydroxide ions and weakening of mass transfer with respect to salt ions. For the surface-modified heterogeneous anion-exchange membranes, no degradation of electrochemical characteristics is observed. The characteristics of the surface-modified MA-41P-M membrane remain stable: after long-term operation of the energized membrane, the partial currents with respect to hydroxide ions are close to zero and the mass transfer with respect to salt ions is considerably intensified. The dependences of the thickness of the hydrolyzed layer of a strongly basic anion-exchange membrane on the time of its exposure to solutions of high pH are determined. An original method is developed for determination of the hydrolyzed layer thickness for strongly-basic anion-exchange membranes, which is based on the copper ability to form stable complex compounds with weakly basic amino groups of anion-exchange membranes.

Selective removal of nitrate ions by controlling the applied current in membrane capacitive deionization (MCDI)

The use of membrane capacitive deionization (MCDI) to selectively remove nitrate ions from a mixed solution of chloride and nitrate ions was investigated in this study. Adsorption equilibrium experiments were conducted to determine the selectivity of the AMX membrane toward the nitrate ion. Desalination experiments were also performed on a mixed solution (5mM NaCl+2mM NaNO3) using a MCDI cell consisting of carbon electrodes, a Neosepta AMX membrane, and a CMX membrane. The AMX membrane had a selectivity coefficient (kNO3−/Cl−) of 4.37, indicating high selectivity of the AMX membrane toward the nitrate ion. The desalination experiments were performed by applying a constant current to the MCDI cell until the cell potential reached 1.0V. The total amount of adsorbed ions was not affected by changes in the applied current. However, the mole fractions of the nitrate ions relative to all the adsorbed ions were inversely proportional to the applied current: 31.7, 33.7, and 41.0% at 30, 20, and 10mA applied current, respectively. This result is attributed to the increase in the mole fraction of nitrate ions adsorbed by the AMX membrane at lower applied currents.

Use of electrochemical impedance spectroscopy for determining the diffusion layer thickness at the surface of ion-exchange membranes

Three ion-exchange membranes (an AMX homogeneous anion-exchange membrane, a MK-40 heterogeneous cation-exchange membrane, and a Nafion-117 homogeneous cation-exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion boundary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the “ideal” one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δLev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK-40 and Nafion membranes markedly differs from the “ideal” behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low-frequency range of a Warburg type finite-length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δLev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK-40) and, especially, high hydrophobicity of the (Nafion-117) surface facilitate the development of electroconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal.