离子交换膜的发展态势与应用展望

基于离子交换膜的电膜技术,由于其独特的离子传递特性,可以用于离子物系的分离、分级,在清洁生产、节能减排、能量转换等方面有着广泛的应用前景。本文综述了离子交换膜的制备、应用过程以及组件设计等方面的前沿性进展,并对亟待解决的问题和未来的发展方向作了展望。关于膜的制备,提出了从二相到三相、从致密到微孔的新型离子交换膜结构,开发了电纳滤膜并用于一价或多价离子的分离,通量和选择性均得到提高,实现了膜功能的多样化。在应用过程中,实现了扩散渗析和电渗析过程的集成,分离效果优越,生产成本降低。同时对膜组件进行优化,设计开发出新型的卷式组件,克服了传统板式组件的诸多缺陷。值得一提的是,离子分离膜的应用领域也由初级水处理扩展到复杂物料的分离与纯化。以上研究成果将为离子交换膜的发展提供指导,加快其工业化进程。     

Water transport in ion exchange membranes

Water transport across ion exchange membranes has been studied experimentally. Water transference numbers were obtained from streaming potential measurements for cation and anion exchange membranes. At low salt concentration the water transference number reaches a limiting value which for the cation exchange membranes seems to be closely correlated with the cation—water friction in infinite dilute solutions.The observed water transference number for a given anion exchange membrane does not seem to be much dependent on the type of coion even at concentrations where the membrane is no longer perfectly permselective.     

Boron removal by ion exchange membranes

Boron removal from aqueous solution was studied through Neosepta-AHA membrane by Donnan dialysis (DD) method as a function of concentration, pH, conduct time, membrane structure and effect of accompanying ions. The feasibility of the system was examined and the highest boron removal was obtained when the pH of the feed phase was 9.5. On the other hand, it was determined that reverse flow transition was accelerated by counter ions and HCO₃⁻ ion was found to be more effective than Cl⁻ and SO₄⁻² ions. It was determined that obtained results of AHA membrane was compared with Neosepta AFN and AMH membranes and the flow rate of boron through Neosepta AHA, AFN and AMH anion exchange membranes were found to be between 3.10⁻⁸−3232.10⁻⁸ mmol cm⁻² s⁻¹ depending on concentration of feed and receiver phases, pH as well as counter ion properties. The order for recovery of boron for membranes was found to be as AFN>AMH>AHA. It could be concluded that the DD method is an appropriate method for boron removal from aqueous solution when an appropriate counter anion was chosen at suitable pH value.     

Electron microscopy investigation of ion exchange membranes

Transmission electron microscopy has been used for the recognition of the structural heterogeneity and the distribution of ionogenic groups in the cation exchange membranes. The investigations were performed on the fluorinated membranes (NAFION, MRF) and on the earlier known membranes containing poly(styrenesulphonic acid) (AMF C-322 and PE—PSSA membranes). The electron images of the ultra-thin sections revealed the homogeneous nature with the uniform distribution of small ionogenic group concentrations in the membranes NAFION and MRF—4MB, and the heterogeneous distribution of the groups in the poly(styrenesulphonic acid) membranes (AMF C-322, PE—PSSA membrane) and the membrane MRF-26, which is the consequence of the inhomogeneous nature of the polymer base of these membranes.     

Polarization studies on interpolymer ion exchange membranes

Polarization studies have been conducted for interpolymer ion exchange membranes by following the pH changes during electrodialysis from which the stagnant layer thickness was calculated. The pH changes are normal while an anion exchange membrane is used as a test membrane.     

Electrodialysis processes using ion exchange resins between membranes

Ion exchange resins were introduced between permselective membranes in a laboratory scale electrodialysis unit. It was found that by this method polarization is substantially decreased, and high electrical efficiency can be obtained even when working on highi/c values (1000–5000) and at low linear velocity of solution (1–2 cm/sec) in the cell. Different ion exchange resins were introduced in the diluate and brine cells and the boundary conditions of such a stack were determined.     

Surface modification of composite ion exchange membranes by polyaniline

The aim of this work is to develop new selective cation exchange membranes (CEMs) from bivalent to monovalent selectivity by surface functional groups. So, a novel hybrid cation exchange membrane was prepared by polymerization of polyaniline on a composite membrane, made of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF). Polyaniline was doped with different doping agents and their effect on total salt extraction and selectivity of bivalent to monovalent cations was evaluated. The chemical and morphological properties of hybrid membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ion exchange capacity (IEC), transport number, ion conductivity and water uptake decreased after surface polymerization. Composite membrane has a good selectivity (∼2) for bivalent and excellent removal of both cations. The hybrid membrane, doped with pTSA has a very high selectivity for monovalent ions (∼7.1) and a high removal of monovalent ions.     

Studies on the radiation stability of ion exchange membranes

This article deals with some aspects of γ-radiation stability of (phenolsulfonic acid, PSA) cation exchange membranes synthesized in the laboratory. The changes in cation exchange capacity, moisture content, coion sorption, concentration potential, area resistance, counterion transference number, and solute flux of the membranes were evaluated before and after irradiation. The membrane samples were irradiated under wet conditions using a ⁶⁰Co source up to a total dose of 18.65 Mrads. The experimental data point out the possibility of degradation of PSA membranes beyond a dose of 9.78 Mrads, resulting in the loss of functional groups from membrane matrix.     

离子交换容量可控自交联型阴离子交换膜的制备

以N-溴代丁二酰亚胺(NBS)为溴化试剂,对十氟二唑与四甲基双酚A(TM-BPA)聚合的聚合物主链进行溴化,随后以N-乙烯基咪唑作为双功能化试剂进行功能化,制备了含有双功能化试剂的氟化聚二唑芳醚阴离子交换膜。其中,功能化试剂(作为离子交换的功能基传导离子及作为膜内分子的交联点)改善膜物理性质。制备的阴离子交换膜在80℃下的离子传导率为9.02×10-2 S·cm-1,甲醇渗透率为1.89×10-7 cm2·s-1(20℃),在碱性燃料电池中具有应用潜力。     

Use of bipolar membranes for ion exchange resin regenerant production

The utilisation of electrodialysis bipolar membranes to prepare hydrochloric acid and sodium hydroxide from a sodium chloride solution was tested at West Thurrock Power Station. The product chemicals were then used to regenerate full scale ion exchange units on the site and the results compared to normal operation with commercial regenerants. The large scale pilot plant was found to operate satisfactorily and the quality of produced chemicals was in line with predictions. The adoption of on-site, on-demand production of regenerants could offer immediate advantages for remote installations which would otherwise be dependent on the transport of dangerous chemicals over long distances. In the longer term further advances in the selectivity of commercially available membranes and the prevailing cost of commercial regenerants are considered the key factors governing the future rate of implementation of this novel process.     

Preparation of ion exchange membranes by plasma polymerization. I

Plasma polymerizations of three kinds of amines, γ-aminopropylethoxydimethylsilane (APEMS), allylamine (AA), and pyrole (PY), were investigated by IR and XPS analysis. Plasma-polymerized films were deposited on porous substrates, and ion exchange abilities of the composite membranes were measured. When APEMS were used as the monomer, the polymer retained the chemical structure of the monomer, amino groups. However, when AA and PY were used as the monomers, the plasma polymers contained a larger amount of amido structures than did the polymer of APEMS. Each membrane had ion exchange ability. In particular, the membrane prepared from APEMS showed superior ion exchange ability, anion permselectivity and conductivity, in acidic solutions. This property was attributed to the chemical structure of the polymer from APEMS retenting amino groups. The ion exchange properties of the membranes depended on the pH of the solution. In particular, the membrane of APEMS showed high membrane potential and low electric resistance only in a narrow pH region due to the weak basicity of amines and the hydrophobic property of the polymer.     

离子交换纤维微观形貌特点及结构修饰研究

利用比表面积测试仪、扫描电子显微镜等物理化学方法对颗粒与纤维状离子交换剂的链间交联与微观形貌进行了研究,通过刚性交联剂对聚丙烯-苯乙烯-二乙烯基苯(PP-ST-DVB)离子交换纤维骨架结构进行了修饰改造。结果表明:离子交换纤维由于不能通过交联剂与致孔剂参与下的单体聚合构建具有丰富微孔的高分子骨架,所以不存在丰富孔结构和高比表面积特征,其内部为连续凝胶相结构;外比表面积大、传质距离短、交联键分布均匀是其反应动力学和渗透压稳定性能优异的主要原因;刚性交联剂在PP-ST-DVB纤维上的附加交联反应使其比表面积从原来的0.5 m~2/g提高到200 m~2/g以上,附加交联反应所形成的分子尺度微孔孔径分布窄,具有超高交联树脂的结构特点。     

Aqueous pore structure and proton dynamics in solvated Nafion membranes

Molecular dynamics computer simulations of water/Nafion mixtures using an all-atom model were performed as a function of temperature and humidity. The simulations are aimed at investigating processes and structures on the picosecond to nanosecond time scale in the nano-phase-separated material with its technological relevance for low temperature fuel cells. Characteristic differences in aqueous pore structure were observed for systems whose water content was varied between 5 and 10 molecules per acid group in the polymer. As expected, proton transport increases significantly with increasing humidity, its mechanism is dominated by the Grotthus structural diffusion mechanism in accordance with earlier studies in simplified model pores. On the simulated time scale no unambiguous conclusions on the role of polymer dynamics for the transport in dry membranes can be drawn.     

Permselectivity of ion exchange membranes from sorption data and its relation to nonuniformity of membranes

Transport numbers of the hydrogen ion within the cation permselective membranes equilibrated with sulfuric acid solutions have been calculated employing sorption data. Calculations were performed according to the equation of Arnold and Swift. The validity of this equation and resulting t _+(app) has been verified by independent determination of transport numbers by the EMF method. The equation has been derived which relates the permselectivity of membranes to the parameters describing their nonuniformity, i. e. to the nonuniformity factor Z and volume fraction of nonselective domains (Δ?).     

Effects of silica sol on ion exchange membranes: Electrochemical characterization of anion exchange membranes in electrodialysis of silica sol containing-solutions

The fouling potential of the negatively charged silica sol in electrodialysis (ED) by adsorption on the surface of an anion exchange membrane was investigated. Since the fouling potential is related to the physical and electrochemical properties of the silica sol and anion exchange membranes, it is important to characterize the properties of silica sol and membranes. The surface charge of silica sol was investigated by the electrophoretic mobility and its isoelectric point was determined as pH 3. The commercial anion exchange membranes were characterized in terms of exchange capacity, water content, the zeta potential and the electrochemical properties of the membranes using impedance spectroscopy to predict the effects on the electrodialysis performances. Among the characterized properties, exchange capacity and some electrochemical properties of the anion exchange membranes were rather improved after ED experiments. In the electrodialysis of solution containing silica sol, deposition of the silica sol did not decrease the desalting performance of the anion exchange membranes because of loosely packed cake layer on the membrane surface.     

Effects of a fuel cell test on the structure of irradiation grafted ion exchange membranes based on different fluoropolymers

The role of the fluoropolymer matrix in the stability of irradiation grafted proton conducting membranes under fuel cell conditions is investigated. The structure of a series of membranes with poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene), poly(tetrafluoroethylene-co-hexafluoropropylene), and poly(ethylene-alt-tetrafluoroethylene) matrices with poly(styrene sulfonic acid) side chains is studied before and after a fuel cell test using X-ray scattering techniques and confocal micro-Raman spectroscopy. All tested membranes suffer from a loss of poly(styrene sulfonic acid) leading to a decrease in conductivity. Changes in crystallinity, lamellar period, orientation and thickness of the membranes are reported and compared to corresponding properties of the initial polymer films and the pristine membranes. The membranes where most severe changes in the structure of the matrix polymer can be observed have the shortest lifetimes in the fuel cell.     

Molecular structure and ion exchange of amidoximated cellulosic materials

Modification of lignocellulosic materials, e.g., cotton stalks, bagasse, and rice straw, by incorporation of amidoxime group for ion exchange is investigated. The uptake of metal ions Cu, Cr, Ni, and Fe by these modified lignocellulosic materials is measured. Amidoximated bagasse has higher efficiency toward metal ions uptake than amidoximated cotton and rice straw. The effect of different variables, e.g., metal ion concentration, temperature of metal ion solution, and time of steeping, on the efficiency of the amidoximated (bagasse raw material, unbleached bagasse, and bleached bagasse) ion exchangers toward metal ions uptake is tested. The molecular structure of the prepared amidoxime from bagasse and its pulps is studied by using infrared spectroscopy. New bands appeared at 1661 and 916 cm^?1 in the spectra of amidoximated bagasse and its pulps ion exchangers due to the formation of C?N and N? OH groups. The thermal properties of these ion exchangers is also studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 303–311, 2006     

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.