Physicochemical investigation of radiation‐grafted poly(acrylic acid)‐graft‐poly(tetrafluoroethylene–ethylene) copolymer membranes and their use in metal recovery from aqueous solution

ET‐g‐PAAc membranes were obtained by radiation grafting of acrylic acid onto poly(tetrafluoroethylene–ethylene) copolymer films using a mutual technique. The ion selectivity of the grafted membranes was determined toward K⁺, Ag⁺, Hg²⁺, Co²⁺, and Cu²⁺ in a mixed aqueous solution. The ion‐exchange capacity of the grafted membranes was measured by back titration and atomic absorption spectroscopy. The Hg²⁺ ion content of the membrane was more than that of either the K⁺ or Ag⁺ ions. The presence of metal ions in the membranes was studied by infrared and energy‐dispersive spectroscopy measurements. Scanning electron microscopy of the grafted and metal‐treated grafted membranes showed modification of the morphology of the surface due to the adsorption of K⁺ and Ag⁺ ions. No change was observed for the surface of the membrane that was treated with Hg²⁺ ions. The thermal stability of different membranes was improved more with Ag⁺ and Hg²⁺ ions than with K⁺ ions. It was found that the modified grafted membranes possessed good hydrophilicity, which may make them promising candidates for practical applications, such as for cation‐exchange membranes in the recovery of metals from an aqueous solution. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2692–2698, 2002     

Cost effective cation exchange membranes: A review

This paper will look at developments of new polymer electrolyte membranes to replace high cost ion exchange membranes such as Nafion^®, Flemion^® and Aciplex^®. These perfluorinated polymer electrolytes are currently the most commercially utilized electrolyte membranes for polymer electrolyte fuel cells, with high chemical stability, proton conductivity and strong mechanical properties. While perfluorinated polymer electrolytes have satisfactory properties for fuel cell applications, they limit commercial use due to significant high costs as well as reduced performance at high temperatures and low humidity. A promising alternative to obtain high performance proton-conducting polymer electrolyte membranes is through the use of hydrocarbon polymers. The need for inexpensive and efficient materials with high thermal and chemical stability, high ionic conductivity, miscibility with other polymers, and good mechanical strength is reviewed in this paper. Though it is difficult to evaluate the true cost of a product based on preliminary research, this paper will examine several of the more promising materials available as low cost alternatives to ion exchange membranes. These alternative membranes represent a new generation of cost effective electrolytes that can be used in various ion exchange systems. This review will cover recent and significant patents regarding low cost polymer electrolytes suitable for ion exchange membrane applications. Promising candidates for commercial applications will be discussed and the future prospects of cost effective membranes will be presented.     

Streaming effect of single electrolyte mass transfer in nanofiltration: potential application for the selective defluorination of brackish drinking waters

This work deals with electrical properties of nanosurfaces in contact with electrolyte solutions. Single halide ion solutions were studied by streaming potential (SP) measurements and observed retention (R_obs) of the F⁻, Cl⁻, and Br⁻ ions across nanofiltration (NF) membranes. The detailed understanding of an electrolyte solution mass transfer requires an intimate knowledge of the physicochemical interactions occurring between nanoporous materials and electrolyte solutions across the first-generation composite membranes called NF55, NF70 and NF90. These membranes are composed of a polysulfone mesoporous sublayer and a microporous skin layer in polyamide. In order to get a better understanding of these effects, it seems attractive to compare the mass transfer permeation of the monovalent ions F⁻, Cl⁻, and Br⁻ with the electrokinetic characterizations deduced from a properly developed SP apparatus. SP measurements is a very simple method to show the intrinsinc charges on membrane pore walls. The membrane's electrical properties are studied with SP design modeling pH, ionic strength and kind of electrolyte solutions. We have observed that the isoelectric point (IEP) of the membrane materials is both dependent on the ionic strength and on the kind of electrolyte solution. The IEP in the presence of KCl is 4.4 at 0.0001 mol/L and 5.8 at 0.001 mol/L, showing an increasing adsorption of the cation K⁺ by increasing its solution concentration. For a fixed concentration, the effect of the electrolyte solution has shown that a higher adsorption of Ca⁺⁺ occurs in comparison to K⁺ and Na⁺. But the adsorption of these electrolyte solutions is essentially reversible as observed under dilution conditions. Furthermore SP measurements were used for the first time to characterize the transmembrane pressure ranges where a convective and/or a diffusional mass transfer occurs. Such an approach was developed to correlate the R_obs of the halide ions F⁻, Cl⁻ and Br⁻ with the kind of mass transfer (diffusional and/or convective) occurring predominantly under transmembrane pressure variations. Thus the NF70 membrane shows at low pressure (under 3 bar) the order of R_obs following the hydrated ionic radius: R_obs.(F⁻)>R_obs.(Cl⁻)>R_obs.(Br⁻). For a higher pressure (> 3 bar) an inversion occurs between Cl⁻ and Br⁻, but F⁻ was not affected. These results open a new prospective area for selective defluorination of brackish drinking waters using NF membranes under low transmembrane pressure.     

Adsorption from aqueous solution by δ-manganese dioxide II. Adsorption of some heavy metal cations

The adsorption isotherms of M²⁺ ions (M²⁺ = Ni²⁺, Co²⁺, Cd²⁺, Zn²⁺ and Mn²⁺) on the K⁺-form of δ-MnO₂, at pH 6 and at different temperatures, fitted the Langmuir equation and an apparent heat of adsorption, Q, was found to be - 78, - 33, - 34, - 19 and - 15 kJ mol^?1 respectively. The adsorption capacity of δ-MnO₂ increased in the series: Ni²⁺ < Co²⁺ < Cd²⁺ ± Zn²⁺ < Mn²⁺. This was nearly the order of decrease in the radii, r′, of the hydrated ions, estimated from hydration enthalpies. An ionexchange mechanism between hydrated K⁺ ions in the outer Helmholtz layer and hydrated M²⁺ ions in the solution, suggests positive entropy contributions which offset the endothermic Q. The proposed mechanism is in agreement with the observed sequence of adsorption capacity and with the decrease in Q in the above series, except for Co²⁺ adsorption (possibly complicated by the oxidation of Co²⁺ by δ-MnO²). The adsorption of the cations is probably accompanied by the exchange with Mn ions from the solid. There was evidence of specific adsorption below the point of zero charge (pH 3.3). The adsorption isotherms of Mn²⁺ ions at pH 7 were higher than those at pH 6 and Q was found to be - 19 kJ mol^?1. As the ionic strength increased, the adsorption isotherm of Mn²⁺ ions at pH 7 and 298 K shifted to lower values. Adsorption isotherms of Cu(II)ions at pH 3.5 and of Fe(III) at pH 2 represent specific adsorption and Q was found to be - 74 and - 13 kJ mol^?1 respectively.     

Effect of ultraviolet irradiation on the selective transport of alkali metal ions through 2,3-epithiopropyl methacrylate–methacrylic acid copolymer membrane

Cationic exchange membranes were prepared with 2,3-epithiopropyl methacrylate (ETMA)–methacrylic acid (MAc) copolymer. Transport of alkali metal ions against their concentration gradient through the membranes was investigated by using the system which contains HCl (left side) and alkali metal solution including two kinds of alkali hydroxides (right side). The effect of ultraviolet (UV) irradiation on the selective transport of alkali metal ions through the ETMA–MAc copolymer membranes was investigated. The membranes were irradiated with a 6-W low pressure mercury lamp at a distance of 10 cm at room temperature in air. The transport selectivity could be increased by using UV-irradiated membranes and the selectivity increased with increasing irradiation time up to 2–3 h, although the transport rate of alkali metal ions decreased with increasing time of UV irradiation. The maximum selectivity of K⁺/Na⁺, Na⁺/Li⁺, and K⁺/Li⁺ were 1.7, 2.0, and 4.2, respectively. In order to explain this phenomena, the effect of UV irradiation on the properties of the membranes was studied. It was concluded that the increase of the selectivity is attributed to the formation of the dense membrane by photocrosslinking of the membrane by UV irradiation.     

Adsorption from aqueous solution by δ-manganese dioxide I. Adsorption of the alkaline-earth cations

The adsorption isotherms of M²⁺ ions (M = Mg, Ca, Sr or Ba) were determined at pH 7.0 and at different temperatures. The adsorbent, δ-MnO₂, was converted to the K⁺ form prior to adsorption and about 1.5 mol K⁺ ions were released per mol of M²⁺ ions adsorbed. The adsorption capacity at a given temperature increased in the series: Mg²⁺ < Ca²⁺ ≦ Sr²⁺ < Ba²⁺. This was explained by an ion exchange mechanism between hydrated ions: K⁺ ions in the outer Helmholtz layer and M²⁺ ions in the bulk of the solution. The radii of the hydrated ions decreased in the series: Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺. The adsorption of M²⁺ ions at pH values below the point of zero charge (pH 3.3) was significant for Mg²⁺ ions only. Although adsorption was not strictly reversible, the results fitted the Langmuir isotherm and ‘apparent heats of adsorption’, Q, were calculated. The endothermic heats (Q = 20,18, 11 and 5 kJ mol^?1 for Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ adsorption respectively) indicated positive entropy contributions which are expected for the adsorption mechanism suggested. The decrease in Q down the alkaline-earth group was correlated to the entropy effects and to the hydration numbers of the cations.     

Adsorption and Diffusion of VO2+ and VO2 + across Cation Membrane for All‐Vanadium Redox Flow Battery

A method based on a selectivity coefficient and the Nernst‐Planck equation is proposed to determine diffusion coefficients of vanadium ions across a cation exchange membrane in VO²⁺/H⁺ and VO₂ ⁺/H⁺ systems. This simplified method can be applied to high concentrations of vanadium ions. Three cation exchange membranes were studied. The logarithmic value of the selectivity coefficient was linearly dependent on the molar fraction of vanadium ions in solution. The diffusion coefficient of vanadium ions decreased with decreasing water content. The membrane with the lowest diffusion coefficient was selected as a battery separator and showed the lowest capacity loss of the studied membranes.     

Separation and extraction of some heavy and toxic metal ions from their wastes by grafted membranes

Preparation and characterization of a series of ion‐exchange membranes for the purpose of separation and extraction of some heavy and toxic metal ions from their wastes were studied. Such ion‐exchange membranes were prepared by γ‐radiation grafting of acrylonitrile (AN) and vinyl acetate (VAc) in a binary monomer mixture onto low‐density polyethylene (LDPE) using the direct technique of grafting. The reaction conditions at which the grafting process proceeds successfully were determined. Many modification treatments were attempted for the prepared membranes to improve their ion‐exchange properties. The possibility of their practical use in waste‐water treatment to remove some heavy and toxic metal ions such as Pb²⁺, Cd ²⁺, Cu²⁺, Fe³⁺, Sr²⁺, and Li⁺ were investigated. These grafted membranes showed great promise for possible use in the field of extraction and removal of some heavy and toxic metals from their wastes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 849–860, 2001     

质子传导膜内受限空间离子扩散过程

以具有纳米尺度孔径的聚偏氟乙烯(PVDF)质子传导膜为对象,研究电解质溶液中水合离子在受限空间内的传递行为,证明使用纳米尺度多孔膜代替离子交换膜用于液流电池过程的可行性。利用渗透实验分别研究浓度场、压力场,以及不同渗透压条件下膜中离子扩散和水迁移现象,分析传质行为与膜结构和组成之间的关系。结果表明离子在纳米尺度孔径的PVDF膜中的扩散过程与溶液中类似,表观离子扩散系数不受浓度差推动力的影响;离子交换膜中的表观离子扩散系数随浓度差推动力提高而增加。在渗透压作用下,自制PVDF纳米孔膜的水迁移速率低于Nafion 117膜,水迁移带来的负面影响更小;对于H⁺/VO²⁺的离子选择系数超过300,有效透过H⁺而阻止VO²⁺,适用于全钒液流电池过程。     

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities. For this purpose polybenzimidazole (PBI) membranes have been prepared and H₃PO₄-doped in a doping range from 300 to 1600 mol %. Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient. Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level. At 160 °C a conductivity as high as 0.13 S cm^–1 is obtained for membranes of high doping levels. Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties. At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide.     

Gel electrolyte membranes derived from co-continuous polymer blends

Polymer gel electrolyte membranes were prepared by first casting films of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and poly(ethylene glycol) (PEG) monomethacrylate and dimethacrylate macromonomers. Polymerization of the macromonomers initiated by UV-irradiation then generated solid films having phase-separated morphologies with a microporous PVDF-HFP phase embedded in PEG-grafted polymethacrylates. Gel electrolyte membranes were finally prepared by allowing the films to take up solutions of LiTFSI in γ-butyrolactone (γ-BL). The PEG-grafted polymethacrylate in the membranes was found to host the largest part of the liquid electrolyte, giving rise to a highly swollen ionic conductive phase. Results by FTIR spectroscopy showed that the Li⁺ ions preferentially interacted with the ether oxygens of the PEG chains. The properties of the membranes were studied as a function of the ratio of PVDF-HFP to PEG-grafted polymethacrylate, as well as the degree of crosslinking, LiTFSI concentration, and liquid electrolyte content. The self-supporting and elastic gel membranes had ionic conductivities of 10⁻³ S cm⁻¹ and a mechanical storage modulus in the range of 2.5 MPa in the tension mode at room temperature. Variation of the salt concentration showed the greatest effect on the membrane properties.     

Enhanced mechanical strength and conductivity of PVFM based membrane and its supporting polymer electrolytes

Polyvinyl formal based polymer electrolyte membranes are prepared via the optimized phase inversion method with poly(ethylene oxide) (PEO) blending. The physical properties of blend membranes and the electrochemical properties of corresponding gel polymer electrolytes (GPEs) are characterized by field emission scanning electron microscopy, X‐ray diffraction, differential scanning calorimetry, mechanical strength test, electrolyte uptake test, AC impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge test. The comparative study shows that the appearance of PEO obviously enhances the tensile strength of membranes and the ionic conductivity of corresponding GPEs. When the weight ratio of PEO is 30%, the tensile strength of membrane achieves 12.81 MPa, and its GPE shows high ionic conductivity of 2.20 × 10⁻³ S cm⁻¹, wide electrochemical stable window of 1.9–5.7 V (vs. Li/Li⁺), and good compatibility with LiFePO₄ electrode. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41839.     

Sulfonated graphene oxide‐doped proton conductive membranes based on polymer blends of highly sulfonated poly(ether ether ketone) and sulfonated polybenzimidazole

Simultaneously improving the proton conductivity and mechanical properties of a polymer electrolyte membrane is a considerable challenge in commercializing proton exchange membrane fuel cells. In response, we prepared a new series of miscible polymer blends and thus the corresponding crosslinked membranes based on highly sulfonated poly(ether ether ketone) and sulfonated polybenzimidazole. The blended membranes showed more compact structures, due to the acid‐base interactions between the two constituents, and improved mechanical and morphological properties. Further efforts by doping sulfonated graphene oxide (s‐GO) forming composite membranes led to not only significantly elevated proton conductivity and electrochemical performance, but also better mechanical properties. Notably, the composite membrane with the filler content of 15 wt % exhibited a proton conductivity of 0.217 S cm^?1 at 80 °C, and its maximum power density tested by the H₂/air single PEMFC cell at room temperature reached 171 mW cm^?2, almost two and half folds compared with that of the native membrane. As a result, these polymeric membranes provided new options as proton exchange membranes for fuel‐cell applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46547.     

Ion exchange characteristics of ceric tungstate: Kinetics of exchange

The kinetics of exchange of Cs⁺, Ag⁺, Zn²⁺ and Sc³⁺ on ceric tungstate were studied under particle diffusion control conditions at different temperatures. At the studied temperatures, the mobility of the monovalent ions in the prepared exchanger is higher than that of the multivalent ones, presumably due to weaker electrostatic interaction with the exchange sites. The activation energy data point to a substantial or complete dehydration of the ions diffusing in the narrow pores of the exchanger. The highest activation energy is displayed by Cs⁺ having the largest crystallographic radius.     

Studies on the Separation of Cadmium from Solutions by Foam Separation. I. Foam Separation of Cadmium Cations

Abstract

The efficiency of Cd²⁺ foam separation with sodium lauryl sulfate and sodium laurate from sodium sulfate and sodium nitrate solution was tested. Foam fractionation of Cd²⁺ with lauryl sulfate in the form of a hydrated ion pair 2C₁₂H₂₅SO₄, Cd²⁺ is ineffective because of a low recovery and high hydration of the foam. But ion flotation of Cd²⁺ as a low-solubility salt (C₁₂H₂₅COO)₂Cd is highly effective. The presence of electrolyte in the solution has a negative influence on Cd²⁺ foam fractionation with lauryl sulfate because of an increase of inert salt concentration which causes competition for the collector between colligend Cd²⁺ and the added Na⁺ ions, and because of simultaneous increase in the thickness of water sheaths around the gas bubbles. In the case of ion flotation with sodium laurate, the presence of electrolyte improves Cd²⁺ recovery and decreases the thickness of the water sheaths around the gas bubbles. The interpretation of the results is based on the exchange of the collector counterions as well as on the properties of the reaction products.     

Studies on syntheses and permeabilities of special polymer membranes: 50. Transport of metal ions against their concentration gradient through water-insoluble poly(styrene sulphonic acid) membrane

Water-insoluble cation exchange membranes were prepared by heat treating membranes made of poly(styrene sulphonic acid) and poly(vinyl alcohol). Transport of metal ions through the above cation exchange membrane against their concentration gradient was investigated under various conditions. The transport in this system, where one side of the membrane in a diaphragm cell was acidic and the other alkaline, was influenced significantly by the initial H⁺ ion concentration on the acidic side. The selectivity of metal ions in diffusive transport depended on the size of their hydrated ions and that in transport against their concentration gradient was due to the affinity between the metal ions and the carrier fixed to the membrane.     

ION-EXCHANGE EQUILIBRIUM OF THE Cu2+/H+, Zn2+/H+ AND Pb2+/H+ IONS ON HYDRATED FERRIC OXIDE

ABSTRACT

The ion exchange of trace amounts of Cu²⁺, Zn²⁺ and Pb²⁺ with the hydrogen-form of hydrated ferric oxide as a function of nitric acid concentration has been studied at 25, 30,35 and 60^°C. Ion exchange of these cations increases with increasing temperature in the investigated temperature range. From these results, equilibrium constants for the Cu²⁺/H⁺, Zn²⁺/H⁺ and Pb²⁺/H⁺ ion exchange on hydrated ferric oxide and thermodynamic quantities for these reactions were calculated, and some predictions made for the exchange process taking place in this material. The selectivity series is Zn²⁺ > Cu²⁺ > Pb²⁺.     

Estimation of ion-site association constants in ion-selective electrode membranes by modified segmented sandwich membrane method

A method aimed at potentiometric estimation of the association of ions with ion-exchanger sites and charged ionophores in ion-selective electrode membranes is proposed. The method relies on the measurements of segmented sandwich membrane potentials. It is shown theoretically that the quantification of ion association requires use of weakly associated ionic additive whose concentration in the working segment of the sandwich must be varied. This is in contrast with well-established technique of ion to neutral ionophore complexation measurements. The advantages and limitations of the novel method are critically evaluated. Association of ions in plasticized poly(vinylchloride) membranes is studied experimentally. Experimental results are provided related to the association of K⁺, Na⁺, Cs⁺, and NH₄⁺, and also Ca²⁺ with commonly used sites: tetra(p-Cl-phenyl)borate anion and calcium-selective lipophilic ion-exchanger bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate.     

Pervaporation separation of water–ethanol mixtures using metal‐ion‐exchanged poly(vinyl alcohol) (PVA)/sulfosuccinic acid (SSA) membranes

Poly(vinyl alcohol)/sulfosuccinic acid (PVA/SSA) membranes in the hydrogen form were converted to monovalent metal ion forms Li⁺, Na⁺, and K⁺. The effect of exchange with metal ions was investigated by measuring the swelling of water–ethanol (10/90) mixtures at 30 °C and by the pervaporative dehydration performance test for aqueous ethanol solutions with various ethanol concentrations at 30, 40, and 50 °C. In addition, electron spectroscopy for chemical analysis (ESCA) analysis was carried out to study the quantity of metal ions in membranes. From the ESCA analysis, the lithium ion quantity in the resulting membranes is greater than that of any other metal ions in question because of the easy diffusion of a smaller metal ion into the membrane matrix. The swelling ratio was in the following order: PVA/SSA‐Li⁺ > PVA/SSA‐Na⁺ > PVA/SSA‐K⁺ membranes. For pervaporation, the PVA/SSA‐Na⁺ membrane showed the lowest flux and highest separation factor for all aqueous ethanol solutions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1867–1873, 2002     

Separation of Counterions during Pressure-Driven Transport of Electrolyte Mixtures across Charged Porous Membranes

Abstract

It has been previously predicted theoretically that a substantial separation of counterions will occur during the pressure-driven transport of electrolyte mixtures across charged porous membranes. This presumption has been supported experimentally using negatively charged sulfonated polysulfone and ternary electrolyte solutions with a simple coion like Cl^?. In dilute solutions the selectivity has approached values between 8 and 10 and sometimes even greater. The experimental findings of the relationship of selectivity to feed ionic strength and feed composition agree fairly well with the theory. Moreover, the theoretical prediction is also supported by the correlation of selectivity with mobilities of counterions and transmembrane volume flow. The less mobile counterions of symmetrical mixtures like KCl/LiCl have a rejection inferior to the more mobile ones when porous charged membranes are used. The reverse effect was observed when more dense membranes of the same polymer and degree of substitution were employed. However, some deviations from normality were noticed when the KCl/MgCl₂ mixture was used such as changes in the sign of the selectivity logarithm and the nonmonotonic dependences of the selectivity on the feed ionic strength and transmembrane volume flow. This leads to the assumption that the mobility of the Mg²⁺ ion in the membrane phase is lower than that of the K⁺ ion, which is just opposite of their bulks. This relative decrease in the Mg²⁺ ion's mobility has been interpreted in terms of stronger electrostatic interactions with membrane fixed charges. Moreover, the lack of anomalies when the LiCl/MgCl₂ solution is used leads to the assumption that the mobility of the Mg²⁺ ion in the membrane phase is between that of Li⁺ and K⁺ ions.