Monovalent Cation Permselectivity Research Articles

Optimizing functional layer of cation exchange membrane by three-dimensional cross-linking quaternization for enhancing monovalent selectivity

Monovalent cation perm-selective membrane (MCPMs) allow fast and selective transport of monovalent cations, and they are promisingly required for extraction of special ions, such as lithium extraction, acid recovery and sea salt production. Herein, we report a novel strategy to design the critical functional layers of MCPMs with both space charge repulsion and cross-linked dense screenability. The in-situ deposition polymerization of pyrrole was carried out on the surface of sulfonated polyphenyl sulfone (SPPSU) substrate membrane followed by cross-linking quaternization of the polypyrrole (PPy) layer with diiodinated functional molecules, thus, the membrane obtained more excellent selective permeability and stable transport properties of monovalent cations. It confirms that the designed PPy layers with charged surface and cross-linking structure improved the hydrophilicity, facilitated cation transport and increased ion flux. Meanwhile, for the dense PPy layer, the charged cross-linked structure endowed the functional layer with the synergistic characteristics of Donnan exclusion and pore size sieving for positively charged ions, which improved the monovalent cation perm-selectivity of the membranes. At a constant current density of 5.1 mA/cm2, the optimal membrane exhibited superior perm-selectivity (PMgNa=2.07) and monovalent cation flux (JNa+ = 2.80 × 10 −8 mol cm−2 s−1) during electrodialysis.

Preparation of monovalent cation perm-selective membranes by controlling surface hydration energy barrier

Cationic separating the with different valence states is greatly desirable but technically challenging. Achieving such a precise separation using membranes requires accurately distinguishing tiny differences of cations on physico-chemical properties. Here, inspired by the ion dehydration effect, a series of cation exchange membrane with adjustable surface hydrophobicity based on quaternized polypyrrole were prepared. We demonstrate that improving hydrophobicity of modified layer substantially hinder cations with higher hydration energy to enter the modified layer due to their stronger binding ability with water molecules. When applied in electro-driven cation separation, the optimal membrane exhibited an outstanding perm-selectivity between monovalent and divalent cations (PNaMg = 12.95, PLiMg = 1.28) than commercial monovalent cation perm-selective membrane (PNaMg = 2.46, PLiMg = 1.07). Overall, our results indicate that the hydrophobicity of the modified layer governs the cations dehydration behavior through the polymeric pores due to ion-pore wall interactions, providing the scientific basis for the construction of membranes with high monovalent cation perm-selectivity.

Novel electrodialysis membranes with hydrophobic alkyl spacers and zwitterion structure enable high monovalent/divalent cation selectivity

Development of monovalent cations perm-selective membranes (MCPMs) for electrodialysis with the ability to improve cation flux, selectivity and reduce membrane swelling are the major concerns in different industrial applications, such as seawater desalination and wastewater purification. We designed and fabricated the novel MCPMs of quaternized poly(2,6-dimethyl phenylene oxide)s containing different lengths of alkyl spacers which were grafted directly to the nitrogen-centered cations connected to hydrophilic carboxylic and sulfonic acids groups. Pendant chains were in charge of tuning the membrane hydrophobicity and promoting the microphase separation. We observed that during changing the hydrophobicity of MCPMs, higher cation flux, greater perm-selectivity and more dimensional stability of monovalent cations perm-selective membrane can be achieved. The cations selectivity of prepared MCPMs was measured from electrodialysis in a Na+/Mg2+ system and compared with the commercial monovalent cation selective NeoseptaTM CIMS membrane. Highest perm-selectivity (PMg+2Na+) that reached up to 25.26 was observed for the MCPMs with the longest hydrophobic pendant chain which demonstrated that enhancement of membranes hydrophobicity considerably improved the monovalent cation perm-selectivity. These observations are quite interesting to expedite the improvement of monovalent cations perm-selective membranes for electrodialysis.

Electro-Driven in Situ Construction of Functional Layer Using Amphoteric Molecule: The Role of Tryptophan in Ion Sieving

The permselective separation of monovalent ions from the solution with the coexistence of multivalent ions was required in the industry of the wastewater treatment and resource recycling. Here, in this work, a novel electro-driven in situ modification method was utilized in the positively charged tryptophan solution to prepare highly permselective cation exchange membrane (CEM). We have optimized the process conditions and discussed the modification mechanism by drawing upon the fouling phenomenon in the membrane separation processes. A series of conventional characterization methods such as scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and electrochemical impedance spectroscopy (EIS) were used to investigate the structure and performance changes after the modification. The SEM-energy-dispersive X-ray (SEM-EDX) was introduced for analyzing the transfer of Na+ and Mg2+ in the functional modification layer and the membrane matrix, thus illustrating the sieving mechanism for the modified membranes. The resultant membranes were observed to have the changed interstructure and the multivalent-ion-repulsive modification layer. Due to the channel filling and electrostatic repulsion effect, the electro-driven tryptophan endowed the pristine CMX with superior monovalent cation permselectivity (PMg2+Na+: 6-35), which was higher than that of pristine CMX and the commercial Neosepta CIMS. It confirms that the proposed electro-driven tryptophan in situ modification method could effectively help improve the membrane permselectivity by structurally forming stable crystallization on and within the membrane, which provides a feasible choice for the cation exchange membrane modification.

Surface-functionalized cation exchange membrane by covalent immobilization of polyelectrolyte multilayer for effective separation of mono- and multivalent cations

ABSTRACTThe existence of numerous non-conductive regions retards the further functionalization of heterogeneous membrane surface. In this report, a modification scheme for covalent immobilization of polyethyleneimine (PEI) multilayer on heterogeneous cation exchange membrane (CEM) was developed by means of diazonium-induced carboxyl group grafting. Of particular note are the influences of some important modification conditions, including the molecular weight of PEI and its deposition times, which were explored and understood. Series of electrodialysis experiments confirmed the achievement of the monovalent cation permselectivity. Furthermore, the results also indicated that the PEI coating can also effectively improve surface homogeneity and then alleviate concentration polarization behaviour during the electrodialysis process.

The improvement of comprehensive transport properties to heterogeneous cation exchange membrane by the covalent immobilization of polyethyleneimine

In this work, a three-step modification scheme was devised to covalently immobilize the polyethyleneimine (PEI) multilayers for improving the comprehensive transport properties of a heterogeneous cation exchange membrane (CEM), including the grafting of carboxyl groups, the PEI immobilization by amidation reaction and glutaraldehyde-induced PEI multilayers deposition. Firstly, the changes in surface chemical composition and morphology after modification were investigated by attenuated total reflection-Fourier transform infrared spectroscopy and scanning probe microscope. Then, the membrane properties corresponding to different modification steps were comprehensively evaluated by the measurements of ion exchange capacity, water content, salt permeability coefficient, membrane electrical resistance, limiting current density and so on. Especially, in order to simulate the possible applications in the reclamation of the industrial effluents from the hydrometallurgy processes and the comprehensive utilization of seawater, respectively, some electrodialysis experiments for H+/Zn2+ system and Na+/Mg2+ system were performed to estimate and compare the monovalent selectivity of the membrane samples. Furthermore, the alteration of surface heterogeneity after modification was explored by current–voltage curves and chronopotentiometric curves. The experimental results showed that the modification scheme proposed in this work can make significant contributions to a heterogeneous CEM, such as the endowment of monovalent cation permselectivity, the improvement of surface homogeneity for ameliorating concentration polarization, the ability to impeding salt diffusion and even the possibility of extending membrane life-span.

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.

Modification of Transport Properties of Ion Exchange Membranes.XV. Preparation and Properties of Cation Exchange Membranes Having a Single Cationic Charged Layer on the Membrane Surface by Sulfonyl–Amide Bonding

Abstract In order to prepare an excellent monovalent cation permselective cation exchange membrane, a polyethyleneimine (PEI) layer was formed with sulfonyl–amide bonding between PEI and sulfonyl chloride groups on one surface of the membrane. Two methods were tried: 1) one surface of the membrane reacted with PEI and the remaining sulfonyl chloride groups were hydrolyzed, and 2) after one surface of the sulfonyl chloride groups had been hydrolyzed, the other surface of the membrane was reacted with PEI and the remaining sulfonyl chloride groups were hydrolyzed. The cation exchange membranes, which had been prepared using both methods, had excellent monovalent cation permselectivity, low electrical membrane resistance and high current efficiency for the long-term electrodialytic concentration of seawater. The reacted amount of PEI on the membrane was about 10% of the cation exchange capacity.

Modification of transport properties of ion exchange membranes, XIII. Surface‐modified cation exchange membranes prepared from polyamines and a copolymer membrane containing p‐stryenesulfonyl chloride

AbstractA membrane precursor was prepared by the copolymerization of p‐styrenesulfonyl chloride and divinylbenzene in the presence of a poly(vinyl chloride) supported cloth. The surface of the membrane was treated with various amines, and then the sulfonyl chloride groups in the internal part of the membrane were hydrolyzed with an aqueous sodium hydroxide solution in organic methanol or acetone solvent. Hydrolysis conditions were examined for the introduction of the sulfonic acid groups. The membrane modified by ethylenediamine exhibited slightly bipolar properties during the electrodialysis of seawater. Surface‐modified membranes using tetraethylenepentamine exhibited excellent monovalent cation permselectivity, low membrane electric resistance, and long performance durability during the electrodialysis of seawater.

Enhancement of the monovalent cation perm-selectivity of Nafion by plasma-induced surface modification

In this paper, we focus on improvement of the monovalent cation perm-selectivity of a perfluorinated cation-exchange membrane, Nafion 117, by depositing an anion-exchange layer using a plasma surface modification process. The anion-exchange layer was deposited from 4-vinylpyridine monomer vapor followed by quaternization with 1-bromopropane. The transference number of divalent cation (Fe2+) through the membrane, tFe, decreased with increasing thickness of the plasma polymer layer at the expense of enhanced membrane resistance. A large interfacial resistance was observed between Nafion and the plasma polymer layer which was ascribed to the implantation of cationic species containing nitrogen. To avoid the formation of an interfacial layer, a novel method of plasma-induced surface modification was devised. After a Nafion 117 sheet was placed on an RF (radio-frequency) electrode and sputtered with an oxygen or argon plasma in order to produce active sites on the Nafion, 4-vinylpyridine or 3-(2-aminoethyl)aminopropyltrimethoxysilane vapor was introduced into the reactor to react with radical sites. tFe decreased with increasing RF power. tFe through Nafion modified with 3-(2-aminoethyl)aminopropyltrimethoxysilane was lower than that for Nafion modified with 4-vinylpyridine, probably due to its weak Si-C bond. Nafion treated by the plasma surface modification method exhibited a very high monovalent cation perm-selectivity compared with Nafion treated by the plasma polymerization method.

Enhancement of Monovalent Cation Perm-Selectivity of Nafion by Plasma-Induced Surface Modification

Abstract Novel monovalent cation perm-selective membranes were prepared by the surface modification of perfluorosulfonate cation exchange membranes, Nafion. Nafion was sputtered with oxygen plasma in order to produce radical sites on its surface. Ultra-thin cationic layers was produced on the Nafion surface by reactions between the radical sites and 4-vinylpyridine vapor. The resultant membranes showed high monovalent cation perm-selectivity (transference number of divalent cation = 2 × 10−2) and low membrane resistance of 2 Ω cm2.

Modification of ion-exchange membrane surface by plasma process. Part 2. Monovalent cation permselective membrane from perfluorosulfonate cation exchange membrane

The monovalent cation perm-selectivity of a perfluorosulfonate cation-exchange membrane, Nafion® 117, was enhanced by depositing an ultra-thin anionic exchange layer on its surface. The anion exchange layer was prepared by plasma polymerization of 4-vinylpyridine followed by quaternization with 1-bromopropane. The resulting layers were about 0.2 μm thick, pinhole-free, and of uniform thickness. The influence of the plasma parameter of polymerization pressure on the deposition rate, membrane resistance, and monovalent cation transference number were investigated. The monovalent cation perm-selectivity of the plasma-modified Nafion membrane was determined by measuring t Fe, the transference number of Fe 2+ through the membrane, in the Li +-Fe 2+ system. Plasma-modified Nafion membranes exhibited a high lithium ion perm-selectivity. This perm-selectivity was enhanced with decreasing polymerization pressure at the cost of high membrane resistance.

Modification of an Ion-Exchange Membrane Surface by Plasma Process Part 3: Interfacial Resistance of Monovalent Cation Perm-Selective Membrane from Nafion®

Abstract The monovalent cation perm-selectivity of Nafion® 117, perfluoro-sulfonate cation-exchange membrane, was enhanced by depositing on its surface an ultrathin anion-exchange layer prepared by plasma polymerization. The anion exchanger was deposited from a 4-vinylpyridine monomer vapor followed by quaternarization with 1-bromopropane. The resulting anion-exchange layers were found to be pinhole-free and of uniform thickness. The monovalent cation perm-selectivity of the plasmamodified Nafion membrane was determined by measuring the Fe 2+ ion transference number, tFe in a Li+–Fe2+ system. The influence of the thickness of the plasma layers on the tFe and the membrane resistance was investigated. The tFe decreased with increasing thickness of the plasma polymer layer, but at the cost of enhanced membrane resistance. The resistance of treated Nafion was separated into bulk resistance and interfacial resistance. A large interfacial resistance was observed between Nafion and the plasma polymer layer, and was ascribed to the implantation of cation species.

Modification of transport properties of ion exchange membrane. XI. Electrodialytic properties of cation exchange membranes having polyethyleneimine layer fixed by acid–amide bonding

AbstractFor the purpose of preparing the monovalent cation permselective membrane for electrodialysis, the membrane having—SO2Cl groups reacted with the polyethyleneimine to form a cationic polyelectrolyte layer on the membrane surface by using the acid–amide bonding. Reaction conditions, i.e., reaction temperature, concentration of the polyethyleneimine solution and the addition of tertiary amines to the polyethyleneimine solution, were all examined in detail. As a result, all the different kinds of polyethyleneimines were effective in obtaining the monovalent cation permselective membrane if the correct reaction conditions were selected. The durability of the monovalent cation permselectivity was evaluated in a continuous electrodialytic concentration of sea water. The monovalent cation permselectivity decreased gradually during the electrodialysis in the cation exchange membrane with the polyethyleneimine layer. Therefore, the cation exchange membrane with the cationic polyelectrolyte layer was further treated with crosslinking reagents, i.e., form aldehyde–hydrochloric acid, epichlorohydrin, etc. or other alkylating reagents. The monovalent cation permselectivity of the treated membranes was almost constant for two months. The crosslinking reaction of polyethyleneimine was more effective than alkylation in maintaining the high permselectivity constantly.

Modification of transport properties of ion exchange membranes, X formation of polyethyleneimine layer on cation exchange membrane by acid‐amide bond

AbstractIn order to give the monovalent cation permselectivity to the cation exchange membrane permanently, polyethyleneimine layer was formed on the membrane surface by acid‐amide bond between polyethyleneimine and the SO2Cl groups of a styrene‐divinylbenzene copolymer membrane. After the reaction, the SO2Cl groups remaining in the inner part of the membrane were hydrolyzed by immersing the membrane into an aqueous sodium hydroxide solution.The resultant cation exchange membranes were evaluated by electrodialysis of sea water from the point of view of the monovalent cation permselectivity, electric resistance of the membrane, current efficiency, and transport number of sodium ions calculated by membrane potential in connection with reduced viscosity of the various commercial polyethyleneimines, ATR‐IR measurements on the membrane surface suggested that the acid‐amide bond was stable for severe hydrolysis reaction.

Modification of the transport properties of ion exchange membranes. IX. Layer formation on a cation exchange membrane by acid-amide bonding, and transport properties of the resulting membrane

In order to modify the transport properties of sulfonic acid-type cation exchange membranes, chlorosulfonyl groups were introduced as reactive groups into the membrane composed of a styrene-divinylbenzene copolymer and poly (vinyl chloride) fabric. The chlorosulfonated membrane was then reacted with various amino compounds. Suitable reaction conditions of the copolymer membrane with chlorosulfonic acid were examined in detail. After the chlorosulfonated membrane had been immersed in an aqueous solution of amino compounds to form a different kind of layer by acid amide bonding, mainly on the membrane surface, the membrane was immersed in aqueous sodium hydroxide solution to hydrolyze unreacted chlorosulfonyl groups to sodium sulfonate groups. The amino compounds used were p-aminosalicylic acid, L-tyrosine and polyamino compounds such as triethylenetetramine, polyethyleneimines etc. Formation of the polyamino compound layer on the surface of the cation exchange membrane by acid amide bonding conferred stable monovalent cation permselectivity on the membrane in electrodialysis. All reactions were confirmed by measurements of attenuated total reflectance infrared spectra.

Modification of properties of ion exchange membranes. II. Transport properties of cation exchange membranes in the presence of water-soluble polymers

Electrodialytic transport properties of commercial cation exchange membranes in the presence of various water-soluble polymers were observed. The transport properties measured in this report were the relative transport number of calcium ion to sodium ion ( P Na Ca), the current efficiency of cations and the electric resistance of the membrane during the electrodialysis. Water-soluble polymers used were polyethyleneimine, poly- N,N-dimethylethyleneammonium sulfate, poly- N-vinylpyrrolidone and polyvinylalcohol. When electrodialysis was carried out using the cation exchange membrane in the presence of cationic polyelectrolytes, P Na Ca of most membranes decreased remarkably while the electric resistance of the membrane during the electrodialysis scarcely increased and the current efficiency increased or decreased slightly. For example, P Na Ca of the cation exchange membrane, Neosepta CL-25T, was 0.18 when the membrane had been exposed to the mixed salt solution containing 27.9 ppm of poly- N,N-dimethylethyleneammonium sulfate for 24 hr while P Na Ca of Neosepta CL-25T in the absence of it was 2.52. However, P Na Ca of the same membrane increased in the presence of poly- N-vinylpyrrolidone or polyvinylalcohol. In the case of polyethyleneimine, it was confirmed that monolayer type adsorption or ion exchange occurred on the surface of the membrane, about 90% of imino groups of polyethyleneimine which adsorbed on or ion exchanged with the membrane did not ion exchange with sulfonic acid groups of the membrane and P Na Ca decreased much more when pH of the mixed salt solution used in electrodialysis was low. Moreover, P Na Ca decreased remarkably in the presence of strongly basic polyelectrolyte more than weakly basic polyelectrolyte. The membrane property of monovalent cation permselectivity by adsorption of or ion exchange with polyelectrolyte was not easily lost. These membranes are preferentially permeable to sodium ion because the electrostatic repulsion force between calcium ion and the cationic surface charge of the membrane is larger than that between sodium ion and the cationic surface charge of the membrane. However, the increase of P Na Ca in the presence of poly- N-vinylpyrrolidone and polyvinylalcohol was not sufficiently explained in this report.