Gérald Pourcelly

Comparison of transport properties of monovalent anions through anion-exchange membranes

Abstract Transport properties of chloride, fluoride and nitrate ions through AFN, AMX, ACS, ACM anion-exchange membranes (from Tokuyama Soda) and MA-40 membrane (from the Institute of Plastic Materials, Moscow) have been investigated by studying electroconductivity, transport numbers and current–voltage characteristics. On the basis of the micro-heterogeneous model proposed by Nikonenko and Zabolotsky describing the microstructure of the membrane material, the volume fractions of the different phases and the electroconductivity of the joint-gel phase have been determined. Good correlations have been founded between the electroconductivity of the anion-exchange membranes (AEMs) and their structural properties. Recommendations are provided to determine which kind of membrane has to be selected for a given application. For all the membranes, increase of the counter-ion mass transfer is observed above the limiting current.

Sulfonated polyimides as proton conductor exchange membranes. Physicochemical properties and separation H+/Mz+ by electrodialysis comparison with a perfluorosulfonic membrane

The properties of new sulphonated polyimide membranes (SP) – ion exchange isotherms, electrical conductivity, selectivity and proton–cation electrotransport – are compared with that of perfluorinated Nafion® membrane. Both membranes when in contact with H+/Mz+ aqueous solutions (MzM+=Na+, Cu2+, Cr3+) present an affinity to cations which increases with their valencies; however the affinity of one of the SP membranes for protons is approximately 10% higher than that of Nafion®. The proton transport number is also 10% higher for this SP membrane than for Nafion®. Using SP membranes for electrodialysis of H+/Cu2+ solutions produces solutions about 10% more concentrated in H+ and about 40% less concentrated in Cu2+ ions than with Nafion® membranes in the same conditions. The difference in conductivity may be explained by differences in ionic clustering because of differences in the polymer structure of the two membranes, block copolymer for SP and statistic copolymer for Nafion®. The difference in properties for the two SP membranes is explained in terms of the chemical structure of the non-sulfonated diamine groups used in the polymer synthesis. These SP membranes seem to be interesting materials for electromembrane processes in acid media.

Chronopotentiometric response of an ion-exchange membrane in the underlimiting current-range. Transport phenomena within the diffusion layers

Abstract The chronopotentiometric response of a cation exchange membrane in contact with a symmetrical 0.01 M NaCl solution was investigated as a function of the applied current density below the limiting one. Taking into account classical assumptions such as the variation of concentrations associated to the Teorell-Meyer-Sievers (TMS) segmentation model, the concept of the Nernst layer model, the quasi-electroneutrality condition within the diffusion boundary layers (DBL) and the validity of the Donnan equation at the solution-membrane boundaries, theoretical values of the transmembrane potential difference were obtained which fit very well the experimental V − t responses. Moreover, this method allows the calculation of the DBL thickness. In addition, a decrease of the DBL thickness was observed when the current density increases. This result was attributed to the occurrence of convection effects inside the DBL.

Polarization phenomena at the interfaces between an electrolyte solution and an ion exchange membrane. Part I : Ion transfer with a cation exchange membrane

Abstract Polarization phenomena occurring with ion exchange membranes have been investigated for the Selemion AMV anion exchange membrane in contact with NACl solutions. Recording of the current density—voltage responses combined with pH measurements and with individual ion flux measurements allow the determination of which phenomena are at the basis of the current transport. The concentration and potential profiles within the depleted unstirred layer have been calculated. Water splitting is indicated beyond the limiting current density. Analysis of the different overvoltages leads to a comparison between the interfacial transfer overvoltages of both the AMV anion exchange membrane and the CMV cation exchange membrane.

Recovery of spent acid by electrodialysis in the zinc hydrometallurgy industry: Performance study of different cation-exchange membranes

Abstract A performance study of four different cation-exchange membranes was conducted on laboratory and pilot scale (172 cm 2 /membrane, model CS-0 from Asahi Glass Co.) electrodialysis cells, using Neosepta CMS from Tokuyama Soda Co.; Morgane CRA from Solvay; Selemion CHV from Asahi Glass Co. and Nafion 117 from DuPont, coupled with a Morgane ARA 17-10 anion-exchange membrane. Comparisons were made of the sulphuric acid recovery rate, water transport, metal leakage and energy intake for these membranes. Proton permselectivity and metal leakage, co- and counter-ion transport numbers of the CMS membrane were investigated using Hittorfs method and radiotracer methods. This study shows that electrodialysis is suitable for the treatment of zinc hydrometallurgy effluents. However, R & D work must still be done in order to improve and optimize the technology for its use in future industrial applications. In particular, research efforts must concentrate on the synthesis of affordable and stable cation-exchange membranes showing high selectivity towards protons.

Transport competition between monovalent and divalent cations through cation-exchange membranes. Exchange isotherms and kinetic concepts

Separation of Ca2+ or Cu2+ ions from binary solutions with Na+ or protons by electrotransport through strong cation-exchange membranes (CEM) has been studied in the case where no sorbed species is present within the membrane material. The study was achieved on two conventional CEM and two CEM manufactured to be preferentially permselective to monovalent cations. Without any applied driving force, the exchange isotherms exhibit no significant increase in permselectivity for the modified CEM. Divalent cations are preferred by the membranes due to their higher electrostatic attraction with the oppositely fixed charged exchange sites. When the CEM are submitted to an electrical field, radiotracer measurements exhibit the difference of permselectivity of these two types of CEM. The apparent rate constants of the ionic interfacial transfer were calculated on the basis of unidirectional fluxes and concentration profiles of cations within the unstirred layer. The values of these kinetic constants were always greater for monovalent than for divalent cations but the difference was more important for modified membranes. Lastly, comparison between the potential profiles showed that the potential decrease within the depleted unstirred layer was more important for the solutions less concentrated in protons.

Self diffusion and conductivity in NafionR membranes in contact with NaCl+CaCl2 solutions

Abstract The ion-exchange isotherm of sodium and calcium ions is drawn for the Nafion R 117 membrane in contact with [NaCl] + 2[CaCl 2 ] = 0.1 M. The self-diffusion coefficients of sodium and calcium ions, obtained from steady-state self-diffusion ionic fluxes by a radiotracer method are independent of the ionic composition of the membrane phase. They are compared to those obtained from high frequency electrical resistance by means of the ac impedance technique. The values of electric conductivity of the membrane can be derived from a simple additivity law.

Electrotransport of Cr(VI) through an anion exchange membrane

The electrotransport of Cr(VI) oxo anions through an anion exchange membrane (AEM) is investigated as functions of the CrO3 concentration and pH on both sides of the membrane. This transport strongly depends on the polychromate ion formation within the membrane, which is favoured by both the very high concentration compared to that of the bulk solution and the proton leakage usually encountered when an AEM is in contact with acid solutions. Their formation is closely connected to the pH of the donor compartment of the electrodialysis cell. Their presence can lead to an inversion of the AEM permselectivity. The increase of the transport number of Cr(VI) species is associated with the reduction of the proton leakage.

Influence of cations on the proton leakage through anion-exchange membranes

Abstract In the recovery of acids from wastewaters or the regeneration of acids and bases from salts by electromembrane processes, the most important phenomenon which limits the current efficiency is the transport of protons through the anion-exchange membrane (AEM). In this work, the proton leakage through an AEM is studied with a system containing hydrochloric acid or sulfuric acid on the cathodic side and the mixture of acid with one homoanionic salt (Li + , Na + , K + , Cr 3+ , NH 4 + , (CH 3 ) 4 N + and (C 2 H 5 ) 4 N + ) on the anodic side. The proton leakage is quantified from the value of the proton transport number. The results are analyzed assuming that the rate determining step of proton leakage is the interfacial transfer reaction of protons from the aqueous anodic solution to the membrane. The proton leakage is enhanced by the polarizing power of the cation. The transfer of protons into the membrane seems to be catalyzed by the presence of a layer of adsorbed cations on the surface of the membrane. The presence of salt decreases the proton leakage but it is always greater with H 2 SO 4 solutions compared to HCl solutions.

Dependence of composition of anion-exchange membranes and their electrical conductivity on concentration of sodium salts of carbonic and phosphoric acids

Abstract Concentration dependencies of electrical conductivity of AFN, AMX, ACS and ACM anion-exchange membranes (from Tokuyama Soda), equilibrated with solutions of sodium salts of carbonic and phosphoric acids as well as with sodium sulfate solutions, are studied. Analysis of the data on electrolyte desorption from the membranes equilibrated initially with solutions of the weak-acid salts as well as the application of the microheterogeneous model allowed to explain the change of the conductivity with a concentration growth by the effect of two factors: (i) a change (an increase, in the most cases) of the conductivity of the electroneutral solution forming the membrane ‘inter-gel’ phase, (ii) a variation of the conductivity of the membrane ‘gel’ phase caused by hydrolysis reactions of the weak-acid anions in the pore solution.