Arlette Lindheimer

Selective permeability of a perfluorosulphonic membrane to different valency cations. Ion-exchange isotherms and kinetic aspects

Transport competition between protons and Ag+, Co2+and Ce3+ ions across a Nafion™ cation exchange membrane (CEM) has been studied in the Donnan exclusion domain. In such conditions the membrane does not contain any mobile anion but only cations (the counter-ions) which electrically equilibrate the sulphonic sites. In the frame of the gel-model, the ion transport flux depends on both the composition of the membrane resulting from the solution–membrane exchange equilibrium and the mobility of the counter-ions within the membrane. As the charge of the ion increases, so does its affinity for the membrane which is greater for multivalent counter-ions than for protons due to their larger electrostatic attraction towards the exchange sites of opposite polarity. On the other hand, as the charge of the ion increases, its mobility in the membrane is reduced. Modelling of this transport takes into account these two aspects as well as ionic concentration variations in the diffusion layer.

Electrical transport of sulphuric acid in nation perfluorosulphonic membranes

Abstract The electrical transport properties of sulphuric acid in a Nation 117 perfluorosulphonic membrane have been studied. Using a mercury cell, the conductivity of the membrane was determined by ac impedance measurements as a function of the degree of swelling and of the amount of sorbed species. The excellent penmselectivity of the Nation 117 membrane to protons was confirmed. By means of radiotracer measurements, the transport number of chemical species bearing a sulphur atom was determined; it was lower than 2% regardless of the amount of sorbed acid or the applied current density. The electro-osmotic flux of water was measured by means of an electro-electrodialysis device. The coefficient of the first dissociation step of molecular sulphuric acid to HSO − 4 within the membrane was calculated as a function of the amount of sorbed acid species; it was found to be less than those observed for hydrohalogenic acids. This dissociation coefficient decreased from 0.42 to 0.22 when the concentration of the sorbed sulphuric acid varied from 0.005 to 0.16 mole per sulphonic site.

A study of the superselectivity of Nafion perfluorosulfonic membranes

Abstract The factors resulting in the superselectivity of Nafion perfluorosulfonic membranes in contact with NaCl solutions are investigated by means of a set of experimental measurements: zero-current membrane potential, determination of the amount of sorbed electrolyte, co-ion and counter-ion self-diffusion fluxes, co-ion fluxes under a constant current and membrane electrical conductance. The influence of swelling is pointed out by comparing the results obtained with two samples of the same membrane, one in the expanded form after boiling and the other remaining untreated. The amount of sorbed electrolyte is higher in expanded membranes, but as the swelling increases by the same ratio, the molality of the sorbed electrolyte remains unchanged. The loss of superselectivity associated with greater swelling is due to a decrease in the ratio of the mobility of counter-ion over co-ion.

Transport properties of anion exchange membranes in contact with hydrochloric acid solutions. Membranes for acid recovery by electrodialysis

Abstract The transport numbers of chloride ion as well as unidirectional fluxes under a constant current are measured by the radiotracer technique. The studied anion exchange membranes are in contact on both sides with hydrochloric acid at the same concentration. The membranes are the commercial membranes AAV Selemion and ARA Morgane, specially designed for the recovery of acids by electrodialysis. In addition, measurements of the electrical conductances of the membrane and of the amount of sorbed electrolyte at equilibrium have been carried out. The analysis of the results suggests a low dissociation degree of the acid present in the membrane phase.

Electrodialysis of acid solutions with metallic divalent salts : cation-exchange membranes with improved permeability to protons

Abstract The electrodialysis of acid solutions (chlorhydric and sulfuric) in the presence of metallic divalent salts (chloride and sulfate) are studied. The anion-exchange membranes used in this process have been selected from several membranes manufactured by Solvay because of their low water content and their good anionic selectivity. Two kinds of cationic exchange membranes have been tested: the No. 9, a homopolar prototype membrane, manufactured by Solvay, which was selected after several tests; and a commercial cation-exchange membranes (CDS membrane), which has been modified on its surface by adsorption of polyelectrolyteimine. The obtained results show that HCl+CuCl 2 or NiCl 2 and H 2 SO 4 +CuSO 4 , NiSO 4 or ZnSO 4 can be treated by electrodialysis using the selected CEM. In all cases the acid is not only more concentrated but also purer than when traditional CEM are used. Some problems appeared with the mixture HCl+ZnCl 2 because of complexes formation between Zn 2+ and Cl − ions.

Conductivity of sorbed hydrohalogenic acid in Nafion perfluorosulfonic membranes

Abstract The electrical transport properties of hydrohalogenic acid in a Nafion 117 perfluorosulfonic membrane are studied. The conductivity of the membrane material is determined by ac impedance measurements. In membranes previously equilibrated with aqueous solutions, the swelling is modified and the amount of sorbed acid species is evaluated. For higher swelling, the presence of sorbed HF does not affect the membrane conductivity, whereas the presence of HCl and HBr increases it strongly when the fraction of sorbed acid molecules over fixed sulfonic groups is less than 5% for HCl and 20% for HBr. For larger amounts of sorbed HC1 and HBr, the membrane conductivity remains constant. Radiotracer flux measurement of chloride ions shows that the transport number of this anion does not exceed 0.02. These results suggest that the superselectivity of this membrane to protons has two distinct origins: a reduced dissociation of sorbed acid in the membrane phase and an increase of the proton mobility resulting from the sorption of acid.

Selectivity of ion transfer in carboxylic ion exchange membranes: Part I. Transport numbers of K+, Na+ and Cl− in the S18 saft membrane

Abstract The SAFT S18 membrane, which consists of a polyethylene film with carboxylic groups, is of the cationic exchange type. Its transport properties with respect to Na+, K+ and Cl− ions have been studied by measuring individual transport numbers using radiotracers. With an apparent pKA value of 4.1 for the carboxylic groups, this membrane behaves, as stated, as cationic exchange substance in neutral media, but as an uncharged diaphragm at lower pH. In neutral NaCl solutions the Cl− ion rejection is total in the 0–2 M concentration range, while with KCl the Cl− transport number increases from 0.02 to 0.4 as the KCl concentration passes from 0.1 to 1.0 M. This difference in the transport selectivity of the Na+ and K+ cations can be attributed to a difference in the ion association processes within the membrane phase. For NaCl there is complete ion pair formation while in the case of KCl it would appear that the observed results must be due to a more complex ion association than a simple ion pair.

Transport properties of electrodialysis membranes in the presence of Zn2+ complexes with Cl−

The aim of this paper is to investigate the properties of anion exchange electrodialysis membranes in contact with aqueous solutions containing zinc chloride complexes. Measurements of electric resistance, water content, quantity of sorbed electrolytes, self-diffusion and electrotransport ionic fluxes were performed as a function of the composition of the external solutions. The results were analysed in relation to the complex formation inside the solutions.

Transport of proton in polymeric ionic exchange membranes in relation with the dissociated sorbed acid

Abstract The transport of proton in ion exchange membranes in contact with HCl and H 2 SO 4 solutions is studied. The membranes are the Nafion® 117 cation exchange membrane and, on the other hand, the Selemion® AAV and the Morgane ARA anion exchange membranes. Sorption and water content measurements combined with the radiotracer technique point out the low dissociation degree of the acid present in the membrane phase. This low dissociation leads to the excellent permselectivity towards proton of the Nafion membrane, and it is also the factor which decreases the proton leakage in the two studied anion exchange membranes.

Loss of permselectivity of anion exchange membranes in contact with zinc chloride complexes

In this paper we compare the properties of anion-exchange membranes in contact with aqueous solutions containing zinc chloride complexes and with uncomplexed metal chloride solutions. Electric resistance measurements were performed as a function of the composition of the external solutions, which are mixtures of hydrochloric acid with sodium, zinc or nickel chloride. The transport numbers of proton and zinc through anion-exchange membranes were measured, and the obtained results showed the influence of the complexed species on the loss of permselectivity of the studied membranes. These results were analyzed on the basis of Raman spectra of the membranes equilibrated with the considered solutions.