Jean Molenat

Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes

Too many or too few fluoride ions in drinking water are harmful to the consumers health. The acceptable fluoride concentration is generally in the range of 0.5 to 1.5 mg.L−1. In the present study, Donnan dialysis (DD) with an anion-exchange membrane (AEM) was applied for the defluoridation of diluted NaF solutions. The initial concentration of the feed solution was maintained at 10−3 mol.L−1, corresponding to a 19 mg.L−1 fluoride concentration. Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested. First, membrane properties were studied at equilibrium. The values of the exchange capacity of the membranes in Cl− and F− form, water content, selectivity coefficient for the Cl−F− exchange, diffusion coefficient of Cl− and F− ions in the membrane, were determined for each membrane. DD experiments, performed using a laboratory cell, showed that the DSV membrane is the most effective AEM, despite its electrolyte leakage. Subsequently, a pre-industrial pilot with a total membrane area of 1760 cm2 was used to study the different physico-chemical and hydrodynamic parameters of the process. As the driving ion, the chloride ion is more efficient than the sulfate ion. At flow rates lower than 0.6 L.h−1, the fluoride concentration remains lower than the permitted values despite the presence of others anions generally present in ground water such as chloride, sulfate and bicarbonate ions.

Competitive diffusion of hydrochloric acid and sodium chloride through an acid dialysis membrane

Abstract Competition between HCl and NaCl in dialysis across the anion exchange membrane Neosepta AFN was studied by measuring the diffusion flux of each electrolyte from aqueous solutions containing the two electrolytes. The acid flux was in all cases higher than the salt flux. These results are analyzed on the basis of the Nernst-Planck equation and the Teorell-Meyer-Sievers (TMS) theory. Measurements of the amounts of acid and salt in the membrane at sorption equilibrium were performed by using radioactive tracers. The results of such measurements, in conjunction with the results for the membrane electrical resistance, enabled us to obtain calculated values of the flux. These values were in satisfactory agreement with the experimental values. The two factors forming the basis of the greater diffusion flux for the acid than for the salt are, on the one hand, the higher mobility in the membrane phase of protons with respect to the other cations and, on the other hand, the higher degree of sorption of the acid in the membrane.

Nitric acid and sodium hydroxide generation by electrodialysis using bipolar membranes

An electrodialysis process with bipolar membranes was used to generate HNO3 and NaOH from NaNO3 which can be found in industrial waste waters. The current efficiency of this process is limited by proton leakage through the anion exchange membrane (AEM), co-ion leakage through bipolar membranes (BPM) and water transport through the ion exchange membranes. Three cell configurations using three or two compartment cells with different anion or cation exchange membranes (CEM) in stack series were used and compared. Electrodialysis with three compartments gives the best current efficiencies for nitric acid and sodium hydroxide production from sodium nitrate.

Elimination of nitrate from drinking water by electrochemical membrane processes

The performance of three membrane processes (Donnan dialysis, electrodialysis, electrodeionization) using an ADS-Morgane anion-exchange membrane, was compared to remove nitrate from drinking water. The influence of concentration, current density and flow rate was studied with a 3.10−3 M nitrate solution to determine the best experimental conditions. Then the membrane processes were applied for the treatment of Montpellier drinking water where the nitrate concentration was increased up to a concentration of 80–100 ppm. In all cases the results showed extraction ratios higher than 80%. With the electrodialysis and electrodeionization techniques, the extraction rate of nitrate was faster than with Donnan dialysis, and a deionized water of high purity was obtained. A nitrate elimination ratio of 99% has been reached with electrodeionization.

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.

Transfer of H2SO4, Na2SO4 and ZnSO4 by dialysis through an anion exchange membrane

Abstract Competitive diffusion between H2SO4 and its salts (Na2SO4 and ZnSO4) was performed using a Neosepta AFN anion exchange membrane. The stronger affinity of sulfuric acid and the higher mobility of protons in the membrane with respect to sodium and zinc cations explain the great difference in the diffusivity between acid and salts through this membrane. Coexisting sodium and zinc sulfates have no significant effect on H2SO4 diffusion flux, ion content and electrical resistance. The results are analyzed with acid sulfuric solutions on the basis of the Nernst-Planck equation and Goldman hypothesis. Calculated values are compared with experimental values. This work shows that diffusion dialysis allows one to recover sulfuric acid from effluents containing sodium and zinc ions.

Transport and separation of Ag+ and Zn2+ by donnan dialysis through a monovalent cation selective membrane

Donnan Dialysis of Ag(+) and Zn(2+) was investigated through a cation exchange membrane (CMS Neosepta) when a proton concentration difference was maintained between the two sides of the membrane. Developed for the production of brine from sea water, CMS Neosepta showed a higher permeability to monovalent than to bivalent cations. Several physico-chemical parameters have been determined (electrical resistance, membrane potential, sorption of electrolytes, Zn(2+) and Ag(+) diffusion coefficients). The flux of Ag(+) and the diffusion potential in the membrane increase with HNO(3) concentrations. Ag(+) and Zn(2+) can be separated because of the preferential membrane transfer for Ag(+).

Desorption of counter-ions followed with labelled ions and water dissociation in perfluorinated quaternary ammonium membranes and Nafion 125 membranes

Abstract Desorption and exchange phenomena have been studied in perfluorinated quaternary ammonium membranes (QAM) by means of radiotracers. In these anion exchange membranes, counter-ion desorption followed with 36 Cl gave higher results than co-ion desorption followed with 22 Na. The cation exchange Nafion 125 membrane gave similar results. When these two kinds of membranes, equilibrated in 0.1 M NaCl at pH 7, are desorbed in water at this pH, pH decreases with QAM and increases with Nafion 125. These shifts in pH values are observed over a wide range of pH around 7. These results can be explained by a larger water dissociation due to the catalyzing action of the exchange sites.

Desorption followed with radioisotopes and membrane potentials in perfluorinated vinyl4pyridine and quaternary ammonium membranes

Abstract A radiotracer technique used to follow desorption and exchange processes with vinyl4pyridine (V4P) and quaternary ammonium membranes (QAM) allowed one to compare the properties of these two types of membranes and to point out the deprotonation process in V4P membranes. Exchange capacities obtained with this technique were compared with those obtained by microanalysis titration of nitrogen from V4P exchange sites. The deprotonation process enabled one to explain the important differences obtained with the two methods. With radiotracers we could set up a new technique to determine the pKa of these membranes. This work was completed by measuring swelling ratios and membrane potentials in uncross-linked and cross-linked V4P and QAM membranes. These measurements allowed one to study the cross-linking and hydration effects with NaCl and HCl solutions and to point out the importance of the deprotonation process in water and in NaCl solutions. Potential measurements show for these two types of membranes an important loss of permselectivity in concentrated HCl or NaCl solutions.

New possibilities for obtaining more accurate results of self-diffussion coefficients by adapting the capillary tube technique

The method of Wang and Mills for the determination of self-diffusion coefficients can be greatly improved by the use of flat-bottomed capillaries, reinjecting the labelled solution at the top of the capillaries once they are immersed and choosing diffusion times corresponding to adequate values for C0/C. This leads to greater accuracy and rapidity in the determination and to a saving in the quantities of some of the chemicals used for the preparation of the solutions and in the counting.