B. Auclair

Sorption and pervaporation of dilute aqueous solutions of organic compounds through polymer membranes

Abstract Pervaporation of dilute aqueous binary mixtures of four organic compounds (benzene, chloroform, acetone and ethanol) through nitrile—butadiene and styrene—butadiene copolymers was investigated. A pervaporation device has been built, which allows measurement of the pervaporation flux and selectivity of a membrane as a function of the upstream composition of the feed and the downstream total pressure of the pervaporate. In order to relate pervaporation results to equilibrium properties of the membranes, the sorption of water and dilute aqueous solutions was mainly investigated. The pervaporation of dilute aqueous solutions of benzene and chloroform has been extensively studied, including the separation of traces of chloroform, and is modelled through a “sixcoefficients exponential model” [1]. This model is derived from a solution—diffusion analysis of the selective transfer, assuming an exponential dependence of both diffusivities on concentrations of both permeants. Semi-quantitative information about the potential interactions existing in the system solute i —solvent j —membrane and about the concentration profiles at steady-state may be derived from these coefficients.

Application of chronopotentiometry to determine the thickness of diffusion layer adjacent to an ion-exchange membrane under natural convection

A brief review of the evolution of the diffusion boundary layer (DBL) conception inspired by the works of Nernst, Levich and Amatore is presented. Experimental methods for studying the DBL in electrode and membrane systems are considered. The electrochemical behaviour of a CM2 cation-exchange membrane in NaCl and KCl solutions is studied by chronopotentiometry at constant under-limiting current. Chronopotentiometric curves are described theoretically by applying the Kedem-Katchalsky equations in differential form to a three-layer system including the membrane and two adjoining DBLs. The conductance coefficients entering the equations are found by treating the results of membrane characterisation: the electrical conductivity, transport numbers of ions and water, electrolyte uptake, as functions of the equilibrium electrolyte solution. The two-phase microheterogeneous model is used for this treatment resulting in presentation of the conductance coefficients as functions of (virtual) electrolyte solution concentration in the membrane. The steady-state DBL thickness (delta) is found by fitting experimental potential drop at sufficiently high times. It is found that delta is proportional to (Delta c)(-0.2), where Delta c is the difference between the electrolyte concentration in the solution bulk and at the interface. This result differs from the Levich equation, which gives the power equal to -0.25 for Delta c. This deviation is explained by the fact that the theory of Levich does not take into account microscopic chaotic convection motion recently described by Amatore et al. It is shown that the treatment of experimental chronopotentiometric curves with the model developed allows one to observe the role of streaming potential in the membrane. Different mechanisms of streaming potential and their effect on the shape of chronopotentiograms are discussed. A simple analytical solution of Navier-Stokes equations applied to natural convection near an infinite vertical ion-exchange membrane is found. It is shown that the formation of DBL induced by electric current is quasi-stationary. This fact allows the empirical expression found earlier and linking delta with Delta c under steady-state conditions to be used in transient regimes. The numerical solution of the non-stationary Kedem-Katchalsky equations together with this empirical expression results in quantitative description of the potential difference (pd) and delta as functions of time in chronopotentiometric experiments. The comparison of theoretical and experimental chronopotentiometric curves shows an excellent agreement, especially for the part after switching off the current. The reasons of a small deviation observed just before the curves attain steady state under a constant current applied are discussed.

Correlation between transport parameters of ion-exchange membranes

Abstract A deduction of a relation between transport coefficients of ion-exchange membranes is considered by comparison the Kedem–Katchalsky and Onsager forms of transport equations in the framework of irreversible thermodynamics. This relation is analysed by using the experimental data of Narebska et al., Berezina et al., and the results of other authors. Transport equations generalising the Nernst–Planck equation with the coefficients determined directly from the practical transport characteristics are obtained either taking into account or disregarding the above mentioned relation. The Nernst–Einstein relation and its generalizations are discussed.

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.

A simplified procedure for ion-exchange membrane characterisation

A new procedure for the characterisation of the transport properties of ion-exchange membranes (IEM) is proposed. Only three characteristics have to be measured: the electrical conductivity, the diffusion permeability coefficient and the apparent transport number. To complete the set of parameters, two complementary characteristics, the true counter-ion transport number and the water transport number, are calculated from the Scatchard equation and from a novel equation deduced earlier. The possibilities to reduce the number of initial measurements are discussed in three cases.

Elimination des fluorures par la dialyse ionique croisée

The rarefaction of drinking water resources led certain populations of different countries to use water which has a high fluoride content, above the W.H.O. limit, resulting in dental and osseous fluorosis. The present work has proved that the Donnan dialysis could be a suitable method for removing the fluorides. The systematic study of the different parameters involved showed that stirring and the membrane characteristics are the main variables affecting the transmembrane flow. The rough computation of a defluoridation unit shows that the process is viable.

Transport of weak-electrolyte anions through anion exchange membranes: Current–voltage characteristics

Transport of salts of phosphoric and carbonic acids through anion exchange membranes (AEM) is studied by means of current–voltage responses. Concentration polarisation of such membrane systems is accompanied by a change in the electrolyte concentration at the membrane–solution interface due to hydrolysis reactions. Current–voltage characteristics (CVC) on HPO42− or PO43− ion-containing systems with modified AEM selectively permeable to mono-charged anions present two waves like in electrode multi-component systems with multi-stage electrochemical reactions. This phenomenon does not occur with nonmodified AEMs. It is also the case with all the AEMs studied for solutions containing HCO3−or CO32− salts because of close values of electrical resistances of the membranes in these both ionic forms.

From the multi-ionic to the bi-ionic potential

Abstract When an ion exchange membrane separates two electrolyte solutions, an electric potential is set up. In the particular case where the two solutions contain the same co-ion and different counter-ions at the same concentration, Co, this potential is named the bi-ionic potential (BIP). The ion interdiffusion fluxes transform every bi-ionic system to a multi-ionic one. We have set up an experimental device which permits us to obtain reliable measurements of BIPs. A cation exchange membrane (CM 2 ) has been used to study the BIP as a function of C 0 . The comparison of experimental results with the Helfferich equations shows that the former is only confirmed at high or low concentrations. The important role of diffusion layers, co-ion flux and/or influence of membrane structure are proved.

Theoretical study of the bi-ionic potential and confrontation with experimental results

A theoretical study of the bi-ionic potential (BIP) has been carried out using the extended Nernst‐Planck equation in the case of a mixed control of the interdiffusion process by the ion-exchange membrane (IEM) and the diffusion boundary layers (DBLs), a non-zero co-ion flux, a non-zero water flux, a variable selectivity coefficient and an affinity coefficient different from unity. The numerical integration of the two coupled differential transport equations allowed, for a given common concentration C0, the computation of the BIP from the values of 12 parameters required. Three of these 12 parameters are taken from the literature, the others are determined from independent experiments except the DBL thickness and the co-ion diffusion coefficient in the membrane. We have developed a procedure to deduce these two parameters from the experimental curves of the BIP vs. C0. For the NaCl/CM2/LiCl bi-ionic system, the DBL thickness changes from 59 mm at high stirring rate to 196 m mi n the absence of stirring. The chloride diffusion coefficient in the CM2 membrane has been estimated to be equal to 2.110 ˇ7 cm 2 s ˇ1 . Using these values, we have studied theoretically the influence of each of the three parameters: affinity coefficient, selectivity coefficient and water flow. We have shown that the affinity coefficient has the most important contribution but the selectivity coefficient and the water flow influence only the BIP at high common concentrations. # 1999 Elsevier Science B.V. All rights reserved.

Mass transfer characterization in Donnan dialysis

Abstract We have established and verified an equation relating the flux of migrant ions to their concentrations in both compartments in an ion-exchange process. This equation is based on a mechanism which considers that the transfer is completely controlled by the liquid films adhering to both sides of the membrane.