FACILITATED EXTRACTION AND FACILITATED TRANSPORT OF NONIONIC PERMEANTS THROUGH ION-EXCHANGE MEMBRANES - INFLUENCE OF THE STABILITY OF THE PERMEANT CARRIER COMPLEXES

Coupling of transport and reaction can be carried out by using ion-exchange membranes and selecting a counter-ion as carrier reacting with the permeant. Their strong exchange capacity allows these membranes to support high concentration gradients of ionic species and consequently confers them a high permeability, while the selectivity is given by the reaction specificity. A model of the transport is propounded assuming that the reaction kinetics are fast. The membrane separates a concentrated and a dilute gaseous phase of permanent S (case 1), a concentrated and a dilute non-ionic solution of S (case 2), a non-ionic solution of S and an ionic carrier solution (case 3). In case 1, polarization phenomena can be neglected. In cases 2 and 3, the concentration polarization is taken into account in order to define the interfacial concentrations. The main factor of the transport-reaction coupling is the stability of the complexes S(p)T(q) involved. Facilitated transport arises in cases 1 or 2 when the values of the stability constants of S(p)T(q) are neither too low nor too high. In the other cases the flux density takes the minimum value of an uncoupled flux: either S(p)T(q) does not form or the membrane is saturated with S(p)T(q) which cannot dissociate. Facilitated extraction arises in case 3 as soon as a stable S(p)T(q) is formed in the membrane and in the downstream phase were the permeant is trapped in the complexed form. This theoretical analysis is supported by experimental results which concern diffusion, facilitated extraction and facilitated transport of weak acids and bases (S = ammonia, acetic acid, boric acid, alpha-alanine, carbon dioxide) through various ion-exchange membranes separating aqueous or gaseous phases. According to the carrier used, the transport is coupled either to a neutralization reaction (T(Z) = OH- or H+) or to the formation of more or less stable complexes (alpha-alanine in cationic form, carbamate zwitterion, ammine complexes, polyborate ions). Enhancement factors near to 20 have been obtained in facilitated transport and up to 100 in facilitated extraction.