Influence of electrostatic interactions in electrophoretic membrane contactors

In electrophoretic separators, a porous membrane is used to put into contact two flowing liquids between which an electrically driven mass transfer takes place. As far as charged solutes are concerned, the mass transfer can be affected by electrostatic interactions taking place at the membrane solution interface. The influence of these interactions on the solvent and solute transfer is investigated by associating a theoretical and an experimental work, carried out with buffered solutions of different solutes, chosen with respect to their size or electrical charge. Experimental variations of the electroosmotic flux as well as those of the solute concentrations are used to get the values of the characteristic parameters involved in the model. Results obtained with binary solutions are then compared to those obtained with single-solute solutions so as to point out the mass transfer limitation.

In this paper, the mass transfer mechanisms involved in electrophoretic membrane contactors were studied by associating a theoretical approach with an experimental study, which was carried out with an original prototype apparatus and buffered solutions of different composition. The variations of the electroosmotic flux, linked to the apparent electrical charge of the membrane, were studied. The partition coefficient, the value of which reveals the strength of the membrane/solute interactions, was determined by fitting calculated and experimental variations of the solute concentration. The strong mass transfer limitation due to electrostatic interactions was thus emphasised. For single solute solutions, both solvent and solute transfer were found to be governed by the electrical charge of the solute. Moreover, for binary solutions, it was demonstrated that the electroosmotic flux is identical to that obtained with the solution containing the single solute featuring the highest electrical charge. This increasing flux, which reveals a higher apparent charge of the membrane, was found to lower significantly the solute mass transfer, as far as it is governed by electrostatic interactions. Finally, it was observed that for some conditions, this enhancement of electrostatic interactions is such that the good selectivity expected from single solute solutions is reduced to zero.