On the impact of ethanol on the rejection and transfer mechanism during ultrafiltration of a charged macromolecule in water/ethanol

Ultrafiltration (UF) is a sustainable membrane separation technique. It could be useful for the concentration/purification of bio-sourced molecules that are extracted either by pure ethanol or by water/ethanol mixtures. Nevertheless, the process optimization requires an in-depth understanding of the transfer mechanisms of solute through membranes, especially for charged solutes, that are nowadays not sufficiently documented. Previous studies achieved in aqueous media have shown that the rejection of charged solutes by an UF membrane involves at least three mechanisms: convection, diffusion and electrostatic interactions. The present study aims at a systematic analysis of the transfer mechanisms of a model protein (lysozyme) in water/ethanol mixtures (100/0-70/30 v/v) during UF by a zirconia inorganic membrane. The influence of the pH varying in the 4-9 range and of the ionic strength (I) is also discussed. The ionic strength I can be adjusted by addition of an indifferent electrolyte (NaCl) only aiming at the screening of the electrostatic interactions or by addition of a selectively adsorbed electrolyte(KH2PO4) that is able to change the isoelectric pH of the protein and thus to modulate the electrostatic interactions in a different way when compared to NaCl. Of course, both salts have an impact on the protein rejection in UF. The results are analysed using the CDE model previously developed in our group to explain the behaviour of a single protein during UF in water and accounting for convection, diffusion and electrophoretic migration. The applicability of the CDE model in water/ethanol mixtures up to 70/30 v/v is finally shown.