Diffusive flows in ultrafiltration and their effect on membrane retention properties

In modelling the retention of ultrafiltration membranes, diffusive fluxes across the membrane have usually been neglected, mainly due to evidence derived from using symmetric track-etched membranes. The present paper reexamines this matter specifically for the case of “real” asymmetric membranes. A critical literature review on the use of irreversible thermodynamic (IT), hydrodynamic and Stefan-Maxwell (S-M) models is presented. It is shown that all three approaches yield the same basic retention equation for the case of non-negligible diffusive solute-flux. It is also shown that, for membranes with a coefficient close to one, a much simpler equation gives results almost identical to the more rigorous basic equation just mentioned. Furthermore an overview of available literature data indicates that diffusive fluxes do play a non-negligible role in the functioning of asymmetric ultrafiltration membranes. Further work is needed in order to predict solute transport properties in hydrodynamic terms. A simplified Stefan-Maxwell approach seems the appropriate tool for future work in studying multicomponent solutions.