Characterisation of ultrafiltration and nanofiltration membranes from rejections of neutral reference solutes using a model of asymmetric pores

The ability of predicting intrinsic rejections is of central importance in solute transport modelling in ultrafiltration and nanofiltration, especially for applications in membrane development and process optimisation. The essential features of a recently developed asymmetric pore model (APM) that can be used for describing hindered solute transport in this type of membranes are discussed here in detail. The effect of membrane asymmetry on rejections through mathematical simulations and the practical application of this APM by characterising a set of commercial membranes were studied. The structure of the membranes is defined in this model using two empirical parameters, the pore radius at the membrane surface, r_ps, and an asymmetry parameter, H. The simulations show that quantifying the asymmetry can be more important when pores are smaller; for large pores and high permeation fluxes, solute retention at the pore entrance is the main effect responsible for rejection. In the experimental part, a set of ultrafiltration membranes, ranging from 1 to 100 kDa molecular weight cut-off, and also a nanofiltration (MWCO near 200 Da) membrane were characterised, with the parameters r_ps and H being determined for the various membranes, using in each case a set of different reference solutes. Tests were performed in two different filtration systems, a plate-and-frame unit from DSS/AlfaLaval and a stirred cell from Amicon. The experimental results show that, for all the membranes tested, H is not negligible, and the introduction of this parameter enables a better characterisation and predictive capability than previous models based on hindered transport in cylindrical pores (thus symmetric pores), specially at low permeate fluxes, as predicted by the developed theory.