Influence of coion and counterion size on multi-ionic solution nanofiltration

Nanofiltration of a solution containing four monovalent ions has been carried out to assess the Donnan steric and dielectrical exclusion pore model (DSDE-PM). The sequences of the ions rejections were Li+ > Cl− > IO3− > K+ and IO3− > K+ > Li+ > Cl− when membrane was positive or negative charged, respectively. Several types of ionic radii were implemented either in the interfaces equilibrium or in transport equations and the Stokes radius gave the best fitting, followed by a model that takes cavity radius for DE and crystal radius for the rest of hindrance factors. Analysis of the model equations showed that for high charge membrane, counterion with the highest mobility inside the pores is more rejected than the slowest one. The sequence of coions rejections is more difficult to predict as it depends on the relative contribution of mobility and several hindrance factors as well.