Ion Rejection Properties of Nanofiltration Membranes with Inhomogeneous Charge Distributions

Ion rejection by nanofiltration (NF) membranes with bipolar fixed charge distributions have been investigated theoretically. Salt flux rectification has been predicted as a result of the broken symmetry of the fixed charge distribution on the pore walls. The existence of an optimal volume flux (i.e. leading to the highest salt rejection) has been put in evidence when there is a region within the membrane pores that is more repulsive than the pore entrance with respect to a given electrolyte. For low Peclet numbers, our results show that bipolar membranes can lead to close rejections for both 2-1 and 1-2 asymmetric electrolytes. These performances result from specific properties of bipolar membranes and cannot be achieved with homogeneously charged membranes. The present work benefits the design of nanoporous membranes with targeted distribution of ionizable surface groups for advanced separations.