Separation of Aqueous Electrolyte Solutions with Asymmetric Membranes Containing One Charged Layer

The Nernst-Planck equation and fine-pore membrane model are applied to describe the ultra- and nanofiltration of electrolyte solutions through a inhomogeneous membrane containing one charged layer. Concentration and electric potential distributions, as well as dependences of electrolyte rejection coefficient (selectivity) and streaming potential on system parameters are determined. Asymmetry effect is revealed with respect to the rejection coefficient and streaming potential at different orientations of the selective charged layer relative to the direction of the filtration flow. The cases of 1 : 1 and 1 : 2 electrolytes are investigated in detail. Theoretical calculations demonstrate that the rejection coefficient of a bi-layer membrane rises in the following series of binary electrolytes: 1 : 2 < 1 : 1 < 2 : 1, when the first layer is positively charged, and in the opposite series of these electrolytes, when the first layer is negatively charged.