Bulk-Phase Criteria for Negative Ion Rejection in Nanofiltration of Multicomponent Salt Solutions

Abstract

An expression for calculating the change in the rejection of a given ion under nanofiltration conditions when adding other salts to the bulk solution has been derived using the extended Nernst-Planck equations. This expression gives a simple means for calculating in which direction the change of the rejection will go from knowledge of the equivalent conductance and the concentration changes of the ions added to the bulk solution. Using the multisalt system NaNO₃-Na₂SO₄-HCl as an example, the nitrate rejection plane was calculated from the derived model. The experimental nitrate rejection data fit very nicely into this plane, showing increasing negative rejections by increasing the Na₂SO₄ concentration which can be reversed by the addition of HCl to the bulk solution.