Nanofiltration of complex mixtures: The effect of the adsorption of divalent ions on membrane retention

Abstract

In nanofiltration (NF), solution and membrane chemistry determine the membrane charge density. In multi-component electrolytes solutions, it is important to consider the interaction between ions and NF membranes. For instance, in the presence of divalent ions, charge reversal can take place due to the interaction of counterions with the charged functional groups of the membrane. This paper presents a model based on state-of-the-art mean-field theory that includes the effect of divalent counterions on the membrane charge density. By using a Langmuir equation with two model parameters, we consider the adsorption of divalent ions (e.g., Ca2+ and Mg2+) in the polyamide active layer of the membrane and calculate the effective membrane charge density. Two different sets of experimental data from literature are used to evaluate the model. Contrary to the statement of a recent study on NF, we show that mean-field theory can predict the rejection of all types of ions in a multi-component electrolyte solution when the effect of divalent counterions on the membrane charge density is included. Moreover, the results in this study reinforce the idea that adsorption of counterions plays a fundamental role in the performance of nanofiltration.