Thermodynamic perspective on negative retention effects in nanofiltration of concentrated sodium chloride solutions

Transport mechanisms for nanofiltration membranes are usually studied for solutions containing low solute concentrations, whereas practical applications of nanofiltration often use highly concentrated or even saturated solutions. These solutions are amongst others obtained from anti-solvent crystallization processes. This study shows that the NaCl retention for nanofiltration of a solution saturated in NaCl and containing either ionic or neutral anti-solvents can be predicted from thermodynamic considerations. The membrane resistance of nanofiltration membranes during NaCl transport through the membrane for solutions with a high NaCl concentration in the concentrate is negligible. Furthermore, the NaCl retention can be predicted based on simple anti-solvent crystallization experiments, provided that the concentrations of the anti-solvent in the permeate and concentrate are known. The concept has been proven for a range of ionic and neutral anti-solvents and a broad range of nanofiltration membranes.