The use of nanofiltration (NF) in water treatment has been proposed to improve the quality of the produced water and extend the options of concentrate valorization, taking the benefit of its different ions selectivity patterns. However, there is a need to understand and optimize the rejection not only of major components (e.g. NaCl or MgSO4) but also of minor components specially in brackish waters. The selectivity of ion rejection by NF membranes has been studied theoretically and experimentally. Theoretical analysis has been carried out within the scope of the solution-diffusion-film model (SDFM) recently extended to electrolyte mixtures. In this study, experimental ion rejection data of typical cationic and anionic species present in surface waters at various trans-membrane pressures and cross-flow velocities have been obtained with a NF membrane (NF270). Several combinations of dominant salts (NaCl, MgCl2, MgSO4, Na2SO4) with trace ions (Na+, Cl−, Mg2+, SO42−) have been used. The rejection of ions crucially depends on their environment. The dramatic differences in the rejections can be explained by the spontaneously arising electric fields generated in the membrane phase. Their effect gives rise to negative rejections of singly charged inorganic ions present as small additions to well-rejected dominant salts. As a result of theoretical interpretation the intrinsic membrane permeabilities to ions have been estimated for different aqueous solutions compositions

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