Effect of Ion Sizes on Separation Characteristics of Nanofiltration Membrane Systems

Abstract. The effect of the ion size on salt rejection is investigated using the Donnan steric pore model (DSPM). This model accounts for the transport phenomena occurring inside the membrane through the appropriate ion sizes. Four hydrated ions (Na+, Mg2+, C1−, S042−) are considered. The ion radius is determined from Stokes-Einstein relationship (Stokes radius), the Born’s theory (Born’s effective radius), and the strength of hydration (Pauling radius). Experimental data validate predictions of ion rejections. It was found that ionic hydration has strong influence on the diffusion properties of ions, which affects significantly the ion rejections. The charge density across the surface of the membrane varies according to the choice of radius of solute, causes remarkable variations in the model prediction. It was also found that the charge density should be predicted accurately to obtain the surface potential which will aid in the improvement of membrane performance and also give approximate index for fabrication of thin film composite layer membrane. Keywords: Membrane, Mathematical modeling, Nanofiltration, Ion siz