Nanofiltration of Multi-Ion Solutions: Quantitative Control of Concentration Polarization and Interpretation by...

For effective use of advanced engineering models of nanofiltration quality of experimental input is crucial, especially in electrolyte mixtures where simultaneous rejections of various ions may be very different. In particular, this concerns the quantitative control of concentration polarization (CP). This work used a rotating disklike membrane test cell with equally accessible membrane surface, so the CP extent was the same over the membrane surface. This condition, which is not satisfied in the conventional membrane test cell, made possible correcting for CP easily even in multi-ion systems. Ion rejections were studied experimentally for several dominant salts (NaCl, MgCl₂, Na₂SO₄ and MgSO₄) and trace ions (Na⁺, NH₄⁺, Cl^- and NO₃^-) using NF270 membrane. The solution-diffusion-electro-migration model was used to obtain ion permeances from the experimental measurements. The model could well fit the experimental data except in the case of NH₄⁺. The correlations between the ion permeances and type of dominant salt are discussed in the context of the established mechanisms of NF such as Donnan and dielectric exclusion. The obtained information contributes to the systematic transport characterization of NF membranes and may be ultimately useful for computational fluid dynamics simulations of the performance of the membranes in various applications.