Prediction of permeate fluxes and rejections of highly concentrated salts in nanofiltration membranes

In this work, the Donnan-steric-pore-model-dielectric-exclusion (DSPM-DE) model was used to predict permeate fluxes and rejections of various single salts solutions in NF90 nanofiltration membrane. The salts used were NaCl, KCl, MgCl₂, Na₂SO₄, Na₂CO₃, MgSO₄ and CaSO₄. The concentration of salts used in this study was generally higher than in previous reported studies. The concentrations were chosen to represent typical ions concentrations in seawater environment. At such higher concentration, the osmotic pressure difference lead to flux decline, which was quite significant when compared to the pure water flux. The osmotic pressure was calculated using the Pitzer equation as well as Vant Hoff equation. It was found that both equations managed to estimate the osmotic pressure difference with negligible difference for the range of concentrations used in this study. The Vant Hoff equation was then incorporated into the DSPM-DE model to allow for prediction of permeate flux in addition to salt rejections. The DSPM-DE model was able to predict the permeate fluxes and rejections with reasonable agreement for some of the salts by just using fitting parameters obtained using the NaCl rejection data.