Prediction of physical properties of nanofiltration membranes for neutral and charged solutes

Two commercial nanofiltration (NF) membranes viz., NF 300 MWCO and NF 250 MWCO were used for neutral and charged solute species viz., glucose, sodium chloride and magnesium chloride to investigate their rejection rates using Donnan steric pore model (DSPM) and DSPM-dielectric exclusion (DSPM-DE) model. The estimated pore radii are 0.62 nm and 0.56 nm, respectively for the above models, while permeabilities of both the membranes are almost identical. Charge densities were computed using Stokes-Einstein, Born effective and Pauling radii, while dielectric exclusion was assessed for NF 250 membrane. The effect of pH for sodium chloride was studied to evaluate isoelectric point of NF membrane, which was around pH 5. Correlations were attempted between charge density, concentration and pH for each radius. Charge density decreased drastically for sodium chloride when dielectric exclusion was included in the calculations, thus showing real physical characteristics of the membranes, whereas divalent magnesium chloride showed a shift from positive to negative charge density. The importance of charge density and pore radius in understanding fouling propensity of NF membranes was discussed. © 2011.