Effect of salt mixture concentration on fractionation with NF membranes

Desal-5 DK, NF 270, NF (Dow), NF 20 and ESNA-1-LF nanofiltration membranes were tested for their selectivity, when single and mixed salt solutions (NaCl, NaCl:Na2SO4 1:1 ratio) of different concentrations were filtered at a constant permeate flux. Streaming potential measurements were used to characterise the new NF 20 (Sepro) and NF (Dow) membranes. The membrane surface charge increased the retentions when dilute salt solutions were filtered. The best selectivity was generally achieved using low permeate fluxes and high salt concentrations. At higher salt concentrations the charge effects were diminished. Negative NaCl retention values were obtained for all but the ESNA-1-LF membrane, when concentrated mixed salt solutions were filtered. The use of higher permeate fluxes increased salt retentions in general. The measured isoelectric points (IP) for the NF (Dow) and the NF 20 (Sepro) membranes were 5.1 and 6.6, respectively. Owing to the close proximity of the IP of the NF 20 membrane to the filtration conditions, the effect of the surface charge on the separation was limited and salt fractionation was efficient at all tested salt concentrations. Of the studied membranes, the NF (Dow) and the Desal-5 DK showed the best selectivity. According to the salt retention data, the ESNA-1-LF membrane should not be considered a nanofiltration membrane, but rather a slightly open reverse osmosis membrane.