Nanofiltration of single and mixed salt solutions: Analysis of results using principal component analysis (PCA)

Abstract

The retention of single (NaCl) and mixed (Na 2SO 4 + NaCl) salt solutions by nanofiltration membranes was analysed using principal component analysis (PCA). Seven variables—five different membranes (Desal-5DK, NF 270, NF Filmtec, NF 20 and ESNA-1-LF), flux and salt concentration—were taken into account in the analysis. The dependences of NaCl retention on these variables for single and mixed solutions were evaluated by PCA. Differences and similarities between the obtained results and those from a previous study based on a univariate method were observed. The PCA results from single and mixed salt solution experiments indicated that the NF 20 membrane differs from the others; a finding that is in accordance with earlier observations. The difference could be explained by the fact that the NF 20 membrane was not charged at the filtration pH (6) and by its bigger pore size. For single salt solutions, the results for the NF Filmtec and NF 270 membranes (manufactured by Filmtec) resembled each other when using the PCA method; a finding which could not be shown using a univariate method. These membranes did not resemble each other when filtering mixed salt solutions. Analysis of the results when using the PCA method for the single and mixed salt solutions indicated that the retention of the ESNA-1-LF membrane behaved differently from all the other tested membranes. While a negative retention for NaCl in a mixed solution was observed with the other membranes, the ESNA membrane showed a positive retention which was close to the retention of a reverse osmosis membrane (above 80%). The results showed that both salt concentration and permeate flux had major effects on the salt retentions of the NF 20, NF Filmtec, NF 270 and Desal-5DK membranes.