Relating nanofiltration membrane performance to membrane charge (electrokinetic) characteristics

The performance (i.e., water flux and solute rejection) of a thin-film comp osite aromatic polyamide nanofiltration membrane and its relation to membra ne surface charge (electrokinetic) characteristics were investigated. Membr ane performance and streaming potential measurements were carried out as a function of pH for several solution chemistries, including an indifferent e lectrolyte, humic acid, and anionic and cationic surfactants. Performance r esults for the membrane were interpreted by relating the water flux and sal t/ion rejection to the membrane charge characteristics. In the case of the indifferent electrolyte (NaCl), water flux and salt passage were maximal at the membrane pore isoelectric point (pH 5) primarily due to decreased elec trostatic repulsion and increased pore volume (size) in the cross-linked po lymer network, ion rejection is directly related to the membrane pore charg e and is attributed to cc-ion electrostatic repulsion (exclusion). At low p H, negative rejection of protons was observed, demonstrating the classical behavior of a more mobile co-ion in a mixture of electrolytes (NaCl and HCl ). Suwannee River humic acid was found to have very little effect on the sh ortterm performance of the membrane, despite its significant influence on m embrane zeta-potential. Sodium dodecyl sulfate, on the other hand, had sign ificant effects on the water flux and salt rejection. Association of the su rfactant molecules (i.e., hemimicelle formation) at the membrane-solution i nterface was analyzed in terms of membrane charge characteristics. It is pr oposed that the adsorbed surfactant molecules in the form of hemimicelles o r a bilayer provide an additional filtration layer that results in reduced water flux and increased salt rejection.