Relating Nanofiltration Membrane Performance to Membrane Charge (Electrokinetic) Characteristics

The performance (i.e., water flux and solute rejection) of a thin-film composite aromatic polyamide nanofiltration membrane and its relation to membrane surface charge (electrokinetic) characteristics were investigated. Membrane performance and streaming potential measurements were carried out as a function of pH for several solution chemistries, including an indifferent electrolyte, humic acid, and anionic and cationic surfactants. Performance results for the membrane were interpreted by relating the water flux and salt/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 electrostatic repulsion and increased pore volume (size) in the cross-linked polymer network. Ion rejection is directly related to the membrane pore charge and is attributed to co-ion electrostatic repulsion (exclusion). At low pH, 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 short-term performance of the membrane, despite its significant influence on membrane ζ-potential. Sodium dodecyl sulfate, on the other hand, had significant effects on the water flux and salt rejection. Association of the surfactant molecules (i.e., hemimicelle formation) at the membrane−solution interface was analyzed in terms of membrane charge characteristics. It is proposed that the adsorbed surfactant molecules in the form of hemimicelles or a bilayer provide an additional filtration layer that results in reduced water flux and increased salt rejection.