Surface charge characterization of nanofiltration membranes by potentiometric titrations and electrophoresis: Functionality vs. zeta potential

The surface charge properties (i.e., functionality and zeta (ζ) potential) of two nanofiltration (NF) membranes were characterized by potentiometric titrations and electrophoresis to predict the electrostatic transport at the membrane surfaces affecting their salt rejection and fouling propensities. The ζ potential was not suitable for evaluating the rejection of Na+ (NE20 membrane = 21-25%; NE70 membrane = 65-70%) and Cl− ions (NE20 membrane = 19-22%; NE70 membrane = 60-63%), and the fouling propensities of organic materials in the NF membranes due to its inherent measurement inaccuracies (∆ ζ potential = − 1.8 × (∆ amount of desorbed organic foulants) + 45.9, R2 = 0.07). The functionality accurately predicted both the rejection of NaCl and the fouling propensities of the organic materials, as the charge densities of the membranes determined by the functionality measurements (only TFC membranes are applicable) truly reflected the acid dissociation constants of the carboxylic and amine functional groups and the points of zero charge values. These results indicate that potentiometric titrations may provide valuable insights into the electrostatic transport at the membrane surface influencing the salt rejection and fouling mechanisms of the NF membranes.