Intrinsic pKa of Nanofiltration Membrane Surfaces to Assess Fouling and Cleaning Behaviors Induced by Foulant-Membrane Electrostatic Interactions

The fouling and cleaning behaviors of m-phenylenediamine (MPD), coumarin-3-carboxylic acid (CCA), and D-(+)-glucose (DG) on polyamide nanofiltration (NF) membrane surfaces were investigated with a focus on the two intrinsic equilibrium constants (pK(a,intr)) of carboxylic and amine functional groups determined using potentiometric titration. The charged foulants (MPD and CCA) strongly influenced the pK(a,intr) of the membrane surface after the fouling layer formed via electrostatic interactions (Virgin = 3.4 and 9.2; MPD-fouled = 4.1 and 8.1; CCA-fouled = 1.5 and 12.4). Moreover, the pK(a,intr) of electrostatically fouled membranes substantially recovered when using cleaning agents that released electrostatic interactions (cleaned MPD-fouled = 3.5 and 9.0; cleaned CCA-fouled = 3.3 and 9.6). In contrast, the neutral foulant (DG) did not affect the pK(a,intr). (DG-fouled = 3.5 and 9.2); however, the zeta-potential of DG-fouled membrane was closer to zero than the virgin membrane (Virgin = -28.1 mV and DG-fouled = -7.2 mV at pH 7). The pK(a,intr) value accurately represented the electrostatic interactions between organic foulants and membrane surfaces. Potentiometric titration is a facile method of determining the pK(a,intr) that gives an in-depth understanding of the electrostatic interactions at the membrane surface associated with the membrane fouling and cleaning mechanism.