Surface charge on loose nanofiltration membranes

The main objective of this work was to study the effect of salt concentration and pH on the surface charge of loose nanofiltration (NF) membranes through streaming potential measurements. A Dow pilot unit equipped with a plate-and-frame module allowed the simultaneous test of two commercial NF membranes: Celgard N30F and Celgard NF-PES-10. Permeation of 0.2–18 mol/m3 NaCl solutions was carried out at constant flow rate and ambient temperature, 2.5–12.5 bar, whereas pH ranged from 5 to 7. The ion exchange capacity (IEC) of the membranes was assessed by potentiometric titration. The streaming potential developed in NaCl solutions (0.2–5 mol/m3; pH = 5–7) flowing along a thin rectangular slit formed by two parallel identical membranes was measured by an EKA electrokinetic analyser, which automatically calculated the zeta potential through the Helmholtz-Smoluchowsky equation and corrected it for the membrane surface conductance. The membrane surface charge was determined through the Gouy–Chapmann equation. Both membranes bear low negative charge densities, the absolute magnitude of which decreased for decreasing pH, as the membrane isoelectric point was approached. The absolute magnitude of the membrane surface charge increased for increasing concentrations, due to the progressive adsorption of anions onto the membrane surface, though it was impossible to glimmer a Freundlich adsorption isotherm, due to the scattering of streaming potential data at concentrations above 5 mol/m3. The ion exchange capacities of the membranes permselective layers were in the range of weak ion exchange membranes. However, IEC data were one order of magnitude higher than the maximum charge densities assessed by electrokinetic measurements.