The role of membrane charge on nanofiltration performance

The zeta potential along the surface and through the pores of a commercial nanofiltration membrane was studied with several electrolyte solutions (including monovalent and divalent hardness ions, KCl, CaCl2 and MgSO4) to investigate the influence of salt type and pH on the charge of the membrane surface and in the membrane pores. The membrane negative charge increased with increasing pH, the membrane surface being more negatively charged than the pores, but having the same isolectric point (pH 4.2 ± 0.2). The membrane was less negatively charged in the presence of divalent salts. The membrane performance evaluated in terms of flux and retentions showed an inverse behaviour: flux decreased with pH, whereas retention increased. Results were explained by membrane charge variation and its effect on membrane pore size and by electroviscous and osmotic effects. For higher salt concentrations (higher ionic strength) flux and retention decrease, this decrease being more pronounced for the highly rejected MgSO4 salt. Negative proton retentions were obtained at low pH due to the behaviour of the more mobile co-ion H+ in a mixture of electrolytes.