The role of pH and concentration on the ion rejection in polyamide nanofiltration membranes

Volumetric membrane charge values corresponding to each operative condition investigated are calculated through the Donnan Steric Pore Model and Dielectric Exclusion. Membrane charge is obtained as an increasing function both with pH and with electrolyte concentration in the feed side. A Langmuir-type behaviour is apparent at constant pH values; amphoteric behaviour is re-confirmed. The points of zero charge approximately correspond to the pH values at which NaCl rejections approach their minimum values.