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

The role of pH and concentration is studied on the ion rejection of Desal5-DK polyamide NF membranes. Various samples are used taken from two different batches (DK99 and DK02). Extensive experimentation at room temperature on HCl-water (0.001-1 mol/m3) as well as on NaCl–water (1-50 mol/m3) solutions is presented to characterize the membrane as a function of electrolyte concentration, at feed pH values in the range from 3 to 6.5. The applied pressure in the feed side is varied from 3 to 30 bar. The effect of feed pH is studied both on salt rejection and on total volume flux, and general trends are obtained. DK02 membranes are less pH sensitive than DK99’s. Na+ and H+ rejections are greatly affected by pH. Na+ rejection goes through a minimum value as the feed acidity increases, and correspondingly H+ rejection goes through a maximum value. Minima are approximately located at pH values in the range between 4.5 and 5. 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.