Effect of pH on Total Volume Membrane Charge Density in the Nanofiltration of Aqueous Solutions of Nitrate Salts of Heavy Metals

The separation efficiencies of aqueous solutions containing nitric salts of Zn, Cu, Fe or Pb at various pH in process of nanofiltration have been investigated experimentally. These results were used to obtain the total volume membrane charge densities, through mathematical modelling based on the Donnan–Steric partitioning Model. The experimentally obtained retention values of individual heavy metal ions varied between 36% (Zn(2+) at pH = 2), 57% (Pb(2+) at pH = 2), 80% (Fe(3+) at pH = 9), and up to 97% (Cu(2+) at pH = 9). The mathematical modelling allowed for fitting the total volume membrane charge density (X(d)), which yielded values ranging from −451.90 to +900.16 mol/m(3) for different non-symmetric ions. This study presents the application of nanofiltration (NF) modelling, including a consideration of each ion present in the NF system—even those originating from solutions used to adjust the pH values of the feed.