Influence of surface charge and solution pH on the performance characteristics of a nanofiltration membrane

Acetyl cellulose nanofiltration membrane (NF) was manufactured by a dry/wet phase inversion process and studied on its performance characteristics for the electrolyte solutions NaCl, MgCl2, Na2SO4 with a concentration range of 10−3-10−2 N at various pH values. It was shown that the multivalent cation Mg2+ or sulfate anion SO42− present in non-symmetrical electrolytes makes the membrane more positively or negatively charged as a result of their adsorption on the amphoteric pore surface, which greatly increases the membrane selectivity up to 85-95% for these electrolyte solutions. Meanwhile, with respect to 1:1 electrolyte solution NaCl the rejection capacity is low (R=30-40%) because there is essentially no adsorption on the membrane surface. For a feed source made of the RO retentate the experimental data showed that the fabricated membrane possesses high rejection capacity for multivalent ions such as SO42− (R=85.7%) and Mg2+ (R=93.5%) and low selectivity for monovalent salts such as NaCl (R=33.7%). The following salts rejection sequence with respect to symmetric and non-symmetric electrolyte solutions has been set up: RNa2SO4>RNaCl>RMgCl2 and RMgCl2>RNa2SO4>RNaCl at pH≥7 and pH≤7, respectively. The fact that at pH values above 7 rejection of sodium chloride is higher than that of magnesium chloride, although its ion size is smaller than that of the latter, can be explained only by charge exclusion effect. From these results it can be concluded that Donnan exclusion effect is the predominant mechanism for salt removal by NF membranes.