Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions

The zeta-potentials (ζ-potentials) of a polyethersulfone nanofiltration (NF) membrane (PES 10, Celgard GmbH) were determined in single salt solutions (Na2SO4 and KCl-electrolytes) at different concentrations and pH values and results were compared with measured rejections rates of the electrolytes in the same aqueous medium. With increasing Na2SO4 concentration the rejection rate reaches a maximum at 8 × 10−5 mol/l, which corresponds to the negative maximum of the ζ-potential at rising electrolyte concentrations. Selective adsorption of the SO42− anions were found to be responsible for increasing rejection rate in the lower concentration range (up to 1 × 10−3 mol/l). However, the absolute value of the ζ-potential not directly relates to the determined rejection rate of Na2SO4. Increasing the Na2SO4 concentration results in a sign reversal of the measured ζ-potential, which indicates the specific adsorption of Na+. This was also proved by the shift of the isoelectric point (i.e.p.) to higher pH-values with increasing Na2SO4 concentration. At the membrane’s i.e.p. the observed rejection rates were not minimal. For a KCl solution it was shown that there is no preferential adsorption of the ions, neither Cl− nor K+, on the membrane surface, which leads to the conclusion that in this case the electrostatic properties of the membrane are only affected by the dissociation of the surface sulfonic acids.