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

The zeta-potentials (zeta-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 x 10(-5) mol/l, which corresponds to the negative maximum of the zeta-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 x 10(-3) mol/l). However, the absolute value of the zeta-potential not directly relates to the determined rejection rate of Na2SO4. Increasing the Na2SO4 concentration results in a sign reversal of the measured zeta-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 KCI 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. (C)2000 Elsevier Science B.V. All rights reserved.