Journal of Membrane Science, Vol.165, No.2, 251-259, 2000
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.