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 solutio ns (Na2SO4 and KCl-electrolytes) at different concentrations and pH values and results were compared with measured rejections rates of the electrolyte s in the same aqueous medium. With increasing Na2SO4 concentration the reje ction 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 r elates to the determined rejection rate of Na2SO4. Increasing the Na2SO4 co ncentration results in a sign reversal of the measured zeta-potential, whic h indicates the specific adsorption of Na+. This was also proved by the shi ft of the isoelectric point (i.e.p.) to higher pH-values with increasing Na 2SO4 concentration. At the membrane's i.e.p. the observed rejection rates w ere not minimal. For a KCI solution it was shown that there is no preferent ial adsorption of the ions, neither Cl- nor K+, on the membrane surface, wh ich leads to the conclusion that in this case the electrostatic properties of the membrane are only affected by the dissociation of the surface sulfon ic acids. (C)2000 Elsevier Science B.V. All rights reserved.