Experimental analysis of negative salt rejection in nanofiltration membranes

The rejections of mixtures of polyelectrolyte (sodium polyacrylate) and a m onovalent salt (NaCl) were measured as a function of flux for two nanofiltr ation (NF) membranes. These rejections were corrected for concentration pol arization of both salt and polyelectrolyte. The feed concentration ratio of polyelectrolyte to salt ranged from 2 to 20. The extent of reduction in mo novalent salt rejection in the presence of polyelectrolyte could be predict ed based on the Donnan effect after evaluating the unbound fraction of poly electrolyte counterion, phi (P), in independent dialysis experiments. In th e dialysis experiments it was found that beta was 0.32. From this the value of the Donnan partition coefficient, beta was evaluated allowing for the e ffect of concentration polarization of both polyelectrolyte and salt. The c oncentration polarization of polyelectrolyte resulted in beta changing as a function of flux. (C) 2001 Elsevier Science B.V. All rights reserved.