Recent Achievements in the Zeta Potential Analysis of Membranes for Desalination

Surface charge plays a major role in solute rejection and fouling of polymer membranes used for nanofiltration and reverse osmosis processes. Surface charge is accessible through the membrane zeta potential, which has gained severe interest starting in the early 1990s. Since then a variety of flat sheet membranes but also of hollow fiber membranes have been characterized in terms of the surface zeta potential.

However, what has been neglected in most of these studies was the possible influence of membrane porosity on the zeta potential. Furthermore, the majority of membrane zeta potential data was obtained by using a standard aqueous solution with an ionic strength of 0.001-0.01 mol/l. The boundary conditions for zeta potential analysis thus differ significantly from those experienced by NF and RO membranes for seawater or brackish water desalination, where the membranes are exposed to high salinity conditions.

In this contribution, we address the effects of membrane porosity and water salinity on the zeta potential analysis. Recently, evidence for the contribution of the porous membrane body to the zeta potential was confirmed for flat sheet membranes used in microfiltration. We extend the new approach, i.e., the streaming current measurement at varying distances between samples of a flat sheet membrane, to TFC NF and RO membranes. Although the nonwoven support dominates the total thickness of such membranes, we find little evidence for an influence of its porous structure.

For the second topic, we report on the impact of high ionic strengths on the membrane zeta potential. Streaming potential measurements on a TFC RO membrane were conducted in a wide range of salt concentration. Zeta potential calculated from streaming potential was then extrapolated to an environmentally relevant ionic strength (up to 1 mol/l). Results revealed that the membrane zeta potential approaches a finite equilibrium value at high salinity.