Microscopical characterizations of nanofiltration membranes for the removal of nickel ions from aqueous solution

The nanofiltration (NF) process is electrostatically governed and the surface free energy plays a key role in the separation of particulates, macromolecules, and dissolved ionic species. Streaming potential measurement and the surface charge mapping by Kelvin probe atomic force mircoscopy (AFM) have been carried out. Forces of interaction near the surface of nanofiltration membranes were further studied by a force spectroscopy using atomic force microscopy. The two membranes used are more negatively charged at high pH values; hence the higher the solution chemistry, the higher and faster will be adhesion of ions on the surface of the nanofiltration membranes. It was observed that the three acquired signals from non-contact AFM (contact potential difference, amplitude and phase) were rigorously connected to the surface structure of the nanofiltration membranes. In addition to the surface structure (roughness), electrostatic interactions can also enhance initial particle adhesion to surfaces of nanofiltration membranes. The performance of the NF membranes was further investigated for the removal of nickel ions from aqueous solution, and the results were correlated to the mechanical responses of the nanofiltration membranes obtained from AFM and the streaming potential measurement.