Characterization of active and porous sublayers of a composite reverse osmosis membrane by impedance spectroscopy, streaming and membrane potentials, salt diffusion and X-ray photoelectron spectroscopy measurements

Abstract

Electrokinetic, structural and surface chemical characterizations of both active and porous sublayers of a composite (polyamide/polysulfone) reverse osmosis membrane were carried out by different techniques: (i) electrochemical impedance spectroscopy (EIS) allows both electrical and geometrical characterizations using equivalent circuits as models; (ii) surface chemical composition of the active layer was determined by X-ray photoelectron spectroscopy (XPS); (iii) salt diffusion, tangential streaming potential and membrane potential measurements allow the determination of different transport and electrokinetic parameters such as salt and ionic permeabilities, zeta potential and surface charge density. Electrical and electrokinetic parameters were determined with the membrane in contact with NaCl solutions at different concentrations, although the influence of pH and the use of the membrane on zeta potential values was also studied. Changes in the electrokinetic parameters as a result of the membrane use were obtained and, according to XPS results, they can be attributed to the hydratation of the polyamide active layer. For comparison, electrochemical measurements with a polysulfone ultrafiltration membrane similar to the porous sublayer of the composite RO membrane were also carried out. Good agreement was obtained for the electrical resistance and ionic permeabilities determined for both the ultrafiltration membrane and the porous sublayer of the RO membrane, which confirms the two-layer model assumed for composite membranes.