Characterization of ion transport in thin films using electrochemical impedance spectroscopy. II: Examination of the polyamide layer of RO membranes

The paper presents an experimental study of ion transport in polyamide selective layer of composite reverse osmosis and nanofiltration membranes by means of electrochemical impedance spectroscopy (EIS). The polyamide films attached to a solid electrode were separated from commercial membranes or synthesized directly on the electrode surface. The experiments employed ionic species of two types: inactive salts and electrochemically (redox) active ion couples. For the former, the electrical resistance of the films, a lump parameter closely related to the salt permeability, and membrane capacitance could be determined. A more comprehensive picture of ion transport could be obtained using redox-active Fe(CN)₆ ions, yielding the individual ion permeability and splitting it to diffusion and partitioning coefficients. The results for different polyamide films indicate that the ion exclusion mechanism in polyamide layer involves both Donnan and non-Donnan mechanisms and estimates could be made for RO membranes regarding the fixed charge content in the membrane (about 10^-5 to 10^-4 M) and the non-Donnan partitioning coefficient (about 10^-4 to 10^-3). These low values apparently pertain to a thin and nearly neutral barrier layer inside the film rather than to the whole polyamide layer.