In situ structural and functional characterization of reverse osmosis membranes using electrical impedance spectroscopy

The nascent stages of flux decline and scale formation during the filtration of calcium carbonate scale constituents were monitored in situ as process recovery increased from 0 to 87% using Electrical Impedance Spectroscopy (EIS). Spectra generated by injecting alternating current at frequencies between 1 Hz and 106 showed a decrease in impedance with increasing recovery. Maxwell–Wagner models were fitted to the spectra that resolved five electrically distinct layers that occur in series from the feed side to the permeate side of the membrane as well as concentration/diffusion polarization phenomenon. The models were consistent with the observed changes in electrical conductivity of the permeate which increased from 0.004 to 0.02 Sm−1 as salts accumulated on the membrane surface with increasing recovery. Furthermore, the fitted EIS measurements verified the orders of magnitude of literature values for the thickness of the polyamide layer (ca. 100 nm), polyethersulfone layer (ca. 50 μm) and polyester support (120–150 μm) that constitute the thin film composite reverse osmosis membranes. Temporal changes in conductance of these layers during filtration were greatest for the molecular polyamide/coating layer, a zone positioned immediately above the polyamide active layer which changed from 4.9 to 22 Sm−1 and was attributed to the accumulation of salts on the surface of the membrane. The results indicate that the dielectric model fitted to the EIS spectra characterizes the various structural elements of a reverse osmosis membrane and the functional changes that occur in situ at the membrane surface during filtration.