Impedance spectroscopy and membrane potential analysis of microfiltration membranes. The influence of surface fractality

Abstract

In this work, the proper charge density of a microfiltration membrane has been determined by using two different methods. Firstly, the ionic transport of a KCl solution has been investigated by simultaneous measurements of saline flux and membrane potential (MP) resulting from a concentration gradient through the membrane. A simple model, including all the relevant contributions to the global electrical potential drop, allowed a calculation of transport numbers and membrane charge density. The response of ions inside the membrane to an oscillating electrical potential has been analyzed by impedance spectroscopy (or electrical impedance spectroscopy EIS). A quite simple experimental EIS design allowed, by taking into account MP measurements too, an easy assignation of an equivalent circuit. After a careful analysis of EIS results, it was possible to evaluate the electrical conductivity inside the pores and the charge density. Both were found to be quite similar to the values obtained from MP alone. This agreement of EIS results with the MP ones, that are much simpler to deal with, confirms the accuracy of EIS to study the electrical properties of microfiltration membranes. The influence of electrode roughness and, in our EIS cell, the membrane roughness, on the constant phase element (CPE) of the equivalent circuit has been proved. Within this frame, the roughness fractal dimension of the membrane surface could be determined from EIS measurements. It resulted in fair agreement with the atomic force microscopy (AFM) determination.