Real-time non-destructive monitoring of supported liquid membranes using electrochemical impedance spectroscopy

The degradation of supported liquid membranes (SLMs) due to membrane liquid leaching has been a hurdle for their implementation in resource recovery from waste streams. We present a novel approach utilizing four-electrode mode electrochemical impedance spectroscopy (EIS) for real-time, non-destructive monitoring of SLM degradation, providing detailed analysis of SLM's structural changes. We first showed that SLMs may be depicted as an electrical circuit of resistors and capacitors by EIS. The EIS then revealed that SLM degradation progressed over four different phases as a result of varying rates of thickness and area reduction of the SLM. To demonstrate the effectiveness of EIS-based diagnosis, we prepared SLMs designed for selective medium-chain fatty acid (MCFA) extraction in intact, partially, and completely degraded states, determined by the EIS. The intact SLM showed a high selectivity (∼50) for MCFAs over short-chain fatty acids. The selectivity decreased to ∼22 in the partially degraded SLMs, and less than 1 in the completely degraded SLM, indicating the clear linkage between the degree of degradation and the separation performance. Our four-electrode mode EIS method is expected to serve as a key tool for efficient SLM operation and stable SLM development.