Experimental verification on stability analysis of supported-liquid-membrane separation of metal ions by in-situ electrical impedance spectroscopy

Background: Supported liquid membrane (SLM) is a very promising method for the recovery and separation of valuable or toxic species from aqueous streams; however, stability is always a critical issue in practice. It is highly desired to develop a non-destructive technique to real-time monitor or detect the stability of an SLM process. Methods: The stability of an SLM processes was real-time analyzed by electrical impedance spectroscopy (EIS). The separation of equimolar Zn(II) and Cu(II) from sulfate solution through an SLM containing di(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene was selected as the model system owing to the established knowledge of extraction chemistry. Experiments were performed with two types of membrane supports (polycarbonate and poly(vinylidene) fluoride), different D2EHPA concentrations in the organic phase (0.001–0.1 mol/L), pH values of the feed (3.0–5.0) and strip (1.0–3.0) phases, and stirring speeds (0–200 rpm). Correlations between the membrane impedances analyzed according to a proposed equivalent circuit diagram and the separation factors of Zn²⁺ over Cu²⁺ measured by SLMs were done to verify the progress of the loss of membrane liquid. Significant findings: Under the conditions studied, the variations of membrane impedances and separation factors with time using both types of membrane support followed similar trends in most cases. A stability factor of a SLM process based on membrane impedance data was defined, which enables us to predict its relative stability. The present results also demonstrated that all parameters including membrane type, carrier concentration, aqueous pH values, and stirring speed influenced the transport and separation of Zn²⁺ over Cu²⁺ and SLM stability.