Instability Mechanisms of Supported Liquid Membrane Processes: Establishment and Applications of Real-Time Analysis Technique

The recovery of heavy metal resources from industrial processes and effluents has recently received much attention due to the global crisis of energy and environment. The main techniques for the treatment of metallurgical wastewaters, recovery of valuable metals, as well as separation and purification of rare earth metals include the adsorption by resins or activated carbons, ion exchange, electrolysis, chemical precipitation, solvent extraction, and displacement with iron filing. All of them have the drawbacks of low quality of discharged water, secondary pollution, and large equipment size. Supported liquid membrane (SLM), a highly efficient separation method that combines membrane materials and solvent extraction, has the advantages of low cost (the need of few membrane phase only), low energy consumption, and low mass-transfer resistance. It is the best method to solve the damage of heavy metals to the environments. In the energy fields, SLM is able to recover Li(I) ion without the use of large amount of extractants. However, until now no commercial SLM processes/plants are established and operated. This is because the carriers and organic solvents readily lose from the pores of membrane supports, leading to the decline of SLM flux and the decrease in selectivity. That is, instability inevitably occurs during SLM processes. To solve this problem encountered in SLM processes, it is necessary to deeply understand the instability mechanisms. A real-time detection technique will be highly desired if possible. In this three-year project, electrochemical impedance spectroscopy (EIS) is used for this purpose due to the reliability of the measured results and the feasibility of online monitoring. Phenol, Cu(II) ion, and/or lactic acid are selected as the solutes to be transferred and separated. In the first year, the traditional SLM processes where the membrane phase contains carrier and organic solvent are focused. The SLM containing ionic liquids (the so-called supported ionic liquid membrane) will be studied in the second year because ionic liquid is a green solvent. Due to the understanding of instability mechanisms in SLM processes in the first two years, the modification of membrane supports by plasma polymerization surface coating will be studied to improve the stability. By measuring the variation of the capacitance and conductance of the membrane plus feed and strip phases with frequency, it is possible to deduce the membrane resistances in SLM system. Synthetic conditions including the addition of salts and surfactants in the feed or strip phase are conducted if necessary. Moreover, the variations of mass-transfer coefficient with time will be calculated from the change of solute concentration in the feed phase with time. Finally, the correlation between the measured membrane resistances and the calculated mass-transfer coefficients will be done. It is expected that the present project allows us to realize the application potentials and limitations of EIS techniques in real-time detection of the instability during SLM processes.

Project ID:PB10401-0605
External Project ID:NSC102-2221-E182-076-MY3