Liquid membrane separation combines the solvent extraction and stripping processes (re-extraction) in a single step.

This entry has the objective of introducing the reader to the basic definitions of the liquid membrane field, with classification.

The term liquid membrane transport includes processes incorporating liquid-liquid extraction (LLX) and membrane separation in one continuously operating device. It utilizes an extracting reagent solution, immiscible with water, stagnant or flowing between two aqueous solutions (or gases), the source or feed, and receiving or strip phases. In most cases, the source and receiving phases are aqueous, and the membrane is organic, but the reverse configuration can also be used. A polymeric or inorganic microporous support (membrane) may be used as bearer (as in SLM) or barrier (as in BLM technologies) or not used, as in ELM and layered BLM (see respective entries: SLM, Emulsion Liquid Membrane (ELM), Silver Recovery by Bulk Liquid Membrane (BLM)).

The great potential for energy saving, low capital and operating cost, and the possibility to use expensive extractants, due to the small amounts of the membrane phase, make LMs an area deserving special attention. Liquid membrane systems are being studied extensively by researchers in such fields as analytical, inorganic, and organic chemistry; chemical engineering, biotechnology, and biomedical engineering; and wastewater treatment. Research and development activities within these disciplines involve diverse applications of liquid membrane technology, such as gas separations, recovery of valued or toxic metals, removal of organic compounds, development of sensing devices, recovery of fermentation products, and some other biological systems.

The general properties of liquid membrane systems have been a subject of extensive theoretical and experimental studies. Some general characteristics of LM processes are as follows:

  1. 1.

    Liquid membrane separation is a rate process, and the separation occurs due to a chemical potential gradient, not by equilibrium between phases.

  2. 2.

    LM is defined based on its function, not the material used in fabrication.

Permeation is a general term for the concentration-driven membrane-based mass transport process. Differences in the permeability produce a separation between solutes at constant driving force. Because the diffusion coefficients in liquids are typically orders of magnitude higher than in polymers, a larger flux can be obtained with liquid membranes. Application of a pressure difference, an electric field, or temperature considerably intensifies the process.

There are several different directions in LM separation classifications: according to module design configurations (see SLM, Emulsion Liquid Membrane (ELM), Silver Recovery by Bulk Liquid Membrane (BLM) entries), according to transport mechanisms (see Transport Mechanisms with Liquid Membranes), according to applications, according to carrier type, and according to membrane support type. Below, these types of classifications are described and discussed briefly.

According to configuration definition, three groups of liquid membranes are usually considered (see Fig. 1): bulk (BLM), supported or immobilized (SLM or ILM), and emulsion (ELM) liquid membrane transport. Some authors add to these definitions polymeric inclusion membranes, gel membranes, and dual-module hollow-fiber membranes, but, to my opinion, the first two types are the modifications of the SLM, and the third is the modification of BLM.

Liquid Membrane Separation, Fig. 1
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Three configurations of liquid membrane systems: bulk (BLM), supported (immobilized) (SLM or ILM), and emulsion (ELM). F is the source or feed phase, E is the liquid membrane, and R is the receiving phase

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According to the transport mechanisms, the LM techniques may be divided into simple transport, facilitated or carrier-mediated transport, coupled counter- or cotransport, and active transport.

According to applications, the LM techniques may be divided into (1) metal separation-concentration, (2) biotechnological product recovery-separation, (3) pharmaceutical product recovery-separation, (4) organic compound separation and organic pollutant recovery from wastewaters, (5) gas separations, (6) fermentation or enzymatic conversion-recovery-separation (bioreactors), (7) analytical applications, and (8) wastewater treatment including biodegradation-separation techniques.

Classification according to carrier type is as follows: (1) water-immiscible, organic carriers, (2) water-soluble polymers, (3) electrostatic, ion-exchange carriers, and (4) neutral, but polarizable carriers.

Classification according to membrane support type is as follows: (1) neutral hydrophobic, hydrophilic membranes, (2) charged (ion-exchange) membranes, (3) flat sheet, spiral module membranes, (4) hollow-fiber membranes, and (5) capillary hollow-fiber membranes.

Module design configurations are used as a rule as basic classification.

Practically, there are a lot of opportunities for liquid membrane separation processes in many areas of industry. The most interesting developments for industrial membrane technologies are related to the possibility of integrating various membrane operations in the same industrial cycle, with overall important benefits in terms of product quality and plant compactness.