Liquid membrane extraction requirements

It is valuable to compare liquid membrane extraction with conventional solvent extraction in which the required product first partitions from an aqueous feed into an... 

Two-phase liquid membrane extraction systems can be seen as a variant of SLM, where both the membrane (located in the pores of a hydrophobic porous membrane support) and the acceptor phases consist of an organic solvent. This is mainly suitable for hydrophobic analytes that are non-dissociable and non-charged. These compounds are easily extracted from water to an organic solvent, but they cannot be back-extracted into a second aqueous phase as required by the SLM approach. 

Pertraction can be regarded as a liquid—liquid extraction process in which a membrane is placed between the two phases. Pertraction is a membrane process based on the same separation mechanism as extraction, where both extraction and stripping of the solute are realized in one unit. Membrane extraction requires the installation of a membrane area, which separates extracting liquid from the extractant. The major advantage of the pertraction process is that dispersion of the extractant in the solvent phase is unnecessary. The pertraction process was successfully used for the recovery of butanol during batch and fed-batch fermentations (Groot et al., 1990). However, the pertraction possess has lower mass transfer coefficients compared with liquid-liquid extraction. 

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

In 2003, Smith reviewed newer sample preparation techniques, including pressurized liquid extraction, solid phase microextractions, membrane extraction, and headspace analysis. Most of these techniques aim to reduce the amount of sample and solvent required for efficient extraction. 

The largest research effort in extraction kinetics is likely to be in the development of solvent extraction related techniques, such as various versions of liquid chromatography, liquid membranes, etc. These techniques require a detailed knowledge of the kinetics of the system to predict the degree of separation. 

Liquid membranes of the water-in-oil emulsion type have been extensively investigated for their applications in separation and purification procedures [6.38]. They could also allow extraction of toxic species from biological fluids and regeneration of dialysates or ultrafiltrates, as required for artificial kidneys. The substrates would diffuse through the liquid membrane and be trapped in the dispersed aqueous phase of the emulsion. Thus, the selective elimination of phosphate ions in the presence of chloride was achieved using a bis-quaternary ammonium carrier dissolved in the membrane phase of an emulsion whose internal aqueous phase contained calcium chloride leading to phosphate-chloride exchange and internal precipitation of calcium phosphate [6.1]. 

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