Supported liquid membranes organic solvents

Several manufacturers introduced products amenable for this solid-supported LLE and for supported liquid extraction (SLE). The most common support material is high-purity diatomaceous earth. Table 1.8 lists some commercial products and their suppliers. The most widely investigated membrane-based format is the supported liquid membrane (SLM) on a polymeric (usually polypropylene) porous hollow fiber. The tubular polypropylene fiber (short length, 5 to 10 cm) is dipped into an organic solvent such as nitrophenyl octylether or 1-octanol so that the liquid diffuses into the pores on the fiber wall. This liquid serves as the extraction solvent when the coated fiber is dipped... 

The principle of a three-phase membrane extraction is illustrated in Figure 1.28. An organic solvent is immobilized in the pores of a porous polymeric support consisting of a flat filter disc or a hollow fiber-shaped material. This supported liquid membrane (SLM) is formed by treating the support material with an organic solvent that diffuses into its pores. The SLM separates an aqueous... 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

Membrane Techniques The interest in membrane techniques for sample preparation arose in the 1980s. Extraction selectivity makes membrane techniques an alternative to the typical sample enrichment methods of the 1990s. Different membrane systems were designed and introduced into analytical practice some more prominent examples are polymeric membrane extraction (PME), microporous membrane liquid-liquid extraction (MMLLE), and supported liquid membrane extraction (SEME) [106, 107]. Membrane-assisted solvent extraction (MASE) coupled with GC-MS is another example of a system that allows analysis of organic pollutants in environmental samples [108-111] ... 

Table 3.4 Stability of the supported liquid membranes as a function of the kind of counterion and of the type of organic solvent...

Supported liquid membranes (SLMs) based on organic solvents are usually unstable... 

The stability of a SILM based on [bmim ][BF ] supported in a nylon membrane has been also analysed in other organic solvents, such as n-hexane//eri-butyl methyl ether and n-hexane/dimethyl sulphoxide [29]. The SEM-EDX study of the membranes after continnons operation showed that the stability of the supported liquid membrane increases with the decrease of the polarity of the solvent used. 

Figure 3 A supported liquid membrane device. A solid microporous sheet (e.g., polypropylene) is soaked in the membrane phase until its pores are saturated with the membrane phase. Usually, the membrane phase is an organic solvent and the microporous sheet is a hydrophobic substance, so the membrane fluid is readily retained in the pores by capillary action. Now this supported liquid membrane can be placed between the feed and receiving phases, and the same mechanism of transport as presented in Figs. 1 and 2 applies.

Indeed, the tuneable properties of ILs associated with their environment-friendly perception have increased their investigation as alternative reaction media to replace traditional organic solvents in organic synthesis [11-13], catalytic reactions [12-16], electrochemical applications [17-19], biochemistry [20-24], and material engineering [25], It has also been reported that ILs can be used in extraction and as liquid phase in supported liquid membranes (SLMs) for the separation and recovery of organic compounds, metals, and gases [26-31]. 

Various substances such as amino acids, organic acids, NaOH, NaCl, carbon dioxide, oxygen, metals, and various ions, such as Cd(II), Cu(II), Co(II), and Fe(III), can be separated by using suitable carrier agents in liquid or solid composite membranes. Liquid membranes behave like double liquid-liquid extraction systems where the usage of organic solvent is minimized. Such devices are generally prepared as bulk liquid, emulsion liquid, and supported liquid membranes. 

Depending on their structure, symmetric membranes can be catalogued as porous and non-porous or dense (polymeric swollen-network), while asymmetric membranes for desalting applications (NF and RO, basically) consist of a dense and thin active layer and a thick porous sublayer for mechanical stability (usually an UF membrane). Moreover, supported liquid membranes (SLMs), and aetivated membranes (AMs), which basically consist in the immobilization of speeifie agents (organic solvent, carrier or ionic liquid at room temperature) in the pores/structure of a support membrane, have been developed for selective separation of valuable/contaminant compoimds [8-11]. 

In liquid-phase microextraction (LPME), a liquid membrane is used to enrich and isolate analytes from a complex sample. The liquid membrane, which is immiscible with water and the sample matrix, is immobilized in the pores of a porous hollow fiber. Such a liquid membrane is referred to as a supported liquid membrane (S LM). Immobilization of the SLM is achieved by simply dipping the hollow fiber in an organic solvent allowing the pores to be filled. Figure 9.11 shows a schematic representation of LPME. 

Supported Liquid Membranes. Supported liquid membranes (SLMs) consist of a hydrophobic, porous plastic sheet or hollow fiber (usually polypropylene or polysulfone). The pores are filled with an organic solvent in which the carrier is dissolved. This membrane separates the aqueous source and receiving phases (Figure 8c). The liquid-containing pores allow transport of the target species via the dissolved carrier, while the plastic sheet offers support for the liquid membrane. Pore sizes generally range from 0.02 to 1.0 pm. 

Supported liquid membrane extraction techniques employ either two or three phases, with simultaneous forward- and back-extraction in the latter configuration. The aqueous sample phase is separated from the bulk organic or an aqueous receiver phase by a porous polymer membrane, in the form of either a flat sheet or a hollow fiber that has been impregnated with the organic solvent phase. The sample phase is continuously pumped, the receiver phase may be stagnant or pumped, and the organic phase in the membrane pores is stagnant and reusable [8-10]. 

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