Carrier-facilitated extraction

Carrier-facilitated extraction may be described by the following generic stoichiometric equation ... 

The partition coefficient technically refers to the quotient of the solute concentration in the organic phase divided by the concentration of un-ionized solute in the aqueous phase, whereas the distribution ratio counts both ionized and un-ionized solute in the aqueous phase. This distinction is important in systems where the extracted species is present as a neutral and an ionized species in the aqueous phase and only the neutral species can be extracted into the organic phase. In carrier-facilitated extraction, the distinction is less important as solutes must be complexed by the extractant or form an ion-pair with it prior to extraction into the organic phase the solute is often in its ionized form prior to complexation. 

In the case of carrier-facilitated extraction, the receiving phase is generally only permeable to the solute of interest after complexation or ion-pair formation with the extractant. This means that in many SX processes, the reaction of the solute with the extractant is one of the most important factors determining the extractive performance of the system. 

Figure 10.2. Schematic description of carrier-facilitated extraction and transport of a cation (M" ") across a PIM with a cation exchange carrier (Nghiem et at., 2006, with permission from Elsevier).

Carrier facilitated transport processes often achieve spectacular separations between closely related species because of the selectivity of the carriers. However, no coupled transport process has advanced to the commercial stage despite a steady stream of papers in the academic literature. The instability of the membranes is a major technical hurdle, but another issue has been the marginal improvements in economics offered by coupled transport processes over conventional technology such as solvent extraction or ion exchange. Major breakthroughs in performance are required to make coupled transport technology commercially competitive. 

FIGURE 37 Mechanisms of carrier-facilitated immobilized liquid membrane extraction, also referred to as coupled transport. The species, R, refers to the carrier component responsible for complexation. 

Carrier-facilitated transport is used successfully to extract various organic and inorganic substances from a feed mixture in liquid membranes. Liquid membranes are employed as bulk liquid membranes, emulsion liquid membranes, and supported liquid membranes. 

Type 2 facilitation is also known as carrier facilitated transport, since a carrier compound, that is, an extractant or complexing agent, solubilized in the organic phase is used to assist transfer across the membrane. In this simation, the solute of interest reacts with the carrier to form a complex that is only soluble in the membrane phase. The solute is de-complexed by a stripping solution contained in the internal phase. An example of such a process is the removal of a metal ion such as copper or zinc from wastewater by the extractant DEHPA (di-2-ethyUiexyl phosphoric acid, represented as HE) as shown in Figure 25.2. In this case, the carrier also enhances the selectivity as most extractants are specifically designed to extract particular metal ions... 

Carrier (TOPO)-mediated transport of uranium(Vl) has been studied by Akiba and Hashrmoto [94] who have observed that uranium was extracted in the liquid membrane as U02(N03)2 2TOPO and stripped into the carbonate solutions as U02(C03)3. Using TOPO and HDEHP mixmre, liquid membrane technique was apphed for the recovery of uranium from WPPA [95]. Carrier-facilitated Pu(IV) pertraction through an SLM was standardized for its decontamination from oxalate wastes employing a commercially available Cyanex-923 (TOPO analog) in dodecane as the receptor [96,97]. More than 95% of plutonium could be easily recovered from Pu oxalate wastes solution during Pu reconversion operations. 

Joshi P, Joshi N, Sharma U. Extraction and carrier-facilitated transport of amino acids using synthetic non-cyclic receptors through bulk hquid membrane. Indian J Biochem Biophys 2006 43 323-326. 

Similar results were obtained by Volkel el al.M who also used LIX 64N as the carrier. Their extraction rates were somewhat slower than those reported by Cahn and coworkers, probably because of greater membrane viscosity. Volkel and coworkers also developed a fairly complicated mathematical model for membrane transport from which Ibey deierminnd mass transfer parameters. These ranged from I X 10 3 to I X I O 1 s 1. These are comperable to the values obtained for the extraction ofpbenoi (type 1 facilitated transfer in which no carrier is used) by the same group.16... 

Facilitated mass transfer Similarly to LLE, the selectivity and efficiency of the liquid membrane separation process can be considerably improved if a suitable extractant with a high selectivity for the analyte of interest is used. This extractant, often referred to as the carrier, facilitates the mass transfer of the analyte between the feed and receiver solutions. The liquid membrane phase in this case usually consists of a suitable liquid extractant or an extractant dissolved in an organic solvent (diluent). The extractant facilitates the transport of the analyte from the feed phase to the liquid membrane phase by chemically interacting with it. This interaction leads to the selective extraction of the analyte into the... 

Carrier-facilitated membrane extraction and transport therefore can be viewed as a combination of solvent extraction and membrane diffusion processes and the extent to which each process contributes to the overall process is an area of particular importance to PIM technology. Figure 10.2 illustrates the principles of carrier-facilitated membrane separation of a cation (M ). In this example the stripping reagent (X+) is in sufficiently high concentration to allow the target cation to be transported quantitatively from the source to the receiving solution. 

Blends (lactose carrier) Advantages Powder can be easily extracted from packaging Inclusion of the carrier phase usually facilitates dispersion... 

Quantitative measurement of diffusional uptake and carrier-mediated transport of nutrients and drugs in experimental animals was greatly facilitated with the introduction of Olden dorfs brain uptake index (BUI) [42].Test and reference tracers are injected as an intraarterial bolus into the carotid artery of the anaesthetized animal. After 5 s the animal is killed and the brain is removed for radioactivity counting. This method measures the ratio of the unidirectional brain extraction, E, of the test substance and of the reference ([ H]-water, [ " C]-butanol), which are labelled with different isotopes, during a single passage through the brain capillary bed ... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

After concentration of the extract by microdistillation [25] or by special procedures [26] to facilitate the identification of the odorants, an aliquot is separated by high-resolution GC and the effluent is split into a flame ionisation detector (FID) and a sniffing port [27]. The positions of the odorants in the gas chromatogram are assessed by sniffing the carrier gas as it flows from the port. This procedure is denoted GC-O. 

In SLM extraction, the transport mechanism is influenced primarily by the chemical characteristics of the analytes to be extracted and the organic liquid in the membrane into which the analytes will interact and diffuse. Analyte solubility in the membrane and its partition coefficient will have the main impact on separation and enrichment. Analyte transport in SLM extraction can be substantially categorized into two major types one is diffusive transport (or simple permeation) and the other covers facilitated transport (or carrier-mediated transport).73... 

---