Macrocyclic carriers for liquid membranes

A great deal of research in recent years has examined methods to separate and recover metal cations from waste streeims and leached ore deposits. The limited amount of natural resources and the toxicity of some metals provide compelling reasons for this research. One method under study is the use of macrocycles as cation carriers in liquid membrane systems(1-6). The emulsion liquid membrane (ELM) has been particularly effective in macrocycle-medlated metal cation separation experiments. 

WaUcowiak, W. and Kozlowski, C.A. 2009. Macrocycle carriers for separation of metal ions in liquid membrane processes A review. Desalination 240 186-197. 

Natural receptors, such as valinomycin or beauverin, can be used as carriers. Also synthetic receptors have been developed. The first generation of synthetic carriers were the crown ether macrocycles, which selectively bind alkali metal ions 4). Since their discovery in 1967, many other types of macrocycles have been synthesized and used for the selective recognition of neutral, charged, or zwitter-ionic species. Many have been used as carriers in liquid membranes (5). 

In another study, dicyclohexano-18-crown-6 (DC18C6) was shown to be an excellent carrier in liquid membranes due to its high selectivity for both U(VI) (77) and Pu (72) even in the presence of several other undesirable radiotoxic elements Table 1). Maximum transport through the macrocycle-based BLM and SLM systems (0.2 M DC18C6 in toluene as the membrane phase) was attained from a 3 M nitric acid feed solution for Pu(IV) and 7 M nitric acid for U(VI). The maximum fluxes for Pu(IV) and U(VI) were 1.6 X 10 and 2.9 X 10 mol/mVs, respectively (77,72). Enrichment factors of 6 or greater were readily achieved by proper manipulation of the feed solution/strip solution volume ratios (72). 

Many other cyclic and noncyclic organic carriers with remarkable ion selectivities have been used successfiilly as active hosts of various liquid membrane electrodes. These include the 14-crown-4-ether for lithium (30) 16-crown-5 derivatives for sodium bis-benzo-18-crown-6 ether for cesium the ionophore ETH 1001 [(R,R)-AA -bisd l-ethoxycarbonyl)undecyl-A,yVl-4,5-tctramcthyl-3,6-dioxaoctancdiamide] for calcium the natural macrocyclics nonactin and monensin for ammonia and sodium (31), respectively the ionophore ETH 1117 for magnesium calixarene derivatives for sodium (32) and macrocyclic thioethers for mercury and silver (33). 

Two or more macrocyclic entities can be linked in several different ways, as shown by (144) and (145).241 Linked macrocyclic systems can bind simultaneously two or more metal ions, giving rise to complexes with unusual electronic, catalytic, and/or redox properties.242 Macrocycles incorporating phenanthroline moieties (146) are useful sensing agents for ion-selective electrodes and fluorimetry, and suitable carriers for selective ion transport in liquid-membrane systems.243,244... 

This volume Is divided Into three sections theory, carrier chemistry, and applications. The theory section Includes chapters which thoroughly describe the theory and analysis of various liquid membrane types and configurations (107-110) The carrier chemistry section contains two articles on the use of macrocycles for cation separations (111-112). The applications section begins with a survey article which thoroughly reviews the liquid membrane applications In the literature and discusses both potential and commercial aspects of liquid membrane technology. The remaining articles discuss both gas phase (113-115) and liquid phase transport (116-117). 

Carrier Chemistry. The use of structurally modified macrocycllc polyethers (crown ethers) as CcU rlers In bulk, emulsion, and Immobilized liquid membranes Is the subject of the chapter by Bartsch et al. (111). They discuss the use of lonlzable crown ethers for the coupled transport of alkali metal cations. The lonlzable carboxylic and phosphonlc acid groups on the macrocycles eliminate the need for an anion to accompany the catlon-macrocycle complex across the liquid membrcuie or for an auxiliary complexlng agent In the receiving phase. The influence of carrier structure on the selectivity and performance of competitive alkali metal transport across several kinds of liquid membranes Is presented. 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

Wlakowiak and Kozlowski " recently reviewed the application of macrocycle carriers including crown ethers, calixarenes, calixcrowns, and CDs in liquid membrane processes. The role of macrocyclic ligands as ion carriers for cations such as alkali and alkaline metals, and heavy metals such as Zn ", Cd +, Hg + and Pb " " was summarized. Mutihac reviewed the calixarenes as membrane... 

When acidic macrocycles are used as membrane carriers of cations, the ionized macrocycle serves as the counterion in the ttansport process. For example, Bartsch s group has synthesized and studied cation separations using a series of acidic lariat ethers in liquid membranes. Lariat ethers are crown ether derivatives with one or more side functional groups. The ligand 26 in Figure 23 was used as a carrier in BLMs to transport alkali metal cations. Several membrane solvents such as dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,1-trichloroehane,... 

Transport Selectivity. For separation purposes, the selectivity of transport by macrocyclic carriers through supported liquid membranes has been investigated extensively. The transport selectivity of a carrier towards two different cations M/ and is obtained by competition experiments in which both M", and were present in the receiving phase. 

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