Macrocycle carriers for separation of metal ions in liquid a review membrane processes

A review with more than 60 references covering macrocycle and macromolecular compounds, i.e. crown ethers, ionizable lariat ethers, calixarenes, calix crowns, as well as macrocycle polymers (for example, cyclodextrins), and macromolecular compounds, i.e. acyclic polyethers used as ion carriers in transport across liquid membranes for metal ions selective removal will be presented. The recent literature data as well as our own research results (published in the last 8 years) on metal ions removal and separation will be summarized. Recent developments of the lariat ethers application as ion carriers for alkali metal and alkaline earth metal cations, as well as heavy metal ions removal and separation will be presented in this review. The effect of structural studies of crown ring size variation, lipophilic and acidic group attachments for lariat ethers with carboxylic and other acidic groups on the selectivity and efficiency of metal cations transport will also be presented. Also, the effect of a counter anion presence will be performed. The examples of metal ions selective removal using various other ion carriers such as calixarenes, calixcrown ethers, cyclodextrins and their polymers as well as acyclic polyethers will also be shown.

One of the most important factors, which have been found to strongly influence the liquid membrane’s performance, is the ion carrier presence in the membrane for selective metal ions transportation. As we have documented, there is a respectable number of published papers in the last 8 years (about 60), which concern the application of macrocycle and macromolecular compounds as ion carriers in liquid membranes. The used macrocycle and macromolecular compounds were found to be efficient and selective ion carriers for removal of cationic and anionic metal species, including alkali metal cations, alkaline earth metal cations, and heavy metal cations and anions. The addition of large anions, such as dinonylnaphtalenesulfonic one causes the high increase of metal cation fluxes in membrane transport. The importance of macrocycles as the novel generation of ion carriers for metal ions was markedly enhanced by the introduction of lariat ethers and calixarenes, which bear pendant proton-ionizable groups, such as carboxylic or N -(X)sulfonyl carbamoyl functional groups. Attaching a protonionizable sidearm to the crown ether ring and other macrocycle or macromolecular structure eliminate the need to transfer aqueous phase anions into the organic phase. Another advantage of these lariat ethers as carriers is coupling of metal ions transport with back transport of H' ions and thus a pH gradient provides the potential for metal ions transport. In the lariat ether and similar macrocycle molecules, lipophilic group can be attached to the functional side arm, or to the other place. To develop such macrocycle compounds systematic variation of the proton-ionizable lariat ethers structure include ring size, length of side arm, attachment site of the lipophilic group, rigidity of the polyether ring, and the identity of the proton-ionizable group must be developed. Although the used macrocyclic reagents are expensive, their employing as ion carriers in liquid membranes is more feasible in comparison with the liquid extraction since the smaller reagent is required and is used in repeated transport experiments.