IL EXTRACTIONS OF METAL IONS The hydrophobic character of some ILs allows them

Il extractions of metal ions the hydrophobic

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IL EXTRACTIONS OF METAL IONS The hydrophobic character of some ILs allows them to extract hydrophobic compounds in biphasic separations. Metal cations tend to stay in the aqueous solution being hydrated. Therefore, in order to remove metal ions from the aqueous phase into hydrophobic ILs, extractants (or chelators, ligands) are normally needed to form complexes to increase the metal’s hydrophobicity. 18–20 The solvation of crown-ether complexes in ILs is more thermodynamically favored than conventional organic solvents. 19 This is a key advantage of using ILs for extractions of metal ions. Table 1 summarizes some representative examples of IL extractions of metal ions, including alkali, alkaline earth, heavy, and radioactive metals. To achieve optimal extraction efficiency, there are several controlling factors: (1) Varying the structure of an IL (especially the side chain of its cation) to change its hydrophobicity can improve the partition coefficients of metal ions. 18 , 21 (2) Types of extractants (such as crown ethers) can be modified to achieve optimal selectivity for a specific application. 22 Correspondence to: Hua Zhao, Chemistry Program, Department of Natural Sciences and Mathematics, Savannah State University, Savannah, GA 31404, USA E-mail: [email protected] ( Received 10 February 2005; revised version received 30 March 2005; accepted 7 April 2005 ) Published online 26 May 2005 2005 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2005/$30.00 1089
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H Zhao, S Xia, P Ma Table 1. Examples of IL extractions of metal ions Substances IL Extractant/ligand/ metal chelator Reference Alkali metals Li + , Na + , K + , Rb + , Cs + [C n MIM][PF 6 ] ( n = 4–9 ) DC18C6 21,37 Na + , Cs + [C n MIM][PF 6 ] ( n = 4, 6, 8) 18C6, DC18C6, Dtb18C6 18 Cs + [C n MIM][Tf 2 N] ( n = 2, 3, 4, 6, 8) BOBCalixC6 38 Na + , K + , Cs + [C n MIM][Tf 2 N] ( n = 2, 4, 6, 8) DC18C6, N -alkyl aza-18-crown-6 ethers 39 Alkaline earth metals Mg 2 + , Ca 2 + , Sr 2 + , Ba 2 + [C n MIM][PF 6 ] ( n = 4–9 ) DC18C6 37 Sr 2 + [C n MIM][PF 6 ] ( n = 4, 6, 8) 18C6, DC18C6, Dtb18C6 18 Sr 2 + [R 1 R 2 MeIM][PF 6 ] , [R 1 R 2 MeIM][Tf 2 N] DC18C6 19 Sr 2 + [C n MIM][Tf 2 N] ( n = 2, 4, 6, 8) DC18C6, N -alkyl aza-18-crown-6 ethers 39 Heavy and radioactive metals Pb 2 + [C n MIM][PF 6 ] ( n = 4–9 ) DC18C6 37 Cu 2 + , Ag + , Pb 2 + , Zn 2 + , Cd 2 + , Hg 2 + [C 4 MIM][PF 6 ] Dithizone 23 Cd 2 + , Co 2 + , Ni 2 + , Fe 3 + , Hg 2 + [C 4 MIM][PF 6 ] , [C 6 MIM][PF 6 ] PAN, TAN 20 Ag + [C n MIM][PF 6 ] ( n = 4, 6, 8) Calyx[4]arene-bearing pyridine 40 Cu 2 + , Cr 6 + , Zn 2 + [C n MIM][BF 4 ] ( n = 1, 3, 6, 8, 10) [C n MIM][PF 6 ] ( n = 6, 10) None 41 Hg 2 + , Cd 2 + TSILs None 28,35 Lanthanides (Nd 3 + , La 3 + , Er 3 + , Ce 3 + , Sm 3 + , Eu 3 + , Gd 3 + , Ho 3 + ) [C 4 MIM][PF 6 ] CMPO 32 Actinides (Th 4 + , U 2 + , Pu 4 + ) [C 4 MIM][PF 6 ], [C 4 MIM][NO 3 ], Dtb18C6, CMPO, TBP 29,34 Others Al 3 + [C 4 MIM][Tf 2 N] , [C 6 MIM][PF 6 ] [C 8 MIM][PF 6 ] n/a 42,43 Note: C n MIM = 1-alkyl-3-methylimidazolium; DC18C6 = dicyclohexano-18-crown-6; 18C6 = 18-crown-6; Dtb18C6 = 4,4 -(5 )-di-( tert -butylcyc- lohexano)-18-crown-6; BOBCalixC6 = calix[4]arene-bis( tert -octylbenzo-crown-6); Tf 2 N = bis[(trifluoromethyl)sulfonyl]amide; R 1 R 2 MeIM = 1-R 1 -2- R 2 -3-methylimidazolium (R 1 = Bu, Et, or Pr; R 2 = H, or Me); PAN = 1-(2-pyridylazo)-2-naphthol; TAN = 1-(2-thiazolylazo)-2-naphthol; CMPO = octyl(phenyl)- N , N -diisobutylcarbamoylmethyl phosphine oxide; TBP = tri- n -butylphosphate.
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