Metal extraction using micellar

Cloud point extraction, using a surfactant to form micellar structures which associate with metal chelates and achieving phase separation through elevation of temperature, has also been evaluated for analysis of noble metals in digested biological samples (da Silva et al. 2001). 

An efficient UF recovery of Am from its low-concentration solutions can be achieved using micellar solution of Tergitol + 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (H2A2). Almost complete recovery of Am(III) is accomplished using SDS micellar solution at and above pH 3. Total back extraction of Am(lll) from micellar pseudophase is achieved by adjusting the acidity of metal-loaded... 

The metal ion can conveniently be recovered from the acid micellar solution by ultrafiltration. Ismael and Tondre demonstrated the use of ultrafiltration in the back extraction of metal ions from micellar solutions containing Ni + and Cu + using the extractants 8-hydroxyquinoline (HQ) and an alkylated derivative (Cn-HQ) in the presence of cationic micelles of cetyltrimethylammonium bromide and 1-butanol. Decreasing the pH to pH = 3 led to effective extraction of Ni " in the presence of Cu + but, for Cu + extraction, it was necessary to reduce the pH further to less than 1. It is interesting to note that for these systems cationic surfactant systems were used rather than the anionic systems described previously. 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

On the basis of data obtained the possibility of substrates distribution and their D-values prediction using the regressions which consider the hydrophobicity and stmcture of amines was investigated. The hydrophobicity of amines was estimated by the distribution coefficient value in the water-octanole system (Ig P). The molecular structure of aromatic amines was characterized by the first-order molecular connectivity indexes ( x)- H was shown the independent and cooperative influence of the Ig P and parameters of amines on their distribution. Evidently, this fact demonstrates the host-guest phenomenon which is inherent to the organized media. The obtained in the research data were used for optimization of the conditions of micellar-extraction preconcentrating of metal ions with amines into the NS-rich phase with the following determination by atomic-absorption method. 

Another of the new techniques for extractive preconcentration, separation, and/or purification of metal chelates, biomaterials, and organic compounds is based on the use of surfactant micellar systems. 

Chemical procedures that produce less waste or less hazardous waste are said to be green because they reduce harmful environmental effects. In chemical analyses with dithizone, you can substitute aqueous micelles (Box 26-1) for the organic phase (which has traditionally been chloroform, CHC13) to eliminate chlorinated solvent and the tedious extraction.2 For example, a solution containing 5.0 wt% of the micelle-forming surfactant Triton X-100 dissolves 8.3 X 10 5M dithizone at 25°C and pH < 7. The concentration of dithizone inside the micelles, which constitute a small fraction of the volume of solution, is much greater than 8.3 X 10 5M. Aqueous micellar solutions of dithizone can be used for the spectrophotometric analysis of metals such as Zn(II), Cd(Il), Hg(Il), Cu(ll), and Pb(II) with results comparable to those obtained with an organic solvent. 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

A new potentially exciting development in this area of extractions concerns the use of different reversed micellar systems in countercurrent extractions of different rare earth metals. A mathematical model was developed in order to help optimize the different parameters of this new mode of extraction (364). This should facilitate the further development and utilization of this approach to metal ion separations. 

In contrast to these post-synthetic modifications, it is also possible to functionalize the pore walls directly during the synthesis, as was first shown by Mann and co-workers [7,8] and Stucky and coworkers [9], who used trialkoxysilanes R-Si(OR )3. In our approach, such R Si(OR )3 molecules substitute for part of the TEOS. After hydrolysis, they serve as additional framework components during the hydrothermally induced condensation. An essential condition for this approach is that the trialkoxysilane does not destroy the micellar arrangement of the surfactant, which gives rise to the mesostructure. In mesostructures produced in this way, the R residues should be covalently linked to the silica walls. After the synthesis, the organic surfactant molecules can be removed by extraction so that a modified mesoporous material should remain. For example, when using phenyltrimethoxysilane (PTMOS), phenyl groups may become attached to the walls of the mesopores these can be utilized for further modifications, e.g. the immobilization of metal complexes. 

An elegant experiment by Crooks showed that the hydrophobic modification of PAMAM dendrimers by noncovalent interactions could also result in macromolecules that behave like inverted micelles (118). The spontaneous assembly between the fatty acids and amino periphery of the PAMAM dendrimer was driven by ionic interactions (Fig. 32). These dendrimers were shown to be capable of extracting hydrophilic dyes such as methyl orange fi om water into toluene. Similarly, these dendrimers were also shown to be excellent molecular containers for catalytically active metal nanoparticles. The inverted micellar nature of various dendrimers have been used by Crooks and others for the preparation of a variety of nanoparticles (119-122). A related macromolecule, but an architecture with less of a control, is a hyperbranched polymer. Hydrophobically modified hyperbranched polymers have also been shown to be capable of acting as inverted micelles (123,124). 

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