Resins metal-selective chelating

Resins can also be prepared with chelating functional groups which show selective affinities for certain metals. Iminodiacetic acid, vinylacetylacetone, glyoxal-thiophenol and 8-hydroxy quinoline have all been used to produce these so-called chelating resins. Their selectivities are similar to those of the free reagent. 

The cost of resins is a major consideration. Heavy-metal-selective chelating resins are the most expensive. Table 4 provides some cost ranges for commercially available resins. 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

In Table 4.13, the relative selectivity of a commercial chelating resin for heavy metal cations is shown. The relative selectivity is based on the selectivity for calcium ions. For... 

Chelating cation resin relative selectivities for metal ions (relative selectivity is based on Ca2+)... 

The early development of ion exchange as a unit operation in hydrometallurgy was slow, mainly because of the lack of selectivity of the resins under operating conditions, and the limited capacities of the earlier commercial resins. Consequently, ion exchange found applications only in processes where the concentration of metal ions in solution was very low, and where the resin could be used to upgrade the solution prior to some final purification step. Recent developments, however, particularly the introduction of chelating resins, have considerably broadened the scope of resins in hydrometallurgy. 

The first chelating resins that were found to be really suitable for application in the field of selective cation absorption were those based on the aminodiacetate functional group.380 The first commercial resin based on this functional group, Dowex Al, was shown381 to have an affinity for a range of metals which was similar to the order of dissociation constants of the metal complexes with ethylenediaminetetraacetic acid (EDTA), i.e. 

Colloidal palladium or platinum supported on chelate resin beads were employed for the stereoselective hydrogenation of olefins 86). Colloidal palladium supported on iminodiacetic acid type chelate resin beads was prepared by refluxing the palladium chloride and the chelate resin beads in methanol-water. Using the resin-supported colloidal palladium as a catalyst, cyclopentadiene is hydrogenated to cyclopentene with 97.1% selectivity at 100 % conversion of cyclopentadiene under 1 atm of hydrogen in methanol at 30 °C. Finely dispersed metal particles ranging from 1 to 6 nm in diameter are the active species in the catalyst. 

Pohl and Prusiz [120] used and compared the performance of the chelating resin Metalfix Chelamine and the strong cation exchanger resin Dowex 50W-X4 for the off-line preconcentration of Cd, Co, Cu, Ni, and Zn. The cation exchanger was selected as the best alternative for the analysis of commercial beer samples by ICP-AES. The preconcentration procedure allowed the selected metals to be determined at concentrations in the order of 1 ng ml-1. 

Ordinary anion and cation ion-exchange resins are of limited use for the analytical concentration of trace elements from water, because of their lack of selectivity. This is especially so with strong electrolytes such as seawater. In this case the major ions sodium, magnesium, calcium and strontium, are retained preferentially. However, the recent advent of commercial chelating resins based mainly on iminodiacetic acid-substituted cross-linked polystyrene, makes it possible to concentrate trace elements from waters. In consequence, a number of researchers have used chelating resins for trace-metal preconcentration from seawater and natural waters. 

Pb was sorbed on a resin [27] and polyurethane foam [28] modified with the crown ether, 18-crown-6, and its derivatives. The sorption of Pb (and other metals) on a chelating resin containing Arsenazo I was studied in water-organic solvent media [29]. A silica gel, modified with 2-mercaptobenzothiazole [30] or 2-mercaptobenzimidazole [31 ] was used for concentration of Pb and other heavy metals from aqueous solutions. Pb was selectively separated from Al, Mg, and Fe(III) on a cation exchanger modified with tin(IV) antimonate [32]. 

Step gradient elution is also well suited for group, or ionic class separations. For example, a chelating resin might be specific for the platinum-group metals. With this resin, a step-gradient scheme could be devised to take up, and later elute, only the selected group of metal ions. 

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