Synergistic extractant systems

In addition to systems of the above type, i.e. involving adduct formation, various other types of synergistic extraction systems are recognised and have been reviewed.4 An example is the synergistic influence of zinc in the extraction and A AS determination of trace cadmium in water.5... 

Synergistic extraction systems include various new types of interesting interfacial reactions. These systems should be studied more extensively. 

Moyer, B.A., McDowell, W.J., Ontko, R.J., Bryan, S.A., Case, G.N. Complexation of strontium in the synergistic extraction system dicyclohexano-18-crown-6, versatic acid, carbon tetrachloride. Solvent Extr. Ion Exch. (1986), 4(1), 83-93. 

The thio-derivative 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-thione (BMPPT), gives good americium/europium separation factors in synergistic extraction systems (Smith et al. 1987, Ensor et al. 1988). 

These equations do not provide complete definition of the reactions that may be of significance in particular solvent extraction systems. For example, HTTA can exist as a keto, an enol, and a keto-hydrate species. The metal combines with the enol form, which usually is the dominant one in organic solvents (e.g., K = [HTTA]en i/[HTTA]]jet = 6 in wet benzene). The kinetics of the keto -> enol reaction are not fast although it seems to be catalyzed by the presence of a reagent such as TBP or TOPO. Such reagents react with the enol form in drier solvents but cannot compete with water in wetter ones. HTTA TBP and TBP H2O species also are present in these synergistic systems. However, if extraction into only one solvent (e.g., benzene) is considered, these effects are constant and need not be considered in a simple analysis. 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

Synergistic extraction, in the system DEHPA and TOPO, is quite interesting for its ability to extract uranium from high concentrations of phosphoric acid (38). This finds application in the recovery of uranium from dilute phosphoric acid medium in... 

The new features are detailed in this review most using diamide as a model system and illustrated with published data on other extractant systems. Figure 7.1 summarizes the structure of the different extractants cited in this chapter. The mixture of extractants as synergistic systems and extractants, such as the N-polydentate ligands BTP (39, 40), BTBP (41, 42), or bis-malonamide (43), will not be treated here. These extractants have low solubility in alkane and are generally used in chlorinated solvent or octanol or diamide/alkane solution to enhance their solubility. 

The hexameric scandium decanoate extracted in benzene is different from the hexameric Al(III), Ga(III), and In(III) decanoates in that the former is neither hydrated nor hydrolyzed (154). Galkina and Strel tsova (36), in the butanoic acid/iso-butanol system, attempted to separate Sc from rare earths and the other metals. They proposed the monomeric Sc(III) butanoate, ScA3, as the extracted species. In this extraction system, the polymerization of scandium butanoate in the organic phase seems to be prevented by solvation with iso-butanol. In the study of the synergistic effect of various amines on the extraction of lanthanum and scandium with hexanoic and a-bromohexanoic acids in chloroform, Sukhan et al. (138) proposed LaA3(HA)3 and ScA3(HA)3 as the extracted species. 

Yang ZF, Guha AK, and Sirkar KK. Simultaneous and synergistic extraction of cationic and anionic heavy metaUic species by a mixed solvent extraction system and a novel contained hquid membrane device. Indust Eng Chem Res, 1996 35(11) 4214-4220. 

In many cases, synergists are added to HTTA extraction systems to enhance the separation of actinide ions. One example is the addition of the crown ethers (CE) dibenzo-18-crown-6, dicyclo-hexyl-18-crown-6, dibenzyl 24-crown-8, and benzyl-15-crown-5. These crown ethers have been shown to synergistically enhance extraction into benzene and the increase follows Eu > 1102 " > Th". The extraction equilibrium for crown ether/HTTA systems for the separation of Th is shown in Equation (23). The binding of the crown ether in the extracted complex seems to be a function of crown ether basicity and steric effects2 ... 

Addition of a synergist to the DEFIPA/FICI extraction system such as tributylphosphate (TBP) has a two-sided effect in improving extraction at high acidity and hindering extraction at low acidity. At low acidity, as in the analogous sulfuric acid system, the presence of TBP disrupts the creation of polymeric complexes between and DEFIPA. ... 

Synergistic mixtures. A solvent extraction system is said to be synergistic when the distribution coefficient obtained frcm a mixture of extractants is greater than the sum of distribution coefficients of each extractant alone. Such systems are usually... 

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