Synergic extraction

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extract-able complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. 

Besides presenting a general review of the literature it has been our objective to draw the attention of reseachers to this field which is both of practical and basic interest. Generally the compounds involved in synergic extraction are characterized employing the extraction methods alone. Thus, it is felt that there is a rich field here for structural chemists, especially those who can separate and purify the complexes, determine their structures and other physicochemical properties and then, through spectroscopic techniques, connect the isolated compounds with those in solution. Such studies appear promising as can be inferred from some recent work reported. 

Thus, there is hardly any extractant combination which is free from mutual association and this factor has to be taken into account in any quantitative study on synergic extraction. 

Thorium is known1135 to get extracted by oxine and several 1 1 neutral donor adducts of thorium oxinate were synthesised1145. Thus, mixtures of oxine and neutral extractants should provide a good combination for the synergic extraction of thorium. 

It was reported115,1165 that by the use of MIBK or n-BA as diluents, the extraction of thorium by cupferron can be accomplished at a lower pH, which suggests that cupferron and neutral extractants can be good partners for the synergic extraction of thorium. 

Earlier attempts75 on synergic extraction of U(IV) were unsuccessful as there was difficulty in preventing the oxidation of U(IV) to U(VI) during extraction. An attempt1175 to prepare the neutral donor adducts of U(IV) betadiketonates did not succeed and hence it was concluded that such adducts are not possible with U(IV) due to its inability to reach nona coordination. 

Table 21. Equilibrium constants in the synergic extraction of U(VI) by mixtures of dialkylphos-phoric acids and neutral donors...

The addition mechanism was also considered by those1 who assumed Eq. (29) to be responsible for U(VI) extraction by HA. Here the synergic extraction equilibrium becomes,... 

The diluents employed influence significantly the extent of synergic extraction of metal ions by different combinations of extractants. Generally with relatively polar diluents such as chloroform synergic enhancement is much less than that observed with non-polar solvents like hexane or cyclohexane. With the former diluents the metal species extracted... 

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