SOLVENT-EXTRACTION OF THE TRIVALENT LANTHANIDES AND YTTRIUM BY SOME 2-BROMOALKANOIC ACIDS

The solvent extraction of the trivalent lanthanides and yttrium from nitrate media by 0.5M solutions of four different 2-bromoalkanoic acids (HA) in xylene was studied. The extraction of these metals was found to take place in a pH range about one pH unit more acidic than with the parent alkanoic acids. The order of selectivity towards the individual lanthanides and yttrium was found to depend markedly on the structure of the 2-bromoalkanoic acid. Thus, for the most sterically hindered compound used (2-bromo-2-ethylhexanoic acid) the pH(0.5) values decrease more or less continuously from lanthanum to lutetium, although the overall difference is quite small (pH(0.5)(La) = pH(0.5)(Lu) = 0,30 pH unit). With this extractant the behaviour of yttrium most closely resembles that of the light lanthanide, neodymium. For the least sterically hindered extractant used (2-bromodecanoic acid) the pH(0.5) values decreased only in the interval from lanthanum to praseodymium, following which the values increase more or less continuously through to lutetium, with the overall change representing a small selectivity towards lanthanum (pH(0.5)(La) - pH(0.5)(Lu) = -0.12 pH unit). With 2-bromodecanoic acid and analogous compounds of low steric hindrance (2-bromo-3-cyclohexylpropanoic and 2-bromo-3,5,5-trimethylhexanoic acids), the extraction of yttrium was found to be less than that of any of the lanthanides. Slope analysis studies showed that 2-bromo-2-ethylhexanoic acid extracts monomeric complexes (such as EuA(3)(HA)(2) or EuA(3)(HA)(3)) throughout the series of lanthanides (and yttrium), whereas with the less sterically hindered bromo-acids dimeric complexes are formed with the lighter lanthanides (such as (LaA(3)(HA)(3))(2)) and monomeric species are formed with yttrium and the heavier lanthanides (such as LuA(3)(HA)(3)).