The effect of the pseudohalide ligand on solvent extraction of trivalent lanthanides, yttrium and americium by tri-n-butyl phosphate

Solvent extraction of Y, Am(III) and some lanthanides from thiocyanate, selenocyanate, cyanate and azide solutions by tri-n-butyl phosphate was studied. It has been found that the distribution ratio, D, is high for thiocyanates and selenocyanates and very low for azides and cyanates. At the same time the separation factor DAm/DY is about 12 for the NCS- and NCSe-ligands, while for the NCO- and N-3 ligands the separation factors between Am and Y, and between all the elements studied are very close to one. The division of the isoelectronic pseudohalide ligands into two classes with respect to D and to separation factors corresponds with the division of the respective hydracids into moderately strong (HNCS and HNCSe) and weak (HNCO and HN3). The results are discussed in terms of the effect of the pseudohalide anion, X-, on the stability and transfer of the inner-sphere MX3 complexes, and on the equilibrium between the inner- and outer-sphere complexes in the aqueous phase. (orig.)