The effect of hydrophobic picrate on the extraction and separation of lanthanides(III) with carbamoylmethylene phosphine oxide

The effect of a hydrophobic anion, such as picrate (pic(-)), on the extraction and separation of Ln(III) ions with a carbamoylmethylene phosphine oxide (CMPO) from aqueous nitric acid solutions into benzene has been examined in the present study. The metal extraction with the CMPO was greatly enhanced by adding a small amount of pic(-), which did not coordinate with the metal ions but only played the role of a counter-anion in the ion pair extraction. Furthermore, the separation of the Ln(III)-series was also improved, which is a great advantage of the anion effect: On the contrary, a synergism by using two extractants, both of which can coordinate with a metal ion, generally decreases such selectivity. From analyses of the distribution data of Ln(III) ions and picric acid, the mechanism of the cooperative effect was clarified. The strong cooperative effect originates from the difference in the hydrophobicity between pic(-) and NO3-. Indeed, the Eu(III)-CMPO complex cation paired with pic(-) was much less hydrated than that paired with NO3-; besides, the binding number of CMPO to the Ln(III) ions also seemed to be affected by the hydration. From these results it is concluded that the metal ions can be dehydrated by the effect of hydrophobic pic(-), to a great extent, when its ion pairs are formed. This can be the origin of the great enhancement of the extractability by adding pic(-). Metal dehydration can also produce the improved selectivity for the Ln(III) ions, i.e., the coordination of the CMPO can be performed more effectively with much fewer water molecules hydrated to the metal ions, and, thus, the inherent selectivity of the extractant can be exhibited more clearly. Metal dehydration can be induced under the influence of pic(-), which is quite inert in coordination, electrostatic interaction, and hydration properties.