Oligomer Formation Effects on the Separation of Trivalent Lanthanide Fission Products

The assessment of trivalent lanthanide yields from the fission of uranium-235 is currently achieved using LN (LaNthanide) resin, di(2-ethylhexyl)orthophosphoric acid immobilized on a solid support. However, coelution of lighter lanthanides into terbium (Tb³⁺) fractions remains a significant problem in recovery of analytically pure fractions. In order to understand how the separation of trivalent lanthanides and yttrium (Ln³⁺) with LN resin proceeds and how to improve it, their speciation with the organic extractant HDEHP must be fully understood under aqueous conditions. A comprehensive luminescence analysis of aqueous solutions of Ln³⁺ in contact with HDEHP, along with infrared spectroscopy, elemental combustion analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and mass spectrometry, was used to indicate that an intermediate species is responsible for the coelution; where similar Ln³⁺ centers (e.g., Eu³⁺ and Tb³⁺) are bridged by the O-P-O moiety of deprotonated HDEHP to form large heteronuclear oligomeric structures with the general formula [Ln₂(DEHP)₆]. Energy transfer from Tb³⁺ to Eu³⁺ in this structure confirms that lanthanide centers are within 10 Å and was used to propose that the oligomeric [Ln₂(DEHP)₆] structure is formed rather than a dimeric Ln₂(DEHP)₆ structure. The effect of this speciation on LN resin column elution is investigated using luminescence spectroscopy, confirming that the oligomeric [Ln₂(DEHP)₆] species could disrupt regular elution behavior and cause the problematic bleeding of lighter lanthanides (Sm³⁺ and Eu³⁺) into Tb³⁺ fractions. Resin luminescence measurements were used to propose that the bleeding of the organic extractant HDEHP from its solid support causes the formation of the disruptive oligometallic species.