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 (Tb) fractions remains a significant problem in recovery of analytically pure fractions. In order to understand how the separation of trivalent lanthanides and yttrium (Ln) 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 Ln 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 Ln centers (e.g., Eu and Tb) are bridged by the O-P-O moiety of deprotonated HDEHP to form large heteronuclear oligomeric structures with the general formula [Ln(DEHP)]. Energy transfer from Tb to Eu in this structure confirms that lanthanide centers are within 10 Å and was used to propose that the oligomeric [Ln(DEHP)] structure is formed rather than a dimeric Ln(DEHP) structure. The effect of this speciation on LN resin column elution is investigated using luminescence spectroscopy, confirming that the oligomeric [Ln(DEHP)] species could disrupt regular elution behavior and cause the problematic bleeding of lighter lanthanides (Sm and Eu) into Tb 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.