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 (HDEHP) immobilised on a solid support. However, co-elution 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⁺ contacted with HDEHP, alongside infrared spectroscopy, elemental combustion analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry was used to indicate an intermediate species is responsible for the co-elution; where similar Ln³⁺ centres (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]_n. Energy transfer from Tb³⁺ to Eu³⁺ in this structure confirms that lanthanide centres are within 10 Å and was used to propose that the oligomeric [Ln₂(DEHP)₆]_n 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]_n species could disrupt regular elution behaviour and cause the problematic bleeding of lighter lanthanides (Sm³⁺ and Eu³⁺) into Tb³⁺ fractions. Resin luminescence measurements were used to propose that bleeding of the organic extractant HDEHP from its solid support causes the formation of the disruptive oligometallic species.