# Oligomer Formation Effects on the Separation of Trivalent Lanthanide Fission Products

## TL;DR

This study explains how oligomer formation affects the separation of lanthanide fission products using LN resin, leading to coelution issues.

## Contribution

The study identifies oligomeric structures formed by lanthanides and HDEHP as the cause of coelution during separation.

## Key findings

- Oligomeric structures [Ln2(DEHP)6]n disrupt regular elution behavior in LN resin separations.
- Luminescence analysis confirms that lanthanide centers are within 10 Å in these oligomers.
- Bleeding of HDEHP from the resin support causes formation of disruptive oligometallic species.

## Abstract

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]n. 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]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 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.

The oligomeric structure [Ln2(DEHP)6]n was identified in aqueous systems relevant to
separation of trivalent lanthanide fission products using LN resin
(HDEHP (di(2-ethylhexyl)orthophosphoric acid) on a polymer support).
Luminescence analysis of [LnaLnb(DEHP)6]n solids and solutions showed a change in the nitric
acid concentration at which these oligomeric complexes break up, dependent
on the lanthanide present. This altered elution behavior led to the
diagnosis that the oligomeric species is responsible for undesired
co-elution when using LN resin.