# Extraction, Characterization, and Stability Studies of Bistriazinyl-Derived Carboxylic Acids

## TL;DR

This paper introduces a new water-soluble ligand, BTPOA, that can selectively extract Am(III) and Cm(III) ions from nuclear waste, offering a potential alternative to existing methods.

## Contribution

The study presents a novel CHON-based ligand, BTPOA, with high selectivity and stability for actinide separation in nuclear reprocessing.

## Key findings

- BTPOA shows good extraction and high selectivity for Am(III) over Eu(III) ions.
- Conditional stability constants for BTPOA complexes with Cm(III) and Eu(III) were determined in HClO4 and HNO3.
- Radiolytic rate coefficients suggest BTPOA's longevity increases with HNO3 concentration.

## Abstract

Separation of An­(III) and Ln­(III) ions will benefit the
recycling
of used nuclear fuel (UNF). For this purpose, many ligands have been
tested over the years, and several separation processes have been
successfully demonstrated on the laboratory scale. Current research
aims at the development of new ligands that are built only with carbon,
hydrogen, oxygen, and nitrogen (CHON), as they can be incinerated
completely without secondary waste production. Here, we tested a new
class of water-soluble ligands, the bistriazinyl-octa-carboxylic acids.
One member, in particular, 2,6-bis-[5,6-di­(3,4-dicarboxyphenyl)-1,2,4-triazin-3-yl]-pyridine
(BTPOA), was found to be suitable for the selective separation of
Am­(III) and Cm­(III) ions from Ln­(III) ions and may act as a CHON alternative
to its sulfonated analogue (SO3-Ph-BTP). BTPOA exhibited
good extraction results and a high selectivity for Am­(III) over Eu­(III)
ions. This ligand’s complexation of metal ions was further
studied using potentiometric spectroscopy, as well as time-resolved
laser-induced spectroscopy with Cm­(III) and Eu­(III) in aqueous HClO4 and HNO3 media. Conditional stability constants
of each formed species were determined. In the HClO4 system,
Cm­(III) formed three species (1:1, 1:2, and 1:3) through the stepwise
addition of a single BTPOA molecule. On the other hand, in HNO3, Cm­(III) formed two 1:2 complexes and one 1:3 complex, while
the stepwise formation of three species was observed for Eu­(III).
The stability constants are comparable to the values for SO3-Ph-BTP. The radiolytic behavior of BTPOA was also investigated using
electron pulse irradiation measurements to determine absolute rate
coefficients (k) under ambient temperature conditions
for the reaction of BTPOA with typical UNF reprocessing radical radiolysis
productsthe hydrated electron (eaq
–, k = (1.60 ± 0.02) × 1010 M–1 s–1), the hydrogen atom (H•, k = (2.17 ± 0.03) × 109 M–1 s–1), and hydroxyl
(•OH, k = (6.95 ± 0.06) ×
109 M–1 s–1) and nitrate
(NO3
•, k = (0.37 ±
0.02) × 107 M–1 s–1) radicals. These rate coefficients indicate that the radiolytic
longevity of BTPOA should increase with HNO3 concentration,
owing to the consumption of eaq
–/H•, by nitrate anions, and the replacement of •OH by the less reactive NO3
•.