The synthesis and evaluation of new extractants for spent nuclear fuel reprocessing are described. New bitopic ligands constituted of phenanthroline and 1,3,5-triazine cores functionalized by picolinamide groups were designed. Synthetic routes were investigated and optimized to obtain twelve new polyaza-heterocyclic ligands. In particular, an efficient and versatile methodology was developed to access non-symmetric 2-substituted-4,6-di(6-picolin-2-yl)-1,3,5-triazines from the 1,3,5-triazapentadiene precursor in the presence of anhydride reagents. Extraction studies showed the ability of both ligand series to extract and separate actinides selectively at different oxidation states (U(VI), Np(V,VI), Am(III), Cm(III), and Pu(IV)) from an acidic solution (3 M HNO3). Phenanthroline-based ligands show the most promising efficiency for use in the group actinide extraction (GANEX) process due to a higher number of donor nitrogen atoms and a suitable pre-organization of the dipicolinamide-1,10-phenanthroline architecture.

In the TALSPEAK liquid–liquid distribution process, dissolved lanthanides can be separated from actinides using a complexing agent such as N-(2-hydroxyethyl)­ethylenediamine-N,N′,N′-triacetic acid (HEDTA, CAS Reg. No. 150-39-0) in a low pH buffered aqueous phase in contact with an organic phase containing a suitable extractant. This study focuses on the chemical speciation of HEDTA, citrate pH buffer, and Eu3+ in aqueous solutions of 1:1 Na+ salts (mainly NaNO3) as a function of ionic strength and pH. New measurements of stoichiometric protonation constants of HEDTA, and the HEDTA complex of Eu3+, in aqueous NaNO3 are reported for ionic strengths from 0.5 to 4.0 M at 25 °C. A Pitzer activity coefficient model of the aqueous mixture has been developed based upon these measurements, available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the HEDTA and buffer speciation, and complexation of Eu3+ by both HEDTA and citrate, to be calculated for different solution compositions and pH values. The model of the citrate buffer, which is based on an extensive range of data for NaCl and NaNO3 media, should also be useful in other practical applications.

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