Complex formation between actinide (An = Pu(IV), Th(IV), Am(III) and U(VI) and a hydrophilic anionic form of SO3-Ph-BTP4– ligand, L4–, in water was studied by liquid-liquid extraction experiments performed over a range of the ligand and HNO3 concentrations in the aqueous phase, at a constant concentration of nitrate anions at 25°C. The competition for An ions between the lipophilic TODGA extractant and the hydrophilic L4– ligand leads to the decrease in the An distribution ratios, D, with an increasing L4– concentration.
The model of the solvent extraction process used accounts – apart from An complexation by TODGA and SO3-Ph-BTP4– – also for An complexation by nitrates and for the decrease in the concentration of the free L4– ligand in the aqueous phase, due to its protonation, bonding in the An complex and the distribution between the two liquid phases.
The analysis of the results, based on the formal correction of free ligand concentrations, points to the formation of 1:x An – SO3-Ph-BTP complexes in the aqueous phase. The conditional formation constant of
the 1:x complex has been determined.