Solvent extraction of TODGA in the presence of phase modifiers of TBP, n-octanol, and DHOA dissolved in n-dodecane: Thermodynamics, kinetics and aggregation studies

Tri-n-butyl phosphate (TBP), n-octanol and N,N-dihexyloctanamide (DHOA) are often used as phase modifiers to prevent the formation of the third phase during the extraction of trivalent lanthanides and actinides by N,N,N′,N′-tetraoctyl diglycolamide (TODGA) in spent fuel reprocessing process. In this context, the effects of these three modifiers on the thermodynamic and kinetic behaviors of 0.20 mol·L⁻¹ TODGA in n-dodecane toward Nd³⁺ in nitric acid solution were studied and compared together for the first time. A systematic comparison of the effects of the three modifiers on the extraction distribution ratio, stripping percentage, loading capacity, aggregation and extraction kinetics was carried out with the aim of selecting a phase modifier for the TODGA-based process that would work optimally and to elucidate the impact of the modifiers in the TODGA extraction system. It is found that modifier reduces the capacity of TODGA to extract Nd³⁺ in the order of DHOA > TBP > n-octanol, slightly hinders the stripping of Nd³⁺, while significantly enhancing the loading capacity of TODGA. With increasing the modifier concentration, the limiting organic phase concentration (LOC) for Nd³⁺ increases. At the modifier concentration below 0.30 mol·L⁻¹, n-octanol has the greatest promotion effect on the LOC value, while at that above 0.50 mol·L⁻¹, TBP has the best effect. Meanwhile, through the dynamic light scattering analysis for the loaded organic phase, it is also found that the particle size of the aggregates in organic phase decreases significantly upon the addition of modifier. Furthermore, the extraction kinetics studies in a Lewis cell indicate that the addition of modifier can prolong the time to reach extraction equilibrium, and decrease the extraction rate. Although the modifier also exerts some influence on the reaction order and extraction rate constant to some extent, it does not alter the underlying regime of the extraction reaction.