Radiolytic stability and effects on metal extraction of N,N,N′-trioctyldiglycolamide, an important TODGA degradation product

Abstract

N,N,N′-Trioctyldiglycolamide (TrODGA) is an important degradation compound of N,N,N′,N′-tetraoctyl diglycolamide (TODGA), one of the most successful extractants under study for minor actinide recovery. Due to the lipophobicity and structural similarity to TODGA, easy removal of this degradation compound is not expected. Once formed, TrODGA can be accumulated or broken into smaller degradation compounds during process operation. In this study, the stability of TrODGA and its possible implications on TODGA-solvents were evaluated by studying its resistance to radiation, its contribution to the formation of degradation compounds, and the consequent extraction behavior. For that, relevant experimental conditions of the process operation, such as the presence of nitric acid and metals, were chosen. TrODGA samples pre-equilibrated with a solution containing the nitric acid and Eu concentration expected in a real process were subjected to gamma radiation. The results showed that TrODGA has a similar stability behavior against radiolysis to TODGA. The weakest bond of TrODGA is C–O_ether resulting in the formation of 2-hydroxy-N-octylacetamide as the main product. Metal complexation during irradiation indicates a protective effect where no change in the principal degradation pathway was observed. Evaluation of the TrODGA degraded samples revealed a reduction in the extraction efficiency of lanthanides and actinides and an undesirable coextraction increase of fission products, which could be attributed to the extraction properties of its degradation compounds. This study demonstrates the importance of thoroughly studying the effects of TrODGA and its degradation compounds for the applicability of TODGA-based solvents and the design of washing strategies to recycle them as much as possible.