An experimental and computational look at the radiolytic degradation of TODGA and the effect on metal complexation

Abstract

Diglycolamide extractants, and in particular N,N,N′,N′-tetraoctyl diglycolamide (TODGA), are currently under investigation for use in nuclear fuel reprocessing by liquid–liquid separations. Several processes, such as ARTIST, i-Sanex, and EURO-GANEX processes, have been developed around the TODGA extractant. The solvent typically combines TODGA with a modifier in an alkane diluent. Due to the high radiation produced by the actinides and fission products present in the fuel, radiolytic degradation can greatly alter the chemistry of these solutions. The radiolysis and hydrolysis of the diglycolamide-based extractant was investigated. The effects of the presence of 1-octanol as phase modifier in organic phase and nitric acid concentration in aqueous phase were investigated. Nitric acid provides a slight protective effect, whereas the presence of 1-octanol seems to have a slight sensitizing effect. Fukui function calculations were done to supplement the experimental data and give an improved understanding of the behaviour of TODGA organic solutions after radiolysis. An increase in nitric acid concentration protects TODGA from radiolysis as nitrate absorbs the radicals. The slight sensitizing effect of 1-octanol is due to a combination of several competitive effects: a protective effect from TODGA–octanol adducts and a sensitizing effect from an increase in the water concentration in the organic phase, which produces ˙OH radicals. Lanthanide extraction was performed with irradiated solutions to identify the degradation products that complex metal ions. Among the identified degradation products in solution, compounds retaining the diglycolamide skeleton and [2-(dioctylamino)-2-oxoethoxy]acetic acid participate in the lanthanide complexation to form mixed Ln-degradation product-TODGA complexes. In contrast, 2-hydroxy-N,N-dioctylacetamide acts more like a phase modifier rather than directly complexing lanthanide ions.