Modeling and Speciation Study of Uranium(VI) and Technetium(VII) Coextraction with DEHiBA

The N,N-dialkylamide DEHiBA (N,N-di-2-ethylhexyl-isobutyramide) is a promising alternative extractant to TBP (tri-n-butylphosphate) to selectively extract uranium(VI) from plutonium(IV) and spent nuclear fuel fission products. Extraction of technetium, present as pertechnetic acid (HTcO4) in the spent fuel solution, by DEHiBA was studied for different nitric acid and uranium concentrations. The uranium(VI) and technetium(VII) coextraction mechanism with DEHiBA was investigated to better understand the behavior of technetium in the solvent extraction process. Uranium and technetium distribution ratios were first determined from batch experiments. On the basis of these data, a thermodynamic model was developed. This model takes into account deviations from ideality in the aqueous phase using the simple solution concept. A good representation of uranium and technetium distribution data was obtained when considering the formation of (DEHiBA)i(HNO3)j(HTcO4)k complexes, as well as mixed (DEHiBA)2(UO2)(NO3)(TcO4) and (DEHiBA)3(UO2)(NO3)(TcO4)(HNO3) complexes, where one pertechnetate anion replaces one nitrate in the uranium coordination sphere in the two complexes (DEHiBA)2(UO2)(NO3)2 and (DEHiBA)3(UO2)(NO3)2(HNO3). Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) enabled full characterization of the formation of mixed uranium-technetium species (DEHiBA)2(UO2)(NO3)(TcO4) in the organic phase for the first time. The structural parameters of this complex are reported in the paper and lead to the conclusion that the pertechnetate group coordinates the uranyl cation in a monodentate fashion in the inner coordination sphere. This study shows how combining a macroscopic approach (distribution data acquisition and modeling) with molecular-scale investigations (FT-IR and X-ray absorption analysis supported by theoretical calculations) can provide a new insight into the description of a solvent extraction mechanism.