Physical properties of N,N,N′,N′-tetramethyl diglycolamide and thermodynamic studies of its complexation with zirconium, lanthanides and actinides

Abstract

The crystal structure, melting range, decomposition temperature, solubilities in aqueous and organic phase, and distribution ratio in biphase system of N,N,N′,N′-tetramethyl diglycolamide (TMDGA) have been studied in the present work. Water-soluble TMDGA with very low solubility in kerosene or 30 % trialkyl phosphine (TRPO)/kerosene meets the primary requirement to act as the stripping agent in TRPO process which has been developed for actinides partitioning from high level waste. Slope analysis, electrospray ionization mass spectrometry (ESI–MS) and spectrophotometric titration indicate that in weak and moderate acidic aqueous solution, up to three TMDGA molecules form complexes with Zr4+, trivalent lanthanides (Ln3+), trivalent actinides (An3+) or tetravalent actinides (An4+). Their complex formation constants determined by liquid–liquid extraction method follow the order predicted by the electrostatic model of 4f and 5f elements and the covalent model of 4d transition elements: Ln3+ ~ An3+ < An4+ ≪ Zr4+. Hydrolysis of actinide cations, especially An4+, in the media of low acidity, reduces the complexation stability. In addition, the effect of aqueous acidity and temperature has been investigated. No obvious influence of temperature on Np4+ complexes has been found. However, the influence on Am3+ and Pu4+, especially on their 1:1 or 1:2 complex species with TMDGA, is relatively considerable. Strong affinity of TMDGA to An3+ and An4+ implies that TMDGA has potential to strip An3+ and An4+ from TRPO.