Complexants for actinide element coordination and immobilization

The goal of this project is to develop inorganic metal oxide clusters known as polyoxoanions (POAs) as complexants for the immobilization of actinide (An) ions from high-level waste (HLW). A diverse array of rugged isopolyoxoanions, [M{sub x}O{sub y}]{sup z{minus}}, and heteropolyoxoanions, [X{sub a}M{sub b}O{sub c}]{sup d{minus}}, comprised of M = V, Mo, W and X = Si, P polyhedra will be investigated for their ability to incarcerate An ions. The research combines two objectives--An-POA coordination and An-POA containment. The first involves the synthesis, isolation, and characterization of POAs that can selectively bind An ions to form stable An-POA complexes in alkaline and acidic solutions. The second involves investigations of the thermochemistry of the An-POA complexes under vitrification conditions germane to the formation of proposed HLW forms, such as borosilicate glass. The approach is envisioned to provide two levels of An encapsulation for maximum stability and durability as well as the potential to incorporate higher levels of An ions (particularly Pu) in waste forms than now possible. Such versatility bodes well for the potential application of POAs as An complexants in technology of significance to the environmental management of HLW. This report summarizes work performed since the commencement of the project on 1 October 1998. As an overview of the research progress-to-date (15 June 1999), the strategic point is that selective binding of An ions by POAs depends upon the An valence. Actinide reduction-oxidation (redox) chemistry is a pivotal property in the proposed, rational use of POAs as coordinating ligands for An ions. Because POAs are oxidants and the transuranium elements are multivalent, the combined redox chemistry is rich and, oftentimes, confusing. The evidence of contrasting and conflicting valence behavior makes it difficult, if not impossible, to make broad generalizations about the collective properties of An-POA systems without appropriate experimentation. To illustrate this chemical diversity, the authors have prepared actinide complexes of two well-known series of heteropolyoxoanions--the Preyssler and Wells-Dawson anions. Research highlights are presented

Coordination and valence of europium in [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(W{sub 5}O{sub 18}){sub 2}]{sup 9{minus}}

Europium L{sub 3}-edge X-ray absorption fine structure (XAFS) spectroelectrochemistry was used to determine the valence of europium in [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(w{sub 5}O{sub 18}){sub 2}]{sup 9{minus}}. Dilute solutions of these anions in aqueous supporting electrolytes were examined at ambient temperature and at extreme potentials. In situ XANES (X-ray absorption near edge structure) data revealed that Eu is trivalent in both [Eu({alpha}-2-As{sub 2}W{sub 17}O{sub 61}){sub 2}]{sup 17{minus}} and [Eu(W{sub 5}O{sub 18}){sub 2}]{sup 9{minus}} at rest potential. Furthermore, it was not reduced to Eu{sup 2+} by constant-potential bulk electrolysis at significantly reducing potentials under the electrochemical conditions used herein. These results stand in obvious contrast to the redox behavior of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12{minus}}, in which Eu{sup 3+} is reduced to Eu{sup 2+} under similar electrochemical conditions

Reactivity of HTcO4 with methanol in sulfuric acid: Tc-sulfate complexes revealed by XAFS spectroscopy and first principles calculations

The reaction between HTcO4 and MeOH in 13 M H2SO4 was investigated by 99Tc NMR, UV-visible and X-ray absorption fine structure (XAFS) spectroscopy. Experimental results and first principles calculations show the formation of Tc(+5) sulfate complexes. The results expand the fundamental understanding of Tc in high acid solutions

On the nature of heptavalent technetium in concentrated nitric and perchloric acid

The speciation of Tc(+7) was performed in HClO4 and HNO3 by 99-Tc NMR, UV-Vis and XAFS spectroscopy. The speciation of Tc(+7) depends on the concentration and strength of the acid. Pertechnetic acid, HTcO4, forms above 8 M HClO4 while in concentrated HNO3, [TcO4]− is still the predominant species. EXAFS spectroscopy shows that the structure of HTcO4 in HClO4 is similar to the one in H2SO4. The reactivity of Tc(+7) was analyzed in the frame of the partial charge model. The partial charge calculated on the Tc atoms (ΔTc) indicates that HTcO4 (ΔTc = +057) is more electrophilic than [TcO4]− (ΔTc = +0.52). The difference in the oxidizing properties between [TcO4]− and HTcO4 is given from the reaction of these species with 12 M HCl(aq). In 13 M sulfuric acid HTcO4 is reduced to Tc(+5) while [TcO4]− is not reduced in 6 M H2SO4