Complexants for actinide element coordination and immobilization

Abstract

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.