ABSTRACT
The use of amidoxime-functionalized polymer fibers as a sorbent for uranium has attracted recent interest for the extraction of uranium from seawater. Vanadium is one of the main competing ions for uranium sorption as V(v) species, however, vanadium is also present as V(iv) in seawater. In the present study, the interactions of V(iv) with amidoxime and similar ligands were explored. Attempts were made to synthesize V(iv) complexes of glutaroimide-dioxime, a molecular analogue of polymer sorbents. However, V(iv) was found to react irreversibly with glutaroimide-dioxime and other oxime groups, oxidizing to the V(v) oxidation state. We have explored the reactions and propose mechanisms, as well as characterized the redox behavior of the vanadium-glutaroimide-dioxime complex.
ABSTRACT
This review provides a brief background on the extraction of uranium from seawater as well as recent work by the United States Department of Energy on this project. The world's oceans contain uranium at 3 parts per billion, and despite this low concentration, there has been historical interest in harvesting it, mainly in Japan in the 1980s and the United States in this decade. Improvements in materials, chemistry, and deployment methods have all been made, with the ultimate goal of lower cost. This has been partially realized, dropping from approximately $2000 per kg U3O8 extracted in 1984 to $500 per kg today, although this is not yet competitive with terrestrial uranium. This technology may become cost-competitive if the cost of land-based uranium rises, especially if seawater extraction technology is improved further. The coordination chemistry aspects of the project are described in more detail, exploring the functional groups that are present on typical polymer sorbents as well as small-molecule analogues of these ligands. Selectivity for uranium over other metals, particularly vanadium, remains problematic, and techniques to both quantify binding strength and selectivity in order to overcome this issue are essential for future cost improvements.
ABSTRACT
A non-oxido V(v) complex with glutaroimide-dioxime (H3L), a ligand for recovering uranium from seawater, was synthesized from aqueous solution as Na[V(L)2]·2H2O, and the structure determined by X-ray diffraction. It is the first non-oxido V(v) complex that has been directly synthesized in and crystallized from aqueous solution. The distorted octahedral structure contains two fully deprotonated ligands (L3-) coordinating to V5+, each in a tridentate mode via the imide N (R V-N = 1.96 Å) and oxime O atoms (R V-O = 1.87-1.90 Å). Using 17O-labelled vanadate as the starting material, concurrent 17O/51V/1H/13C NMR, in conjunction with ESI-MS, unprecedentedly demonstrated the stepwise displacement of the oxido V[double bond, length as m-dash]O bonds by glutaroimide-dioxime and verified the existence of the "bare" V5+/glutaroimide-dioxime complex, [V(L)2]-, in aqueous solution. In addition, the crystal structure of an intermediate 1 : 1 V(v)/glutaroimide-dioxime complex, [VO2(HL)]-, in which the oxido bonds of vanadate are only partially displaced, corroborates the observations by NMR and ESI-MS. Results from this work provide important insights into the strong sorption of vanadium on poly(amidoxime) sorbents in the recovery of uranium from seawater. Also, because vanadium plays important roles in biological systems, the syntheses of the oxido and non-oxido V5+ complexes and the unprecedented demonstration of the displacement of the oxido V[double bond, length as m-dash]O bonds help with the on-going efforts to develop new vanadium compounds that could be of importance in biological applications.
