Uranium and vanadium binding studies for the selective extraction of uranium from seawater
An introduction to the uranium from seawater project.
A non-oxo 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-oxo 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. Using 17O-labelled vanadate, concurrent 17O/51V/1H/13C NMR, in conjunction with ESI-MS, unprecedentedly demonstrated the stepwise displacement of the oxo V=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 oxo 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. Because vanadium plays important roles in biological systems, the direct synthesis of the non-oxo V5+ complex and the unprecedented demonstration of the displacement of the oxo V=O bonds may also help with the ongoing efforts to develop vanadium compounds that could be of importance in biological applications.
The kinetics of the binding of uranium, vanadium, and iron with glutaroimide-dioxime as a molecular analogue of polymer sorbents has been studied using stopped-flow and conventional UV-visible absorption spectroscopy to monitor the reactions over a range of time scales. Qualitatively, vanadium reacts the slowest of the three metals despite being able to form a very strong complex, with the 1:2 vanadium/ligand complex forming over weeks, likely due to the slow hydrolysis of the strong oxido ligands, while iron reacts fast and uranyl faster still, despite the presence of carbonate in the uranyl species. Conditional rate constants were determined for the formation of 1:1 glutaroimide-dioxime complexes with the three metal ions. In a narrow and near neutral pH region, a rate equation for the formation of the 1:1 vanadium/glutaroimide-dioxime complex was developed, showing the reaction is the first order with respect to [V], [ligand], and [H+]. These observations, some qualitative and others quantitative, are consistent with previous marine tests of polymer adsorbents, and give mechanistic insight into how glutaroimide-dioxime forms complexes with uranium, iron, and vanadium.
Interactions of vanadium(IV) with amidoximes and similar ligands as molecular analogues of polymer sorbents used to extract uranium from seawater is explored. 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 so this reaction is of interest to U/V selectivity and polymer stability. The synthesis of V(IV) complexes of glutaroimide-dioxime was attempted under a wide variety of conditions, however, V(IV) was found to react irreversibly with glutaroimide-dioxime and other oxime groups which oxidize vanadium to the V(V) oxidation state by transferring an oxygen atom from the ligand or substrate. A mechanism has been proposed for this type of reactivity, and the redox behavior of the vanadium-glutaroimide-dioxime complex has been characterized.
A triazine hydroxylamine ligand, H2bihyat, has been investigated for its potential application to selective uranyl binding for extraction from seawater. The vanadium chemistry of this ligand is known; compared to glutaroimide-dioxime the binding is significantly weaker and it does not form a 2:1 non-oxido V(V) complex. This ligand has a very similar binding group configuration as glutaroimide-dioxime, and through potentoimetry it was found to be comparable in binding ability. NMR techniques were used to confirm the stoichiometry and species proposed by potentiometry over a wide pH range. Additionally, the 1:1 complex UO2(bihyat) was isolated and the crystal structure obtained. Solid-state binding is also similar to glutaroimide-dioxime, further suggesting that this ligand may be a feasible alternative to glutaroimide-dioxime, but with much improved selectivity over vanadium.
As a new strategy to discover new ligands for the selective binding of uranyl, a combinatorially synthesized peptoid library (N-substituted glycine oligomers) was screened for uranyl binding with the goal of identifying high-affinity ligands for use in polymer sorbents. Qualitative screening techniques using a dye, arsenazo III, identified three uranyl-binding sequences, all of which contained only carboxylic acids as the active binding groups. Fluorescence spectroscopy was used to determine a dissociation constant for one of the identified peptoids by monitoring the decrease in peptoid fluorescence upon uranyl binding. Density functional theory calculations were used to model the solution-state binding of these sequences to understand favored binding modes and geometries.