A report on emergent uranyl binding phenomena by an amidoxime phosphonic acid co-polymer

Abstract

The development of technology to harvest the uranium dissolved in seawater would enable access to vast quantities of this critical metal for nuclear power generation. Amidoxime polymers are the most promising platforms for achieving this separation, yet the design of advanced adsorbents is hindered by uncertainty regarding the uranium binding mode. In this work we use XAFS to investigate the uranium coordination environment in an amidoxime-phosphonic acid copolymer adsorbent. In contrast to the binding mode predicted computationally and from small molecule studies, a cooperative chelating model is favoured, attributable to emergent behavior resulting from inclusion of amidoxime in the polymer. Samples exposed to seawater also display a feature consistent with a μ(2)-oxo-bridged transition metal, suggesting the formation of an in situ specific binding site. These findings challenge long held assumptions and provide new opportunities for the design of advanced adsorbent materials.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater

“…From these results, we conclude the cyclic imide-dioxime form is solely responsible for the extremely strong sorption of vanadium by polyamidoxime sorbents. Of equal importance, these results also partially rationalize the previously reported EXAFS data that reveal no cyclic imide-dioxime contribution to the uranium coordination environment 30 , 31 . The much higher stability of the non-oxido V(V) complex compared to the U(VI) complex with cyclic imide-dioxime (Supplementary Table 15 ) can explain why uranium cannot access the imide-dioxime sorption sites that are occupied by vanadium.…”

“…Metal uptake is reported in Supplementary Table 5 . Polymer samples were prepared for XAFS analysis as reported previously 30 , 31 (see Methods), while small molecule standards of crystalline Na[VO 2 (HIDO)], Na[V(IDO) 2 ] and several vanadium oxides were blended with BN and pressed into self-supporting pellets. Samples of Na 3 VO 4 , and V(V) with H 3 IDO in a 0.1 M NaOH solution were also prepared, with the concentration of H 3 IDO selected to afford the [VO 2 (IDO)] 2− and [V(IDO) 2 ] − species, as per potentiometric titration data presented above.…”

“…For instance, it was not clear, which factors determine the selectivity of amidoxime-based adsorbents toward a particular metal ion in seawater. Although many computational and experimental works 17 , 30 , 31 , 40 – 42 have been performed to elucidate mechanisms of binding and accumulation of uranium on the adsorbent, a number of marine studies have shown that vanadium consistently outcompetes uranium and many other cations for amidoxime sorption sites 9 . The vanadium adsorption capacity of ~15 g per kg adsorbent (Fig.…”

Development of Novel Porous Sorbents for Extraction of Uranium from Seawater

“…A series of polymer adsorbents prepared by radiation induced graft polymerization (RIGP) were contacted with aqueous UO 2 2+ and investigated by XAFS to determine the U binding environment. 13 In contrast to results from crystallographic, computational, and in situ XAFS investigations using small molecule analogs, amidoxime displays a distinctly different binding mode when incorporated into a polymer and is thus an example of an emergent phenomenon which can only be induced when included as part of a larger system. Although seeming to possess spectral features indicative of tridentate binding, fitting the XAFS spectrum for a brinecontacted AI-8 adsorbent revealed including the cyclic imide dioxime binding site resulted in a fit which was less statistically rigorous than fits performed without the cyclic binding site ( Figure 4).…”

Aqueous Solution Blow Spinning of Seawater‐Stable Polyamidoxime Nanofibers from Water‐Soluble Precursor for Uranium Extraction from Seawater

“…The adsorption kinetics of PAO/Alg NF adsorbents (15 mg per sample) in 5 L of U-spiked seawater with different U concentrations (8,16, and 32 ppm) are shown in Figure 4b (1)…”