Abstract
Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V5+ complex, exhibiting the highest stability constant value ever observed for the V5+ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.
Highlights
Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater
The combined computational and experimental findings provided here represent a significant step toward a fundamental understanding of the selective adsorption of metal ions from seawater by polymeric adsorbent materials
The majority of studies in this area were mostly focused on the extraction of uranium from seawater, with considerable experimental work done in the 1990 s in Japan, where repeated screening programs identified amidoximefunctionalized polymers as the most promising candidate adsorbent[37]
Summary
Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. In striking contrast to cyclic imide-dioxime (H3IDO), the results for open-chain amidoxime (HAO) revealed that, despite a large excess of the ligand in speciation modeling (Supplementary Fig. 4), no complex formation was observed over the entire pH range, indicating that all identified V(V)/HAO
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