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

Alexander S. Ivanov, Christina J. Leggett, Bernard F. Parker, Zhicheng Zhang, John Arnold, Sheng Dai, Carter W. Abney, Vyacheslav S. Bryantsev, Linfeng Rao

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 V(5+) complex, exhibiting the highest stability constant value ever observed for the V(5+) 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.