Amidoxime Polymers for Uranium Adsorption: Influence of Comonomers and Temperature

Recovering uranium from seawater has been the subject of many studies for decades, and has recently seen significant progress in materials development since the U.S. Department of Energy (DOE) has become involved. With DOE direction, the uranium uptake for amidoxime-based polymer adsorbents has more than tripled in capacity. In an effort to better understand how these new adsorbent materials behave under different environmental stimuli, several experimental and modeling based studies have been employed to investigate impacts of competing ions, salinity, pH, and other factors on uranium uptake. For this study, the effect of temperature and type of comonomer on uranium adsorption by three different amidoxime adsorbents (AF1, 38H, AI8) was examined. Experimental measurements of uranium uptake were taken in 1−L batch reactors from 10 to 40 °C. A chemisorption model was developed and applied in order to estimate unknown system parameters through optimization. Experimental results demonstrated that the overall uranium chemisorption process for all three materials is endothermic, which was also mirrored in the model results. Model simulations show very good agreement with the data and were able to predict the temperature effect on uranium adsorption as experimental conditions changed. This model may be used for predicting uranium uptake by other amidoxime materials

Theoretical Study of Oxovanadium(IV) Complexation with Formamidoximate: Implications for the Design of Uranyl-Selective Adsorbents

Poly­(acrylamidoxime) fibers are the current state-of-the-art adsorbent for mining uranium from seawater. However, the competition between uranyl (UO₂²⁺) and vanadium ions poses a challenge to mining on the industrial scale. In this work, we employ density functional theory and coupled-cluster methods in the restricted formalism to investigate potential binding motifs of the oxovanadium­(IV) ion (VO²⁺) with the formamidoximate ligand. Consistent with experimental extended X-ray absorption fine structure data, the hydrated six-coordinate complex is predicted to be preferred over the hydrated five-coordinate complex. Our investigation of formamidoximate–VO²⁺ complexes universally identified the most stable binding motif formed by chelating a tautomerically rearranged imino hydroxylamine via the imino nitrogen and hydroxylamine oxygen. The alternative binding motifs for amidoxime chelation via a nonrearranged tautomer and η² coordination are found to be ∼11 kcal/mol less stable. Natural bond orbital analysis was performed to understand the nature of the interactions in the VO²⁺ complexes. The difference in the most stable VO²⁺ and UO₂²⁺ binding conformation has important implications for the design of more selective UO₂²⁺ ligands

First-Principles Integrated Adsorption Modeling for Selective Capture of Uranium from Seawater by Polyamidoxime Sorbent Materials

Nuclear power is a relatively carbon-free energy source that has the capacity to be utilized today in an effort to stem the tides of global warming. The growing demand for nuclear energy, however, could put significant strain on our uranium ore resources, and the mining activities utilized to extract that ore can leave behind long-term environmental damage. A potential solution to enhance the supply of uranium fuel is to recover uranium from seawater using amidoximated adsorbent fibers. This technology has been studied for decades but is currently plagued by the material’s relatively poor selectivity of uranium over its main competitor vanadium. In this work, we investigate the binding schemes between uranium, vanadium, and the amidoxime functional groups on the adsorbent surface. Using quantum chemical methods, binding strengths are approximated for a set of complexation reactions between uranium and vanadium with amidoxime functionalities. Those approximations are then coupled with a comprehensive aqueous adsorption model developed in this work to simulate the adsorption of uranium and vanadium under laboratory conditions. Experimental adsorption studies with uranium and vanadium over a wide pH range are performed, and the data collected are compared against simulation results to validate the model. It was found that coupling ab initio calculations with process level adsorption modeling provides accurate predictions of the adsorption capacity and selectivity of the sorbent materials. Furthermore, this work demonstrates that this multiscale modeling paradigm could be utilized to aid in the selection of superior ligands or ligand compositions for the selective capture of metal ions. Therefore, this first-principles integrated modeling approach opens the door to the in silico design of next-generation adsorbents with potentially superior efficiency and selectivity for uranium over vanadium in seawater