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

Spacer Monomer in Polymer Chain Influencing Affinity of Ethylene Glycol Methacrylate Phosphate toward UO₂²⁺ and Pu⁴⁺ Ions

The complexation behavior of ligating groups bearing ethylene glycol methacrylate phosphate (EGMP) units spaced in the polymer chain was studied to understand the coordination ability of segregated EGMP toward UO₂²⁺ and Pu⁴⁺ ions. The EGMP units in the polymer chain were copolymerized with a varying mol proportion of spacer monomer <i>N</i>-isopropylacylamide (NIPA) and methyl methacrylate (MMA) along with a cross-linker. These copolymer gels were characterized by FTIR, SEM/EDS, and thermal analysis. It was observed that the copolymer gels had homogeneity and concentration of phosphate groups systematically decreased with an increase in mol proportion of spacer monomer units. The hydrophlicity of (EGMP-<i>co</i>-MMA) copolymer gels decreased with an increase in mol proportion of MMA units whereas hydrophilicty of EGMP-<i>co</i>-NIPA copolymer increased with an increase in mol proportion of NIPA units in the copolymer gels. It was observed that UO₂²⁺ ions sorption decreased with an increase in MMA units in the polymer chain at higher HNO₃ conc. (3 mol L^–1) but did not affect Pu⁴⁺ ions sorption. This seems to suggest that EGMP units segregated sufficiently in the polymer chain by spacers exhibit a remarkable selectivity toward Pu⁴⁺ ions with respect to UO₂²⁺ ions at high HNO₃ conc. Pu­(IV) ions are known to have high affinity toward nitrate ions that help in the formation of a stable complex at higher acidity with a lesser number of coordination with phosphate groups. The experiments showed that the lower affinity of poly­(EGMP-<i>co</i>-MMA) gel toward UO₂²⁺ ions was not due to dilution of phosphate groups concentration in the copolymer gel but could be attributed to the coordination requirement of UO₂²⁺ ions to form a stable complex at higher HNO₃ concentration. The X-ray photoelectron spectroscopy (XPS) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) studies of poly­(EGMP) loaded with UO₂²⁺ ions from aqueous solution having pH 2 and 3 mol L^–1 HNO₃ indicated that (i) 1:2 and 1:4 (UO₂²⁺–EGMP) complexes are formed at lower acidity and higher acidity, respectively, (ii) UO₂²⁺ ion is coordinated with water molecules at pH range in addition to the ion-exchange, and (iii) the complex formed in pH range has higher stability with respect to that form at 3 mol L^–1 conc. The higher hydrophilic poly­(EGMP-<i>co</i>-NIPA) gels at 25 °C exhibit higher UO₂²⁺ uptake in 3 M HNO₃, as phosphate units in hydrophilic gel are mobile and could form stable complex involving 3–4 phosphoryl oxygen, which is not possible in collapsed state due to hydophobicity of polymer net work above critical temperature. However, there was no effect of hydrophilicty/hydrophobicity on the UO₂²⁺ ions sorption in the copolymer gels from solutions having lower HNO₃ conc. due to a requirement of lesser number of EGMP units to form a stable complex

Elution of Uranium and Transition Metals from Amidoxime-Based Polymer Adsorbents for Sequestering Uranium from Seawater

High-surface-area amidoxime and carboxylic acid grafted polymer adsorbents developed at Oak Ridge National Laboratory were tested for sequestering uranium in a flowing seawater flume system at the PNNL-Marine Sciences Laboratory. FTIR spectra indicate that a KOH conditioning process is necessary to remove the proton from the carboxylic acid and make the sorbent effective for sequestering uranium from seawater. The alkaline conditioning process also converts the amidoxime groups to carboxylate groups in the adsorbent. Both Na₂CO₃–H₂O₂ and hydrochloric acid elution methods can remove ∼95% of the uranium sequestered by the adsorbent after 42 days of exposure in real seawater. The Na₂CO₃–H₂O₂ elution method is more selective for uranium than conventional acid elution. Iron and vanadium are the two major transition metals competing with uranium for adsorption to the amidoxime-based adsorbents in real seawater. Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, 1 M) can remove iron from the adsorbent very effectively at pH around 7. The coordination between vanadium­(V) and amidoxime is also discussed based on our ⁵¹V NMR data

Investigations into the Reusability of Amidoxime-Based Polymeric Adsorbents for Seawater Uranium Extraction

The ability to reuse amidoxime-based polymeric adsorbents is a critical component in reducing the overall cost of the technology to extract uranium from seawater. This report describes an evaluation of adsorbent reusability in multiple reuse (adsorption/stripping) cycles in real seawater exposures with potassium bicarbonate (KHCO₃) elution using several amidoxime-based polymeric adsorbents. The KHCO₃ elution technique achieved ∼100% recovery of uranium adsorption capacity in the first reuse. Subsequent reuses showed significant drops in adsorption capacity. After the fourth reuse with the ORNL AI8 adsorbent, the 56-day adsorption capacity dropped to 28% of its original capacity. FTIR spectra revealed that there was a conversion of the amidoxime ligands to carboxylate groups during extended seawater exposure, becoming more significant with longer exposure times. Ca and Mg adsorption capacities also increased with each reuse cycle supporting the hypothesis that long-term exposure resulted in converting amidoxime to carboxylate, enhancing the adsorption of Ca and Mg. Shorter seawater exposure (adsorption/stripping) cycles (28 vs 42 days) had higher adsorption capacities after reuse, but the shorter exposure cycle time did not produce an overall better performance in terms of cumulative exposure time. Recovery of uranium capacity in reuses may also vary across different adsorbent formulations. Through multiple reuses, the AI8 adsorbent can harvest 10 g uranium/kg adsorbent in ∼140 days, using a 28-day adsorption/stripping cycle, a performance much better than would be achieved with a single use of the adsorbent through a very long-term exposure (saturation capacity of 7.4 g U/kg adsorbent). A time dependent seawater exposure model to evaluate the cost associated with reusing amidoxime-based adsorbents in real seawater exposures was developed. The predicted cost to extract uranium from seawater ranged from $610/kg U to$830/kg U. Model simulation suggests that a short seawater exposure cycle (<15 days) is the optimal deployment period for lower uranium production cost in seawater uranium mining