XAFS investigation of polyamidoxime-bound uranyl reveals an adjacent μ2-oxo-bridged transition metal, suggesting new routes for adsorbent design in radionuclide separations.

The role of different intermolecular interactions in the aggregation of amphiphiles in an organic solvent is studied for systems of relevance to liquid−liquid extraction (LLE), a chemical process used to selectively recover metals from complex mixtures. Of specific interest is the role, or lack thereof, of hydrogen bonding, which is often assumed to be a main driver of the organic phase structural organization that has been linked to separation efficacy. Toward that end, a series of malonamide extractants in n-dodecane have been studied in the absence of any extracted aqueous solutes, including water. The series of extractants includes N,N′-dimethyl-N,N′-dibutyltetradecylmalonamide (DMDBTDMA), two of its homologs, and N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA). This simplified model LLE system enables systematic investigation of the role of dipole−dipole and alkyl tail steric interactions in amphiphile aggregation. Small-angle X-ray scattering (SAXS) profiles computed from molecular dynamics trajectories are in good agreement with experimental SAXS data. Molecular dynamics simulations show that malonamide aggregation results from dipole-driven self-association and lacks characteristic aggregate sizes. Mid-q correlation peaks in the SAXS profiles emerge at high concentration for each malonamide. In those densely packed solutions, the correlation peaks are observed to result from alkyl tail-induced spacing between electron-rich polar head groups, with peak positions determined by the different alkyl tail lengths present in the malonamide molecule. This explanation of the SAXS correlation peaks contrasts with the prevailing literature, which attributes mesoscale features observed in small-angle scattering to the formation of microemulsions. Instead, this work finds that these features are present in the absence of water or any reverse micellar organization of the malonamides. As such, molecular-scale malonamide self-association and packing, rather than microemulsion-based colloidal-scale descriptions, is a more appropriate framework for these LLE systems.

The amidoxime group (-RNHNOH) has long been used to extract uranium from seawater on account of its high affinity toward uranium. The development of tunable sorbent materials for uranium sequestration remains a research priority as well as a significant challenge. Herein, we report the design, synthesis, and uranium sorption properties of bis-amidoxime-functionalized polymeric materials (BAP 1-3). Bifunctional amidoxime monomers were copolymerized with an acrylamide cross-linker to obtain bis-amidoxime incorporation as high as 2 mmol g after five synthetic steps. The resulting sorbents were able to uptake nearly 600 mg of uranium per gram of polymer after 37 days of contact with a seawater simulant containing 8 ppm uranium. Moreover, the polymeric materials exhibited low vanadium uptake with a maximum capacity of 128 mg of vanadium per gram of polymer. This computationally predicted and experimentally realized selectivity of uranium over vanadium, nearly 5 to 1 w/w, is one of the highest reported to date and represents an advancement in the rational design of sorbent materials with high uptake capacity and selectivity.

Informed by density functional theory calculations, a novel bifunctional chelator, (Z)-2-[2-(N′hydroxycarbamimidoyl)phenoxy]benzoic acid, was designed and synthesized for ultrahigh uranium uptake from seawater. Investigation of the ligand for uranium sorption was conducted in artificial seawater (pH = 8.2). An exceptional uranium uptake of 553 mg of uranium (g of sorbent) −1 was obtained with a theoretical saturation capacity of 710 mg g −1 obtained by fitting isotherm data with the Langmuir−Freundlich model. The resulting yellow precipitate was characterized via X-ray absorption fine structure (XAFS) at the U L III -edge, with the extended XAFS spectra best fitted by a model where uranyl is coordinated by monodentate amidoxime, one chelating carboxylic acid, and two water molecules. These results are consistent with the formation of a uranium coordination polymer. The ultrahigh uranium uptake capacity obtained by the bifunctional chelating ligand makes it a promising candidate for deployment as a uranium adsorbent.