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U(IV) Aqueous Speciation from the Monomer to UO2 Nanoparticles: Two Levels of Control from Zwitterionic Glycine Ligands.

The fate of U(IV)O2 in the environment in a colloidal form and its dissolution and growth in controlled environments is influenced by organic ligation and redox processes, where both affect solubility, speciation, and transport. Here we investigate U(IV) aqueous speciation from pH 0 to 3 with the glycine (Gly) ligand, the smallest amino acid. We document evolution of the monomeric to the hexameric form from pH 0 to 3 via UV-vis spectroscopy and small-angle X-ray scattering (SAXS). Crystals of the hexamer [U6O4(OH)4(H2O)6(HGly)12]·12Cl-·12(H2O) (U6) were isolated at pH 2.15. The structure of U6 is a hexanuclear oxo/hydroxo cluster U6O4(OH)4 decorated by 12 glycine ligands and 6 water molecules. The effect of pH and temperature on U6 conversion to UO2 nanoparticles, or simply reversible aggregation, is detailed by transmission electron microscopy imaging, in addition to SAXS and UV-spectroscopy. Because of the zwitterion behavior of glycine, pH and temperature control over U(IV) speciation is complex. Unexpectedly, stability of the polynuclear cluster actually increases with increased pH. Speciation is sensitive to not only metal-oxo hydrolysis but also ligand lability and hydrophobic ligand-ligand interactions.