Effect of citric acid on uranyl(VI) solution speciation, gas-phase chemistry and surface interactions with alumina

Activities related to nuclear weapons production have left a legacy of uranium contamination in the United States. Understanding the chemical interactions that uranium undergoes in the environment is important for prediction of uranium mobility and development of remediation strategies.; A detailed spectroscopic examination of the pH-dependent behavior of the UO22+-citrate system in aqueous solution was completed using Raman, ATR-FTIR, and NMR spectroscopies, combined with ESI-MS. Three structurally-distinct UO22+-citrate complexes, {lcub}(UO2)2Cit2{rcub}2-, {lcub}(UO 2)3Cit3{rcub}3-, and (UO 2)3Cit2 exist in dynamic equilibrium over a pH range from 2 to 9. 17O and 13C NMR data confirm the previously published structure of {lcub}(UO2)2Cit 2{rcub}2- and indicate that {lcub}(UO2)3 Cit3{rcub}3- is a symmetric, fluxional molecule. The (UO2)3Cit2 complex was found to have a rigid structure and two structural isomers.; Chemical interactions of U(VI), citric acid and Al2O 3 were investigated using ATR-FTIR spectroscopy to examine how complexation of U(VI) by citrate affects adsorption of U(VI) to Al2O3. Participation in UO22+-citrate complexes does not significantly affect the ability of citrate to chemisorb to Al2O 3. The UO22+-citrate complexes dissociate upon adsorption, with hydrolysis of UO22+. Adsorption isotherms developed from ATR-FTIR data indicate enhanced citrate adsorption to Al2O3 in the presence of UO22+ , suggesting that UO22+ acts as a central link between two citrate ligands, one of which is complexed to Al2O 3. UO22+-citrate complexes can physisorb to citrate-saturated Al2O3. This study demonstrates how an in-depth infrared spectroscopic analysis of UO22+-ligand complexes both in solution and adsorbed to oxide surfaces can be used to understand the adsorption mechanisms of these complexes.; ESI-MS was investigated for the characterization of U(VI) species in groundwater. Both ion trap and FTICR instruments were used. UO2 2+ forms complexes with ligands such as acetate, trifluoroacetate, and nitrate, which readily react with CH3CN, CH3OH, and H2O to form solvated gas-phase species of the form [(UO 2L)Sn]+, where L represents the ligand, S represents solvent, and 1 ≤ n ≤ 4. n is directly related to the number of available coordination sites on UO22+, providing insight into the coordination environment of UO22+. Solvent exchange and addition reactions readily occur. UO2 2+ also forms coordinately saturated negatively-charged complexes with nitrate. UO22+-carbonate complexes were also investigated.