Speciation and Coordination Chemistry of Uranyl(VI)−Citrate Complexes in Aqueous Solution

• Published in: Inorganic chemistry 2003-10, Vol.42 (21), p.6793-6800
• Main Authors: Pasilis, Sofie P, Pemberton, Jeanne E
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0341800

The pH dependence of uranyl(VI) complexation by citric acid was investigated using Raman and attenuated total reflection FTIR spectroscopies and electrospray ionization mass spectrometry. pH-dependent changes in the νs(UO2) envelope indicate that three major UO2 2+−citrate complexes with progressively increasing UO bond lengths are present over a range of pH from 2.0 to 9.5. The first species, which is the predominant form of uranyl(VI) from pH 3.0 to 5.0, contains two UO2 2+ groups in spectroscopically equivalent coordination environments and corresponds to the [(UO2)2Cit2]2- complex known to exist in this pH range. At pH values >6.5, [(UO2)2Cit2]2- undergoes an interconversion to form [(UO2)3Cit3]3- and (UO2)3Cit2. ESI-MS studies on solutions of varying uranyl(VI)/citrate ratios, pH, and solution counteranion were successfully used to confirm complex stoichiometries. Uranyl and citrate concentrations investigated ranged from 0.50 to 50 mM.