Speciation and coordination chemistry of uranyl(VI)-citrate complexes in aqueous solution

The pH dependence of uranyl(VI) complexation by citric acid was investigated using Raman and attenuated total reflection FIR spectroscopies and electrospray ionization mass spectrometry. pH-dependent changes in the nu(s)(UO2) envelope indicate that three major UO22+-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 UO22+ groups in spectroscopically equivalent coordination environments and corresponds to the [(UO2)(2)Cit(2)](2-) complex known to exist in this pH range. At pH values >6.5, [(UO2)(2)Cit(2)](2-) undergoes an interconversion to form [(UO2)(3)Cit(3)](3-) and (UO2)(3)Cit(2). 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 nM.