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

摘要

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. [References: 45]