Dimeric and Trimeric Uranyl(VI)-Citrate Complexes in Aqueous Solution

Area of Science:

• Inorganic Chemistry
• Environmental Chemistry
• Solution Chemistry

Background:

• Uranyl(VI) (U(VI)) and citric acid interactions have been controversial for nearly 70 years.
• Previous studies proposed various complex structures without definitive evidence.

Purpose of the Study:

• To unambiguously determine the structures of U(VI)-citrate complexes in aqueous solution.
• To investigate the speciation and stability of these complexes across different pH values.
• To revise existing misconceptions in U(VI)-citrate solution chemistry.

Main Methods:

• Multi-spectroscopic approach: 17O NMR, UV-Vis, ATR FT-IR, EXAFS.
• Computational modeling: Density Functional Theory (DFT) calculations.
• Species distribution and stability constant determination.

Main Results:

• First evidence for dimeric (UO2)2(HCit-H)22- existing as syn- and anti-diastereomers, with syn-isomer favored.
• Identification of ternary dimeric and trimeric U(VI)-citrate complexes, including unprecedented stability constants for a 3:3 species.
• Characterization of the 6:6 sandwich complex and its trinuclear uranyl(VI) core.

Conclusions:

• The characterized polynuclear U(VI)-citrate species do not significantly enhance uranium mobility in the environment.
• This research corrects long-standing misconceptions regarding U(VI)-citrate complexation.
• Provides a basis for future studies on U(VI) complexation with generalized isostructural considerations.