Thorium Complexation with Aliphatic and Aromatic Hydroxycarboxylates: A Combined Experimental and Theoretical Study

Area of Science:

• Analytical Chemistry
• Radiochemistry
• Computational Chemistry

Background:

• Hydroxycarboxylic acids like alpha-hydroxyisobutyric acid (HIBA) and mandelic acid (MA) are key eluents in inner transition metal separation.
• Despite similar functional groups, HIBA and MA exhibit distinct retention behaviors for thorium and uranium in liquid chromatography.

Purpose of the Study:

• To elucidate the aqueous phase interaction mechanism between thorium and HIBA/MA.
• To understand how differing complexation affects chromatographic separation of thorium and uranium.

Main Methods:

• Potentiometric titration for speciation and stability analysis.
• Electrospray ionization mass spectrometry (ESI-MS) for identifying Th-ligand species.
• Density Functional Theory (DFT) calculations and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy for mechanistic insights.

Main Results:

• Dominant species identified: Th-HIBA forms ML3, while Th-MA forms ML4.
• Potentiometric speciation confirms thorium's higher affinity for MA over HIBA, forming more stable, higher-stoichiometry complexes.
• Observed complexation differences correlate with reversed retention behavior in chromatographic separations.

Conclusions:

• The distinct complexation stoichiometries (ML3 for HIBA, ML4 for MA) are the primary drivers for the observed differences in thorium and uranium retention during liquid chromatography.
• This study provides a mechanistic explanation for chromatographic separation trends using hydroxycarboxylic acids.