Interactions of thorium(IV), uranium(VI), neptunium(V) and americium(III) with polyoxometalates

The anionic polyoxometalates [NaP₅W₃₀O₁₁₀ ]14−, [As₂W₂₁O₆₉] 6− [P₂W₁₈O₆₂]6− and [P₂W₁₇O₆₁]10− were synthesized according to published procedures, and were characterized by 31P nmr, thermogravimetric analysis and infrared spectroscopy.; Solvent extraction and calorimetric titration techniques were used to measure the stability constants and thermodynamic parameters (ΔG, ΔH, ΔS) of formation of 1:1 complexes of [NaP₅W₃₀O₁₁₀] 14− with the four actinides (Th4+, UO₂ 2+, NpO₂+ and Am3+). The formation of 2:1 and 3:1 complexes of Th4+ with [NaP₅ W₃₀O₁₁₀]14− were measured by solvent extraction and the stability constants calculated using the speciation program HySS.; The stability constants and thermodynamic parameters (ΔG, ΔH, ΔS) for the interaction of Th4+, UO₂2+, NpO₂+ and Am3+ with [As₂W ₂₁O₆₉]6− were also determined. The ΔH for interaction of Th4+, UO₂2+ with [P₂W₁₈O₆₂]6− and [P ₂W₁₇O₆₁]10− were determined and discussed in context of previous measurements. The stability constant of NpO₂+ with [P₂W₁₇O ₆₁]10− was also determined.; The order of magnitude of the metal/polyoxometalate stability constants was Th4+ > Am3+ > UO₂2+, NpO₂+ which was interpreted on the basis of electrostatic repulsion between the terminal oxygens of the actinyl ions and the oxygens of the polyoxometalates. Likely geometries of complexation were proposed using computer simulated molecular modeling.