Complexation of aqueous uranium(IV) with phosphate investigated using time-resolved laser-induced fluorescence spectroscopy

Heavy metals like the actinides possess a high risk potential to the environment not only because of their radiotoxicity but also due to their chemical toxicology. Uranium as one of the major actinide elements has to be considered in particular. Under reducing conditions, tetravalent uranium occurs primarily in the environment. To date, a lack of appropriate analytical techniques that featured sufficient sensitivity made it difficult to study the aqueous phosphate chemistry of uranium(IV) as such complexes show only low solubility. A novel time-resolved laser fluorescence spectroscopy system was set up recently and optimized to do research on uranium(IV). By application of this laser system we could successfully study uranium(IV) phosphate in concentration ranges where no precipitation or formation of colloids occurred. At pH = 1.0, U⁴⁺ and one uranium(IV) phosphate complex existed in parallel in aqueous solution. The complex could be identified as U(H₂PO₄)]³⁺. Determination of its corresponding complex formation constant via two different evaluation methods resulted in the finding of (1) logb₁₂₁ ^° = 2 6. 3 7 ±0. 7 6 \log \beta_{121}^{ \circ } = 2 6. 3 7 \pm 0. 7 6 and (2) logb₁₂₁ ^° = 2 6. 4 3 ±0. 2 3 \log \beta_{121}^{ \circ } = 2 6. 4 3 \pm 0. 2 3 . Both values prove that U(H₂PO₄)]³⁺ is a very stable complex in solution under experimental conditions. As they are in very good agreement with each other, the total complex formation constant was determined by means of the weighted average out of (1) and (2). It was calculated to be logb₁₂₁ ^° = 2 6. 4 2 ±0. 2 2 \log \beta_{121}^{ \circ } = 2 6. 4 2 \pm 0. 2 2 .