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Plutonium(VI) / Reduction / Hydrolysis / Absorption spectroscopy / Capillary cellSummary. The chemical speciation of aqueous plutonium species has been performed by spectrophotometry. The main focus was concentrated on the effect of Pu(V) produced by the reduction of Pu(VI) on the stability of the Pu(VI) hydrolysis species. In order to detect the trace amounts of PuO 2 + ions, a liquid waveguide capillary cell with an optical path length of 100 cm was connected to a spectrophotometer. As a consequence of the improved detection sensitivity, an absorption band at 569 nm being indicative of Pu(V) was observed within one day after the sample preparation from the pure Pu(VI) solution. The first hydrolysis species was (PuO 2 ) 2 (OH) 2 2+ , at plutonium concentrations of 0.1-0.12 mM and at weak acidic conditions (pH 5-6). The formation constant of (PuO 2 ) 2 (OH) 2 2+ was determined to be log * β 22 = −7.34 ± 0.22 at 0.01 M NaClO 4 . This dinuclear hydroxide species was unstable and disappeared within one month after the sample preparation at the present experimental condition. In the neutral pH range, the second hydrolysis species was formed and remained for over one year. The samples showed different behavior for the reduction of Pu(VI). The time dependent distribution of plutonium species and their characteristic features in the absorption spectra are reported.

Plutonium(VI) / Hydrolysis / Absorption spectroscopy / Oxidation condition / Redox potentialSummary. The formation constants of mononuclear hydrolysis species of Pu(VI) were determined by absorption spectroscopy under carefully designed experimental conditions. In order to exclude the formation of polynuclear hydrolysis species, the concentration of plutonium was diluted below 50 µM. To maintain the oxidation state of Pu(VI), NaOCl was applied as an oxidant. The trace amount of Pu(V) was monitored by the measurement of redox potential and absorption spectroscopy by using a liquid waveguide capillary cell with the optical path length of 100 cm. The determined molar absorption coefficients (ε, M −1 cm −1 ) were 272 ± 26 and 436 ± 33 for PuO 2 (OH) + and PuO 2 (OH) 2 (aq) species, respectively. The formation constants of mononuclear hydrolysis species of Pu(VI) at ionic strength of 0.01 M NaClO 4 were determined as follows: log * β 1 (for PuO 2 (OH) + ) = −5.8 ± 0.3 , log * β 2 (for PuO 2 (OH) 2 (aq)) = −13.4 ± 0.2 and log * β 3 (for PuO 2 (OH) 3 − ) = −24.3 ± 0.8 .

This study investigates multiple equilibria related to the formation of the U(VI)-salicylate complex in a pH range of 3.0–5.5 using UV-Vis absorption and fluorescence measurement techniques. The absorbance changes at the characteristic charge-transfer bands of the complex were monitored, and the results indicated the presence of multiple equilibria and the formation of both 1:1 and 1:2 (U(VI):salicylate) complexes possessing bi-dentate chelate structures. The determined step-wise formation constants (log K 1:1 and log K 1:2) are as follows: 12.5 ± 0.1 and 11.4 ± 0.2 for salicylate, 11.2 ± 0.1 and 10.1 ± 0.2 for 5-sulfosalicylate, and 12.4 ± 0.1 and 11.4 ± 0.1 for 2,6-dihydroxybenzoate, respectively. The molar absorptivities of the complexes are also provided. Furthermore, time-resolved laser-induced luminescence spectra of U(VI) species demonstrate the presence of both a dynamic and static quenching process upon the addition of a salicylate ligand. Particularly for the luminescent hydroxouranyl species, a strong static quenching effect is observed. The results suggest that both the UO2(HSal)+ and the U(VI)–Sal chelate complexes serve as ground-state complexes that induce static quenching. The Stern–Volmer parameters were derived based on the measured luminescent intensity and lifetime data. The static quenching constants (log K S) obtained are 3.3 ± 0.1, 4.9 ± 0.1, and 4.4 ± 0.1 for UO2 2+, (UO2)2(OH)2 2+ and (UO2)3(OH)5 +, respectively.