Kyoung-Kyun Park

Effect of reduction on the stability of Pu(VI) hydrolysis species

Abstract 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 PuO2+ 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 (PuO2)2(OH)22+, at plutonium concentrations of 0.1–0.12 mM and at weak acidic conditions (pH 5–6). The formation constant of (PuO2)2(OH)22+ was determined to be log*β′22=−7.34±0.22 at 0.01 M NaClO4. 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.

Spectroscopic studies on U(VI)-salicylate complex formation with multiple equilibria

Abstract 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 K1:1 and log K1:2) 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 KS) obtained are 3.3 ± 0.1, 4.9 ± 0.1, and 4.4 ± 0.1 for UO22+, (UO2)2(OH)22+ and (UO2)3(OH)5+, respectively.

Spectroscopic study on the mononuclear hydrolysis species of Pu(VI) under oxidation conditions

Abstract 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 PuO2(OH)+ and PuO2(OH)2(aq) species, respectively. The formation constants of mononuclear hydrolysis species of Pu(VI) at ionic strength of 0.01 M NaClO4 were determined as follows: logߙ*β′1 ߙ(for PuO2(OH)+) = −5.8±0.3, logߙ*β′2 ߙ(for PuO2(OH)2(aq)) = −13.4±0.2 and logߙ*β′3ߙ(for PuO2(OH)3−) = −24.3±0.8.

Pertechnetate removal from aqueous solution using activated carbon modified with oxidizing and reducing agents

The adsorption mechanism of pertechnetate on activated carbon (AC) has been studied. For this work, the surface of commercial AC was modified using various oxidizing and reducing agents. The changes of surface functional groups were monitored by FT-IR spectroscopy and Boehm titration. The distribution coefficient (Kd) of TcO4− on the modified AC has been determined. In acidic media, the value of Kd slightly increased owing to the reductive treatment of ACs, whereas a large decrease in Kd was observed in case of oxidative treatment of ACs. Based on this work, the previous adsorption mechanisms for TcO4− on AC were examined. As an alternative adsorption mechanism, the adsorption by electrostatic attraction between TcO4− and protonated basic sites on AC surface was proposed.