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Electrochemical reduction of actinide ions in aqueous solution

Applications to separations and some intermetallic compound synthesis

Journal of Radioanalytical and Nuclear Chemistry
Authors: F. David, A. Maslennikov, and V. Peretrukhin

Abstract  

Electrochemical reduction of heavy elements from aqueous solution to amalgams was studied by radiopolarography and radiocoulometry. Mechanism of actinide reduction on a mercury pool is discussed through simulation techniques. Special emphasis is placed on redox reactions and potentials, kinetics of the process and effect of acetate and citrate ions as complexing agents. Three groups of actinides have been found. The first group represents actinium and from uranium to berkelium. Reduction occurs in the experimental conditions via an irreversible 3–0 process. The second group consists of the elements from fermium to nobelium, which are reduced in non-complexing solutions, or with acetate ions, similarly as barium and radium, via a reversible 2–0 reaction. Finally, californium and einsteinium behave as intermediate elements. It is noticeable that such groups are also observed in the actinide series by studying the structure of the trivalent aqua ions. On the basis of the above mentioned investigations of actinides and lanthanides several examples of electrochemical application are presented. Californium has been separated from preceding transuranium and lanthanide elements (except europium) by electrochemical reduction to amalgams in acetic solution. Separation factors from 25–90 are achieved with appropriate cathodic potentials. Similarly, this element could be separated from several heavier actinides with citric media. The electrochemical preparation of mixed uranium-nickel and uranium-tin amalgams from aqueous acetate solutions is investigated. The dependence of redox potentials of mixed amalgams on different atomic ratio UNi and USn in amalgams is measured. The large shift of redox potentials of mixed amalgams to the positive direction is detected when the atomic ratio UNi or USn in amalgams reaches 15. The thermal distillation of mercury from mixed amalgams with different UNi and USn atomic ratios was carried out and the products were identified by chemical analysis and X-ray diffraction. The intermetallics UNi5 and USn3 were prepared from mixed amalgams with the atomic ratios UNi=15 and USn=13. The uranium and neptunium amalgams are prepared by electrolysis of aqueous acetate solutions and are processes into metals or nitrides U2N3, NpN by thermal distillation of mercury in vacuum or in nitrogen atmosphere.

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Abstract  

The capillary electrophoresis method with direct UV detection is proposed for the determination of nitrite and nitrate in high-salt perchlorate solutions issued from uranium carbide dissolution. The isotachophoretic sample stacking was used to compensate for the perchlorate matrix interference. Simple electrolyte composed of 120 mM formiate buffer, pH 3.8 enabled the nitrate and nitrite determination in the presence of up to 1000-fold excess of perchlorate with 2 µM and 4 µM detection limits for nitrate and nitrite, respectively. The proposed method was applied to the determination of nitrate and nitrite in high-salt non-irradiated uranium carbide dissolution samples.

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Abstract  

The simultaneous determination of U(VI), Pu(VI), Pu(V) in 0.5–4.0 M NaOH has been elaborated by means of classical and differential pulse voltamperometry. U(VI) is determined with a dropping mercury electrode (DME) at the half-wave potential of E1/2=–0.89 V vs. Ag/AgCl reference electrode due to reduction to U(V). The limiting current or peak heights are proportional to uranium(VI) concentration in the range of 1.3.10–7–3·10–4 M U(VI). Deviation from proportionality is observed for higher concentrations due to polymerization of uranates. Pu(VI) and Pu(V) are determined with a platinum rotating electrode at E1/2=–0.02 V due to the reaction Pu(VI)+e»Pu(V) and with DME at E1/2=–1.1 V due to the reduction to Pu(III). The limiting currents of both Pu(VI) and Pu(V) are proportional to their concentrations in the range of 4·10–6–1.2·10–3 M Pu. The determination of U(VI), Pu(VI), Pu(V) is not interfered by the presence of the following salts: 2M NaNO3, 2M NaNO2, 1.5M NaAlO2, 0.5M NaF and ions of Mo(VI), W(VI), V(V), Cu(II). The presence of CrO 4 2– and FeO 2 ions disturbs the determination of U(VI) in 1–4M NaOH, however, contribution of the reaction Fe(III)+e»Fe(II) to uranium reduction peak can be calculated from the height of the second peak Fe(II)+2 e»Fe(0).

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