Electroreduction of pertechnetate ions in concentrated acetate solutions

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Highlights

  • Metallic technetium can be electrodeposited from aqueous ammonia-acetate solutions.

  • Electroreduction of pertechnetate ions proceeds stepwise via the route Tc(VII) → Tc(IV) → Tc(III) → Tc(0).

  • The obtained technetium coatings are amorphous.

Abstract

Electrochemical reduction of pertechnetate ions is studied in acetate solutions on both mercury dropping and solid electrodes. It is found that electroreduction of TcO4 ions on the mercury dropping electrode occurs stepwise via the following route Tc(VII) → Tc(IV) → Tc(III) → Tc(0). Ammonium ions affect the reduction of pertechnetate ions by shifting potentials of polarographic waves in a positive direction.

Diffusion coefficients of Tc(VII) species in acetate solutions decreased from ~1 · 10−5 cm2 s−1 in a solution containing 2 mol dm−3 acetate ions to ~1.2 · 10−6 cm2 s−1 in a solution containing 8 mol dm−3 acetate ions. The Stokes radius of pertechnetate ion is 0.19 ± 0.01 nm.

Technetium coatings were characterized using X-ray absorption spectroscopy (XANES/EXAFS). It is shown that technetium is in the metallic state in the electrolytic deposits; however, the obtained metal is amorphous. The Tc-Tc interatomic distances and coordination numbers of Tc atoms in the deposits are determined. The distance between technetium atoms in the electrodeposits is slightly less (0.262 nm) compared to technetium foil (0.272 nm).

Introduction

Electrochemical reduction of pertechnetate ions with formation of solid products is of interest for the development of an efficient technology for the processing of nuclear wastes, since the products of uranium fission contain ca. 6% of technetium [1]. Technetium species are characterized by a high mobility in the environment [2,3], and, therefore, can cause very serious risks in the case of their leakage.

Electroreduction of Tc(VII) usually results in the formation of soluble technetium species in the intermediate oxidation states [4,5]; however, deeper electroreduction of technetium species with formation of metallic technetium is also possible [[6], [7], [8]]. Electrodeposition of technetium is of considerable interest for practice. Technetium coatings can be employed for the production of isotopically pure 99Ru, which is used for Mössbauer studies [9]. Besides, Tc deposits can be applied in the standard β–radiation sources calibrated for the middle energy range (100–300 keV). Electrodeposition can also be used for separation from molybdenum target of the short-lived nuclear isomer 99mTc, which is in high demand in nuclear medicine.

However, the data on electroreduction of technetium species to the metallic state are lacking in the literature. The majority of the published papers are focused on the incomplete reduction of pertechnetate ions with formation of either Tc(IV) or Tc(III) species [4,10,11]. It was found that both the products and routes of electrochemical reduction differ in acidic and alkaline solutions [12]. Electroreduction of pertechnetate ions in acidic solutions proceeds in a stepwise manner and results in the formation of Tc(IV) [13,14] or Tc(III) [[4], [5], [6]] soluble species. The formation of mixed-valence compounds, e.g. Tc2O23+, is also possible [10]. Tc2O23+ ions undergo the further reduction to the hydrated form of TcO(OH) at more negative electrode potentials. The electrode potential of the half-reaction ofTc2O23++2H2O+e2TcOOHaq+2H+was found to be equal to 0.255 V in [3]. Here and after all electrode potentials are given vs. a sat. Ag/AgCl reference electrode.

Some discrepancy in results presented in [4,5] and [14] is probably related to the dependence of the routes of electroreduction of pertechnetate ions on the material of the cathode. Detailed information on the chemical nature of species formed during electroreduction of pertechnetate ions can be obtained by means of spectroelectrochemical methods. These revealed formation of unstable Tc(V) species as intermediate products of electroreduction of pertechnetate ions [15].

Electroreduction of pertechnetate ions to the metallic state takes place at high cathode potentials and is accompanied by vigorous hydrogen evolution [7,8,14]. For this cause, electrodeposition of metallic technetium proceeds with low current efficiency. Moreover, it was suggested that complexation of Tc(IV) with sulfate ions occurring in the solutions of sulfuric acid leads to the formation of species that cannot be reduced to the metallic state [16]. Therefore, the solutions of sulfuric acid are considered by the authors of [16] unsuitable for metallic technetium deposition.

To reduce hydrogen evolution, it is desirable to increase pH of the plating solution. However, an increase in pH can lead to the formation of hydrated technetium(IV) oxide on the cathode and later – in the solution, thus resulting in the conversion of Tc to electrochemically inactive species. It should be noted that alkaline solutions are not suitable for deposition of technetium coatings since hydrated technetium(IV) polymeric oxospecies are the product of pertechnetate ion electroreduction in such case [17]. Their further electroreduction to metal is impossible. Generally speaking, the formation of kinetically inert oxo- and hydroxospecies of technetium in intermediate oxidation states is one of the major obstacles in technetium electrodeposition.

Various ligands could be used to prevent hydrolysis of technetium species in the intermediate oxidation states. However, the complexes formed should be capable of the further reduction to the metallic state. It was found that metallic technetium was deposited from solutions containing an excess of carboxylate ions at pH 4–7. Formate anions were used for the preparation of solutions for metallic technetium deposition in [8].

The possibility of molybdenum electrodeposition from aqueous solutions containing a high concentration of acetate ions have recently been demonstrated in [[18], [19], [20]]. Comparing the electrode potentials of half-reactions with the participation of Mo and Tc species [21], one can conclude that the redox transitions of technetium species occur at more positive potentials. Hence, the electrodeposition of technetium should be facilitated, as compared to molybdenum. This led us to the idea of using electrolyte solutions with the composition similar to [[18], [19], [20]] for electrodeposition of technetium coatings.

The aims of this investigation were: (i) determination of the kinetics of electroreduction of pertechnetate ions on both liquid mercury and solid electrodes; (ii) electrodeposition of technetium coatings, and (iii) characterization of metal deposits obtained.

Section snippets

Materials and methods

Electroreduction of Tc(VII) species and electrodeposition of technetium coatings were studied in concentrated acetate solutions containing 4.0 M ammonium acetate, 4.0 M potassium acetate, and 0.001–0.004 M potassium pertechnetate. Some experiments were carried out in solutions that contained no ammonium ions; in this case, the solution contained 8.0 M potassium acetate. Deionized water (Milli-Q, R > 18.2 MΩ cm, TOC < 3 ppb) was used for the preparation of solutions. pH of electrolyte was

Polarographic behavior of Tc(VII) in concentrated acetate solutions

The possibility of Tc(VII) electroreduction to the zero oxidation state in the solutions of potassium acetate was proved by means of polarography. In fact, there were three waves in polarograms in a solution containing 8 M potassium acetate, which corresponded to a stepwise reduction of pertechnetate ions (Fig. 1). The ratio of the heights of the first two waves was approximately 3:1. This observation allows us to conclude that electroreduction of Tc(VII) proceeds according to the route of

Conclusions

  • 1.

    Pertechnetate ions can be reduced to a metallic state in the concentrated acetate solutions. The formation of Tc(0) occurs at high cathodic potentials E < −1.4 V in the solutions of potassium acetate and at E < −1.15 V in the mixed solutions of potassium and ammonium acetates on the dropping mercury electrode.

  • 2.

    Electroreduction of pertechnetate ions in the acetate solutions occurs via the following route Tc(VII) → Tc(IV) → Tc(III) → Tc(0). The limiting current at E = −0.6 … –1.1 V in

CRediT authorship contribution statement

V.V. Kuznetsov: Conceptualization, Methodology, Writing - original draft. M.A. Volkov: Investigation. K.E. German: Investigation, Methodology, Project administration. E.A. Filatova: Writing - review & editing. O.A. Belyakova: Data curation, Formal analysis, Software. A.L. Trigub: Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the Core Facilities Center of the Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences

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