Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 267-269, 2005

Study on Electrolytic Reduction of Pertechnetate in Nitric Acid Solution for Electrolytic
Extraction of Rare Metals for Future Reprocessing
T. Asakura,*, a S.-Y. Kim,a Y. Morita,a and M. OzawabaTokai Research Establishment, Japan Atomic Energy Research Institute (JAERI), Tokai-mura, Ibaraki
319-1195, JapanbO-arai Engineering Center, Japan Nuclear Cycle Development Institute (JNC), Oarai-machi, Ibaraki
311-1393, Japan
Received: July 31, 2005; In Final Form: September 30, 2005
Electrolytic reduction of Tc(VII) in a nitric acid solution was examined to recover Tc as well as rare metals (Pd,Ru, and Rh) by an electrolytic extraction method, i.e. electro-reductive deposition, from a spent fuel solution for
future nuclear fuel reprocessing. An absorption peak at 482 nm observed after electrolytic reduction of Tc(VII)in 3 M HNO3 at –0.3 and –1.0V vs SSE (standard silver electrode) suggests the reduction of Tc(VII) to Tc(IV).
When Pd is present together with Tc in 1 M HNO3, the concentration of Tc decreased to 85% of the initial concentration by electrolysis at ± 0.0 V vs. SSE for 60 min, suggesting acceleration of Tc deposition by Pd (promotereffect). The Tc concentration, however, increased returning to the initial value after further electrolysis. The Pd
deposit from Tc-Pd-Ru-Rh solution redissolved more easily than that from Pd-Ru-Rh, as a consequence of achange in the oxidation-reduction property of the Pd deposit due to the presence of Tc in the solution.

1. Introduction
In nuclear fuel reprocessing, many potentially useful rare
metals, such as Pd, Ru, Rh, Tc, Te and Se, are vitrified as highlevel waste. If it is possible to recover and utilize these elements, spent fuel reprocessing will be much more attractive. In
the future, it will be strongly demanded to minimize the volumeof radioactive waste arising from reprocessing. In an electrolytic extraction (EE) method,1, 2 i.e. electrolytic reductive deposition, little secondary radioactive waste is produced because noextra chemical reagent is needed in the EE method. Therefore,
the EE method is considered to be promising for recovering
those precious elements. In the present work, we examinedelectrolytic reduction of Tc in a nitric acid solution to recover Tc
together with rare metals (Pd, Ru, and Rh), whereas Re wasused as a simulant for Tc in the previous work.1, 2ver electrode (SSE, Ag/AgCl in 3 M KCl) was used as a reference electrode. All of these electrodes were dipped in anelectrolyte solution. The electrolyte solution was prepared fromthe Tc stock solution described above by adding aqueous solution of HNO3 with desired concentration. Aqueous solution ofPd(NO3) 2 was also added to the electrolyte if necessary. The
concentration of Tc and Pd in the electrolyte solution was determined by ICP-AES. Photo-absorption spectra of the Tc (only)
solutions were measured during electrolysis. The electrolysis
was tentatively stopped after prescribed time and a part of anelectrolyte solution (2 mL) was transferred to a 1 cm quartz cell.
The spectrum of the solution was measured with a Shimadzu U4100 spectrophotometer.
3. Results and Discussion
Figure 1 shows a change of absorption spectra of the Tc
solution by electrolysis at the potential of –0.3 V vs. SSE. The
solution showed a reddish brown color. Three peaks at 368,

2. Experimental
To prepare Tc(VII) stock solution, 185 MBq of NH499TcO4powder purchased from Du Pont Co. was used. The powder
showed a black color suggesting the existence of Tc oxide (probably TcO2 ·xH2O), and was pre-treated to obtain pure NH4TcO4.
In 30 mL of 0.1 M NH3 solution, the powder was mildly heatedwith stirring and 10 M H2O2 solution was added dropwise. Thepowder was completely dissolved in 5 min, and then the solution was evaporated to dryness. The obtained white solid ofNH4TcO4 was dissolved in 10 mL of water (stock solution). The
concentration of Tc in the stock solution was determined to be
0.144 M by induction coupled plasma-atomic emission spectroscopy (ICP-AES).
Constant potential electrolytic experiments were performed
with a BAS CV-660A voltammetric analyzer by a three-electrode technique. Carbon plates of 0.2 mm in thickness were cut
into 1.0 cm squares, and three plates were used as a working
electrode giving a surface area of 6 cm2. A Pt wire of 0.6 mm
in diameter was used as a counter electrode, and a standard sil*Corresponding author. E-mail: asakura.toshihide@jaea.go.jp.
FAX: +81-29-282-6677.

Figure 1. Absorption spectra of Tc solution of electrolysis. Electrolyte: 1.5 mM of initial Tc in 3 M HNO3 aqueous solution. Electrolysis
conditions: Potential of –0.3 V vs. SSE, current density of approximately 2 mA cm−2. Line: a baseline, the spectrum of initial solution,
the spectra of the solution electrolyzed for 10, 20, 30, 40, 50, 60, 90,and 210 min.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005268 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
384, and 482 nm are observed in the spectra. The spectra ofthe solution electrolyzed at –1.0 V vs. SSE were almost thesame as Figure 1. The two peaks at 368 and 384 nm are specific to HNO2 and imply evolution of HNO2 during the electrolysis. From colorimetric measurements, the concentration ofHNO2 was determined to be 6 mM after 60 min of electrolysis.
An additional absorption peak at 482 nm in the spectra is not
attributed to HNO2. Boukis et al. reported the chemical reduction of Tc(VII) to Tc(IV) by hydrazine in a nitric acid solution
and the formation of Tc(IV) complex with DBP−.3 They founda weak shoulder of Tc (IV) at 470 nm. They also found a strong
absorption peak of a Tc(IV) complex with DBP− at 472 nm inpentane. Spitsyn et al. found an absorption peak at 500 nm
after electrolytic reduction of KTcO 4 in H 2SO 4 solution andattributed the peak to the formation of a complex composed ofTc(IV) and SO42−.4 The absorption peak at 482 nm observed in
the present work suggests that Tc(VII) was reduced to Tc(VI).
It is reasonable to consider that the significant absorption peak(483 nm) implies the formation of a Tc(IV) complex.
Figure 2 shows a change of absorption spectra of the Tc solution containing hydrazine, a well known HNO2 scavenger, byelectrolysis at –0.3 V vs. SSE. After electrolysis the solution
became yellowish. In the spectra, only a peak at 430 nm wasobserved. There are no peaks at 368 and 384 nm arising from
HNO2. The spectra of the solution electrolyzed at –1.0 V vs.
SSE were almost the same as Figure 2. The wavelength of 430
nm for the absorption peak observed here is fairly shorter thanthat of Tc (IV).3 In literatures,5, 6 an absorption band at 400–430
nm is reported for a yellowish complex of Tc with isothiocyanate. From the results of elemental and electrochemical analyses, the valence state of Tc in the complex is established to beTc(III).5 The absorption peak (430 nm) observed here may suggest the formation of a Tc(III) complex by electrolytic reduction.
In Figure 3, a concentration change of Tc during electrolysis
in 3 M HNO3 aqueous solution without hydrazine is shown. At–0.3 V vs. SSE, the Tc concentration decreased to 95% of theinitial value after 30 min of electrolysis. After 60 min, however, the concentration increased nearly to the initial value, andthe concentration hardly changed until 120 min. At more noble
potential of –1.0 V vs. SSE, the concentration hardly changed
as shown in Figure 3. Maslenikov et al. reported that the Tc
concentration decreased to 91% of the initial value after 2 h ofelectrolysis at –0.5 V vs. SCE (standard calomel electrode, thepotential value against SCE being roughly the same to thevalue against SSE) with a graphite electrode in 4 M HNO3.7
This value is compatible to the present experiment at –0.3 Vvs. SSE after 30 min. They also reported that the concentration
decreased to 66% at –1.0 V vs. SCE. This result does not agree

Figure 2. Time dependence of concentrations of Tc in electrolyte
solution of constant potential electrolysis determined by ICP-AES.
Electrolyte: 5 mM of initial Tc in 3 M HNO 3 aqueous solution.
Electrolysis conditions: current density approximately 2 mA cm –2 for–0.3 V vs. SSE and 6 mA cm–2 for –1.0 V vs. SSE. Line: a baseline,
the spectrum of initial solution, the spectra of the solution electrolyzed for 10, 20, 30, 40, 50, 60, 90, and 120 min.

Asakurawith the present result for –1.0 V vs. SSE. One possible explanation for this discrepancy is a difference in the current density,
which was not high in the present experiment as compared with
their experiment. As shown in Figure 4, the Tc concentration
decrease was not significant at –0.3 V vs. SSE with co-existing
hydrazine. At –1.0 V vs. SSE, the concentration decreased during 90 min of electrolysis. Although the concentration was 90%
at 90 min, the concentration increased again after 120 min.
Remarkable deposit formation on the electrode was not observedin every case where the concentration decreased.

Figure 3. Absorption spectra of Tc solution of electrolysis.
Electrolyte: 1.5 mM of initial Tc and 0.1 M of hydrazine in 3 M
HNO3 aqueous solution. Electrolysis conditions: Potential of –0.3 Vvs. SSE, current density about 28 mA cm–2.

Figure 4. Time dependence of concentrations of Tc in electrolyte solution of constant potential electrolysis determined by ICP-AES.
Electrolyte: 5 mM of initial Tc and 0.1 M of hydrazine in 3 M HNO3
aqueous solution. Electrolysis conditions: current density approximately
14 mA cm–2 for –0.3 V vs. SSE and 28 mA cm–2 for –1.0 V vs. SSE.

Figure 5. Time dependence of concentrations of Tc and Pd in electrolyte solution of constant potential electrolysis determined by ICPAES. Electrolyte: 1.8 mM of initial Tc and 6.1 mM of initial Pd in 1
M HNO3 aqueous solution. Electrolysis conditions: potential of ±0.0
V vs. SSE, current density about 0.3 mA cm –2.

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Study on Electrolytic Reduction of Pertechnetate in Nitric Acid Solution

Figure 6. Cyclic voltammograms of the deposits under multiple potential scans up to the tenth cycle. Anodic current positive. Initial potential:
1.1 V vs. SSE. Initial scan direction: cathodic. Scan rate: 0.1 V s−1. Potential range: ±0 to 1.3 V vs. SSE. a) Deposit from Tc-Pd-Ru-Rh solution.
Electrolyte: 4.7 mM of Pd, 5.4 mM of RuNO3+ and 5.6 mM of Rh3+ in 3 M HNO3 aqueous solution. Quantity of electricity for deposition: 26.6 mC.
The first and the tenth cycles. b) Deposit from Tc-Pd-Ru-Rh solution. Electrolyte: 5.4 mM of Tc, 7.4 mM of Pd, 5.4 mM of RuNO3+ and 4.7 mM ofRh3+ in 3 M HNO3 aqueous solution. Quantity of electricity for deposition: 378 mC. The first, the second, the forth, the sixth and the tenth cycles.

From previous results for Re,1 it is expected that the co-existence of Pd2+, depositing easily as Pd black, will accelerate theTc deposition (promoter effect). Figure 5 shows concentration
changes of Tc and Pd during the electrolysis in 1 M HNO3
aqueous solution containing both Tc and Pd at the potential of±0.0 V vs. SSE. In this case, the formation of a black deposit
was observed and the Pd concentration decreased to 10% of theinitial value after 240 min of electrolysis. It is supposed thatthe recovery of Pd by EE method is not interfered by co-existing Tc. The Tc concentration also decreased to 85% of the initial value after 60 min. The decrease is larger than thatobserved in the case of Tc only shown in Figures 3 and 4. The
result suggests the promoter effect of Pd on Tc deposition. TheTc concentration, however, increased again in further electrolysis longer than 60 min. It might be supposed that once deposited Tc redissolved.
To examine oxidation-reduction property of the deposit,
cyclic voltammetry (CV) measurements was performed using
3 M HNO3 solution containing HTcO4, Pd2+, RuNO3+ and Rh3+
as an electrolyte solution. As a reference, an electrolyte solution without HTcO4 was also used. Before CV measurements,
a black deposit of Pd-black was formed on the Pt wire (0.6 mmin diameter) working electrode by polarizing it at +0.1 V vs.
SSE for 5 min. The quantities of electricity for the deposition
were 378 mC for Tc-Pd-Ru-Rh solution and 26.6 mC for PdRu-Rh solution. After the deposition, the cyclic voltammetry
measurement with ten continuous sweep cycles was performedin the same electrolyte solution using this Pt electrode withdeposit. Figure 6a shows the CV curves observed for thedeposit from Pd-Ru-Rh solution. The curves hardly changed
as the sweep cycle proceeded. The cathodic peak current at+0.5 V vs. SSE was 1.08 mA and the anodic peak current at+0.89 V vs. SSE was 0.798 mA for the first cycle. As shown inFigure 6a, the curve drastically changed in the case of Tc-PdRu-Rh. The observed current peaks decreased as the potential
sweep cycle proceeded. The cathodic peak current at +0.46 V
vs. SSE was 2.40 mA for the first cycle, and this peak almost
disappeared in the tenth cycle. The anodic peak current at +0.87
V vs. SSE was 6.05 mA for the first cycle, and the current
decreased to 0.511 mA with a slight shift of the peak potential
to +0.81 V vs. SSE for the tenth cycle. After the tenth cycle ofthe measurement, the deposit disappeared from the electrode.
The deposit may redissolve in the solution by oxidation occurring at the potential +1.3 V vs. SSE or lower. To remove thedeposit from the electrode in the Pd-Ru-Rh case, a potential of+1.6 V vs. SSE or higher had to be applied. The Pd deposit fromthe Tc containing solution redissolved more easily than thatfrom the solution containing no Tc.

4. Conclusion
The electrolytic reduction of Tc in nitric acid solution wasperformed to examine the possibility of Tc co-recovery by theEE method. A significant absorption peak at 482 nm observed
as a consequence of the electrolytic reduction of Tc(VII) at–0.3 and –1.0V vs SSE suggests the reduction of Tc(VII) toTc(IV) and a possibility of the formation of Tc(IV) complexes.
On deposition of Tc by electrolysis, remarkable deposit formation on an electrode could not be observed in the cases of Tc
only, despite the Tc concentration decrease to 93% (Tc only, in3 M HNO3, at –0.3 V vs SSE) and 90% (with hydrazine, in 3
M HNO3, at –1.0 V vs SSE) of the initial concentration. In thecase of Tc and Pd electrolyzed at ±0.0 V vs. SSE in 1 M HNO3,a black deposit of Pd was observed and the Tc concentration
also decreased to 85% of the initial one. The co-existence ofPd and Tc in the solution might accelerate the deposition of Tc(promoter-effect). Quantitative deposition of Tc was notachieved, and further studies are needed to recover Tc by electrolysis. It was also found from cyclic voltammetry measurements that Tc affected the oxidation-reduction property of thePd deposit in the course of its redissolution.
Acknowledgements. This work has been done under thecollaboration research, “research on selective separation andrecovery of long-lived fission products from spent fuel,”
between JAERI and JNC.
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