Speciation of Technetium Peroxo Complexes in Sulfuric Acid Revisited

Frederic Poineau1, Konstantin E. German

2, Benjamin P. Burton-Pye

3, Philippe F. Weck

4,

Eunja Kim5, Olga Kriyzhovets

6, Aleksey Safonov

2, Viktor Ilin

2,6, Lynn C. Francesconi

3,

Alfred P. Sattelberger7 and Kenneth R. Czerwinski

1

1 Department of Chemistry, University of Nevada Las Vegas, Las Vegas, USA

2 A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy

of Sciences, Moscow, Russian Federation

3 Department of Chemistry, Hunter College, New-York, USA

4 Sandia National Laboratories, Albuquerque, USA

5 Department of Physics and Astronomy, University of Nevada Las Vegas, Las Vegas,

USA

6 Medical Institute REAVIZ, Moscow Branch, Moscow, Russian Federation

7 Energy Engineering and Systems Analysis Directorate, Argonne National Laboratory,

Lemont, USA.

Abstract. The reaction of Tc(+7) with H2O2 has been studied in H2SO4 and the

speciation of technetium performed by UV-visible and 99-Tc NMR spectroscopy. UV-

visible measurements show that for H2SO4 ≥ 9 M and H2O2 = 0.17 M, TcO3(OH)(H2O)2

reacts immediately and blue solutions are obtained, while no reaction occurs for H2SO4 <

9 M. The spectra of the blue solutions exhibit bands centered around 520 nm and 650 nm

which are attributed to Tc(+7) peroxo species. Studies in 6 M H2SO4 show that TcO4-

begins to react for H2O2 = 2.12 M and red solutions are obtained. The UV-visible spectra

of the red species are identical to the one obtained from the reaction of TcO4- with H2O2

in HNO3 and consistent with the presence of TcO(O2)2(H2O)(OH). The 99-Tc NMR

spectrum of the red solution exhibits a broad signal centered at + 5.5 ppm vs TcO4- and is

consistent with the presence of a low symmetry Tc(+7) molecule.

1. Introduction

The isotope 99-Tc is an important fission product of the nuclear industry. In typical

reprocessing schemes, the spent fuel is dissolved in nitric acid, where technetium is

oxidized to Tc(+7) and separated from the other elements by liquid-liquid extraction [1].

During reprocessing activities, due to the highly radioactive nature of the spent fuel,

hydrogen peroxide could be formed from the radiolysis of the water in nitric acid [2]. The

radiolytic H2O2 may interact with Tc(+7) and produce peroxo complexes. Hydrogen

peroxide is also used in several steps of modern versions of PUREX-type reprocessing

(i.e., Pu refinement in the presence of vanadate peroxide [3]. Therefore, the

understanding of the chemistry of technetium in the presence of H2O2 in acidic solution is

important to predict its behavior in the nuclear fuel cycle. In this context, we decided to

study the reaction between Tc(+7) and H2O2 in mineral acids. Recently, we analyzed the

speciation of technetium in HNO3/H2O2 solutions [4]. UV-visible measurements show

that for HNO3 ≥ 7 M and H2O2 = 4.25 M, Tc(+7) reacts immediately and red solutions

are obtained. The nature of the red Tc species was investigated by computational methods

and results were consistent with the formation of TcO(O2)2(H2O)(OH). Interestingly, the

speciation of technetium peroxo complexes in nitric acid is different from the one in

sulfuric acid. In a previous study [5], technetium peroxo complexes were identified by

UV-visible spectroscopy after the reaction of technetium dioxide with H2O2 in 16 M

H2SO4; the solutions were blue-purple and exhibit spectra different from the one in nitric

acid; it was proposed that these species were Tc(+5) and Tc(+6) complexes and contained

the Tc(O2)x fragment, but no structural characterization was performed. In the present

work, we investigated the reaction between Tc(+7) and H2O2 in sulfuric acid, and

investigated the speciation of the technetium complexes by UV-visible and 99-Tc NMR

spectroscopy.

2. Experimental methods

Caution. Techetium-99 is a weak beta emitter (Emax = 292 keV). All manipulations were

performed in a radiochemistry laboratory designed for chemical synthesis using efficient

HEPA-filtered fume hoods, and following locally approved radioisotope handling and

monitoring procedures. The starting material NH4TcO4 was obtained from the Oak Ridge

Isotope Office and purified prior uses. Hydrogen peroxide solutions (30% and 50 %)

were obtained from Sigma Aldrich and used as received.

UV-visible spectroscopy. UV-visible spectra were recorded at room temperature in a

quartz cell (1 cm) on a Cary 6000i double beam spectrometer. Solutions of H2SO4/H2O2

were used as reference. Technetium stock solutions ([Tc] = 17 mM)) were prepared by

dissolution of NH4TcO4 in 3 M, 6 M, 9 M, 13 M and 18 M H2SO4 and used for the

spectroscopic measurements.

99-Tc NMR spectroscopy. The 99-Tc NMR spectra of solutions were collected on a

JEOL GX-400 spectrometer with 5 mm NMR tubes fitted with Teflon inserts that were

purchased from Wilmad Glass. Chemical shifts (δ) were measured from a 0.2 M

NH4TcO4 solution in D2O as the external reference (δ = 0). Technetium stock solutions

([Tc] = 0.1 M) were prepared by dissolution of NH4TcO4 in 6 M and 13 M H2SO4.

3. Results and discussion

Speciation in 3-18 M H2SO4 and H2O2 = 0.17 M. Solutions of Tc(+7) (Tc = 0.17 mM)

in 3 M, 6 M, 9 M, 13 M and 18 M H2SO4 were prepared from the stock solutions and 1

mL was added in the 1 cm quartz cell. Then, 10 µL of H2O2 (50 %) was added in the cell

with a pipette (i.e., H2O2 = 0.17 M). A blue color was immediately observed for H2SO4 ≥

9 M while the solutions remained colorless for 3 M and 6 M H2SO4. UV-visible

measurements (Figure 1) show that the formation of the blue species is accompanied by

the appearance of bands in the region 500-800 nm; these bands being more intense in 13

M and 18 M than in 9 M H2SO4. The UV-visible spectra in 9 M H2SO4 exhibit bands at

275, 520 and 610 nm; the spectra in 13 M and 18 M H2SO4 exhibit two bands,

respectively, at 520 and 610 nm and at 520 and 650 nm. These results are consistent with

the one previously obtained from the reaction TcO2 with H2O2 in H2SO4; it was shown

that blue-purple complexes with bands centered at 500 nm and 650 nm were observed for

H2SO4 ≥ 12 M and H2O2 = 0.25 M.

In our study in 9 M H2SO4, the band at 275 nm is consistent with the presence of

unreacted TcO3(OH)(H2O)2 while this species is not observed in 13 M and 18 M H2SO4

[6]. The blue solutions are unstable and decomposed by release of gas (oxygen). In 18 M

H2SO4, the spectrum after decomposition (Figure 2) is consistent with the presence of

TcO3(OH)(H2O)2; no other species (i.e., Tc(+5)-sulfate) were detected [7]. Because

hydrogen peroxide (EH2O2/H2O = 1.8 V) is a much stronger oxidizing agent than Tc(+7)

(ETc(+7)/Tc(+4) = 0.7 V), the reduction of Tc(+7) is not expected under our experimental

conditions and the oxidation state of Tc in the H2SO4/H2O2 solution remains +7 during

the course of the experiment.

In previous studies, we have shown that TcO3(OH)(H2O)2 forms in 7 M H2SO4 and is the

predominant species above 13 M H2SO4 [5, 8]. Here, UV-visible measurements show

that the formation of the blue species globally follows the one of TcO3(OH)(H2O)2

(Figure 3), which indicates that the blue species originates from the peroxidation of

TcO3(OH)(H2O)2.

0

0.25

260 460 660 860 1060

Wavelength (nm)

Absorbance

1

2

3

Figure 1. UV-visible spectra of Tc(+7) in H2SO4/H2O2 solution after 3 minutes of

reaction. [Tc] = 0.17 mM, [H2O2] = 0.17 M. Spectra in: 1) 9 M H2SO4, 2) 13 M H2SO4

and 3) 18 M H2SO4.

The UV-visible spectra of the blue Tc species differs from the one of Re(+7) peroxo

species obtained from the reaction of Re2O7 with H2O2 in THF [9]; the Re(+7) peroxide

complex presents a single band at 350 nm (ε(THF) = 1314 M-1

L-1

).

The low stability of the blue solutions precludes their analysis by EXAFS spectroscopy

[10]. In order to get more information, the experimental UV-visible spectrum of the blue

species was compared to the theoretical spectra of the Tc(+7) peroxo species previously

calculated (i.e., Tc(O2)4-, TcO(O2)3

-, TcO3(O2)

-, and TcO2(O2)(H2O)2(OH)) [4].

0

1

220 320 420 520 620 720 820 920

Wavelength (nm)

Absorbance

1

2

Figure 2. UV-visible spectra of Tc(+7) in H2SO4/H2O2 solution. [Tc] = 0.23 mM, [H2O2]

= 0.017 M, H2SO4 = 18 M. Spectra after: 1) three minutes of reaction and 2) ten minutes

of reaction.

0

20

40

60

80

100

0 3 6 9 12 15 18

Absorbance at 520 (nm)

Fraction of TcO

3(OH)(H2O) 2(%)

H2SO4 (M)

0.13 100

1)

2)

Blue species

TcO3(OH)(H2O)2

Blue species

Figure 3. 1) Fraction (%) of TcO3(OH)(H2O)2 in solution as a function of [H2SO4]. 2)

Absorbance at 520 nm as a function of [H2SO4] for the blue species obtained from the

reaction of Tc(+7) and H2O2 ([Tc] = 0.17 mM, [H2O2] = 0.17 M).

The calculated spectra of Tc(O2)4- and TcO3(O2)

- (Table 1) do not exhibit bands in the

region 500-700 nm which is not consistent with the experimental spectra and therefore

precludes their presence in the solution in significant concentration. The experimental

spectrum is more consistent with that calculated for the mono-peroxo

(TcO2(O2)(H2O)2(OH)) or triperoxo (TcO(O2)3-) complexes. Previous studies have shown

that for [H2SO4]> 5 M, peroxosulfuric acid, i.e., H2SO5, is obtained from the reaction of

H2SO4 with H2O2 [11]. Under the conditions of formation of the blue species (i.e., H2SO4

≥ 9M), peroxosulfuric acid should also be present in the solutions and the formation of

Tc(+7) peroxosulfate complexes is then not excluded; further experiments (e.g., EXAFS

measurement on solid frozen solutions, 17-O NMR) will need to be performed to confirm

this hypothesis.

Table 1. Experimental absorption maxima (nm) and extinction coefficient (M-1

L-1

) of the

blue species obtained by reaction of Tc(+7) and H2O2 (0.17 M) in 18 M H2SO4 and

calculated oscillator strengths (nm) for the complexes Tc(O2)4-, TcO(O2)3

-, TcO3(O2)

- and

TcO2(O2)(H2O)2(OH) [4]. Oscillator strength: m (medium) > 0.001; s (strong) > 0.004; vs

(very strong) > 0.01

Complexes Bands maxima (nm) and extinction coefficient (M-1

L-1

)

18 M H2SO4 520 (814) , 650 (816)

Tc(O2)4- 980 (s), 410 (vs), 350 (vs)

TcO(O2)3- 825 (m), 680 (s), 570 (m), 475 (s), 380 (vs)

TcO3(O2)- 415 (m), 320 (vs)

TcO2(O2)(H2O)2(OH) 714 (m), 515 (m), 395 (m), 345 (vs), 311 (s), 303 (vs)

Speciation in 6 M H2SO4 and H2O2 = 0.38 M - 8.5 M. Solutions of Tc(+7) (Tc = 0.34

mM) in 6 M H2SO4/H2O2 (H2O2 = 0.38 M, 2.12 M, 3.30 M and 8.5 M) were prepared. A

red color was observed for H2O2 ≥ 2.12 M. The UV-visible spectra of the red solutions

exhibit a single band (Figure 4) centered at 500 nm; the intensity of the band increases

with the H2O2 concentration. The UV-visible spectra of the red solution are essentially

identical to the one obtained from the reaction of TcO4- with H2O2 in HNO3 ≥ 7 M

(Figure 4.5) and is consistent with the presence of TcO(O2)2(H2O)(OH).

0

0.5

350 450 550 650 750 850

Wavelength (nm)

Absorbance

2

3

4

1

5

`

Figure 4. UV-visible spectra of Tc(+7) in H2SO4/H2O2 solution after 3 minutes of

reaction. [H2SO4] = 6 M, [Tc] = 0.34 mM. Spectra in: 1) 0.38 M H2O2, 2) 2.12 M H2O2,

3) 3.30 M H2O2 and 4) 8.50 M H2O2. The UV-visible spectrum 5) of a red Tc(+7)

solution ([Tc]= 0.26 mM) in 12 M HNO3/ 4.25 M H2O2 is also represented.

In order to get more information on the reaction between Tc(+7) and H2O2, 99-Tc NMR

measurements were performed in 3 M H2SO4/4.9 M H2O2 (solution A) and 6.5 M

H2SO4/4.9 M H2O2 (solution B).

Solutions A and B were respectively prepared after dilution (v:v : 1:1) of cold stock

solutions of NH4TcO4 ([Tc] = 0.1 M) in 6 M and 13 M H2SO4 with cold 30% H2O2. After

the addition of H2O2 in 3 M H2SO4 (solution A), the solution remained colorless while an

intense red color was observed after addition in 6.5 M H2SO4 (solution B). The solutions

were transferred into the Teflon inserts; the inserts were closed with a Teflon cap and

placed in the 5 mm glass tubes which were capped with a rubber septum. In order to

minimize pressure build up in the NMR tube due to gas formation, measurements were

performed at 5 °C. Measurement of 99-Tc spectra (shift and linewidth) provides

information on the oxidation state of the Tc atoms and on the structure of the molecules

[12]. Highly symmetric diamagnetic molecules (i.e., TcO4-) exhibit sharp lines while low

symmetry molecules (i.e., distorted octahedral) exhibit broad lines. The 99-Tc NMR

spectrum of the solution A (Figure 5.1) exhibits a narrow signal (linewidth = 1.25 ppm)

centered at -7.5 ppm-; this signal is consistent with the presence of TcO4

- and indicates

that no reaction occurred between TcO4- and H2O2 under these conditions.

0.00E+00

-40 -20 0 20 40

99Tc shift (ppm) vs TcO4-

1 2

Figure 5. 99-Tc NMR spectra of 1) solution A ([Tc] = 0.05 M, 3 M H2SO4, 4.9 M H2O2)

and 2) solution B ([Tc] = 0.05 M, 6.5 M H2SO4, 4.9 M H2O2).

The 99-Tc NMR spectra of the solution B (Figure 5) exhibits a broad signal (linewidth =

10 ppm) centered at + 5.5 ppm. The signal is consistent with the presence of Tc(+7)

species with a lower symmetry than TcO4-. These results confirm that the peroxidation of

TcO4- in 6.5 M H2SO4/4.9 M H2O2 solutions is accompanied by a change of symmetry of

the molecule.

4. Conclusion.

In summary, the reaction of Tc(+7) with H2O2 was studied in H2SO4 and two different

Tc(+7) species were observed: a blue species is observed for H2SO4 ≥ 9 M and H2O2 =

0.17 M and a red species for H2SO4 = 6 M and H2O2 ≥ 2.12 M. UV-visible spectroscopy

show that the blue species follows the formation of TcO3(OH)(H2O)2 and indicates this

species is the peroxidation product(s) of TcO3(OH)(H2O)2. In 18 M H2SO4, the blue

species is unstable and decomposes back to TcO3(OH)(H2O)2. Various hypotheses can be

formulated, the blue species can either be a Tc(+7) peroxosulfate complex, a Tc(+7)

monoxo or triperoxo complex; further experiments are needed to determine its exact

nature. The UV-visible spectra of the red species are identical to the one obtained from

the reaction of TcO4- with H2O2 in HNO3 and consistent with the presence of

TcO(O2)2(H2O)(OH). The UV-visible spectra obtained from the reaction of Tc(+7) with

H2O2 in H2SO4 are similar to the ones obtained from the reaction of TcO2 with H2O2 and

indicate Tc(+7) peroxo complexes to be the peroxidation products of Tc(+4) by H2O2 in

H2SO4. The reaction between Tc(+7) and H2O2 was also studied by 99-Tc NMR

spectroscopy. In 3 M H2SO4/4.9 M H2O2, the NMR spectrum of the clear solution is

consistent with the presence of TcO4- while in 6.5 M H2SO4/4.9 H2O2, the spectrum of

the red solution is consistent with the presence of a Tc(+7) species with a lower

symmetry than TcO4-. Heptavalent technetium peroxo complexes could also form in other

mineral acids and dominate the speciation of Tc(+7) in presence of H2O2; current in

phosphoric and perchloric acids are in progress and the results will be reported in due

course.

Acknowledgements. Funding for this research was provided by the U.S. Department of

Energy, Office of Nuclear Energy, NEUP grant through INL/BEA, 321 LLC, 00129169,

agreement number DE-AC07-05ID14517. Further supports were provided by the

National Science Foundation (Grant NSF-CHE 0750118 and Grant NSF-CHE-0959617

for purchase of the 400 MHz NMR spectrometer at Hunter College) and the U. S

Department of Energy, Grant DE-FG02- 09ER16097 (Heavy Element Chemistry, Office

of Science) and Grant DE-SC0002456 (Biological and Environmental Research, Office

of Science). Infrastructure at Hunter College is partially supported by Grant RR003037

from the National Center for Research Resources (NCRR), a component of the National

Institutes of Health (NIH). Sandia National Laboratories is a multiprogram laboratory

operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin

Company, for the United States Department of Energy’s National Nuclear Security

Administration under Contract DE-AC04-94AL85000. The authors thank Trevor Low

and Julie Bertoia for outstanding health physics support.

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