R Acta Sulfur Technetium 2014-2369-online

Radiochim. Acta 2015; aop
Konstantin E. German*, Andrey A. Shiryaev, Alexey V. Safonov, Yana A. Obruchnikova,
Viktor A. Ilin, and Varvara E. Tregubova
Technetium sulfide – formation kinetics, structure
and particle speciation
Abstract: Technetium sulfide formation kinetics was stud-
ied in the pH range 8−12 in presence of Na2S and phos-
phate buffer solution. The conditions for separation of Tc
sulfide micro and nanoparticles were found with ultra-
microcentifugation and the values of Tc sulfide solubility
were demonstrated to be dependent on the Na2S concen-
tration as 𝐶(Tc3S10+x) = −9 × 10−5
ln [Na2S]−2 × 10−5
M.
The composition of Tc sulfide precipitate was elucidated
with EXAFS, RBS and chemical analyses as Tc3S10+x or
[Tc3(𝜇3
-S)(S2)3(S2)3/3]n in agreement with recent Lukens
data.
Keywords: Technetium sulfide, environmental chemistry,
ultramicrocentifugation.
DOI 10.1515/ract-2014-2369
Received April 25, 2014; accepted December 17, 2014
1 Introduction
Sulfur isanelementof highenvironmentalimportance, es-
pecially in view of behavior of a number of metals. Tc−S
system is not an exception in this respect. The sulfur, its
different compounds and their derivatives, present miner-
als and rocks, are being leached by dissolution or microbi-
ological transformation into natural waters, which drasti-
cally affect the environmental transportation routes of Tc.
For a better understanding of the Tc behavior in different
natural environments one needs quantitative data on the
*Corresponding author: Konstantin E. German, Frumkin Institute
of Physical Chemistry and Electrochemistry Russian Academy of
Sciences, Moscow, Russia; and Moscow Medical Institute REAVIZ,
Moscow Russia, e-mail: guerman_k@mail.ru
Alexey V. Safonov, Viktor A. Ilin, Varvara E. Tregubova: Frumkin
Institute of Physical Chemistry and Electrochemistry Russian
Academy of Sciences, Moscow, Russia; and Moscow Medical
Institute REAVIZ, Moscow Russia
Andrey A. Shiryaev: Frumkin Institute of Physical Chemistry and
Electrochemistry Russian Academy of Sciences, Moscow, Russia
Yana A. Obruchnikova: Frumkin Institute of Physical Chemistry and
Electrochemistry Russian Academy of Sciences, Moscow, Russia; and
Moscow Medical Institute REAVIZ, Moscow Russia; and Mendeleev
Russian Chemical Technology University, Moscow, Russia
composition and properties of Tc sulfide species that are
formed. This work aims to analyze the progress in the stud-
ies of Tc−Ssystem, with the primary focus on its formation
kinetics and size speciation.¹
2 Earlier published data
Technetium sulfide (it is important to emphasize that
Tc2S7 does not exist as Tc(VII) sulfide and is a complex
sulfide compound) was among the first synthesized Tc
compounds and it was considered [1, 2] as a convenient
route for Tc separation from aqueous solutions. Already
in the early publications its composition was proposed as
Tc2S7, in which Tc was present in its higher oxidation state
of +VII. Rather surprising was that Tc(VII) redox poten-
tials are in contradiction to those of S2−
. Later Spitsyn and
Kuzina confirmed this composition [3]. However, the crys-
tallographic characterization of technetium heptasulfide
was not possible as the precipitate is X-ray amorphous. All
the attempts of recrystallization in solution were unsuc-
cessful and no interpretation of the composition and the
structure was available for more than 45 years[4].
Bondietti, Lee and co-workers in 1979–83 studied the
effects of Fe(II) and of S2−
on the Tc solubility. They had
found that, in the absence of S2−
, the pertechnetate was
reduced by Fe(II) and Tc(IV) hydroxide precipitated from
the solution [5, 6]. In the presence of S2−
, in turn, Tc2S7 is
precipitated and the authors concluded that Tc(VII) was
not reduced with sulfide [6]. When both Fe(II) and S2−
were
present, Tc was reduced and coprecipitated with a FeS
phase as a carrier.
Boyd described preparation of amorphous Tc(IV) sul-
fide by heating Tc2S7 in the absence of O2 [7]. Crystalline
TcS2 in turn was prepared by chemical transport reaction
along a temperature gradient (1423–1353 K) in a sealed
tube. According to Wildervank and Jellinek [8] the pres-
1 Part of the experimental data on reaction kinetics and Tc sulfide
size speciation was reported at the International symposiums on
technetium and rhenium, 2005 O-arai, Japan and 2011, Moscow, Rus-
sia, but are presented for publication just here.
Authenticated | guerman_k@mail.ru author's copy
Download Date | 2/7/15 8:01 AM

2 | K. E. German et al., Technetium sulfide
ence of halogen as a carrier gas improved transport effi-
ciency [8]. Triclinic crystals of TcS2 were formed.
Sodium thiosulphate and thioacetamide were shown
to be able to produce Tc2S7 in treatment of acidic Tc(VII)
solutions [9, 10].
Considerable Tc accumulation by some sulfide miner-
als and rocks have been observed [11–13], the most effec-
tive sorbents being the sulfides with higher solubility or
those possessing reducing metal ions. The mechanism for
Tc removal is differed for various minerals [13].
The Tc2S7 solubility was investigated based on the
measurements of Tc concentration in the aqueous solu-
tions equilibrated with the previously precipitated and
washed technetium sulfide [13]. No permanent thermody-
namic solubility value were established because of tech-
netium sulfide peptisation followed by dissolution and
slow oxidation in course of its dissolution in distilled wa-
ter [3, 4, 13] and references therein.
Theknowledgegap information, structureand Tc spe-
ciation in its sulfide form motivated the re-examination of
this system.
3 Experimental results and
discussion
The preliminary spectrophotometric study on formation
kinetics of technetium sulfide reported in [14] was recon-
sidered and completed in this work, providing new quan-
titative data on the Tc(VII) reaction with sulfide. The
spectrophotometer (Carry 50) was used for the study. All
reagents were of analytical grade or higher. Na2S ⋅ 9H2O
has been recrystallized from saturated solution in bidis-
tilled water and the single crystals as large as approx.
1 × 1 × 1 cm size were selected for solution preparations.
Tc-99 was purchased as KTcO4 from PO Mayak. The stud-
ies on colloidal particle size distribution were done with
ultracentrifuge technique (Ultracentrifuge MP-20 (Poland)
at 15 000 rpm speed and “Sartorius” 5, 10 and 20 kD ul-
tramicrocentrifuge tubes) and were coherent with the data
obtained by Saiki et al. by different method [15]. That en-
abled usto providewithreliableinformationonthedepen-
dence of Tc sulfide solubility vs. sulfide ion concentration
in the solution.
3.1 Spectroscopic kinetics study
The TcO4
−
and S2
−
solutions, being transparent in the vis-
ible region of spectrum, demonstrate strong absorbance in
Fig.1: Typical absorbance spectrum for technetium sulfide (reaction
time 4560 s), [TcO4
−
] = 1 × 10−4
M, [Na2S] = 0.27 M, pH 11.8.
Fig.2: Absorbance at 450 nm for the reaction of pertechnetate with
sodium sulfide as a function of reaction time:
[TcO4
−
]0 = (0.5−2) × 10−4
M, [Na2S] = 0.27 M, pH 11.8.
the UV region. In course of the reaction of the pertechne-
tate with S2
−
, brown color of the solution develops and the
corresponding spectrum is attributed to the formation of
technetium sulfide. For the kinetics study the 𝜆 = 450 nm
wavelength was chosen in this paper as a criterion of Tc
sulfide formation (Figures 1 and 2). The kinetic curve reg-
istered for solution with different initial pertechnetate and
sodium sulfide concentrations in Figure 2.
As the pH of the solution could be affected by sulfide
hydrolysis and oxidation, some tests were conducted in
buffer solutions. In all cases the pH was kept within 8−12
as the decrease of pH enhances the hydrolysis of sulfide
producing hydrogen sulfide ions and its conversion to el-
ementary sulfur. These two products react producing in
turn the disulfide ion.
A typical kinetic curve in a phosphate buffer is shown
in Figure 3. The kinetics is characterized by three stages,
with the first one being induction period (from 13 500 and
up to 35 000 s, depending on the pH). According to the
composition of the final precipitate (see below) the sec-
ond step is most probably a complex reaction. It includes
the reduction of pertechnetate with sulfide giving Tc(IV)
sulfide and formation of 3 S0
atoms per each reduced Tc
formed in brutto reaction Eq. (1), with formation of disul-

K. E. German et al., Technetium sulfide | 3
Fig. 3: Technetium sulfide formation kinetics (registered at 450 nm)
in the reaction of pertechnetate with sodium sulfide at pH 8.2:
[TcO−
4 ]0 = 1.57 × 10−4
M, [Na2S] = 0.09 M, buffer solution 0.14 M
Na2HPO4 + 0.05 M NaH2PO4.
fide S2
−
ligand where sulfur is present in the oxidation
state (−1):
TcO4
−
+ 5H2S = TcS2 + 3S0
= TcS(S2)2
−
(1)
This step takes 14 000–50 000 s, depending on the pH.
The third step according to the formula proposed earlier
by Lukens and co-workers should be trimerisation of the
reaction product Eq. (1) to trinuclear Tc(IV) polydisulfide
[Tc3(𝜇3
-S)(S2)3(S2)3/3]n [16]. The colloidal solutions of the
technetium sulfide formed were found to be rather stable.
Similar stability of the Tc sulfide colloides has been ob-
served for solutions described by Saiki et al. [15].
3.2 Tc sulfide stochiometry and structural
studies
The composition of the precipitate separated at 10 kD
MWCO membranes from the solutions described above
was determined by chemical and radiochemical analyses.
The 𝛽-counting of Tc-99 using Beckman-6500 in GSL liq-
uid scintillation cocktail was used. To determine the sulfur
concentration the oxidation of the sulfide to sulfate with
HNO3(conc) followed by microtitration with Ba2+
was used.
The ratio value Tc : S was established to be 2 : 6.7(1). In-
dependent determination of Tc : S ratio was carried out
by using 4
He+
induced Rutherford Backscattering Spec-
trometry (RBS) at CENBG Radioanalytical and Environ-
mental Chemistry Lab (Bordeaux-Gradignan) in collabora-
tion with Pierre Sue Laboratory, CE de Saclay (France). The
dilute dispersion of Tc sulfide was placed on the polished
aluminum disc, dried and the backscattering of 1 MeV
Fig. 4: Tc𝐾-edge EXAFS spectra (left) and their Fourier transforms
(right) of the Tc sulfide colloidal solution corresponding to the
reaction of pertechnetate with sodium sulfide for [TcO4
−
]0 =
2.0 × 10−4
M, [Na2S] = 0.27 M, pH 11.8.
4
He+
from these samples was analyzed. The result of the
latter method is sensitive to the thickness of the sample so
the resulting value was extrapolated to zero sample thick-
ness as shown in the figure and was equal to Tc : S = 2 :
6.73(25).
All the samples of Tc sulfide precipitate, prepared as
described above, were X-ray amorphous and no crystal
structural data could be obtained. The structural informa-
tion from this precipitate was obtained from EXAFS stud-
ies done at ESRF(Grenoble, France) and Kurchatov Source
of Synchrotron Radiation (Kurchatov Institute, Moscow).
The EXAFS data (Figure 4) support the structure of the
technetium sulfide reported by Lukens et al. earlier [16].
The rate constant 𝐾1 for the second step of Tc(VII)
reaction with sulfide (producing Tc(IV) and polysul-
fide ions) was determined as 7.0 × 10−6
s−1
while 𝐾2 for
the final formation of technetium sulfide (in fact trinu-
clear technetium(IV) polydisulfide) as shown in [16]) was
2.0 × 10−4
s−1
, being by two orders of magnitude higher
than 𝐾1.
The influence of initial technetium(VII) concentration
(within the range of (0.57−2.66) × 10−4
M KTcO4 at con-
stant [Na2S] = 0.3 M) on the reaction rate of Tc and sul-
fide was determined from the data shown in Figure 2. The
rate constant demonstrates almost linear dependence on
the technetium concentration within the studied concen-
tration range.
The explanation of the Tc−S system was a great prob-
lem for 40 years as described by J. Rard and co-workers [4].
It became clear only after the research based on EX-
AFS studies made by Lukens and co-workers[16]. It was
demonstrated that −S−S− disulfide ligands are present in
the structure of technetium sulfide thus explaining the re-
duction of Tc(VII) to Tc(IV) with no notable change in
Tc : S stoichiometric ratio that remained close to the value
within the interval of 3.3−3.5 [16].

4 | K. E. German et al., Technetium sulfide
Fig.5: RBS determination of S/Tc ratio in technetium polydisulfide
as a function of sample thickness (measured in relative units –
counts/mm2
).
Fig.6: Structure unit fragment of [Tc3(𝜇3
-S)(𝜇2
-S2)3(S2)3] (or Tc3S13)
for technetium sulfide according to EXAFS studies [16].
At the same time the presence of S2
2−
ligand in the
compound explains some other properties that were not
well understood before. The determination of the concen-
tration of free Tc species as fractionized in this work by
separation of ionic Tc from colloid-bound Tc with 5 kD
MWCO membrane (Figure 7) indicating colloid formation
that according to [16] should have the composition Tc3S13
(Figure 6). The resulting concentration of truly dissolved
Tc (most probably in form of Tc3S13) was dependent on the
Na2S concentration as shown in Figure 8.
The stability of Tc3S10 in the resulting solution
was dependent on the solution S2−
solution concen-
tration. When Na2S concentration was higher than
0.05 M, further growth of Tc polymeric sulfide particles
Fig.9: The scheme of Tc reaction with sulfide describing precipitation and size speciation tests drawn based on the results of Lukens et
al. [16] and this work.
Fig.7: Concentration of free Tc(IV) species Tc3S13 as fractionized
with separation of ionic from colloidal particles with 5 kD MWCO
membrane.
Fig.8: Solubility of Tc sulfide at 𝑡 = 75–110 h as evaluated for
different [Na2S] by separation of colloidal particles with 5 kD
membrane.
([Tc3(𝜇3
-S)(𝜇2
S2)3(S2)3/3]n) occurred for 𝑡 ≥ 150 h. For
[Na2S] ≤ 0.04 M, the Tc3S13 was reoxidized by ambi-
ent air to Tc(VII) within 175–200 h resulting in the
resolubilization of Tc as TcO4
−
.
The equation for the Tc sulfide solubility
based on these figures was evaluated 𝐶(Tc3S13) =
− 9 × 10−5
Ln[Na2S] − 2 × 10−5
M (Figure 8).
The stoichiometry of technetium sulfide precipitated
from aqueous solutions by sulfide action was recently con-
firmed by Liu et al. [17]. Some important data on the Fe
sulfide ores reducing Tc to Tc(IV) onto Tc environmen-
tal behavior was reported by the same authors [18]. The

K. E. German et al., Technetium sulfide | 5
results of [17–19] support in principle the evidence for the
formulation of common technetium sulfide established by
Lukens and co-workers [16] and confirmed in this work.
We consider that the total data of the latter works provide
a correct and important description of technetium polysul-
fide as a complex compound formed from water solutions
by reaction of pertechnetate with sulfide source ores.
Acknowledgement: The work was carried out in part as
the statutory work of the A.N. Frumkin Institute of Physical
Chemistry and Electrochemistry of the Russian Academy
of Sciences within the grant RFBR 14-03-00067. One of us
(KEG) is grateful to the staff of CENBG Radioanalytical and
Environmental Chemistry Lab (Bordeaux-Gradignan) and
of Pierre Sue Laboratory, CE de Saclay, for the possibility
of carrying out the analyses of Tc sulfide by RBS.
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