i.p.n. - 91406 orsay cedex - inis.iaea.org · 2. electrochemistry of selenium. general information....
TRANSCRIPT
I.P.N. - 91406 ORSAY CEDEX FR9901050
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IPNO.DRE 98-16
SELENIUM ELECTROCHEMISTRY.APPUCATIONS IN THE NUCLEAR FUEL CYCLE.
A.Maslennikov1, F.David2, V.Peretroukhine1,M.Lecomte3.
1 - Institute of Physical Chemistry, Russian Academyof Sciences, Moscow, Russia;2 - Institute of Nuclear Physics Orsay, CNRS3 - DCC/SEMP CEA VALRHO, Marcoule
3 1 - 0 4
IPNO.DRE 98-16
SELENIUM ELECTROCHEMISTRY.APPUCATIONS IN THE NUCLEAR FUEL CYCLE.
A.Maslennikov1, F.David2, V.Peretroukhine1,M.Lecomte3.
1 - Institute of Physical Chemistry, Russian Academyof Sciences, Moscow, Russia;2 - Institute of Nuclear Physics Orsay, CNRS3 - DCC/SEMP CEA VALRHO, Marcoule
Abstract.
Modern state of selenium electrochemistry is reviewed in respect of the application of
electrochemical methods for the study of the behavior of this element and its quantitative
analysis in the solutions of nuclear fuel cycle. The review includes the data on the redox
potentials of Se in aqueous solutions, and the data on Se redox reactions, occurring at
mercury and solid electrodes. Analysis of the available literature data shows that the inverse
stripping voltammetry technique for trace Se concentration and determination seems to be the
most promising in application for the Se determination in PUREX solutions and in radioactive
wastes. The adaptation of the ISV technique for the trace Se concentration and determination
in the solutions of the nuclear fuel cycle is indicated as the most prospective goal of the future
experimental study.
Contents
1. Selenium in nuclear fuel cycle 4-6
2. Electrochemistry of selenium. General information. 6-21
2.1. Redox potentials of selenium. 6-8
2.2. Selenium polarography 8-13
2.3.Selenium electrochemistry at solid electrodes. 13-20
3. Possible applications of the electrochemical methods for the study of Sebehavior in the solutions of the nuclear fuel cycle 20-23
References 23-26
1. Selenium in nuclear fuel cycle
Selenium isotopes with mass numbers from 76 to 79, particularly long-lived, Se-79
(estimated T1/2 from 2*104 [1] to 5*106 years [2], Ep^ = 200 keV), are formed in process of
irradiation of nuclear fuel. The total yield of all selenium isotopes appears to be about 5 g/ton U
for the 33000 MW\J/ton U burn-up) [3].
In spite of low content of selenium in the spent fuel this element seems to play an
important role in the development of the treatment of nuclear fuel and radioactive wastes. The
presence of selenium in fuel may cause the formation of binary and ternary selenides with
transuranium elements and such fission products as Mo, Tc and Ru subgroup metals [4-6].
These compounds are known to be rather stable towards dissolution in nitric acid, thus
increasing the part of insoluble residues in process of fuel dissolution. The procedures of Se
recovery from the solutions of nuclear fuel have not been developed since its concentration in
these solutions does not exceed 5-7*10"4 M and the dissolved Se does not affect significantly
the mechanism and the yield of the extraction of the principal fuel components - uranium and
Plutonium. In the first extraction cycle of PUREX process the principal part of Se remains in the
aqueous phase. The attempt to isolate Se from the rafinate of the first extraction cycle has
been undertaken recently and has been aimed to precise the JV2 value of Se-79. The isotope
dilution of the high level waste solution with selenious acid, containing stable selenium,
followed by co-precipitation with CaSO3 and Se extraction with nitrobenzene has been used
as an experimental method for Se-79 isolation [2].
The hazardous effect of selenium isotopes may appear in process of storage of
nuclear fuel. Due to high value of its half-life period the specific activity of Se-79 will not
undergo significant changes during thousands years. Moreover even the stable selenium
isotopes are known to be chemical poisons. Therefore, the Se monitoring in nuclear fuel cycle
and radioactive waste treatment, including the development of analytical procedures for the
determination of extremely low concentrations of this element in PUREX solutions and
environmental objects seems to be of undoubted importance.
Selenium analytical chemistry is well developed and reviewed in a number of
monographs [1,7] Such instrumental methods as atomic adsorption spectrophotometry using
different methods of sample atomization (for instance ETA AAS) [8-10], and X-ray
fluorescence technique [11,12] and inductively coupled plasma mass-spectrometry (ICP MS)
[13,14] have been successfully applied for Se determination in a wide spectrum of the
samples.
Electrochemical methods such as differential pulse (DPP) and square wave
polarography (SWP) at dropping mercury electrode (DME) [15] and inverse stripping
voltammetry (ISV) techniques using hanging mercury drop electrode (SMDE) [16,17] or solid
electrodes (Pt, Au, glassy carbon (GC), carbon paste electrode) without or with chemical
modifications [18] are widely used for the determination of the transition elements [16-18],
uranium and transuranium elements [19-21] in a vast number of objects, for the chemical
speciation of the elements in the solutions and in the investigations of the mechanism of the
redox reactions [21,22]. A number of the original articles, devoted to the development of the
electrochemical techniques for the Se trace determination, indicate that the sensitivity of
certain electrochemical methods (about 0,3 ppb) may compete with such methods as X-ray
fluorescence and AAS. Along with quantitative determination of selenium utilization of
electrochemical methods of the study of the aqueous solution chemistry may provide the
important information on the chemical speciation of the element of interest in the system and
on the mechanisms of the possible redox reactions occurring in the solutions. Therefore the
study of the electrochemical behavior of selenium in the nitric solutions, simulating the
composition of the solutions of PUREX process seems to be interesting for general
comprehension of the redox behavior of this element in such complex redox systems. The
development of new analytical techniques for trace Se determination in the samples of PUREX
solutions, which could compete with traditional ICP MS and X-ray fluorescence techniques,
seems to be another goal of the proposed study, since the cost of the cost of electrochemical
determination is evidently much less than of the two mentioned instrumental methods.
2. Electrochemistry of selenium. General information.
2.1. Redox potentials of selenium.
In aqueous solutions selenium may exist in four oxidation states (-2), (0), (+4) and (+6),
forming correspondingly Se2'(selenide), SeO32' (selenite) and SeO4
2" (selenate) ions. Oxidation
potentials for the corresponding redox pairs are presented in the Table 1. Dissociation of
selenious acid and hydrogen selenides in aqueous solution the potentials are found to depend
strongly on the hydrogen ions concentration. Only the values of the Se(IV)/Se(0) oxidation
potential have been measured experimentally [25] by measuring the potential of the Pt
electrode covered with the electrodeposited crystalline «gray» selenium in the solution of
selenious acid as a function of the concentration of the latter compound. The equilibrium in the
system was found to be achieved rapidly (from 5 to 60 min, dependent on the Se
concentration) and the obtained potential values had been found in good correlation with
Nernst equation. The values of the oxidation potential, presented in the Table 1, shows, that
selenium in the aqueous solutions should behave as a noble metal i.e. may be deposited at a
variety of electrodes in elementary form and stay stable towards the action of moderate
reducing and oxidizing agents. In spite of the high
Table 1.
Oxidation potentials of different selenium redox pairs
in 1 M HCIO4 and in 1 M NaOH.
Oxidation potentials,V/NHE
1 M HCIO4
IMNaOH
(+6)/(+4)
1,1
1,15
0,03
0,05
(+4)/0
0,74
0,74
-0,36
-0,366
0/(-2)
-0,11
-0,40
-0,67
-0,92
Ref.
23
24
23
24
value of oxidation potential of the Se(VI)/Se(IV) couple the electrochemical reduction is
extremely slow due to the necessity of the anion
SeO42- + 2H* + 2e ~> H2Se03
structure change from this point of view Se(VI) in the aqueous solutions resembles sulfur,
more than tellurium. Homogenous reduction of Se(VI) in the solutions is much faster. For
instance in presence of HCI Se(VI) is reduced to Se(IV) [1] according the reaction:
SeO42' + 2HCI+ 2H* -> H2Se03 + CI2
Elementary selenium may be easily dissolved in the solutions of HNO3 with formation of
selenious acid. Electrolysis of the selenites in alkaline solutions at the potential exceeding 1,0
V / NHE may result in formation of perselenate anions (Se2O82") [24].
2.2. Selenium polarography
The reduction of SeO42" ions at DME in aqueous solutions is very slow and therefore
cannot be studied, using polarographic techniques. The only published results on the
electrochemical reduction of Se(VI) concern neutral solutions of 0.5 M NaF [26] and are non-
reproducible [24]. There are also a few indications, that the reduction of the SeO42' ions may
be catalyzed by chromate ions in weakly acidic aqueous solutions [24].
The electrochemical reduction of selenide [27-29] and selenite [30-33] ions in aqueous
solutions is well studied and reviewed in details in [24]. According to the different original
studies [27-33] polarograms of both Se2" and SeO32" ions in aqueous solutions are irregular
and indicate that the electrode reactions are complicated by the chemical interaction of the Se
species with mercury and the adsorption of the products on the electrode surface.
The data on the Se2'ions electrochemical oxidation, presented in [27-29], show that this
process occurs at the DME potentials from -0,49 V / SCE in 1 M HO to -1,0 V / SCE in 1 M
NaOH [24]. The analysis of the corresponding polarogams enables the conclusion that HgSe
was the principal product of the electrode reaction occurring at DME The reaction paths are
determined by the H+ ions concentration in the supporting electrolyte and may be expressed
by the following reactions [27]
Hg + H2Se -> HgSe + 2H* + 2e
Hg + HSe' -> HgSe + H* + 2e'
Hg + Se2'->HgSe + 2e
It is necessary to mention that at low (less than 5*10^ M) selenium concentrations in the
supporting electrolyte (1 M NH4CI + NH4OH) the waves of Se2" oxidation at DME have the
classical shape. Their analysis indicates the 2-electron transfer in the electrode reaction
under study. However, the increase of Se concentration results in splitting of the Se2"
oxidation wave into two waves and in the loss of linear dependence of wave limiting current
on Se2' concentration in the solution. No data about the limiting step of the kinetics of the
electrode reaction are available. Taking into account the extremely low solubility product of the
HgSe it is easy to suppose that the polarographic waves of the selenide ions oxidation in
different media are complicated by the adsorption of the latter compound at the electrode
surface.
The behavior of Se(IV) at the DME may be characterized by several waves (from 1 to
4) appearing at the polarograms, registered in different supporting electrolytes.
In the solutions of NaOH and alkaline buffer solutions NH4CI + NH4OH with the ionic
strength (|j.) from 0,1 to 1,0 M only one irregular wave was observed at the potentials from -
1,39 to -1,73 dependent on the pH of supporting electrolyte [26,29-31]. Since the value of
diffusion coefficient for SeO32" ions are not available from the literature, the authors [29]
determined the number of the electrons participating in the electrode reaction by comparison
of the limiting current, observed at he Se(IV) polarograms with the corresponding values of
the limiting current, obtained for the monovalent ion electrochemical reduction, recorded in the
same conditions. Therefore the results of the work seem confusing. In the NH4CI + NH4OH
buffer solution (n=1,0 M) Se2+ is expected as the electrode reaction product. The decrease of
the solution ionic strength to (j.=0,1 M causes to the opinion of the authors the change of the
reduction mechanism and Se2' is reported as the product of the Se(IV) reduction at DME It is
necessary to mention that in the classical review of selenium inorganic chemistry there are no
indications on the existence of Se(ll) compounds in the aqueous solutions [34].
In weakly acidic and neutral buffer solutions (0,2 M Na2HPO4) in the pH range from 1,4
to 6,4 two waves were observed on the polarograms of Se(IV). [32]. The values of half-
wave potentials for both the first (E1/2(l) from +0,041 to -0,71 V / SCE) and the second (E1/2
from -0,58 to -1,38 V / SCE) wave were found to shift towards negative values with the
increase of the pH of supporting electrolyte. The first wave was found to be diffusion
controlled, while the second exhibited mixed faradic and adsorption properties. Therefore the
ratio of the diffusion currents of the first and the second wave could not serve for the
determination of the number of the electrons participating in the electrode reaction. Logarithmic
analysis of the first wave indicated its complete irreversibility. The apparent number of the
electrons calculated from the slope of the logarithmic analysis curves in all the cases was
found to be less than 1. It is interesting to mention, that the first wave was regular only in the
range of Se concentrations in the electrolyte from 10"6 to 5*10"4 M Se(VI). The further increase
of its concentration caused the splitting of the wave to two or even three waves and the
appearance of maxima. The linear dependence of the limiting current of the second wave on
selenium concentration in the electrolyte was observed only up to the concentrations 5*10"4 M
Se(VI). Since it was found impossible to propose the mechanism of the Se(IV) reduction at
DME based on the mentioned polarographic data, the coulometric study of the electrochemical
reduction of Se(IV) in 0,2 M Na2HPO4 at pH=2,5 was undertaken. The potentiostatic
electrolysis of this solution at the middle of the plateau of the first wave (E=-0,35 V / SCE) has
been carried out at the Hg pool electrode. At the beginning of the process the formation of red
colloidal amorphous selenium was observed in the bulk of the solution. After a certain period
of time red coloration disappeared and the precipitation of the black crystalline powder was
viewed at the electrode surface. The chemical analysis of this powder has indicated the
formation of the HgSe. The total electric charge, passed through the cell in the mentioned
process corresponded to the transfer of 4 electrons per md of Se. Basing on the visual
observations in combination with the data of coulometric analysis and the chemical analysis of
the reaction products the authors [32] concluded that the electrochemical reduction of Se at
DME had passed according to the reaction:
H2Se03 +4H* + Hg + 4e ? HgSe + 3H2O
The electrolysis of the suspension of black HgSe at E=-0,8 V / SCE (the plateau of the second
wave of Se(IV) reduction in phosphate buffer resulted in the dissolution of the black
precipitate with simultaneous evolution of H2Se from the system. The latter phenomenon was
described with the equation:
HgSe + 2hT + 2e ?Hg + H2Se.
No detailed study of the effect of adsorption of the products of electrochemical
reduction of Se(IV) has been carried out. By the measurements of electrocapillary curves it
has been shown that the introduction of 1,2*10"* M Se(IV) does not introduce any changes to
the electrocapillary curves of 0,2 M Na2HPO4, 0,2 M H2SO4 and 0,1 M HCIO4, thus proving the
absence of the Se(IV) adsorption at DME surface. At the same time, the adsorption of the
intermediate products of the Se(IV) reduction, such as colloidal Se and HgSe, seems to play
an important role in this process. Thus, the conclusions on the mechanism of the electrode
reactions require more sophisticated investigations, which should take into consideration the
adsorption phenomena.
For the application of the electrochemical methods in nuclear fuel cycle the
comprehension of the electrochemical behavior of selenium in acidic solutions seems to be
more important than that for alkaline and neutral ones. In the solutions of 0,1 - 3,0 M mineral
acids (HCI, HNO3, H2SO4, HCIO4) two waves at the potential ranges from +0,1 to -0,5 V/ SCE
and from -0,4 to -1,0 V / SCE are observed. The half-wave potential of the first wave in
H2SO4 was found to vary from -0,22 to -0,38 V / SCE with the increase of the acid
concentration from 0,1 to 3,0 M. The opposite trend was observed in the change of the values
of E1/2 for the second wave in H2SO4. They were found to shift from -0,92 to -0,79 V / SCE,
with the augmentation of the H+ concentration in the electrolyte. The limiting current of the first
wave depended linearly on the Se concentration in the solution up to concentration 10"3 M and
the process was found to be controlled by diffusion. It is necessary to indicate that the
increase of the acid concentration in the solution resulted in the diminution of the limiting
current of the first wave. At the same time this parameter for the second wave remained
constant in all the studied interval of acidity. In the HCI, HNO3, HCIO4 solutions the potentials of
the first reduction wave of Se(IV) were found more positive than in H2SO4 and varied in the
range from +0,02 to +0,08 V / SCE [32]. No significant trends in the change of their values
with the increase of the solution acidity has been observed. It is necessary to mention that in
the HCI and HNO3 solutions the first wave of Se(IV) reduction was overlapping with the
anodic wave of mercury dissolution. Therefore the experimental errors in determination of the
wave parameters in these electrolytes have been found significant. The second wave of
Se(IV) reduction in the HCI, HNO3, HCIO4 solutions had apparently the same nature as in
H2SO4. However, the half-wave potentials of the second wave were found to shift towards
more positive direction with the growth of electrolyte acidity. No dependence of the limiting
current () on the acid concentration was observed. At the same time 60 % increase of the l,im
value was marked in the solutions of HNO3 if compared with other considered mineral acids.
The obtained experimental data do not allow coming to any concrete conclusion concerning
the mechanism of the Se(IV) electrochemical reduction at DME in the solutions with pH<1,0.
The stepwise reduction, including the consequent formation of SeO, Se and H2Se seems to be
confusing, since there is no indications in the literature about the existence and stability of
Se(ll) compounds in aqueous solution. The other possible explanation of the numerous waves
on Se(IV) polarograms and their splitting with increase of Se concentration in the electrolyte
was proposed in [27] and consisted in the simultaneous reduction of different Se species
(H2Se03l HSeO3' and SeO32") which are present in the dynamic equilibrium in the electrolyte.
However, the consideration of the dissociation constants of selenious acid, presented in [23]
and in more recent studies [36] indicates that when the pH of the electrolyte is less than 2,5 all
the Se(IV) in the solution is present in the form of neutral H2Se03.
The description of the available literature data on the selenium electrochemical
reactions occurring at the mercury electrode enables the following conclusions:
electrochemical reduction of Se(VI) at DME is rather slow and therefore the
study of this reaction using polarographic techniques is impossible;
the reduction of Se(IV) in aqueous solutions of different composition results
in the formation of the HgSe of extremely low solubility (SP=10'59);
this compound may react with the other components of the solution under
study, playing the role of the weak reducing agent, disturbing the results of the
quantitative electrochemical analysis with mercury electrode [36];
the adsorption of the latter compound at the electrode surface disturbs the
form of polarographic waves and the linearity of the concentration dependencies
of the limiting current;
therefore, the conclusions on the Se reduction mechanism, which base
only on the polarographic data are often confusing.
2.3. Selenium electrochemistry at solid electrodes.
The value of the standard oxidation potential for the Se(IV)/Se° couple (E°=+0,74 V /
NHE) indicates that elementary selenium may be easily deposited at the solid electrodes which
do not demonstrate high overvoltage for the reaction of Se(IV) ions in the aqueous solutions.
The electrochemical reduction of Se(IV) solutions in neutral or weakly acidic aqueous
solutions at Pt sheet electrode results in simultaneous formation of red selenium sole in the
bulk of the solution, electrodeposition of crystalline or amorphous selenium at the electrode
surface and the evolution of gaseous H2Se. The yields of the products depend on the applied
10
potential. When more negative potential is applied to the electrode the part of the hydrogen
selenide in the reduction products is found to increase [24]. Se red soles were stable in pure
water for a few days [24].
In alkaline solutions the yield of the red selenium soles decreases due to the formation
of polyselenides as the products of the Se(IV) reduction. Their formation is expected to take
place due to the dissolution of the elementary selenium in sole in its reaction with hydrogen
selenide, formed at the electrode. The latter hypothesis is confirmed by the dependence of the
polyselenide concentration in the solution on the time of the electrolysis [24,37]. The amount of
Se atoms in the polyselenide chain was increased from in the 1,6 to 5,0 also with the increase
of the electrolysis time.
The applications of the electrolysis of the aqueous solutions for the selenium
electroplating at different solid electrodes are reviewed in [38]. The selenium metal layers
were reported to be formed at the Pt, C, Pb, Ti, W cathodes. Electrolysis of selenious acid
appeared to be more effective towards electrodeposition, than the analogous processes with
selenides or hydrogen selenide. The experiments with weakly acidic solutions of selenious
acid at Pt sheet electrode have shown, that the electrolysis at low current densities, i.e. in
absence of the hydrogen evolution, at room temperature results in formation of the selenium
layers with a maximum thickness about 5*10"6 mm [39]. However, the illumination of the
cathode and the elevation of the electrolysis temperature to 80-90°C allowed to increase the
latter parameter to 1,0*10"* mm. Amorphous Se was found to be accumulated between the
crystallites of Se metal, growing in process of the electrolysis. 9 M H2SO4 has been found the
optimal electrolyte for carrying out the selenium electroplating [39]. Such a high acidity, to the
opinion of the authors [39] is necessary to maintain Se(IV) in the electrolyte in the form of
electroactive H3Se03+ cations. The potential of Pt electrode, modified with Se should be
maintained at a level, excluding the possibility of hydrogen evolution. The Se concentration in
the electrolyte was maintained constant due to the utilization of the Se - graphite cathode,
which served for alimentation of the electrolyte with Se(IV).
In 70s the process of selenium electrodeposition caused a great interest due to the
development of the industry of semiconductors. The kinetics and mechanism of the process
have been studied in details in Russia [40-42], but the information on these results is hardly
available.
The electrochemical behavior of selenium on the Pt, Au, Au(Hg) and W electrodes has
been compared [43]. Pt and Au electrodes have been found to give the most reproducible
results in different analytical applications. The first detailed study of the Se(IV) behavior at
gold rotating disk electrode (AuRDE) has been carried out by W.Andrews and C.Johnson [44].
n
They have shown, that the electrodeposition of elementary selenium at the electrode surface
from 0,1 M HCIO4 solution becomes possible at the electrode potentials E<0,2 V / SCE. One
peak at E1c=0,2 V / SCE and the wave with E1/2c=-0,1 V / SCE are observed on the cathodic
branch of cyclic voltammetric curves. The peak is attributed to the reduction of Se(IV)
adsorbed on the electrode surface, while the wave is accounted for the diffusion controlled
Se(IV) reduction from the bulk of the solution. On the anodic branch of the corresponding
voltammetric curves three peaks of elementary Se dissolution were detected. The
measurements of the dependence of the peak square (Electric charge = quantity of the
deposited Se) on the deposition time have shown that the first peak (E1a=0,63 V / SCE) had
been associated with the oxidation of the crystalline selenium, deposited at the electrode. The
authors [42] attribute the second peak (E2a=0,815 V / SCE) to the dissolution of the adsorbed
monolayer of elementary Se, while the third one (E^O.95 V / SCE) is expected to reflect the
dissolution of the Se - Au intermetallic compound, formed in process of cathodic deposition of
selenium. The method of inverse stripping voltammetry i.e. accumulative electrolysis at the
potentials from 0,15 to-0,2 V / SCE, followed by a potential scan towards positive direction,
has been applied for the determination of the selenium trace amounts in biological objects. The
detection limit of the method was found to be about 1 ppb within the accuracy of 10 %. Less
noble ions such as Cd(ll), Pb(ll) did not interfere with selenium determination. Adverse effect
of Hg(ll) was eliminated by the preliminary sample treatment [44]. The anodic dissolution of
Cu(ll) occurred unfortunately at the same potential of the AuRDE as the dissolution of
selenium. However, selenium concentration in presence of large (more than 10 fold excess)
Cu(ll) amounts in the sample has been measured, using the peak E2a=0,815 V / SCE. The
linearity of the calibration curve in presence of Cu(ll) was observed in the narrower range of
Se concentrations in the samples, but the detection limit of the method stayed without
changes.
The method of inverse stripping voltammetry (ISV) was developed and successfully
applied for the determination of selenium and tellurium traces in the electrolytic copper [45].
The elimination of the copper matrix from the sample has been carried out by sorption of
Cu(NH3)62+ complexes in the column filled with Chelex-100 resin. The eluate has been acidified
with nitric acid and the ISV determination of Se(IV) has been carried out at AuRDE using the
procedure, close to that, described in [44]. The possibility of carrying out Se determination in
the electrolytes, containing up to 0,6 M HNO3 seems to be the principal result of the study [44],
being of practical importance for the development of electrochemical methods of Se
separation and determination in the solutions of nuclear fuel cycle.
The increase of the detection limit for Se(IV) determination in different environmental
objects to 420 ppt has been achieved recently, due to the utilization of gold
ultramicroelectrode arrays (AuUMEA), prepared using the microlithographic methods. The
12
electrode chip of 3-mm diameter contained 564 disk interconnected Au electrodes of 12 |im
diameter for each electrode. Such an arrangement of the working electrode allowed to work
with extremely diluted background electrolytes. 0,005 M H2SO4 was chosen for the procedure
of ISV Se determination [46]. This choice has been accepted taking into account the results of
the studies [47,48]. The properties of HCI, HNO3 and H2SO4, as the electrolytes for the ISV
determination of Se traces have been compared. The conclusion about the best response of
H2SO4 electrolyte for the anodic dissolution of elementary Se monolayer has been made. In the
other mentioned electrolytes the signals proportional to the Se concentration were either
suppressed, or disturbed. The electrode potential for Se deposition has been chosen in the
range between 0,20 V and 0,0 V / SCE. This potential value is found to be in good correlation
with that, determined in [44], for the adsorption of the elementary selenium on the surface of
the gold electrode. On the other hand some contradictions with the values of the potential of
Se electrodeposition, determined in [49] are observed. The dissolution of Se monolayer while
scanning the AuUMEA potential towards positive direction was observed at the potential
E=0,8 V / SCE, also being in good correspondence with the results, reported in [44]. The
dependence of the Se anodic dissolution peak current was proportional to the deposition time
(tj.) up to ^=200 s for the solution, containing 20 ppb Se(IV). The further increase of the
deposition time caused the decline of the mentioned dependence from the linearity. The
formation of compact crystalline selenium phase on the electrode surface was reported to be
as one of the possible reasons for such deviation [46]. However, changing the deposition
time, the different scale of sensitivity could be achieved. The sensitivity of the Se
determination was improved up to the detection limit 430 ppt, due to the utilization of high
frequency square wave voltammetry technique. The effect of the adverse ions on the Se
determination using the method under study is not discussed in the article [46].
It seems to be interesting to mention in the present review one more study, dealing
with trace selenium determination in the aqueous solutions using ISV technique. To trace the
behavior of the elements, deposited at the electrode, a combination of cyclic voltammetry and
electrochemical quartz crystal microbalance (EQCM) has been applied [50]. The general
concept of this method is described in [51]. It was applied to the investigation of Se behavior
at GC Au and R RDE in the solutions of 0,5 M H2SO4 [52]. The EQCM experiment included
simultaneous recording of the voltam metric curve and the change of the mass of the indicator
gold - quartz crystal electrode, vibrating with sufficient frequency. The latter parameter was
changed with the changes of the electrode mass. The sensitivity of the applied indicator
balance was sufficient to observe the mass changes on the level about 10 ng. 0,1 M HCIO4
was used as an electrolyte in the study [50]. The results of the study indicate that the
utilization of GC electrode was ineffective towards the Se trace determination. The authors
associated the observed low efficiency with low electrochemical activity of selenious acid at
GC electrode. At the Au RDE the distinct wave of Se electrodeposition (Edep=-0,1 V /SCE) and
13
corresponding Se anodic dissolution peak at Ediss =0,9 V /SCE were observed, being in good
agreement with literature data [42,46]. The possibility of formation of Au-Se intermetallic
compound at the electrode surface enabled the attempt of the authors [50] to study the Se(IV)
cyclic voltammetry at GC electrode in presence of Au in the electrolyte. It has been indicated,
that in presence of Au in 0,1 M HCIO4 and in 0,1 M HNO3 Se electrodeposition had taken place
and the corresponding dissolution peaks, have been detected on the anodic branches of the
cyclic voltammetry curves. The increase of the efficiency of Se electrodeposition at the
AuRDE has been observed in presence of Cd, Cu and Pb in the electrolyte, apparently also
due to the formation of the corresponding intermetallic compounds on the electrode surface.
The results of Se determination in different Pharmaceuticals using the method under study
have been compared with the results of the determination carried out ETA AAS technique and
were found in good compliance (1-5 % precision for the 10 mg/l Se concentration [50]).
The ISV method is the reported to be also used for the selenium concentration from
rather diluted samples for further determination either by ICP AAS or ISP MS techniques [53].
The development of such a concentration process appeared to be essential since the
selenium emission lines, used in ICP AAS are relatively weak and are placed in the near
ultraviolet. In this part of spectrum the conventional photomultipliers used in ICP AAS
instrumentation usually exhibit low efficiency [54]. On the other hand the selenium lines in
mass spectra are found to overlap with the masses of such particles as Ar2+ and ArCI+ and
this phenomenon also decreases the sensitivity of determination. In the electrochemical cell,
developed to carry out the Se determination the GC electrode modified with the electroplated
gold was used [55,56]. Taking into account the literature data [44], selenium electrodeposition
was carried out using 0,1 M HNO3 as supporting electrolyte. The deposition potential E^-0,3 V
was chosen for the Se concentration since at these potential values the current peaks,
referring to Se dissolution, were found to be more sharp, in comparison with those, obtained
at more positive deposition potentials. The stripping was carried out at the potential £3=1,1 V
/SCE. In such operating conditions the determination of 20-20 jxg/l Se is reported for the CP
AAS detection. Se recovery from the sample with the help of the method under study was
found to be about 91 %. The following obstacles, while carrying out the analysis are marked.
The calibration curve is found to deviate from the linearity, when more than one monolayer of
Se had been deposited at the electrode. The highest sensitivity of the method was achieved
only with the electrodes with freshly plated gold. The aging of the gold layer resulted in the
unpredicted reduction of the analytical signal. The electrochemical recovery of the spent gold,
followed by the plating of the fresh gold portions, were found to be ineffective. Therefore, the
reproducible results of Se determination were achieved only with freshly (less than one day)
prepared electrodes.
14
The brief consideration of Se(IV) and Se2" electrochemical behavior at solid electrodes
enables the following conclusions:
the mechanism of the redox reactions of Se2" and Se(IV) electrode
reactions at most commonly used electrodes (Pt, Au, GC) are well studied and
reviewed and their products are identified;
the reaction of Se electrodeposition followed by its anodic dissolution may
serve as a powerful analytical instrument for trace selenium determination in
aqueous (particularly, nitric acid) solutions, using either electrochemical detection,
or other analytical techniques (AAS, ICP MS, X - ray fluorescence)
however, the complex influence of the electrolyte composition, of the
electrode material and the mode of its preparation make the routine application of
the ISV Se determination rather difficult.
3. POSSIBLE APPLICATIONS OF ELECTROCHEMICAL METHODS FOR THE STUDY OFSELENIUM BEHAVIOR IN THE SOLUTIONS OF THE NUCLEAR FUEL CYCLE.
The data on the selenium electrochemistry in aqueous solutions, summarized in the
present short review, allow to select the possible directions of the research on the
applications of electrochemical methods for the study of the behavior of this element in the
solutions of PUREX process and in radioactive wastes. Available data on the Se
thermodynamics in aqueous solutions (See 2.1.) indicate, that selenium is present in the most
of solutions of PUREX process in the form of selenious acid H2Se03 or as the cation H3Se03+.
The oxidation of Se(IV) to Se(VI) in the solutions of nitric acid seems to be hardly possible.
However, in presence of strong reducing agents, for example hydrazine or hydroxylamine,
there exists the probability of the reduction of Se(IV) to amorphous elementary selenium,
forming the stable sols in the aqueous solutions [1,24,34]. The conditions and the parameters
of this reaction seem to be interesting to study, since there is now indication on the interaction
of Se(IV) with this agents in literature.
The numerous studies of selenium polarography in aqueous solutions, cited in the
present review [24-33], show that the reduction of the Se(VI) and Se(IV) along with the
oxidation of Se2' in different aqueous electrolytes at DME is well studied. However, only a few
data dealing with the polarographic behavior of selenium in the solutions of nitric acid are
reported [26,32]. The detailed study of Se(IV) electrochemical reduction at dropping mercury
seems to be of a great interest, since it might provide the information on the kinetics and
mechanism of the Se(IV)/(0) reduction and possible effect of NO3" ions on this process.
However, the interpretation of the results of the polarographic studies in nitric acid usually
face with the following processes, which should be taken into account:
15
interaction of mercury in nitric acid and following reactions of the ions
under study with Hg22+ ions [32,57-59];
electrochemical reduction of nitrate ions, catalyzed by the certain films,
adsorbed at the electrode[57,59].
The mentioned processes usually result in the appearance of the additional waves at the
polarograms of the element under study, disturbing or masking the principal electrode reaction.
In case of polarography of selenium(IV) the processes of the oxidation of mercury with HNO3
would be complicated with formation of insoluble HgSe either at the surface of DME or in the
bulk of the solution [27,28,32,34]. This chemical reaction would obviously cause the additional
troubles with the interpretation of selenium polarographic data. It is necessary to mention, that
the practical interest of to study of Se(IV) reduction at DME (possibility of quantitative Se
determination in the solutions of PUREX process) will be rather low. Se concentrations in such
solutions do not exceed 1,5*10'5 M, i.e. close to the detection limit of the modem polarographic
techniques. Taking into account the presence of a lot of adverse ions, which are present in
the solutions understudy in much greater concentrations (from 0,8 to 1,0 M U(VI); from 0,02
to 0,05 M Pu(IV) etc.) it is easy to predict what chemical treatment of sample should be carried
out to obtain reliable results of Se polarographic determination. Therefore, the study of Se
electrochemical reduction at DME, to our opinion seems to be of only scientific interest.
The results of the studies of Se electrochemical behavior at solid electrodes (See
chapter 2.3.) seem to be much more encouraging from the point of view of their application in
the radiochemical practice. The obtained results indicate, that the application of ISV method to
the Se determination in the objects with rather low content of the element of interest was
successful. The sensitive and reproducible analytical procedures were developed. Gold was
found to be the best electrode material for ISV selenium analysis. The possibility of the
application of HNO3, as the supporting electrolyte for Se determination was demonstrated [44].
The application of the ISV is not restricted only by the determination of Se, using the
electrochemical signal, proportional to the Se concentration. The method was also applied for
Se concentration for the further determination, using such advanced instrumentation as CP
AAS and ICP MS [53]. The drastic fall of the method sensitivity connected to the electrode
aging seems to be the only serious obstacle, observed in course of the analysis of the
literature data on the ISV Se determination. The adaptation of the ISV technique for the trace
Se concentration and determination in the solutions of the nuclear fuel cycle is indicated as
the most prospective goal of the future experimental study. This study may include:
the comparative study of Se ISV determination using bulk AuRDE and QC
electrode, modified with the gold film and the choice of the electrochemical
parameters forSe determination in HNO3solutions;
16
the study of the effect of different ions, originating from the PUREX
process, on the process of Se ISV determination and correction of the
electrochemical parameters;
the development of the analytical procedure of ICP MS determination of Se,
including ISV concentration of the element of interest from the nitric acid solutions
generated in the PUREX process or in the processes of radioactive wastes
treatment.
17
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