I.P.N. - 91406 ORSAY CEDEX FR9901050

CO

I

—oc

CO

s

CO

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

REFERENCES

1. Nazarenko 1.1., Yenmakov A.N. Analytical chemistry of selenium and tellurium. AnalyticalChemistry of the elements Series. Russian Academy of Sciences Publ., 1971, p.1-32

2. Li Chunsheng, Guo Jingru, Li Darning. J. Radioanal. Nucl. Chem. Articles, 1997, v.220, N 1,p.69-71.

3. Choppin G., Ridberg J. Nuclear Chemistry. Theory and applications. Pergamon Press Publ.Ney-York - London 1985, p.594.

4. . Noel, H., Daoudi, A., Levet, J.C., Potel, M. Crystal structure and magnetic properties of thehexagonal uranium dichalcogenides US2 and USe2 Materials Research BulletinVolume/Issue, Cover date: 31, 10 , October 1996 1213-1218

5. . Daoudi, A., Potel, M., Noel, H. Crystal structure and magnetic behaviour of the ternaryuranium and thorium-molybdenum chalcogenides UM06S8, UMo6Se8 and ThMo6S8 JournalOf Alloys And Compounds Volume/Issue, Cover date: 232, 1-2 , January 1996 180-185

6. Kochubey D.I., Nikitenko S.G., Parmon V.N., Gruzdkov Yu.A., Tributch H., Alonzo-Vante N.Physica B. Condensed matter 1995, v.208-209 N 1-4, p.694-696.

7. Kolthoff I.M. Lingane J.J. Polarography. Second edition. Volume 2, p.564-572.

8. Benemariya H., Robberecht H., Dellstra H. Sci. Tot. Environ. 1991, v. 105, N 1, p.73.

9. Kubota T., Okutani T. Anal. Chim. Acta 1997, v. 351, p. 319.

10. Kaiser G., Tolg G. Fresenius Z. Anal. Chem. 1986, v.325, p.32.

11. Hill S.G., Pitts L, Worsfold P. J. Anal. Atomic Spectr. 1995, v.10, p.409.

12. Rodrrigues Rodrigues E.M., Sanz Alaejos M., Diaz Romero C. Anal. Chim. Acta, 1996,v.334, p.161.

13. Lozak A., FijalekZ. Acta Polon. Pharm., 1996, v.53, p.83.

14. Lozak A., Fijalek Z. Chem. Anal., 1998, v. 43, N 1, p.1.

15. Eccles G.N. Critical reviews in analytical chemistry. 1991, v. 22, N 5, p. 345-380.

16. Paneli M.G., Voulgaropoulos A. Electroanalysis, 1993, v.5, p. 355-373.

17. Fernandez-BobezC, Fernandez Abidul M.T., Costa-Garsia A. Electroanalysis, 1998, v.10,N 10 p. 701-706.

18. Ryan M. D., Chambers J.Q. Anal. Chem., 1992, v. 64, p.79R-116R.

19. Bourges J. Radiochem. Radioanal. Lett, 1972, v.12, N 2-3, p. 111-119.

20. Abuzwida M., Maslennikov A., Peretrokhine V. Radioanal. Nucl. Chem. Articles, 1991, v.147, N 1, p. 141.

21. Peretroukhine V., David F., Maslennikov A. Radiochimica Acta 1994, v.65, p.161-166.

22. Peretroukhine V., Maslennikov A., Guerman K., Silin V., Gelis A., Troushina V. C.DelegardAnnual rep. forWHC 52 p., 1998 in press

18

23. Standard potentials in aqueous solutions. Ed. by Bard J., Parsons R., Jordan J. MarcelDekker Inc. New-York - Basel, 1981.

24. Zhdanov S.I. Selenium, in: Encyclopedia of the electrochemistry of the elements. MarcelDekker Inc. New-York - Basel 1971. v. IV. p.361-392.

25. Osman-Zade Sh., Vagramyan AT. Sov. Electrochemistry, 1966, v.2, p.82-88.

26. West Ph.Z., Dean J., Breda E.J. Collect. Czeck. Chem. Commun., 1948, v.13, N 1, p.1-8.

27. Lingane J.J., Neidrach W. J. Am. Chem. Soc, 1948, v. 70, p.4115-4119.

28. Toropova V.F. Soviet analytical chemistry, 1956, v. 11, p. 599-603. (Russian version).

29. Schawer L, Suchy K. Czeck. Chem. Commun., 1935, v. 7, N 1, p.25-30.

30. Lingane J.J., Neidrach W. J. Am. Chem. Soc, 1949, v. 71, p.196-204.

31. Desmukh G.S., Asthana O.P. Naturwissershaften, 1961, v. 48, p.477-481.

32. Christian G.D., Knoblock E.C., Purdy W.C. Anal. Chem., 1963, v. 35, p.1128-1135.

33. Lai S., Srivastava S.N., Z. Phys. Chem. 1967, v.236, p. 326-331.

34. Lumbroso H. Selenium. In: Nouveau traite de chimie minerale. Sous la direction de P.Pascal.Tome XIII, p.1651-1912, Masson et C* Editeurs, Paris -1961.

35. Seby F., Potin-Gautier M., Giffaut E., Donard O.F.X. Selenium au sein d'une site destockage de dechets radioactifs. Evaluation critique des constantes thermodynamiquesassociees aux equilibres.

36. Holak W. Anal. Chem., 1980, v.52, p.2189-2192.

37. Le Blanc M., Reichenstein D. Ber. Bunsenges. Phys. Chem., 1909, v.15, p.262.

38. Gmelins Handuch der Anorganichen Chemie, System No. 10, "Selen", 1953, p.253-269.

39. Von Hippel A., Bloom M.C. J. Chem. Phys., 1950, v.18, p.1243.

40. Abrarov O.A. The study of kinetics and mechanism of Se and Te electrodepositionManuscript deposited to the All-Union Institute of Scientific and Technical Information of theUSSR. N 3622-81 Dep. 1981, 17 p.

41. BaeshovA.,A., Zhurinov M.Zh., Zhdanov. In: Electrochemistry of selenium, tellurium andpolonium. Alma-Ata "Nauka" Publ. USSR 1989,176 p.

42. Kamenev A.I., Agasyan P.K., Lunev M.I. Investigations of selenium and telluriumelectrotransformations on stationary electrodes. In: Abstr. All-Union Conf. On thechemistry and technology of the chalcogens and chalkogenides. Moscow - Karaganda1978, p. 141-142.

43. Agassyan L.B., Yurchenko A.B., Ogassyan P.K. Sov. Anal. Chem., 1967, v.22, p.229.

44. Andrews R.W., Johnson D.C. Anal. Chem. 1975, v.47, N 2, p.294-299.

45. Hamilton T.W., Ellis J. Analytica Chimica Acta, 1979, v.110, N 1, p.87-94.

19

46. Tan H.S., Kownaves S.P. Electroanalysis, 1998, v.10, N 6, p. 364-268.

47. Gil E.P., Ostapchuk P. Analytica Chimica Acta 1994, v.293, N 1, p.55-59.

48. Wang J., Lu J. Analytica Chimica Acta, 1993, v.274, N 2, p.219-223.

49. Hrehocik M.H., Lundgren J.S., Boyer W.J. Electroanalysis, 1993, v.5, N 2, p.289-292.

50. Fijatek A., Lozak A., Sarna K. Electroanalysis, 1998, v.10, N 12, p.846-851.

51. Hepel M., Kanige K., Bruckenstein S. J. Electroanal. Chem., 1989, v. 266, N 4, p. 409-414.

52. Wei C, Myung N., Rajeshwar K., J. Electroanal. Chem., 1994, v.375, N 5, p.109-115.

53. Pretty J.R., Blubaugh E.A., Caruso J.A. Anal. Chem., 1993, v.65, N 23, p. 3396-3403.

54. Wingle R.K., Peterson V.J., Fassel V.A. Appl. Spectrosc. 1979, v.33, N 1, p.209-212.

55. Pretty J.R., Evans E.H., Blubaugh E.A., Shen W.L, Caruso J.A. J. Anal. Atomic Spectrom.1990, v.5, N 2, p.437-443.

56. Pretty J.R., Evans E.H., Blubaugh E.A., Caruso J.A. J. Anal. Atomic Spectrom., 1992, v. 7,N 12, p.1131-1138.

57. Heyrowsky Ya., Kuta Ya.. Principles of polarography. Academic Press Publ., London -New-York, p.253, 388 (1966).

58. Zelverte A. Ph.D.Thesis. Centre des Etudes Fontenay aux Roses. Rapport CEA R-5443,April (1988).

59. Maslennikov A., Courson O., Peretroukhine V., Ddavid F., Masson M., in: 4th Int. Conf.Nuclear and Radiochemistry Ext. Abstr. V.2, GO2. St. Malo, France, Sept. 8-13, (1996).

20