june 24, 20023-rd russian - japan seminar on technetium chemistry 1 electrochemical methods for...

June 24, 2002 3-rd Russian - Japan Seminar on Technetium Chemistry

1

ELECTROCHEMICAL METHODS FOR ELECTROCHEMICAL METHODS FOR TECHNETIUM RECOVERY, ANALYSIS AND TECHNETIUM RECOVERY, ANALYSIS AND

SPECIATION IN AQUEOUS SOLUTIONS.SPECIATION IN AQUEOUS SOLUTIONS.

A.Maslennikov1, F.David2, O.Courson2, B. Fourest2, V. Silin1, A. Kareta1 M.Masson3, M. Leconte3, V.Peretroukhine1, C. Delegard4.

1 – Institute of Physical Chemistry, Russian Academy of Sciences2 - Institute of Nuclear Physics, CNRS, Orsay3 - Centre d’Etudes VALRHO, CEA, France4 – US Department of Energy


2

PLAN OF PRESENTATIONPLAN OF PRESENTATION

Physico-chemical properties of Tc-99, its accumulation in nuclear fuel, pathways in PUREX process.

Technetium oxidation potentials in aqueous solutions, chemical forms, complex formation.

Electrochemical reduction of TcO4- ions in non-buffered aqueous solutions at different pH.

General considerations. Mechanism of the electrode reactions.

Tc(VII) electrochemical reduction in the solutions of nitric acid. Interaction of the reduced Tc species with the products of NO3

- electrochemical reduction. Tc recovery and denitration of the additional washing solution.

Tc electrochemical reduction in the solutions of NaOH. Tc(V) stability in concentrated NaOH solutions. Possible analytical applications.

Tc(VII) reduction in presence of complexing agents. Acetate and formate buffer media. Complex formation and electrodeposition. Tc recovery from PUREX wastes.


3

TECHNETIUM-99 RADIOCHEMICAL PROPERTIESTECHNETIUM-99 RADIOCHEMICAL PROPERTIES

1. RADIOCHEMICAL PROPERTIES

Tc-99 is the only long-lived fission product: T1/2 99Tc : 2,13.105 years--emmitting isotope (100%) E max = 293 KeVPrincipal nuclear reaction 99Mo (T1/2 : 67 h.) 99Tc 99Ru (stable)Specific b--activity: 16,9 mCi/g (625.3 GBq/g)Thermal fission yield from 235U : 3.4 % (en atoms)

2. ACCUMULATION IN NUCLEAR FUEL OF DIFFERENT TYPES

Fueltype

Enrichment 235U orPu content, %

Burn-upMwd/tUHM.

Coolingtime,years

99Tc content,g/tUi

UOX1 3.5 33000 3 814

UOX3 4.5 60000 3 1403

MOX 8.3 43500 2 1056

FBR 16.5 125000 3 2453


4

TECHNETIUM BEHAVIOR IN PUREX PROCESSTECHNETIUM BEHAVIOR IN PUREX PROCESS

1. CHEMICAL STATE IN THE FUELWhite inclusions – alloys Mo-Tc-Ru-Rh-Pd, TcO2(?)

2. FUEL DISSOLUTIONTechnetium-99 dissolution is incompleteFor UOX 1 fuel at final uranium concentration equal to 250 g/l :90 % de Tc is dissolved, while 10 % is found in insoluble residues.Tc(TcO2) + HNO3 Tc(VII) + Tc(IV,V)(?) + NOx + H2O

Technetium concentration in the feed solution - 180 mg/lFraction of Tc extracted with TBP/TPH is more than 95 %Tc concentration in rafinate 5 mg/l

3. FIRST EXTRACTION CYCLE

HTcO4(aq,) + 2TBP(org.) HTcO4*2TBP(org.)

UO22+

(aq.) + 2TcO4-(aq.) + 2TBP(org.) UO2(TcO4)2*2TBP(org.)

Zr(IV)(aq.) + 4TcO4--

(aq.) + 2TBP(org.) Zr(TcO4)2*2TBP(org.)4. Tc STRIPPING

Fraction of Tc is stripped from organic solvent by washing is more than 95%Tc concentration in the additional rafinate solution - 220 mg/l + 2-4 M HNO3.

Zr(IV) washing – 0,5 M HNO3

Tc(VII) washing – 4 M HNO3.


5

SELECTED TECHNETIUM REDOX POTENTIALS IN SELECTED TECHNETIUM REDOX POTENTIALS IN AQUEOUS SOLUTIONS.AQUEOUS SOLUTIONS.

Redox couple Experimental conditions Potential, V/NHE

Tc(VII)/Tc(VI) 0,1 – 1,0 MeOH, Cl-, ClO4- SO4

2-

polarography, coulometry-0,52 - -0,64

Tc(VII)/Tc(V) 0,1 – 1,0 MeOH, Cl-, ClO4-

polarography, coulometry-0,54 - -0,63

Tc(VII)/Tc(IV) 1<pH<4 Cl-, SO42-, ClO4

-

potentiometry, emf0,732 - 0,748a

0,5790,016b

Tc(VII)/Tc(III) 1<pH<4 Cl-, SO42-, ClO4

-

potentiometry, emf0,576

a – TcO4- + 4H+ + 3e- TcO2(cr.) + 2H2O.

b – TcO4- + 4H+ + 3e- TcO(OH)2 + H2O


6

CHEMICAL FORMS OF Tc IN DIFFERENT OXIDATION CHEMICAL FORMS OF Tc IN DIFFERENT OXIDATION STATES IN AQUEOUS SOLUTIONSSTATES IN AQUEOUS SOLUTIONS

VII VI V IV III

TcO4- TcO4

2- TcO43- TcO(OH)3

- Tc(OH)3

HTcO4 HTcO4- TcO(OH)3

- TcO(OH)2 Tc3+

H2TcO4 TcO(OH)x(3-x)+ TcO(OH)+

TcnOn(OH)nxn(3-x)+ TcO2+

VI V IV III

TcOCl5- TcOX5

2- TcX62- TcX6

3-

TcOX4- TcX5

- TcX5(OH)3-

TcO2X43- Tc(CO3)(OH)2

TcO(NO3)3 Tc(CO3)(OH)3-

Tc2O2(RCOO)62-

pH

Selected complex ions and compounds

In non-complexig media


7

ELECTROCHEMICAL REDUCTION OF Tc(VII) IONS IN ELECTROCHEMICAL REDUCTION OF Tc(VII) IONS IN AQUEOUS ACID SOLUTIONS. AQUEOUS ACID SOLUTIONS.

Polarographic curves of 3.7*10-4 M TcO4- in 0.5

M KCl at various pH values (1) - bacground current; (2) - pH=7.74, (3) - pH=4.72; (4) - pH=4.21; (5) - pH=3.91, (6) - pH=3.59, (7) - pH=3.47, (8) - pH=3.30, (9) - pH=3.17, (10) - pH=3.17, (11) - pH=2.09.

Three cathodic irreversible reduction waves are observed and attributed to the following electrode reactions:

1. TcO4- + 8H+ + 4e- Tc3+ + 4H2O irr., E = k*[H+]8, n=4

2. Tc3+ + 3e- - Tco(Hg) irr., weakly pH dependent, n=33 2H+ + 2e- He catalytic H+ discharge

pH 0,0 - 3,0

pH 2,0 - 5,0

Two cathodic reduction waves are observed and attributed to the following electrode reactions

1. TcO4- + 6H+ + 4e- TcO2+ + 3H2O irr., E=k[H+]6<x<8, n=3

2. 2H+ + 2e- He catalytic H+ discharge


8

POSSIBLE ELECTRODE REACTIONSPOSSIBLE ELECTRODE REACTIONS

pH – 1.0 – 3.0If HTcO4 is a weak acid If HTcO4 is a strong acid

VII HTcO4 + e- HTcO4- TcO4

- + e- TcO42-

VI HTcO4- + e- HTcO4

2-

2HTcO4- HTcO4

2- + HTcO4

TcO42- + e- TcO4

3-

2TcO42- TcO4

3- + TcO4-

VHTcO4

2- + 5H+ + e- TcO2+ + 3H2O3HTcO4

2- + 10H+ HTcO4 + 2TcO2+ + 6H2OHTcO4

2- + (x-1)H+ TcO(OH)x(3-x)+

HTcO42- + 5H+ + e- TcO2+ + 3H2O

3TcO43- + 12H+ TcO4 + 2TcO2+ + 6H2O

TcO43- + (x-1)H+ TcO(OH)x

(3-x)+

IV TcO2+ + 2H+ + e- Tc3+ + H2OTcO2+ + xH2O TcO(OH)x

(2-x)+

pH – 2,0-7,0

VII TcO4- + e- TcO4

2-

VI TcO42- + e- TcO4

3-

2TcO42- TcO4

3- + TcO4-

VHTcO4

2- + 5H+ + e- TcO2+ + 3H2O3TcO4

3- + 12H+ TcO4- + 2TcO2+ + 6H2O

TcO43- + (x-1)H+ TcO(OH)x

(3-x)+

- electrode reaction; - disproportionation; - hydrolysis


9

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1

-E, V/SCE

Cu

rre

nt

de

ns

ity,

A/m

²

0

12

3

4

0

0.002

0.004

0.006

0.008

0.01

0.012

0 0.2 0.4 0.6

-E, V/SCE

Cu

rren

t d

ensi

ty,

A/m

²

0

1-3

4

0

0.02

0.04

0.06

0.08

0.1

0.00E +00 2.0 0E-04 4.0 0E-04 6.0 0E-04 8.0 0E-04

Tc (V II) co nce n tra tion, M

Cu

rre

nt

de

ns

ity

, A

/m²

1

2

3

4

Zoom: E = 0 - -0.6 V/SCE

Fig.2A

Fig.2B

Tc(VII) ELECTROCHEMICAL REDUCTION AT Hg Tc(VII) ELECTROCHEMICAL REDUCTION AT Hg ELECTRODE IN HNOELECTRODE IN HNO33 SOLUTIONS SOLUTIONS

2A --Polarography of 2.0*10-4 M T c(VII) at HMDE in solutions of nitric acid at different HNO 3 concentrations: 0 - 1.0 M HNO3; [Tc(VII)]=0 (background curve.); 1 - 0.1 M HNO3; 2 - 0.25 M HNO3; 3 - 0.5 M HNO3; 4-1.0 M HNO3.

2B - Limiting currents of Tc(VII)/Tc(III) reduction and catalytic currents of NO3

- ions destruction in HNO3 solutions. 1-Tc(VII)/Tc(III) in 0.1 - 1.0 M HNO3; NO3 destruction in: 2 - 0.1 M HNO3, 3 - 0.25 M HNO3, 4 - 1.0 M HNO3,

At 0 V / SCETcO4

- + 8H+ + 4e- Tc3+ + 4H2O

At –0,7 V / SCE

NO3- + 3H+ + 2e- HNO2 + H2O

Tc(III) + HNO2 +3H+ Tc(IV) + NO+2H+ +H2O4Tc3+ + 2HNO2 + 4H+ Tc4+ +N2O+2H+ +3H2O

2NO + NO3- ++H2O + H+ 3HNO2

Tc4+ + e- Tc3+


10

TECHNETIUM CATALYZED ELECTROCHEMICAL TECHNETIUM CATALYZED ELECTROCHEMICAL DENITRATION AT GRAPHITE ELECTRODEDENITRATION AT GRAPHITE ELECTRODE

2

2.5

3

3.5

4

4.5

0 30 60 90 120

Time, min

HN

O3

con

cen

tra

tio

n,

M

1A

1B

2A

2B

Kinetics of electrochemical denitration of 4 M HNO3 at different potentials of graphite cathode. 1-E=-0.5 V/SCE; 2-E=-1.0 V/SCE, A-[Tc(VII)]=0; B-[Tc(VII)]=5*10-4 M.

Experimental results

-E, V/SCE 0.5 0.65 0.8 1.0 1.2

[Tc(VII)=0 3.76 3.87 3.12 3.18 2.46[HNO3]=2h,

M [Tc(VII)]=2*10-4 M

3.98 3.81 2.49 2.48 2.36

[Tc(VII)=0 2.12 2.65 3.09 5.05 5.44CE, F/1 MHNO3 [Tc(VII)]=

2*10-4 M2.28 2.39 2.79 4.37 4.97

Tc electrodepositionyield, %

9.05 12.4 23.5 34.1 35.0

Formation of the Tc compound possessing the absorption band at =480 nm in the VIS spectra is observed in the solution.


11

Tc(VII) polarography in the solutions of 0,5 – 4.0 M NaOH

Tast-polarography

Tc(VII) – NaOH

[Tc(VII)] – 2*10-4M--- - 0,5 M NaOH;--- - 1.0 M NaOH;--- - 4.0 M NaOH;--- - 4.0 M NaOH.

DPP Tc(VII) – NaOH

[Tc(VII)] – 2*10-4M--- - 0,5 M NaOH;--- - 1.0 M NaOH;--- - 4.0 M NaOH;--- - 4.0 MNaOH

-E, V/SCE 0.5 0.65 0.8 1.0 1.2

[Tc(VII)=0 3.76 3.87 3.12 3.18 2.46[HNO3]=2h,

M [Tc(VII)]=2*10-4 M

3.98 3.81 2.49 2.48 2.36

[Tc(VII)=0 2.12 2.65 3.09 5.05 5.44CE, F/1 MHNO3 [Tc(VII)]=

2*10-4 M2.28 2.39 2.79 4.37 4.97

Tc electrodepositionyield, %

9.05 12.4 23.5 34.1 35.0

Conclusion: application of DPP technique for the Tc(VII) determination may provide detection limit about 5*10-7 M, being compatible with LSC counting of Tc-99.


12

ELECTROCHEMICAL CHARACTERISTICS OF Tc(VII) REDUCTION AT SMDE IN NaOH SOLUTIONS.

Reduction potentials of Tc(VII) in NaOH solutions and calculations of the number of the electrons, engaged to the

electrode reaction

Tast polarography DPP[OH-

], M Е1/2, VAg/AgCl

na nb Ер, VAg/AgCl

nc

0.1 -0.775 1.050.06 3.090.05 -0.775 0.90

0.5 -0.735 1.130.09 2.890.05 -0.733 0.95

1.0 -0.705 1.240.05 2.810.02 -0.715 0.95

2.0 -0.696 1.470.07 2.690.06 -0.700 0.95

4.0 -0.694 2.080.08 2.340.09 -0.690 1.29

a – data of the logarithmic analysis of TP waves;b – calculated, using Cotrell equation;

c – calculated using the ration W1/2 = 90.4 мV/ (W1/2 peak widthat I=0.5Ip)

Suggestions on the Tc(VII) reduction mechanism

Tc(VII) + e- Tc(VI)

fast in 1-4 M NaOH

2Tc(VI) Tc(VII) + Tc(V)

fast in 1-4 M NaOH

2Tc(V) Tc(IV) + Tc(VI)

fast in 1-2 M NaOH


13

DETERMINATION OF U(VI), Tc(VII) AND Cr(VI) IN THE DETERMINATION OF U(VI), Tc(VII) AND Cr(VI) IN THE SOLUTION SIMULATING ALKALINE RADWASTESSOLUTION SIMULATING ALKALINE RADWASTES

0

0.05

0.1

0.15

0.2

0.25

0.00E+00 2.00E-06 4.00E-06 6.00E-06 8.00E-06 1.00E-05

Concentration, M

I, µA

Tc(VII)

Cr(VI)

U(VI)

IONLinearity range,

MR2

Detection

limit, MSr, %

Tc(VII) 1х10 -7 - 1х10 -4 0.998 8х10 -8 5.0-8.0

U(VI) 1х10 -6 - 1х10 -4 0.997 8х10 -7 2.0-4.0

Cr(VI) 3х10 -7 - 2х10 -4 0.998 3х10 -7 3.0-7.0

Metal ionAdded,M·105

Found,M·105 Sr,%

Rec., %

U(VI)** 0.5 0.48±0.06 5 96CTc(VII)=1·10-4 mol l-1 1.0 1.1±0.4 14 110Chydr.=8·10-4 mol l-1

Tc(VII)** 0.2 0.20±0.04 8 100CU(VI)=3·10-4 mol l-1 0.3 0.30±0.04 5 100

0.4 0.43±0.06 5 108Cr(VI)* 1.0 0.95±0.07 3 95

CU(VI)=1.5·10-4 mol l-1

2.0 2.00±0.05 2.5 100

3.0 3.3±0.5 7 110U(VI)* 2.0 2.3±0.3 9 115

CCr(VI)=1·10-5 mol l-1 4.0 4.0±0.2 2,5 1005.0 4.7±0.2 3 94

* - 1 M NaOH**- 2 M NaOH

Metal ionAdded,M·105

Found,M·105 Sr,%

Rec., %

U(VI)** 0.5 0.48±0.06 5 96CTc(VII)=1·10-4 mol l-1 1.0 1.1±0.4 14 110Chydr.=8·10-4 mol l-1

Tc(VII)** 0.2 0.20±0.04 8 100CU(VI)=3·10-4 mol l-1 0.3 0.30±0.04 5 100

0.4 0.43±0.06 5 108Cr(VI)* 1.0 0.95±0.07 3 95

CU(VI)=1.5·10-4 mol l-1

2.0 2.00±0.05 2.5 100

3.0 3.3±0.5 7 110U(VI)* 2.0 2.3±0.3 9 115

CCr(VI)=1·10-5 mol l-1 4.0 4.0±0.2 2,5 1005.0 4.7±0.2 3 94

* - 1 M NaOH**- 2 M NaOH

Results of U(VI), Tc(VII), Cr(VI) DPV determination by standard addition method. (n=3, P=0.95).


14

Element Adverse ion RatioTc(VII) 1/1000*

U(VI)Cr(VI) 1/1

Tc(VII) U(VI) 1/350Cr(VI) U(VI) 1/15

* -After the action of hydrazine

MUTUAL ADVERSE EFFECT Tc(VII)-U(VI) AND U(VI)-Cr(VI)MUTUAL ADVERSE EFFECT Tc(VII)-U(VI) AND U(VI)-Cr(VI)

Tc(VII) reduction with hydrazine is suggested to eleminate the effect of Tc(VII) ions on the U(VI) determination.

Before After

Effect of 8·10-4 mol l-1 hydrazine on DPV curves of 5·10-4 mol l-1 Tc(VII) and 5·10-6 mol l-1 U(VI) in 2 M NaOH.


15

TECHNETIUM(VII) ELECTROCHEMICAL REDUCTION TECHNETIUM(VII) ELECTROCHEMICAL REDUCTION AT SMDE IN COMPLEXING BUFFER MEDIAAT SMDE IN COMPLEXING BUFFER MEDIA

5

10

15

200,0

0,10,2

0,30,4 0,5 0,6 0,7 0,8 0,9 1,0

0,0

0,5

1,0

1,5

2,0

V1''

V1'

V2

V1

-i (µ

A)

-E (V)

t (s)

5

10

15

20 0,91,0

1,11,2

1,31,4

1,5

0

10

20

30V

4

V5

V3

3D polarogram of 10-4 M TcO4- in 0,1 M

acetic buffer solution (pH=4,6).

0 V / Ag/AgCl to -1 V / AgAgCl

V1 - TcO4- + 8H+ + 4e- Tc3+ + 4H20

V1’ – TcO4- + 2e- TcO4

3-

V1’’ - TcO43- + 8H+ + 2e- Tc3+ + 4H20

3D polarogram of 10-4 M TcO4- in 0,1 M acetic

buffer solution (pH=4,6).

1,0 V / Ag/AgCl to –1,5 V / AgAgCl

V3 – Tc3+ + 3e- - Tco(Hg)V4,Vs – 2H+ + 2e- - H2(ads.)


16

SMDE DIFFERENTIAL CAPACITY IN ACETIC BUFFER SMDE DIFFERENTIAL CAPACITY IN ACETIC BUFFER SOLUTION (pH=4,60), CONTAINING Tc(VII).SOLUTION (pH=4,60), CONTAINING Tc(VII).

10

20

30

400,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

20

40

60

80

Cd (µ

F.cm

-2)

-E (V)

t (s)

10

20

30

400,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5

20

40

60

80

A B

Conclusion: there are two ranges of potentials, where the Tc separation from acetic solution is using electrodeposition technique possible.

Differential capacity of SMDE in the 0,1 M acetic buffer solution containing 10 -4 M Tc(VII)A – potential range from 0 to 0,7 V / Ag/AgCl

B – potential range from –0,5 to -1,5 V / Ag/AgCl

TcO43- + xH2O TcO(OH)x

3-x(ads.) + xH+ Tc3+ + 3H2O Tc(OH)x

3-x + xH+


17

PUREX waste solution 200

mg/l Tc(VII)

Catalytic HNO3

destruction

28 M HCOOH

Neutralisation

to pH 6.5-7.5Tc electrodepositionTcOx anodic oxidation

Precipitation of R4NTcO4

R-C2H5, C4H9

Tc metal

Tc carbide

To

FLOWSHEET FOR TECHNETIUM RECOVERY FROM FLOWSHEET FOR TECHNETIUM RECOVERY FROM PUREXPUREX WA WASTESTE SOLUTION SOLUTION..


18

TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONSFORMATE SOLUTIONS.

1. Electrochemical cell design

1 2 33 4 5

6 77 89 9

1. Graphite cathode2. Referecnce SCE electrode3. Pt counter electrode4. Inert gas inlet5. Inert gas outlet6. Cathode compartment7. Anode compartment8. Mechanical stirrer9. Cation exchange membrane

1 2 33 4 5

6 77 89 9


19

TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS.FORMATE SOLUTIONS.

2. Cyclic cvoltammetry measurements

-240

-200

-160

-120

-80

-40

0

40

80

-1000 -500 0 500 1000

Potential, mV / SCE

Cu

rre

nt

de

ns

ity

, A

/m2

v=10 mV/s[Tc(VII)]=0

[Tc(VII)]=2E-3 M

A

-240

-200

-160

-120

-80

-40

0

40

80

-1000 -500 0 500 1000

Potential, V/SCEC

urr

en

t d

en

sit

y, A

/m2

v=10 mV/s1 - [Tc(VII)]=0

2 - [Tc(VII)]=2E-3 M

B

Redox behavior of Tc(VII) at graphite electrode in 1 M formate buffer ( = 1) at different pH.A – pH = 3,95, B – pH = 7.37.


20

TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS.FORMATE SOLUTIONS.3. Stripping voltammetry measurements.

-40

-30

-20

-10

0

0 100 200 300 400

Temps, s.

I, m

A

E=-0.625 V/SCE

E=-0.775 V/SCE

E=-0.825 V/SCE

E=-0.825 V; [Tc(VII)]=0A

-100

0

100

200

300

400

-1000 -500 0 500 1000

Potential, mV / SCE

Cu

rre

nt

de

ns

ity,

A/m

2

[Tc(VII)]=2E-3 M

[Tc(VII)]=0

B

0

100

200

300

400

0,00E+00 1,00E-03 2,00E-03 3,00E-03

Tc(VII) concentration, M

Tc

re

cove

ry, m

C

C

A – evolution of the electrodeposition current as a function of electrolysis time at different potentials of graphite electrode

B – evolution of the current during potential scan in positive direction after electrodeposition at E=-0,825 V / SCE for 60 s.

C – Tc(IV) hydrated oxide recovery at the electrode at different Tc(VII) concentrations in the electrolyte.


21

TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS.FORMATE SOLUTIONS.

4. Determination of the electrodeposition potential and yield at different pH

0

100

200

300

400

600 650 700 750 800 850

-Epr. mV / SCE

Tc

reco

very

, m

C

pH=5.39

pH=7.37

pH=3.95

Edep.=-0,73 V / SCE

0

20

40

60

80

0,6 0,8 1 1,2 1,4

-E, V/SCE

Tc r

eco

very

, %

pH=1.56

pH=3.46

pH=8.5

pH=7.37

pH=5.32

Dependence of technetium recovery from formate buffer solutions with different pH

on the graphite electrode potential.

Tc concentration – 2E-3 M,

electrolysis time – 5 min.

Dependence of the technetium recovery from formate buffer solutions of different pH

values on the electrolysis potential.

[HCOO(Na++H+)] = 1 M, [Tc(VII)]=0=2*10-3 M, T=25°C, Electrolysis time - 30 min.


22

0

0.2

0.4

0.6

0.8

1

300 400 500 600 700

Wavelength, nm

Ab

so

rba

nce

max. = 528 nm

2 mL electrolyte

0.05 mL electrolyte1.95 mL H2O

0

0.1

0.2

0.3

0.4

250 300 350 400

Wavelength, nm

Ab

so

rba

nce

max. = 290 nm

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

450 500 550 600 650 700 750 800

Wavelength, nm

Ab

so

rba

nc

e

0 min

5 min

10 min

15 min

20 min

30 min

TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS.FORMATE SOLUTIONS.5. Electrolyte UV-VIS spectra and possible interpretation.

A,B - Electrolyte spectrum after 30 min electrolysis of 2*10 -3 M Tc(VII) solution in 1 M HCOONa (pH=5.32) at Ecath.=-0.4 V/SCE.

C - Evolution of the Tc formate electrolyte (pH=5.34, µ=1.0) visible spectra after accomplishing the electrodeposition process. Electrolysis conditions: 1 M HCOONa (pH=3.95); Ecath.=-1.4 V/SCE, Electrolysis time - 1 hour.


23

POSSIBLE MECHANISM OF Tc(VII) ELECTROCHEMICAL POSSIBLE MECHANISM OF Tc(VII) ELECTROCHEMICAL REDUCTION IN FORMATE BUFFER SOLUTIONREDUCTION IN FORMATE BUFFER SOLUTION

At E > -0,6 V / SCE

TcO4- + 6H+ + 4e- TcO2+ + 3H2O

2TcO2+ + 6HCOO- Tc2O2(HCOO)6-2

At E < -0,6 VTcO4

- + 6H+ + 4e- TcO2+ + 3H2O

TcTc

O

OO

OO

O

CH

CH

HCOO

HCOO

HCOO

HCOO

2-

TcTc

O

OO

OO

O

CH

CH

HCOO

HCOO

HCOO

HCOO

3-

+ e-

- e-

When electrolyte pH > 5 Tc2O2(HCOO)6

-2 + 4H2O 2TcO(OH)2 + 4HCOOH + 2HCOO-

Conclusion: at the potentials less, than -1,0 V/SCE the latter process leads to the complete hydrolysis of the latter complexes with formation of TcO2-x hydrous oxide deposited at the electrode


24

Electrochemical behavior of Tc at Hg and solid electrodes is rather complex. The majority of the electrode reactions are quite irreversible. The intermediate products of the electrode reactions undergo the fast processes of disproportionation and hydrolysis. Reliable data on Tc standard oxidation potentials in aqueous solutions are hardly available.

In HNO3 solutions electrochemical reduction of Tc(VII) results in formation of Tc(IV,V) species, which are reoxidiszed by the products of NO3

- ions electrochemical reduction. Therefore direct electrodeposition of Tc(IV) from the solutions of nitric acid is not quantitative.

The products of Tc(VII) electrochemical reduction in 0,5-4,0 NaOH solutions depend on the OH - ions concentration in the electrolyte. Its increase to the values more than 2 M results in the slow down the rate of Tc(V) disproportionation, leading to the accumulation of the latter species in the electrolyte.

DPP technique for direct simultaneous determination of Tc(VII), U(VI), Cr(VI) and Fe(II,III) in 2,0-4,0 M NaOH was developed. Method may be characterized by the detection limit 2,0 10 -7 M Tc(VII) and may compete with LSC technique of Tc-99 determination in the alkaline radwastes.

Electrochemical reduction of Tc(VII) in buffer systems, containing complex forming anions is characterized by formation of a variety of Tc(IV,III) polynuclear complexes, stable to hydrolysis, and thus applicable for the development of Tc electrodeposition processes.

Electrochemical technique for Tc-99 recovery from the PUREX wastes solutions, including catalytic denitration of starting solution with HCOOH, neutralization to pH>6,0 and electrodeposition of Tc from the latter solution. Method may be characterized by 95-97 % Tc recovery and by formation of comparatively thick hydrated TcO 2-x

layers (about 3,5 mg/cm2) at the graphite electrode surface.

CONCLUSIONCONCLUSION

june 24, 20023-rd russian - japan seminar on technetium chemistry 1 electrochemical methods for...

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