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June 24, 20023-rd Russian - Japan Seminar on Technetium Chemistry 1 ELECTROCHEMICAL METHODS FOR TECHNETIUM RECOVERY, ANALYSIS AND SPECIATION IN AQUEOUS.

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Presentation on theme: "June 24, 20023-rd Russian - Japan Seminar on Technetium Chemistry 1 ELECTROCHEMICAL METHODS FOR TECHNETIUM RECOVERY, ANALYSIS AND SPECIATION IN AQUEOUS."— Presentation transcript:

1 June 24, rd Russian - Japan Seminar on Technetium Chemistry 1 ELECTROCHEMICAL METHODS FOR TECHNETIUM RECOVERY, ANALYSIS AND SPECIATION IN AQUEOUS SOLUTIONS. A.Maslennikov 1, F.David 2, O.Courson 2, B. Fourest 2, V. Silin 1, A. Kareta 1 M.Masson 3, M. Leconte 3, V.Peretroukhine 1, C. Delegard 4. 1 – Institute of Physical Chemistry, Russian Academy of Sciences 2 - Institute of Nuclear Physics, CNRS, Orsay 3 - Centre dEtudes VALRHO, CEA, France 4 – US Department of Energy

2 June 24, rd Russian - Japan Seminar on Technetium Chemistry 2 PLAN OF PRESENTATION ò Physico-chemical properties of Tc-99, its accumulation in nuclear fuel, pathways in PUREX process. ò Technetium oxidation potentials in aqueous solutions, chemical forms, complex formation. ò Electrochemical reduction of TcO 4 - ions in non-buffered aqueous solutions at different pH. General considerations. Mechanism of the electrode reactions. ò Tc(VII) electrochemical reduction in the solutions of nitric acid. Interaction of the reduced Tc species with the products of NO 3 - electrochemical reduction. Tc recovery and denitration of the additional washing solution. ò Tc electrochemical reduction in the solutions of NaOH. Tc(V) stability in concentrated NaOH solutions. Possible analytical applications. ò Tc(VII) reduction in presence of complexing agents. Acetate and formate buffer media. Complex formation and electrodeposition. Tc recovery from PUREX wastes.

3 June 24, rd Russian - Japan Seminar on Technetium Chemistry 3 TECHNETIUM-99 RADIOCHEMICAL PROPERTIES 1. RADIOCHEMICAL PROPERTIES Tc-99 is the only long-lived fission product: T 1/2 99 Tc : 2, years - -emmitting isotope (100%) E max = 293 KeV Principal nuclear reaction 99 Mo (T 1/2 : 67 h.) 99 Tc 99 Ru (stable) Specific b - -activity: 16,9 mCi/g (625.3 GBq/g) Thermal fission yield from 235 U : 3.4 % (en atoms) 2. ACCUMULATION IN NUCLEAR FUEL OF DIFFERENT TYPES

4 June 24, rd Russian - Japan Seminar on Technetium Chemistry 4 TECHNETIUM BEHAVIOR IN PUREX PROCESS 1. CHEMICAL STATE IN THE FUEL White inclusions – alloys Mo-Tc-Ru-Rh-Pd, TcO 2 (?) 2. FUEL DISSOLUTION Technetium-99 dissolution is incomplete For 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(TcO 2 ) + HNO 3 Tc(VII) + Tc(IV,V)(?) + NO x + H 2 O Technetium concentration in the feed solution mg/l Fraction of Tc extracted with TBP/TPH is more than 95 % Tc concentration in rafinate 5 mg/l 3. FIRST EXTRACTION CYCLE HTcO 4(aq,) + 2TBP (org.) HTcO 4 *2TBP (org.) UO 2 2+ (aq.) + 2TcO 4 - (aq.) + 2TBP (org.) UO 2 (TcO 4 ) 2 *2TBP (org.) Zr(IV) (aq.) + 4TcO 4 -- (aq.) + 2TBP (org.) Zr(TcO 4 ) 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 mg/l M HNO 3. Zr(IV) washing – 0,5 M HNO 3 Tc(VII) washing – 4 M HNO 3.

5 June 24, rd Russian - Japan Seminar on Technetium Chemistry 5 SELECTED TECHNETIUM REDOX POTENTIALS IN AQUEOUS SOLUTIONS. a – TcO H + + 3e - TcO 2(cr.) + 2H 2 O. b – TcO H + + 3e - TcO(OH) 2 + H 2 O

6 June 24, rd Russian - Japan Seminar on Technetium Chemistry 6 CHEMICAL FORMS OF Tc IN DIFFERENT OXIDATION STATES IN AQUEOUS SOLUTIONS pH Selected complex ions and compounds In non-complexig media

7 June 24, rd Russian - Japan Seminar on Technetium Chemistry 7 ELECTROCHEMICAL REDUCTION OF Tc(VII) IONS IN AQUEOUS ACID SOLUTIONS. Polarographic curves of 3.7 * M TcO 4 - 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. TcO H + + 4e - Tc H 2 O irr., E = k*[H + ] 8, n=4 2. Tc e - - Tc o (Hg) irr., weakly pH dependent, n=3 3 2H + + 2e - H e 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. TcO H + + 4e - TcO H 2 O irr., E=k[H + ] 6

8 June 24, rd Russian - Japan Seminar on Technetium Chemistry 8 POSSIBLE ELECTRODE REACTIONS pH – 1.0 – 3.0 pH – 2,0-7,0 - electrode reaction; - disproportionation; - hydrolysis

9 June 24, rd Russian - Japan Seminar on Technetium Chemistry 9 Tc(VII) ELECTROCHEMICAL REDUCTION AT Hg ELECTRODE IN HNO 3 SOLUTIONS - 2A -Polarography of 2.0*10 -4 M T c(VII) at HMDE in solutions of nitric acid at different HNO 3 concentrations: M HNO 3 ; [Tc(VII)]=0 (background curve. ); M HNO 3 ; M HNO 3 ; M HNO 3 ; M HNO 3. 2B - Limiting currents of Tc(VII)/Tc(III) reduction and catalytic currents of NO 3 - ions destruction in HNO 3 solutions. 1- Tc(VII)/Tc(III) in M HNO 3 ; NO 3 destruction in: M HNO 3, M HNO 3, M HNO 3, At 0 V / SCE TcO H + + 4e - Tc H 2 O At –0,7 V / SCE NO H + + 2e - HNO 2 + H 2 O Tc(III) + HNO 2 +3H + Tc(IV) + NO +2H + +H 2 O 4Tc HNO 2 + 4H + Tc 4+ +N 2 O +2H + +3H 2 O 2NO + NO H 2 O + H + 3HNO 2 Tc 4+ + e - Tc 3+

10 June 24, rd Russian - Japan Seminar on Technetium Chemistry 10 TECHNETIUM CATALYZED ELECTROCHEMICAL DENITRATION AT GRAPHITE ELECTRODE Kinetics of electrochemical denitration of 4 M HNO 3 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 Formation of the Tc compound possessing the absorption band at =480 nm in the VIS spectra is observed in the solution.

11 June 24, rd Russian - Japan Seminar on Technetium Chemistry 11 Tc(VII) polarography in the solutions of 0,5 – 4.0 M NaOH Tast-polarography Tc(VII) – NaOH [Tc(VII)] – 2*10 -4 M ,5 M NaOH; M NaOH; M NaOH; M NaOH. DPP Tc(VII) – NaOH [Tc(VII)] – 2*10 -4 M ,5 M NaOH; M NaOH; M NaOH; MNaOH 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 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 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 13 DETERMINATION OF U(VI), Tc(VII) AND Cr(VI) IN THE SOLUTION SIMULATING ALKALINE RADWASTES Results of U(VI), Tc(VII), Cr(VI) DPV determination by standard addition method. (n=3, P=0.95).

14 June 24, rd Russian - Japan Seminar on Technetium Chemistry 14 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 15 TECHNETIUM(VII) ELECTROCHEMICAL REDUCTION AT SMDE IN COMPLEXING BUFFER MEDIA 3D polarogram of M TcO 4 - in 0,1 M acetic buffer solution (pH=4,6). 0 V / Ag/AgCl to -1 V / AgAgCl V 1 - TcO H + + 4e - Tc H 2 0 V 1 – TcO e - TcO 4 3- V 1 - TcO H + + 2e - Tc H 2 0 3D polarogram of M TcO 4 - in 0,1 M acetic buffer solution (pH=4,6). 1,0 V / Ag/AgCl to –1,5 V / AgAgCl V 3 – Tc e - - Tc o (Hg) V 4,V s – 2H + + 2e - - H 2(ads.)

16 June 24, rd Russian - Japan Seminar on Technetium Chemistry 16 SMDE DIFFERENTIAL CAPACITY IN ACETIC BUFFER SOLUTION (pH=4,60), CONTAINING Tc(VII). 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 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 TcO xH 2 O TcO(OH) x 3-x (ads.) + xH + Tc H 2 O Tc(OH) x 3-x + xH +

17 June 24, rd Russian - Japan Seminar on Technetium Chemistry 17 PUREX waste solution 200 mg/l Tc(VII) Catalytic HNO 3 destruction 28 M HCOOH Neutralisation to pH Tc electrodeposition TcO x anodic oxidation Precipitation of R 4 NTcO 4 R-C 2 H 5, C 4 H 9 Tc metal Tc carbide ToTo FLOWSHEET FOR TECHNETIUM RECOVERY FROM PUREX WASTE SOLUTION.

18 June 24, rd Russian - Japan Seminar on Technetium Chemistry 18 TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS. 1. Electrochemical cell design

19 June 24, rd Russian - Japan Seminar on Technetium Chemistry 19 TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS. 2. Cyclic cvoltammetry measurements 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 20 TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS. 3. Stripping voltammetry measurements. 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 21 TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS FORMATE SOLUTIONS. 4. Determination of the electrodeposition potential and yield at different pH 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 June 24, rd Russian - Japan Seminar on Technetium Chemistry 22 TECHNETIUM ELECTROCHEMISTRY IN AQUEOUS 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 E cath. =-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); E cath. =-1.4 V/SCE, Electrolysis time - 1 hour.

23 June 24, rd Russian - Japan Seminar on Technetium Chemistry 23 POSSIBLE MECHANISM OF Tc(VII) ELECTROCHEMICAL REDUCTION IN FORMATE BUFFER SOLUTION At E > -0,6 V / SCE TcO H + + 4e - TcO H 2 O 2TcO HCOO - Tc 2 O 2 (HCOO) 6 -2 At E < -0,6 V TcO H + + 4e - TcO H 2 O When electrolyte pH > 5 Tc 2 O 2 (HCOO) H 2 O 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 TcO 2-x hydrous oxide deposited at the electrode

24 June 24, rd Russian - Japan Seminar on Technetium Chemistry 24 èElectrochemical 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. èIn HNO 3 solutions electrochemical reduction of Tc(VII) results in formation of Tc(IV,V) species, which are reoxidiszed by the products of NO 3 - ions electrochemical reduction. Therefore direct electrodeposition of Tc(IV) from the solutions of nitric acid is not quantitative. èThe 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. èDPP 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, M Tc(VII) and may compete with LSC technique of Tc-99 determination in the alkaline radwastes. èElectrochemical 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. èElectrochemical 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 % Tc recovery and by formation of comparatively thick hydrated TcO 2-x layers (about 3,5 mg/cm 2 ) at the graphite electrode surface. CONCLUSION


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