Redox Chemistry of Neptunium in Solutions of Nitric Acid

Transcription

1 Redox Chemistry of Neptunium in Solutions of Nitric Acid Alena PAULENOVA Martin PRECEK, Kyle HARTIG, Nathan KNAPP

2 Neptunium Redox Chemistry Coexistence of three oxidation states in acidic solutions NpO 2+, NpO 2+ 2, Np 4+ Disproportionation of NpO 2+ in strongly acidic solutions NpO H + NpO H 2 O In UREX reprocessing solutions NpO 2+ 2 and Np 4+ are readily extracted NpO 2+ remains in the aqueous phase Oxidize Np to NpO 2+ 2 with V(V) Reduce/Scrub NpO 2+ 2 with AHA

3 4 M HNO 3 Np(III, IV, V, VI) =f(hno 3 ) Spectra by Friedman H A, Toth L M. 1980, J. Inorg. Nucl. Chem. V. 42 pp M HNO 3 Np(V) Np(IV)

4 Vanadium (V) with Np(V) Reduction potentials : VO 2+ /VO 2+ = 1.0 V and NpO 2 2+ / NpO 2+ =1.15 V This redox reaction is reversible: NpO + VO + 2H NpO + VO + H O Np( VI ) + V ( IV ) Np( V ) + V ( V ) d x dt = [ Np V ] d ( ) dt dnpv [ ( )] α = k [ Np( V)] [ V( V)] 1 dt dnpv [ ( )] γ + = k [ Np( VI)] [ V( IV)] 2 dt δ β d x d t = k Np V V V k Np VI V IV [ ( )] [ ( )] [ ( )] [ ( )] 1 2

5 Reaction order with respect to concentration of Np(V) and V(V) -Δ[Np(V)] (M) 3.E-04 2.E-04 1.E-04 0.E mm 1.80 mm 3.60 mm Reaction Time (s) Effect of increasing initial [V(5+)] on reaction rate lg (Reaction rate, M s -1 ) V(V) Np(V) slope V(V) = 0.98 slope Np(V) = lg (Concentration, M) : c V(V) = mm; c Np(V) = mm : c Np(V) =0.360 mm, c V(V) = mm

6 Initial rates of oxidation of Np(V) as f(h + ) 6.E-06 5.E-06 4M HNO3 1 M HNO3 + 3M LiNO appar. rate const., k1'' appar. equilibrium const., K' initial rate (M s -1 ) 4.E-06 3.E-06 slope = E-06 1.E-06 slope = E+00 0.E+00 5.E-07 1.E-06 2.E-06 c Np(V) c V(V) (M 2 ) lg k 1" (lg M -1 s -1 ) slope = 1.21 slope = lg [H + ] (lg M) lg K' Initial rates of oxidation Effect of[ H + ] NpO + VO + 2H NpO + VO + H O

7 Hydrolysis of Vanadium (V) in Acid Fast protonation equilibria of VO 2 + Transformation into two additional forms VO(OH) 2+ and VO 3+ : VO 2+ + H + VO(OH) 2+ VO H 2+ VO 3+ + H 2 O A simple electron transfer to VO 3+ leads to oxidation of Np(V): NpO + VO NpO + VO Speciation of V(V) =f[h 3 O + ], [VO 2+ ], VO(OH) 2+ and VO 3+ are given by: K 1 = 2+ [ VO( OH ) ] [ VO ] [ H ] K = 3+ [ VO ] [ VO ] [ H ] 2

8 Constant rate of oxidation Assuming: NpO 2+ is the only form of Np(V) in acidic (HClO 4 or HNO 3 ) solution Speciation/hydrolysis of V(V) to VO 3+ The forward (oxidation) reaction in a non-complexing acid is: d[ Np( V )] k [ H ] = k NpO VO = Np V V V dt 1 K [ H ] K [ H ] [ ] [ ] [ ( )] [ ( )] Koltunov: K 1 =0.79 M -1 and K 2 =0.30 M -2 "true" rate constant k = 245 ± 15 M -3 min -1 (4.08 ± 0.25 M -3 s -1 ) Precek [OSU]: "true" rate constant k = 4.35 ± 0.18 M -3 s -1 ) where K 1, K 2 are constants for V(V)

9 Thermodynamic of activation reaction Np(V) + V(V) Data source C 4M HNO 3 [OSU] E A kj/mol H* kj/mol S* J/K.mol G* kj/mol ±1 53 ±1-55.8± ±1.6 4M HClO 4 [Koltunov] ±6 53± 6-59± ±.2 2M HNO 3 [Dukes] Eyring equation: Boltzmann=k B = JK -1 Planck = h= J R =8.314 J K -1 mol -1 T = K = 25 C ±12 n/a n/a n/a ln k E RT A 1 = + ln kt 1 e e e e A B kt k = = h h A ΔG* ΔS* E - RT B R RT

10 Reduction of Np(VI) by HNO NpO + HNO + H O NpO + H + NO catal. HNO2 2 2 Standard redox potentials (in volts) for U, Pu, Np and 1M HNO 2 /HNO 3 [Miles, 1990; Drake, 1990]

11 REDUCTANT T ( o C) Rate Const (s 1 ) Source Rate constant Acetohydroxamic Acid nd order Formohydroxamic Acid nd order Phenyl hydrazine U(IV) N,N ethyl(hydroxyl)ethyl hydroxylamine hydroxylethyl hydrazine Hydroxylamine Dimethyl hydroxylamine Acetaldoxime ,1 dimethyl hydrazine Methyl hydrazine Isopropyl hydroxylamine Methyl hydroxylamine Diethyl hydroxylamine Hydrazine Isobutyraldehyde Butyraldehyde A Study of the Kinetics of the Reduction of Neptunium(VI) by Acetohydroxamic Acid in HClO4, Mat Sci Eng, A Preliminary Study of the Reduction of Np(VI) by Formohydroxamic Acid using Stopped Flow Near Infrared Spectrophotrometry, Radioch Acta, Organic Derivates of Hydrazine and Hydroxylamine in Future Technology of Spent Nuclear Fuel Reprocessing, GLOBAL '95, Separation of Plutonium and Uranium, Science and Technology of TBP, volume III, The Reduction of Plutonium(IV) and Neptunium(VI) ions by N,N ethyl(hydroxyl)ethyl hydroxylamine in Nitric Acid, Radiochimica Acta. 6 The Reduction of Neptunium and Plutonium Ions by Acetaldoxime in Nitric Acid, Radiochimica Acta. 7 Nuclear Technology, 102, 341, 1993.

12 Reduction of Np(VI) by HNO NpO + HNO + H O NpO + H + NO catal. HNO2 2 2 Radiolysis of HNO 3, aq mainly gaseous compounds: H 2, O 2 and N 2 significant amounts of HNO 2 and H 2 O 2 HNO2 + H2O2 HNO3+ H2O Hydrogen peroxide: not present in post irradiated solutions, if the production of HNO 2 is significantly higher i.e., it happens in solutions with concentration of [HNO 3 ] > 0.5 M

13 Rad Production of HNO 2 Radiation yield of HNO 2 in aq solutions: proportional to concentration of nitrate anion decreases with acidity [H+] In 0.1M HNO M NaNO 3 3 x higher than in 6M HNO 3 low LET radiation like β and γ has radiation yields around 0.25 mm/kgy in 1M HNO 3 α radiation yields are approximately two times lower a, b a) Savel ev, Yu. I.; Ershova, Z. V.; Vladimirova, M. V. Alpha-Radiolysis of Aqueous Solutions of Nitric Acid. Soviet Radiochemistry. 1966, 9 (2), b) Kazinjian, A. R. et al. Radiolysis of Nitric Acid Solutions: LET Effects. Trans. Faraday Soc. 1970, 66,

14 Managing Np Oxidation State NpO + HNO + H O NpO + H + NO catal. HNO2 2 2

15 Acetamide O O C H 3 NH NH 2 H 3 C NH 2 Methylurea Acetamide Acetamide: similar to the reaction of other primary ] amide (e.g. urea) production of N 2 (gas) rate of the reaction slow* considerably slower scavenger of HNO 2 than methylurea * Hughes, M. N.; Stedman G. The mechanism of the deamination of acetamide, J. Chem. Soc., 1964, CH3CONH2 + HNO2 CH3COOH + N2 + H2O

16 Nitrosation Reaction C H 3 Mechanism: NH O NH 2 +HNO 2 -H 2 O MU (methylurea) protonation of HNO ] 2 intermediate NO + NO + attacks the carbonyl oxygen followed by deprotonation and rearrangement of the NO group NMU is not reactive towards Np(VI) anymore MU functions as an efficient inhibitor of Np(VI) reduction by HNO 2 C H 3 N N O O NH 2 NMU nitrosomethylurea Arndt, F.; Amstutz, E. D.; Myers, R. R. Organic Syntheses, 1943 Coll. Vol. 2, 461 (1935, Annual Vol. 15, 48 Plimmer, H. A. P. The action of nitrous acid upon amides and other amino compounds. J. Chem. Soc. Trans. 1925, 127,

17 Nitrosation and irradiation of MU absorption coefficient, M 1cm NMU HNO2 in HNO wavelength, nm absorbance mm MU 10 mm MU 20 mm MU 100 mm MU 500 mm MU wavelength, nm Absorption spectrum of HNO 2 and nitrosomethylurea (NMU) in HNO 3 after addition of methylurea (MU). Spectrum of 10mM NaNO 2 and variable [MU] after irradiation with 40.5 kgy. The spectra are clearly distinguished. 4M HNO 3

18 Effect of addition of vanadium V Observed: Irradiated, V(V) had no effect on the final redox speciation of neptunium, No Np(VI), even with a 5 fold excess of V(V) Conclusion: Vanadium (V) was also reduced to V(IV) during the irradiation, but not directly by HNO 2 ; More likely by Np(V) generated by reduction of Np(VI) by HNO 2 (or another intermediate products of radiolysis)

19 Effect of combined additives on Np redox Irradiated solutions: 2 mm Np(VI) in 4 M HNO 3, various [V(V)] and[mu] (10 mm=5x[np]) Dose: 0 61 kgy. Initial ratio of Np(VI):Np(V) = 95:5. Synergistic effect expected: Scavenging of HNO 2 by MU undergoes simultaneously with oxidizing Np(V VI) by V(V): If any HNO 2 is produced, it is scavenged by MU If any Np(V) produced/reduced by HNO 2, it is re oxidized back to Np(VI) by V(V).

20 Effect of combined additives on Np after irradiation Observed: Neither the presence of MU or V(V), nor their combination did affect the reduction of Np(VI) to Np(V) at all with respect to the case with no MU and V(V) addition. The ratio Np(VI):Np(V)=95:5 decreases to 50:50. Results suggest: [HNO] 2 larger than estimated studied scavengers interact also with other intermediate products of radiolysis [Kazinjian, 1970] Higher concentrations of the selected additives (in the order of 100mM) should be applied to prevent reduction of Np(VI)

21 Reference: non irradiated 100% 50 (...) and 100 ( ) mm methylurea reference p l N ta to in n ctio F ra 80% 60% 40% 20% 0% Time equivalent to dose level (kgy) Ratio Np(VI):Np(V) dropped to ~75:25 Np IV Np V Np VI Np IV Np V Np VI

22 40kGy 100 mm MeU

23 However, higher concentrations of the selected additives (in the order of 100mM) may have an opposite effect than desired: Reduction of Np(VI). At 60kGy Np(VI):Np(V):Np(IV) = 0:10:90 100% 50 (...) and 100 ( ) mm methylurea irradiated p l N ta to in n ctio F ra 80% 60% 40% 20% Np IV Np V Np VI Np IV Np V 0% Dose (kgy) Np VI

24 Radiolytic decrease of [MU]

25 Conclusion MU is gradually decomposed with a rate 0.7 mm/kgy (approx.) Higher concentration of MU scavenger can promote reduction of Np(VI), even up to Np(IV) However, if it is manageable; Np(IV)can work, too. Acknowledgment Funding: US DOE/NERI program

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27 xas 1.0 V for VO 2+ /VO 2+ and 1.15 V for NpO 2 2+ / NpO V for PuO 2 2+ /Pu 4+