The forward (oxidation) reaction in a non-complexing acid is: .... The action of
nitrous acid upon amides and other amino-compounds. J. Chem. Soc. Trans.
1925 ...
Redox Chemistry of Neptunium in Solutions of Nitric Acid
Alena PAULENOVA Martin PRECEK, Kyle HARTIG, Nathan KNAPP
Neptunium Redox Chemistry • Coexistence of three oxidation states in acidic solutions – NpO2+, NpO22+, Np4+ • Disproportionation of NpO2+ in strongly acidic solutions NpO2++ 4H+ ↔ NpO22+ +2H2O • In UREX reprocessing solutions – NpO22+ and Np4+ are readily extracted – NpO2+ remains in the aqueous phase • Oxidize Np to NpO22+ with V(V) • Reduce/Scrub NpO22+ with AHA
4 M HNO3
Np(III, IV, V, VI) =f(HNO3) Spectra by Friedman H A, Toth L M. 1980, J. Inorg. Nucl. Chem. V. 42 pp 1347‐1349
1 M HNO3
• Np(V) • Np(IV)
Vanadium (V) with Np(V) • Reduction potentials : VO2+/VO2+ = 1.0 V and NpO22+/ NpO2+ =1.15 V • This redox reaction is reversible: + 2
+ 2
+
2+ 2+ ↔ NpO + VO + 2 H NpO2 + VO + H 2O
⎯⎯ → Np (V ) + V (V ) Np (VI ) + V ( IV ) ←⎯ ⎯ dx dt
dx dt
=–
−
d [ Np (V ) ] dt
+
d [ Np (V )] dt
d [ Np (V )] dt
= k1′′ ⋅ [ Np (V )]α ⋅ [V (V )]β
= k 2′′ ⋅ [ Np (VI )]γ ⋅ [V ( IV )]δ
= k1′′ ⋅ [ Np (V ) ] ⋅ [V (V ) ] – k 2′′ ⋅ [ Np (VI ) ] ⋅ [V ( IV ) ]
Reaction order with respect to concentration of Np(V) and V(V) 0.72 mM
1.80 mM
-5
3.60 mM
lg (Reaction rate, M s -1)
-Δ[Np(V)] (M)
3.E-04
2.E-04
1.E-04
slope V(V) = 0.98 -5.5 V(V) Np(V)
-6 -6.5
slope Np(V) = 1.00
0.E+00 0
100 200 300 Reaction Time (s)
400
-7 -4.5
-4
-3.5
-3
-2.5
lg (Concentration, M )
Effect of increasing initial [V(5+)] on reaction rate
: cV(V)‐ = 0.375 mM; cNp(V)= 0.113‐0.504 mM : cNp(V)=0.360 mM, cV(V)‐= 0.266‐6.91 mM
-2
Initial rates of oxidation of Np(V) as f(H+) 2.9
4M HNO3 1 M HNO3 + 3M LiNO3
2.4
4.E-06
lg k 1 " (lg M-1 s-1 )
initial rate (M·s -1)
5.E-06
slope = 4.2
3.E-06 2.E-06
2.9
appar. rate const., k1'' appar. equilibrium const., K'
2.4
1.9
1.9 slope = -2.31
1.4
1.4
0.9
0.9 slope = 1.21
0.4
0.4
1.E-06
slope = 0.95 0.E+00
0.E+00
5.E-07 1.E-06 cNp(V) · cV(V) (M 2)
2.E-06
+ 2
-0.1
-0.6
-0.6 -0.4
-0.2
0.0
0.2
0.4
0.6
lg [H +] (lg M)
Effect of[ H+]
Initial rates of oxidation + 2
-0.1
+
2+ 2+ ↔ NpO + VO + 2 H NpO2 + VO + H 2O
lg K'
6.E-06
Hydrolysis of Vanadium (V) in Acid • Fast protonation equilibria of VO2+ • Transformation into two additional forms VO(OH)2+ and VO3+:
VO2+ + H+ ↔ VO(OH)2+ VO2+ + 2H2+ ↔ VO3+ + H2O A simple electron transfer to VO3+ leads to oxidation of Np(V):
NpO2+ + VO 3+ → NpO22 + + VO 2 + Speciation of V(V) =f[H3O+], [VO2+], VO(OH)2+ and VO3+ are given by:
K1 =
[VO (OH ) 2 + ] + 2
+
[VO ] ⋅ [ H ]
K2 =
[VO 3+ ] [VO2+ ] ⋅ [ H + ]2
Constant rate of oxidation Assuming: •NpO2+ is the only form of Np(V) in acidic (HClO4 or HNO3) solution •Speciation/hydrolysis of V(V) to VO3+ The forward (oxidation) reaction in a non-complexing acid is:
−
d[ Np (V )] dt
= k ′′ ⋅ [ NpO2+ ] ⋅ [VO 3+ ] =
k ⋅ [ H + ]2 +
+ 2
1 + K1 ⋅ [ H ] + K 2 ⋅ [ H ]
Koltunov: K1=0.79 M-1 and K2=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 K1, K2 are constants for V(V)
⋅ [ Np (V )] ⋅ [V (V )]
Thermodynamic of activation reaction Np(V) + V(V) °C
Data source 4M HNO3 [OSU] 4M HClO4 [Koltunov] 2M HNO3 [Dukes]
∆H* EA kJ/mol kJ/mol
∆S* J/K.mol
∆G* kJ/mol
55±1
53 ±1
-55.8±1.3 69.3±1.6
25.5-46 56 ±6
53± 6
-59±21
49±12
n/a
n/a
10-50
24-50
71.4± .2 n/a
− EA ′′ ln k1 = + ln A R ⋅T Eyring equation: 1.38⋅10-23
Boltzmann=kB = Planck = h=6.63⋅10-34 J R =8.314 J⋅K-1mol-1 T = 298.15K = 25°C
J
K-1
k BT ′′ ⋅e k1 = h
ΔG* RT
k BT = e⋅ ⋅e h
ΔS * R
⋅e
-
EA RT
Reduction of Np(VI) by HNO2 NpO
2+ 2
1 1 3 + 1 + − ⎯⎯⎯⎯ → + HNO2 + H 2 O ←⎯⎯⎯⎯ + + NpO H NO 2 3 catal . HNO2 2 2 2 2
Standard redox potentials (in volts) for U, Pu, Np and 1M HNO2/HNO3 [Miles, 1990; Drake, 1990]
REDUCTANT Acetohydroxamic Acid Formohydroxamic Acid Phenyl hydrazine U(IV) N,N‐ethyl(hydroxyl)ethyl hydroxylamine hydroxylethyl hydrazine Hydroxylamine Dimethyl hydroxylamine Acetaldoxime 1,1‐dimethyl hydrazine Methyl hydrazine Isopropyl hydroxylamine Methyl hydroxylamine Diethyl hydroxylamine Hydrazine Isobutyraldehyde Butyraldehyde 1 2 3 4 5 6 7
T (oC) Rate Const (s‐1) 10 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22
2500 1174 44 15.7 4.5 4 3.5 3.2 3 1.6 0.69 0.52 0.45 0.28 0.17 0.05 0.001
Source Rate constant 1 2 3 4 5 3 3 3 6 3 3 3 3 3 3 7 7
2nd order 2nd order
A Study of the Kinetics of the Reduction of Neptunium(VI) by Acetohydroxamic Acid in HClO4, Mat Sci Eng, 2009 A Preliminary Study of the Reduction of Np(VI) by Formohydroxamic Acid using Stopped‐Flow Near‐Infrared Spectrophotrometry, Radioch Acta, 2000 Organic Derivates of Hydrazine and Hydroxylamine in Future Technology of Spent Nuclear Fuel Reprocessing, GLOBAL '95, 1995. Separation of Plutonium and Uranium, Science and Technology of TBP, volume III, 1990. The Reduction of Plutonium(IV) and Neptunium(VI) ions by N,N‐ethyl(hydroxyl)ethyl hydroxylamine in Nitric Acid, Radiochimica Acta. The Reduction of Neptunium and Plutonium Ions by Acetaldoxime in Nitric Acid, Radiochimica Acta. Nuclear Technology, 102, 341, 1993.
Reduction of Np(VI) by HNO2 NpO
2+ 2
1 1 3 + 1 + − ⎯⎯⎯⎯ → NpO H NO + HNO2 + H 2 O ←⎯⎯⎯⎯ + + 2 3 catal . HNO2 2 2 2 2
Radiolysis of HNO3, aq → mainly gaseous compounds: H2, O2 and N2 significant amounts of HNO2 and H2O2
HNO2 + H 2O2 → HNO3 + H 2O Hydrogen peroxide: •not present in post‐irradiated solutions, if the production of HNO2 is significantly higher •i.e., it happens in solutions with concentration of [HNO3] > 0.5 M
Rad Production of HNO2 Radiation yield of HNO2 in aq solutions: • proportional to concentration of nitrate anion • decreases with acidity [H+] • In 0.1M HNO3 + 5.9M NaNO3 3 x higher than in 6M HNO3 • low LET radiation like β and γ has radiation yields around 0.25 mM/kGy in 1M HNO3 • α radiation yields are approximately two times lowera, 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), 225-230 b) Kazinjian, A. R. et al. Radiolysis of Nitric Acid Solutions: LET Effects. Trans. Faraday Soc. 1970, 66, 2192–2198
Managing Np Oxidation State NpO
2+ 2
1 1 3 + 1 + − ⎯⎯⎯⎯ → NpO H NO + HNO2 + H 2 O ←⎯⎯⎯⎯ + + 2 3 catal . HNO2 2 2 2 2
Acetamide O
O
H 3C NH
Methylurea
NH2
H 3C
NH2
Acetamide
Acetamide: ] • similar to the reaction of other primary amide (e.g. urea) • production of N2(gas) • rate of the reaction – slow* •considerably slower scavenger of HNO2 than methylurea *
Hughes, M. N.; Stedman G. The mechanism of the deamination of acetamide, J. Chem. Soc., 1964, 5840 - 5844
CH 3CONH 2 + HNO2 → CH 3COOH + N 2 + H 2O
Nitrosation Reaction O O
H 3C
H 3C NH
NH2
+HNO2
N
-H2O
N
NH2
O
MU (methylurea)
NMU nitrosomethylurea
Mechanism: + ] • protonation of HNO2 → 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 HNO2 • •
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, 2651 – 2659
Nitrosation and irradiation of MU 0.50
140
5 mM MU 10 mM MU 20 mM MU 100 mM MU 500 mM MU
0.40 0.35
100 absorbance
absorption coefficient, M‐1cm‐1
0.45 120
80 60
0.30 0.25 0.20 0.15
40
0.10
NMU
20
0.05
HNO2 in HNO3
0.00
0 320
340
360 380 400 wavelength, nm
420
440
Absorption spectrum of HNO2 and nitrosomethylurea (NMU) in HNO3 after addition of methylurea (MU). The spectra are clearly distinguished.
320
340
360 380 400 wavelength, nm
420
440
Spectrum of 10mM NaNO2 and variable [MU] after irradiation with 40.5 kGy. 4M HNO3
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 HNO2; • More likely by Np(V) generated by reduction of Np(VI) by HNO2 • (or another intermediate products of radiolysis)
Effect of combined additives on Np‐redox Irradiated solutions: • 2 mM Np(VI) in 4 M HNO3, 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 HNO2 by MU undergoes simultaneously with oxidizing Np(V→VI) by V(V): – If any HNO2 is produced, it is scavenged by MU – If any Np(V) produced/reduced by HNO2, it is re‐oxidized back to Np(VI) by V(V).
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)
Reference: non‐irradiated 50 (...) and 100 (‐) mM methylurea ‐ reference 100% p N l at o t n i n o it ca rF
80%
Np‐IV Np‐V
60%
Np‐VI
40%
Np‐IV
20%
Np‐V Np‐VI
0% 0
20
40
Time equivalent to dose level (kGy)
Ratio Np(VI):Np(V) dropped to ~75:25
60
40kGy
0.1 100 mM MeU
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
900
800
700
600
500
400
300
‐0.01
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
50 (...) and 100 (‐) mM methylurea ‐ irradiated 100% p N l at o t n i n o it ca rF
80%
Np‐IV Np‐V
60%
Np‐VI
40%
Np‐IV
20%
Np‐V Np‐VI
0% 0
20
40 Dose (kGy)
60
Radiolytic decrease of [MU]
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
xas 1.0 V for VO2+/VO2+ and 1.15 V for NpO22+/ NpO2+ 1.1 V for PuO22+/Pu4+
