ウラン廃棄物の除染法開発に向けた硝酸‐イオン液体二相系

First SACSESS International Workshop
Warsaw, Poland, April 22-24, 2015
Studies on Extraction Behaviour of U(VI) to
[bmim][NfO] Ionic Liquid and Its Recovery
Using Hydrogen Peroxide
Research Laboratory for Nuclear Reactors
Tokyo Institute of Technology
Koichiro Takao* (Associate Prof.)
Kohei Arakawa, Takahiro Mori, Masayuki Harada, Yasuhisa Ikeda
*E-mail: [email protected]
1
2/14
Background
 Uranium-Contaminated Wastes (Low-Level Wastes)
from Nuclear Fuel Fabrication
Current Problem
•
Extremely Long Lifetime of Uranium
Isotopes
•
Raising Total Radioactivity after 104
Years by Daughters of U
How to Treat & Dispose Them ?
(Shallow Depth?)
To reduce load to waste repository…
Decontamination
Chemical Decontamination Using
Ionic Liquids
+
Chemical Traps (NaF, Al2O3)
from Enrichment Process
in Nuclear Fuel Fabrication
105 Drum Cans (200 L)
2
3/14
Background
What is Ionic Liquid?
A Salt melts below RT or 100°C
• Typical Traits:
• Non-volatile, Incombustible, Durable towards Electrolysis,
Solubility for Various Metal Ions
• Designable Chemical Structure to Give Specific Functions
3
4/14
Background
 1-Butyl-3-methylimidazolium Nonafluorobutanesulfonate ([bmim][NfO])
•
Solubility for U(VI)
•
Highly Hydrophobic: Immiscible with Water
Chemical Decontamination for
U-Contaminated Wastes
Extraction
U-Wastes
U
U
U
U
U
Decontaminated
Wastes
H2O2
U
U
U
[bmim][NfO]
Decontaminated
Wastes
U
Recovery of
U(VI) as UO4
[bmim][NfO]
[bmim][NfO]
U
U
U
Dissolution of
U(VI) in HNO3(aq)
U
U
U
U
U U
Extraction
Recycle of
[bmim][NfO]
Chemical Decontamination Process Using [bmim][NfO] (planned)
4
—: Direct, —: Indirect
5/14
Objectives
1. Extraction Behaviour of U(VI) in HNO3(aq)-[bmim][NfO] System
2. Recovery of Extracted U(VI) from [bmim][NfO] by Addition of H2O2
１
Extraction
U-Wastes
U
U
U
U
U
Decontaminated
２
Wastes
U
U
U
[bmim][NfO]
Decontaminated
Wastes
U
U
Dissolution of
U(VI) in HNO3(aq)
U
U
Recovery of
U(VI) as UO4
[bmim][NfO]
[bmim][NfO]
U
H2O2
１
U
U
U
U U
Extraction
Recycle of
[bmim][NfO]
Chemical Decontamination Process Using [bmim][NfO] (planned)
5
—: Direct, —: Indirect
6/14
Experimental
 Extraction Behaviour of U(VI) in HNO3(aq)-[bmim][NfO] System
Mechanical Shaking
at 298 K, 1 h
HNO3(aq)
0.01-3.00 M
+ 18.6 mM U(VI)
1 mL
Determination of
U(VI) by ICP-AES
[bmim][NfO]
1 mL
Evaluation
E = ([U(VI)]init – [U(VI)]aq)/[U(VI)]init × 100
D = ([U(VI)]init – [U(VI)]aq)/[U(VI)]aq
E: Extractability (%)
D: Distribution Ratio
[U(VI)]init: Concentration of U(VI) at Initial State
[U(VI)]aq: Concentration of U(VI) in Aq Phase
after Extraction
6
7/14
Extraction Behaviour of U(VI) in HNO3(aq)-[bmim][NfO]
Extractability and Distribution Ratio of
U(VI) as a Function of [HNO3]
[HNO3]/M
0.010
0.100
0.500
1.00
3.00
E /%
80.3
60.0
25.2
13.3
17.1
D
4.08
1.50
0.336
0.154
0.206
 Extractant-Less Extraction of U(VI)
•
Prominent Role of [bmim][NfO]
NaNO3
HClO4
HNO3
 [H+] = 0.01-3.00 M
•
Hydrolysis of UO22+ can be ignored.
 [H+] Dependency
•
Fig. Distribution ratio (D) as a function of [H+] or [NO3-].
Competition with NfO- + H+ = NfOH
 [NO3-] Dependency
•
Coordination of NO3- to UO22+ → Decrease
in E and D
•
Extractable Species: UO22+
Origin of U(VI) Extraction by [bmim][NfO] ?
Further Investigation at [HNO3] = 0.01 M,
7
the Simplest System
8/14
Role of NfO- in U(VI) Extraction
 [bmim][Tf2N]
•
U(VI) Not Extractable (Panel (a))
•
As “Solvent”
•
[bmim][NfO] as “Extractant”
+
[bmim][Tf2N]
[bmim][NfO]
 [NfO-] Dependency
•
•
Linearity of log D-log[NfO-] Plot
(panel (b))
Slope: 4.2 ± 0.3
4 NfO- is accompanied with U(VI).
Coordination? Not Likely…
Fig. Extraction behavior of U(VI) in HNO3(aq)-[bmim][Tf2N] systems. In
panel (a), extractability is compared with that in HNO3(aq)-[bmim][NfO].
Panel (b) shows dependency of D as a function of [NfO-] at [HNO3] =
0.010 M together with the best fit line resulted from the least-squares
regression.
8
What happens in this extraction?
9/14
Interaction between UO22+ and NfO UV-vis Absorption Spectra
• 0.01 M HClO4(aq)-[bmim][NfO] System
 aq. vs. [bmim][NfO]
• Characteristic Fine Structure@420 nm
 LMCT Band of UO22+ with Vibronic Str.
• Identical to Those of [UO2(H2O)5]2+
• No Coordination Interaction
• [UO2(H2O)5]2+ in Both Phases Even after
Extraction
Formation of Outer-Sphere Complex
as an Extractable Species
Fig. UV-vis absorption spectrum of U(VI) in 0.01 M HClO4(aq)[bmim][NfO] extraction system (blue: aq, red: [bmim][NfO])
together with that of [UO2(H2O)5]2+ in 0.01 M HClO4(aq) (black).
[UO2(H2O)5]2+(aq) + 4 NfO-(IL)
9
= [UO2(H2O)5]2+.4(NfO-)(IL)
10/14
On Extraction Mechanism
 Charge Compensation
•
Charge balance in each phase must be compensated.
 Possible Mechanisms
• Cation Exchange
 2 [bmim]+ per [UO2(H2O)5]2+ move from [bmim][NfO] to aq.
• Ion Pairing
 2 NO3- are also extracted to [bmim][NfO] together with [UO2(H2O)5]2+.
 Net Conc. of [bmim]+ transferred to Aq. was ONLY 5 mM, while
13.3 mM U(VI) was extracted.
Aq.
Reliable Extraction Mechanism
NO
YES
Cation
Exchange
2 [bmim]+
[bmim][NfO]
Ion Pair
2 NO3-
10
[UO2(H2O)5]2+
NfONfO2+
NfO- [UO2(H2O)5] NfO-
11/14
Experimental
 Recovery of Extracted U(VI) from [bmim][NfO] by Using H2O2
30 wt% H2O2(aq)
0.5 or 1 mL
Mechanical Shaking
at 298 K, 20 min
Back-Extraction of U(VI) by
3 M HNO3(aq) 0.5 mL
H2O 0.9 ml
[bmim][NfO] from
Extraction Expt., 0.9 mL
Determination of
U(VI) by ICP-AES
[bmim][NfO]
0.5 mL
Precipitation Yield (Y/%)
Y = 100 × (1 – D[U(VI)]sup,b/[U(VI)]IL)
D: Distribution Ratio at [HNO3] = 3 M (D = 0.21)
[U(VI)]IL: U(VI) Conc. in [bmim][NfO] after Extraction Expt.
[U(VI)]sup,b: U(VI) Conc. in Aq of Back-Extraction
11
Pale yellow ppt was formed.
12/14
Precipitation Behaviour of U(VI) from [bmim][NfO]
 Characterization of Precipitate
 XRD Pattern (Cu Ka, l = 1.5418 Å)
 Identical to [(UO2)(O2)(H2O)2]·2H2O
(studtite, dotted line)
Expt.
UO22+ + H2O2 →
[(UO2)(O2)(H2O)2]·2H2O↓ + 2 H+
Sim.
Fig. Powder XRD pattern of (a) pale yellow precipitate collected from
[bmim][NfO] after addition of 30 wt% H2O2(aq) together with (b) that of
[(UO2)(O2)(H2O)2]·2H2O (studtite) calculated on the basis of its
crystallographic data reported by Burns et al. (fwhm = 0.3°, Am.
Mineralogist, 2003, 88, 1165-1168).
12
13/14
Precipitation Behaviour of U(VI) from [bmim][NfO]
 Dependency of Precipitation Yield (Y)
 Low [HNO3] in Extraction → High Y
 No Dependency on Volume of 30 wt%
H2O2(aq)
 pH in Aq.
•
High pH → High Y
•
Lower Efficiency Compared with
Aqueous System
UO22+ + H2O2 →
[(UO2)(O2)(H2O)2]·2H2O↓ + 2 H+
Fig. Precipitation yield (Y) as functions of [HNO3] (a) and pH at equilibrium
(pHeq, (b)) after addition of 30 wt% H2O2(aq). In panel (a), the transverse
axis represents [HNO3] used in the preceding extraction experiments.
• At [HNO3] = 0.01 M, E = 80.3%, Y = 96.2%
• Low [HNO3] is preferable for both extraction and recovery
of U(VI)
13
Guide to Chemical Decontamination Process
14/14
Conclusion
1. Extraction Behaviour of U(VI) in
HNO3(aq)-[bmim][NfO] System
 U(VI) is extractable by [bmim][NfO]
 Applicable to Both Direct and
Indirect Decontamination of UContaminated Wastes under an
Optimized Condition
2. Recovery of Extracted U(VI) from
[bmim][NfO] by Addition of H2O2
 Feasibility of U(VI) Recovery from
[bmim][NfO] by H2O2(aq)
 Recyclability of [bmim][NfO] in
Decontamination Process
 Strong Dependency of Ppt. Efficiency on pH
This chemical decontamination
Process should be operated at
low [HNO3].
Further Investigation…
•
•
•
Dissolution of Uranium Oxides
Selectivity
Actual Decontamination
of
14
Simulated U-Wastes
Forum Article, Inorg. Chem. 2013, 52, 3459-3472
Dziękuję!
Thank you for your attention!
http://www.nr.titech.ac.jp/~ktakao/
15
NO3－のU(VI)の抽出挙動への影響
NO3－による影響
・UO22+にはNO3－が配位する。
UO22+ ＋ n NO3－
UO2(NO3)n2-n
硝酸濃度の増加に伴い形成される錯体
[UO2NO3]+
[UO2(NO3)3]－
[UO2(NO3)2]
・ [HNO3] = 0.01 M の場合UO22+のモル分率
が最も大きい。
各種硝酸濃度におけるウラニルイオ
ン
の錯体分布図([HNO3] = 0.01～10 M)*
NO3－のUO22+への配位が、分配比の減少に関与。
*；佐々木琴江、芝浦工業大学大学院修士論文、2012
16
硝酸濃度0.01 MにおけるU(VI)の抽出メカニズムの考察
硝酸(0.01 M)-[BMI][NfO]系における抽出機構としては、次の2種類の可能性が考えられる。
イオン対形成による抽出
UO22+(aq) + 2A－(aq) + 4[NfO]－(IL)
[UO2(NfO)4]2－(IL) + 2A－(IL)
カチオン交換による抽出
UO22+(aq) +
4[BMI]＋(IL) + 4[NfO]－(IL)
[UO2(NfO)4]2－(IL) + 2[BMI]＋(aq)
水相
[BMI][NfO]相
UO22+
2A－
4[NfO]－
4[BMI]+
水相
[BMI][NfO]相 4[BMI]+ [UO2(NfO)4]2－2A－
水相
UO22+
[BMI][NfO]相
4[NfO]－
水相
[BMI][NfO]相
4[BMI]+
2[BMI]+
[UO2(NfO)4]2－
17
2[BMI]+
硝酸濃度0.01 MにおけるU(VI)の抽出メカニズムの考察
抽出機構の判別を行うため、NMRを用いて抽出操作後
に水相に含まれる[BMI]+の定量を行った。
抽出操作時の水相の硝酸濃度と抽出操作後の水相の[BMI]+の濃度の関係
[HNO3]/M
[BMI+]aq/M
0.01
0.1
0.5
1
3
3.01×10－2
4.80×10－2
8.06×10－2
9.86×10－2
0.162
[BMI+]aq (ブランク)
/M
2.55×10－2
3.20×10－2
7.01×10－2
9.17×10－2
0.136
[BMI+]aq濃度の差/M
[UO22+]IL /M
4.60×10－3
1.60×10－2
1.05×10－2
6.00×10－3
2.60×10－2
1.33×10－2
1.10×10－2
5.64×10－3
2.85×10－3
3.09×10－3
カチオン交換による抽出が起きる場合1個のU(VI)が抽出されると2個の[BMI]+ が水相に移行する
[HNO3] = 0.01 Mにおいて、そのような量的関係は見られない。
カチオン交換は否定される。
イオン対形成が妥当。
18
[BMI][NfO]系におけるU(VI)の沈殿機構
UO22+ + H2O2 → UO2(O2)↓ + 2H+
[BMI][NfO]系において沈殿試験前後のpHを測定し、pHと沈殿率の関係を調べた。
沈殿試験前後におけるpH変化と沈殿率
pHpre-equ
pHeq
発生したH+の
モル数/mol
Y/%
2.86
1.87
1.30
1.06
0.37
2.08
1.70
1.20
0.91
0.43
1.04×10－5
1.58×10－5
4.32×10－5
9.36×10－5
1.36×10－4
96.2
92.2
85.1
77.5
48.1
ウラニルイオンの沈殿により水素イオンが生成す
る。
UO2(O2)生成反応式と一致
19
U(VI)の沈殿率
・各硝酸濃度(0.01, 0.1, 0.5, 1, 3 M)でU(VI)の溶媒抽出実験を行った後、[BMI][Nｆ
O]相に対して沈殿試験を行った。
・ U(VI)の沈殿率の抽出時の硝酸濃度依存性を調べた。
・U(VI)の沈殿率は溶媒抽出実験で用いた
100
水相の硝酸濃度の増加とともに減少
Y /%
80
60
40
0.001
沈殿率の減少には[BMI][NfO]中に溶解し
た硝酸が影響を与えている。
30 wt% H2O2
: 0.5 mL
: 1 mL
0.01
0.1
[HNO3] /M
1
10
硝酸-[BMI][NfO]系におけるU(VI)の溶媒抽出実
験を行った際の硝酸濃度と沈殿率の関係
・30 wt%過酸化水素水添加量0.5 mL以上
では沈殿率の変化見られなかった。
20
U(VI)の沈殿率のpH依存性
U(VI)の沈殿率のpH依存性を調べた。比較のため水溶液系におけるU(VI)の沈殿試験も
行った。
・水溶液系：U(VI)の沈殿率は pH 0.43～1.34の
範囲において、96%以上になる。
80
・[BMI][NfO]系：U(VI)の沈殿率は水相の
pHの減少に伴い減少する。
Y /%
100
30 wt% H2O2
: [BMI][NfO]
: H2O
60
・水相のpH 1.7以上の領域では[BMI][NfO]中
における沈殿率は90%以上に達する。
40
0.5
1.0
1.5
pHeq
2.0
2.5
U(VI)を含む水溶液と[BMI][NfO]相に
おける沈殿率とpHの関係
水相のpHを高くすることでU(VI)をより
多く沈殿として回収できる。
21

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