interactions of radionuclides with organic ligands: implications for their mobility in nuclear waste...
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Interactions of Radionuclides With Organic Ligands: Implications For Their Mobility In Nuclear Waste
Cleveland J. Dodge
Environmental Sciences Department
Brookhaven National Laboratory
NCSS Symposium
Nuclear Science in the Environment
July 18, 2006
Introduction• Actinide contamination of the environment results from nuclear
fuel processing, reactor fuel storage, and defense related activities.
• The presence of naturally-occurring (e.g. citric acid, catechol, oxalate) and synthetic organic ligands (e.g. EDTA, NTA) with the actinide may result in its complexation and solubilization with subsequent migration from the site.
• Elucidation of the coordination chemistry of these complexes under various environmental conditions (e.g. pH, [ligand], ionic strength) will result in a better understanding of the fundamental processes affecting actinide mobility.
• This knowledge can then be used for the design and development of viable treatment strategies.
Scope of Presentation
• Types of metal-ligand complexes
• Structural characterization of complexes
- single metal / single ligand
- mixed metal / single ligand
- single metal / mixed ligand
• Effect of environmental parameters on complexation
- pH
- ionic strength
- ligand concentration
Historical Photo of Oak Ridge National Laboratory Waste Site
Waste site is highly acidic (pH 3.4) and influences complexation of actinide U.
Contaminated Barrels From Brookhaven National
Laboratory Boneyard
Presence of radionuclides with other metals (e.g. Fe) may influence complexation.
• Transuranic waste (>100 nCi/g) is currently disposed of in deep geological bedded salt formations.
• EPA certified that WIPP will be safe for at least 10,000 y.
•Little is known of the influence of high ionic strength on complexation.
Design capacity = 175,600 m3 of waste
Waste Isolation Pilot Plant, Carlsbad, NM
Types of Metal-Organic Complexes
• A metal-organic complex consists of a metal covalently bonded to citric acid by means of the functional groups COOH, OH, NH2.
• The type of metal-organic complex formed can include bidentate, multidentate, mononuclear, binuclear, and polymeric forms.
• The metal-organic complex formed is dependent upon the metal, its oxidation state, its concentration in the solution, ionic strength, and pH.
Molecular Structures for Selected Ligands
OH
OH
Catechol
O O
OH COOH
OH
OH
Citric acid
Naturally-occurring
Synthetic
CH2COOH
CH2COOH
HOOCH2C
HOOCH2C
EDTA
NCH2CH2N
COOH
COOH
Oxalic acid
N
CO OH
HOOCCOOH
NTA
Complexation of ligand to metal may occur through the carboxylate or hydroxyl functional groups. N ligands interact with the metal through the lone-pair electrons.
Uranyl ion consists of 2 double-bonded oxygen atoms in axial plane at 1.76 Å.There are 4 to 6 atoms at 2.30 to 2.45 Å in the equatorial plane.
OO
O
UO
O
O
O
Structure for Uranyl Ion (U6+)
U U
Metal Ligand Complexes
(C) Six-coordinate Fe-EDTA complex (D) Dinuclear U-citrate complex
(B) Bidentate Fe(acac)3 complex(A) Uncomplexed citric acid
U-citrate U-catechol U-salicylate U-protocatechuate
Complexes adjusted to pH 6.0, equilibrated overnight, and filtered through 0.22 um filter. The variation in color is the result of electronic transitions in the f shell.
Effect of Ligand on Absorption Characteristics of Complex
Structural Characterization of Uranium Citrate Complex
Potentiometric Titration and UV-vis Spectrophotometry of 1:1 U:Citric Acid Complex
Titration of citric acid shows release of 3 protons in overlapping steps, while addition of U shows two inflection points due to dissociation of 3 protons during complexation and formation of polymer at pH 7.5.
UV-vis spectrophotometry of citric acid shows no absorption in the visible region, while the U-citrate complex shows fine structure indicating interaction of U with the citric acid.
EXAFS Analysis of a 1:1 U:citric Acid Complex at pH 6.0
U-U
Fourier transform of 1:1 U:citric acid complex shows a U-U interaction at approx. 3.8 Å.
U U
3.8 Å
A
B B
B B
Schematic Diagram for the EXAFS Back-Scattering Process
Structural information on the sample is obtained by analysis of the signal resulting from backscattering of the photoelectron (B) following excitation of the target atom (A).
0
100
200
300
400
500
600
700
800
0 100 200 300 400 500
Tot
al c
ount
s
m/z
155 239
254
326353
409
A
0
100
200
300
400
500
600
0 100 200 300 400 500T
otal
Cou
nts
m/z
B
223 252352
Fe(acac)2-2CH
3
Fe(acac)2-H Fe(acac)
3-H
TOF-SIMS Analysis of Ferric Acetylacetonate
Fragmentation permits analysis of the mass components which make up the complex. Figure A shows + ions and Figure B shows the – ion fragments. The predominant signals are due to the formation of Fe(acac), Fe(acac)2 fragments and the molecular ion peak (Fe(acac)3.
TOF-SIMS Analysis of 1:1 U:citric Acid Complex
U
Tot
al c
ount
s
300 400 500 600 700 0
200
400
600
800
460 685
401
341
325
534
B
m/z 800
U
U
FeU
200 220 240 260 280 0
1000
2000
3000
4000
5000
Tot
al c
ount
s
239 281 270 221
207
254
m/z
UH+
UO+
UO2+
A
300
Fragmentation of the 1:1 U:citric acid in the (+) A and (-) B modes confirms the involvement of the two terminal carboxylate groups of citric acid as well as the -hydroxyl group.
Structural Characterization of Plutonium Citrate Complex
EXAFS Analysis of a 1:2 Pu:citric Acid Complex at pH 6.0
EXAFS analysis indicates the Pu-citrate complex is mononuclear.
EXAFS Structural Parameters for 1:2 Pu(IV):citric acid Complex
(N) coordination number; (R) interatomic distance; and (2) disorder parameter.
Sample Atom N R(Å) 2 E0
1:4 Pu(IV):citric acid
Pu-O 3.5±1.2 2.26±0.01 0.008±0.002 6.5±1.5
Pu-O 5.6±1.5 2.41±0.02 0.008±0.002 5.0±1.2
Pu-O 1.5±0.8 2.69±0.02 0.005±0.002 5.0±2.3
Pu-C 4.2±1.5 3.27±0.02 0.010±0.003 5.0±1.3
0
20
40
60
80
100
300 600 900 1200 1500
Rel
ativ
e ab
unda
nce
(%)
m/z
mode (-) A190.9
0
20
40
60
80
100
300 600 900 1200 1500R
elat
ive
abun
danc
e (%
)m/z
mode (+) B
215.1
473.0686.9
709.0
731.0
966.9
827.1
[Pucit(H
2O)Na]
+
The dominant peaks in the + mode are due to the formation of a monomeric Pu-citrate complex at m/z 473.0 [Pucit(H2O)Na]+ and a biligand complex at m/z 686.9 [Pu(H2cit)2NO3]+, m/z 709.0 [Pu(Hcit)(H2cit)NaNO3]+, and m/z 731.0 [Pu(Hcit)2Na2NO3]+. The presence of a dimeric complex is denoted at m/z 966.9 [Pu2(Hcit)(cit)(NO3)2]+.
ESI-MS Analysis of 1:2 Pu:citric Acid Complex at pH 6.0
Citric acid
Pu
Proposed Structure for 1:2 Pu:citric Acid Complex at pH 6.0
The complex consists of a biligand [Pu-cit2] structure, similar to the structure suggested by Metivier and Guillaumont, Radiochem. Radioanal. Lett., 1972.
Structural Characterization of Uranium-Catechol Complex
COOH
OH
O
O
COOH
OH
QuinoneProtocatechuic
acidSalicylic acid
Catechol
OH
OH
Resorcinol
OH OH
OH
OH
OH
Hydroquinone Pyrogallol
OH
OH
OH
COOH
COOH
Phthalic acid
Selected Molecular Structures for NOM Analogs
Phenolic compounds are analogs for naturally-occurring organic compounds such as humic and fulvic acids.
Idealized Structure for Humic Acid
Natural organic matter (NOM) contains aromatic hydroxyl and carboxyl groups.
phthalate
catechol
salicylate
Catecholcatechol(+)50-1000 #1 RT: 0.02 AV: 1 NL: 7.40E5T: + c ESI Full ms [ 50.00-1000.00]
100 200 300 400 500 600 700 800 900 1000
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Re
lativ
e A
bu
nd
an
ce
109.1
125.1
185.1
149.0
192.9
97.0 201.0257.3 269.2 985.5
298.4 964.2323.9 391.0 897.2572.8 677.7522.1 603.6 765.9409.6 708.1 828.093.2 476.0
OH
OH
Electrospray ionization-mass spectrometry (ESI-MS) spectra for 10-3 M catechol.
1:1 U:catechol (pH 3.8)U-catechol(+)50-1000(pHunadj) #2 RT: 0.03 AV: 1 NL: 1.08E7T: + c ESI Full ms [ 50.00-1000.00]
100 200 300 400 500 600 700 800 900 1000
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Re
lativ
e A
bu
nd
an
ce
414.9
397.0
457.9287.3
851.5
734.5 853.6365.0504.7270.2109.1 323.0 806.5779.4
454.7763.6
488.8 505.9 835.6718.7 854.7110.1 898.4148.0 671.6556.9 579.0 925.897.2 239.3 974.0
O
O
U
O
O
OH2
OH2
ESI-MS spectra for 10-3 M 1:1 U:catechol complex at pH 3.8 shows presence of mononuclear complex.
1:1 U:catechol (pH 5.0)U-catechol(+)50-1000(pH5) #1 RT: 0.01 AV: 1 NL: 1.39E6T: + c ESI Full ms [ 50.00-1000.00]
100 200 300 400 500 600 700 800 900 1000
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Re
lativ
e A
bu
nd
an
ce
414.9
397.0
930.5287.3109.1
792.7
972.3
396.1
270.3 323.0855.3718.8 810.5
365.0 763.8935.6140.0
882.7332.0 700.8594.9415.9 834.7 996.3149.0 504.8 689.6255.2 603.8591.1183.297.2
O
O
U
O
O O
O
U
O
O
O
O
ESI-MS spectra for 10-3 M 1:1 U:catechol complex at pH 5.0 shows presence of mononuclear as well as dinuclear complex.
1:1 U:catechol (pH 6.0)
U-catechol(+)50-1000(pH6) #1 RT: 0.03 AV: 1 NL: 4.49E5T: + c ESI Full ms [ 50.00-1000.00]
100 200 300 400 500 600 700 800 900 1000
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Re
lativ
e A
bu
nd
an
ce
930.6
109.1
140.0
240.9
341.8
414.9
972.6
719.8 912.8397.0 820.6
792.6718.8543.8442.7 830.5 958.6287.1 882.9
396.1149.2 721.9270.1 618.8 716.9 975.7833.0452.9 644.6346.5 535.7 791.0171.2 609.9325.8 506.0100.1
ESI-MS spectra for 10-3 M 1:1 U:catechol complex at pH 6.0 suggests break-up of complex as shown by increase in catechol peak at 109 and a possible U-polymer peak at 930.
U-catechol polymer
Structural Characterization of a Ternary Fe(III)-U(VI)-Citric Acid Complex
EXAFS Spectra for Fe-U-Citrate Complex at the Uranium LIII Edge
2 4 6 8 10 12 14 16
k3 X(k
)
k(Å-1)
Uranyl acetate
1:1:2 U:Fe:citric acid (s)
1:1:2 U:Fe:citric acid (15 mM)
A
0 1 2 3 4 5 6 7 8
Fo
uri
er
tran
sfo
rm m
ag
nit
ud
eRadial distance (Å)
Uranyl acetate
1:1:2 U:Fe:citric acid (15 mM)
1:1:2 U:Fe:citric acid (s)
B
Uranium forms a mononuclear complex; bidentate coordination of the equatorial oxygen with carbon is also noted.
2 4 6 8 10 12 14 16
k3 X(k
)
k(Å-1)
Ferric acetylacetonate
1:1:2 Fe:U:citric acid (s)
1:1:2 Fe:U:citric acid (15 mM)
A
0 1 2 3 4 5 6 7 8F
ouri
er
tran
sfo
rm m
ag
nit
ud
eRadial distance (Å)
Ferric acetylacetonate
1:1:2 Fe:U:citric acid (15 mM)
1:1:2 Fe:U:citric acid (s)
B
EXAFS Spectra for Fe-U-Citrate Complex at the Iron K Edge
EXAFS spectrum shows iron forms a dinuclear core with coordination of a Na atom to the iron core.
U
U
Fe
NaFe
Proposed Structure for 2:2 Fe:U:Citric Acid Complex
Structural Characterization of A Mixed –Ligand Complex
Eu
Structure for the 1:1:1 Eu:EDTA:Ox Complex
Atoms: black, Carbon; white, Oxygen; blue, Nitrogen; green, Hydrogen.
Structure for the 1:1:2 Eu:EDTA:Ox Complex
Fate of Uranium Citrate Under Anaerobic Conditions
Cell Morphology of Clostridium sp.
• Strict anaerobic, spore-forming, fermentative bacteria commonly found in soils, sediments, and wastes.
• Reduce iron (Fe3+ to Fe2+)
manganese (Mn4+ to Mn2+)
technetium (Tc7+ to Tc4+)
uranium (U6+ to U4+)
• U(VI)*aq U(IV)s
*uranyl carbonate, uranyl nitrate
Serum Bottles for Growing Clostridium sp.
Prereduced uranyl nitrate is added through the stopper using a needle and syringe.
Anaerobic Bacterial Reduction of Uranium Complexed With Citric Acid
Clostridium sp. reduced U(VI) complexed to citric acid only in the presence of carbon source. The reduced U remained in solution associated with the citric acid as the U(IV)-citrate complex.
The change in spectrum of U(VI)-citrate following bioreduction indicates theU(VI) was reduced to U(IV).
XPS and XANES Analysis of Uranium Following Anaerobic Bacterial Activity
XPS analysis of the treated sample shows a 1.6 eV decrease in binding energy to 380.6 eV compared to uranyl ion (382.0 eV); XANES spectra at the MV absorption edge shows shift in sample absorption peak to 3550.1 eV from 3551.1 eV for U(VI). These complementary techniques confirm bacterial reduction of uranyl ion to U(IV). Francis et al. 1994. Environ. Sci. Technol. 28:636-639.
Proposed Structure for U(IV)-Citrate Complex
EXAFS analysis indicates the binuclear U(VI)-citrate complex is transformed to a mononuclear biligand complex following reduction of U(VI) to U(IV).
bacteriaelectron donor
U U
U
Francis. A.J.; G.A. Joshi-Tope; C.J. Dodge; J.B. Gillow. 2002. Biotransformation of uranium and transition metal citrate complexes by Clostridia. J. Nucl. Sci. Technol. Suppl. 3:935-938.
Summary
• Organic ligands citrate, catechol, oxalate, NTA, EDTA form stable complexes with actinides
• Uranium forms a dinuclear complex with citric acid involving two carboxylate groups and the -hydroxyl group.
• Plutonium forms a mononuclear biligand complex with citrate.
• Complexation of uranium with catechol is dependent on the pH of the medium.
• Iron and uranium form a mixed-metal complex with citric acid.
• Europium forms a mixed-ligand complex with EDTA and oxalic acid.
• Uranium is reduced by anaerobic bacterial activity and forms a soluble biligand U(IV)-citrate complex.
Acknowledgements
Brookhaven National LaboratoryA.J. Francis J. Gillow
Florida State UniversityP. ThakurJ.N. MathurG. Choppin