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