# Presentation: 897702

## 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 

- Contaminated Barrels From Brookhaven National
- Laboratory Boneyard

- Presence of radionuclides with other metals (e.g. Fe) may influence complexation.

## Waste Isolation Pilot Plant, Carlsbad, NM

-  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 m****3**** of waste**

## 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.

## 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.

## Metal Ligand Complexes

- (C) Six-coordinate Fe-EDTA complex

- (D) Dinuclear U-citrate complex

- (B) Bidentate Fe(acac)3 complex

- (A) Uncomplexed citric acid

## Effect of Ligand on Absorption Characteristics of  Complex

- 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.

## Structural Characterization of Uranium Citrate Complex

## Potentiometric Titration and UV-vis Spectrophotometry 

- 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.

- 3.8 Å

## 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).

## 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

- 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 |
|  |  |  |  |  |  |

## ESI-MS Analysis of 1:2 Pu:citric Acid Complex at pH 6.0

- 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]+.

- Citric acid

## 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

## 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

## Catechol

- Electrospray ionization-mass spectrometry (ESI-MS) spectra for 10-3 M catechol.

## 1:1 U:catechol (pH 3.8)

- 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)

- 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)

- 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

- Uranium forms a mononuclear complex; bidentate coordination of the equatorial oxygen with carbon is also noted.

## 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.

## Proposed Structure for 2:2 Fe:U:Citric Acid Complex

## Structural Characterization of A Mixed –Ligand Complex

## 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.

- 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 the
- U(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).

- bacteria

- electron 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 Laboratory_
- A.J. Francis
- J. Gillow
- _Florida State University_
- P. Thakur
- J.N. Mathur
- G. Choppin