Interaction of Actinide Species with Microorganisms & Microbial Chelators: Cellular Uptake, Toxicity, & Implications for Bioremediation of Soil & Ground Water. Page: 4 of 12
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Amino carboxylate ligands are nonspecific chelating agents with strong binding affinity for a
variety of metal ions. Upon binding to metal ions, the chelating agents change the composition of the
inner coordination shell of the metal ions and thereby alter their chemical speciation and behavior.
Because of these properties, ethylenediaminetetra-acetic acid (EDTA) and other amino carboxylate
ligands have been extensively used in the processing of radionuclides. As a consequence EDTA is
codisposed with radionuclides and co-located with radionuclide contamination. If EDTA is released to
the environment, it has the potential to significantly affect the solubility and overall behavior of
radionuclides.' This is particularly true for low-valent actinides, such as plutonium, that would
generally hydrolyze and precipitate or sorb to mineral surfaces in water and soils in the absence of
strong chelators. The first step in predicting how EDTA would affect the environmental behavior of
plutonium and impact the safe disposal of nuclear waste is determining the nature of the complexes
formed. Previous work on EDTA complexation of plutonium was focused on acidic conditions and
interpretation of bulk solubility studies.2 Our approach was to determine the speciation, and
thermodynamic stability of species formed, more directly and under near-neutral pH, and excess
Plutonium can exist in aqueous solution as ions in a range of oxidation states III-VI that can be
complexed by EDTA. The specific affinity of aminocarboxylate ligands for Pu(IV) will tend to drive
the complexation reaction toward the formation of Pu(IV) complexes through redox processes.
Specifically, Pu(IV) EDTA complexes can form through the reduction of the higher oxidation states, as
for Pu(VI) and Pu(V), and through oxidation of Pu(III). Plutonium(IV) is known to form a 1:1
complex with EDTA in aqueous solution; but this hexadentate ligand does not fully encapsulate the
metal ion. Two to four waters complete the coordination sphere, providing coordination sites for
hydrolysis, polymerization or the formation of mixed ligand complexes.
We have determined the stoichiometry and the thermodynamic parameters for several
Pu(IV)EDTA complexes using potentiometric and spectrophotometric titration methods (Figure 1).
12 - Figure 1. Potentiometric titrations of
10 Pu(IV)-EDTA complexes. (1) [Pu(IV)] = 2.5 mM;
[EDTA] = 2.53 mM, (2) [Pu(IV)] = 2.49 mM;
[EDTA] = 5.0 mM, (3) [Pu(IV)] = 2.49 mM;
6 4 [EDTA] = 2.5 mM, [citrate] = 2.52 mM and (4)
4 [Pu(IV)] = 2.49 mM; [EDTA] = 2.5 mM,
0 alytical and Nuclear Chemistry (2001), 250(1), 47-53.
0 2 4 6 8 10 1, G. R. Radiochim. Acta (2001), 89, 67-74. (b)Cauchetier, P.; Guichard,
juichard, C. J. Inorg. Nucl. Chem. (1975), 37, 1771-8.
mn eq base
DFB 600 2500
Al(III)(Cit) 81 3000
Ba(II) 500 3600
DFB 1200 4200-5000
DFB 1600 29000
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Hakim Boukhalfa Mary, P. Neu Alvin Crumbliss. Interaction of Actinide Species with Microorganisms & Microbial Chelators: Cellular Uptake, Toxicity, & Implications for Bioremediation of Soil & Ground Water., report, March 28, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc877623/m1/4/: accessed May 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.