The Kinetics of Direct Enzymatic Reduction of Uranium (VI): Effects of Ligand Complexation and U(VI) Speciation (Ainsworth NABIR, PNNL Scope # 42335)
EMSL Project ID
6294
Abstract
Experimental studies of U(VI) reduction by AH2DS have shown that the aqueous uranyl ion and its hydrolysis products are reduced at significantly slower rates compared with Fe(III). While reduction rates of U(VI)-organic chelate complexes vary with the speciation of uranyl, rates are dependent on solution pH, even if the speciation remains the same. While uranyl fluorescence spectra at cryogenic temperatures suggested energy transfer from AH2DS/AQDS to uranyl ion, which is evidence of encounter complex formation, the reaction mechanism appears far more complicated compared with Fe(III). Overall, the mechanism appears to be a sequential two-electron transfer process whose first transfer is the reaction’s slow step. Current research is focusing on single electron reductants of U(VI), such as hydroxylamine·HCl, hematin, and vitamin B12. Current modeling efforts involve adapting and applying the Marcus Theory model for the reduction of U(VI)-aquo, -carbonato, and -hydroxamate complexes. Recent experimental work has shown that UO22+ reduction rates vary significantly as a function of complexation, but simpler interpretations in terms of ligand structure, reorganization energy, or driving force do not appear to be possible. In addition, dependence of the internal reorganization energy on the presence of equatorial ligands in the inner coordination sphere varies unpredictably with ligand number and structure. Therefore, it is anticipated that the current experimental and modeling efforts will bring significant insight on the physical quantities that control the reduction rate.Work authorization text added 08/11/05:
Experimental studies of U(VI) reduction by AH2DS have shown that the aqueous uranyl ion and its hydrolysis products are reduced at significantly slower rates compared with Fe(III). While reduction rates of U(VI)-organic chelate complexes vary with the speciation of uranyl, rates are dependent on solution pH, even if the speciation remains the same. While uranyl fluorescence spectra at cryogenic temperatures suggested energy transfer from AH2DS/AQDS to uranyl ion, which is evidence of encounter complex formation, the reaction mechanism appears far more complicated compared with Fe(III). Overall, the mechanism appears to be a sequential two-electron transfer process whose first transfer is the reaction's slow step. Current research is focusing on single electron reductants of U(VI), such as hydroxylamine"HCl, hematin, and vitamin B12. Current modeling efforts involve adapting and applying the Marcus Theory model for the reduction of U(VI)-aquo, -carbonato, and -hydroxamate complexes. Recent experimental work has shown that UO22+ reduction rates vary significantly as a function of complexation, but simpler interpretations in terms of ligand structure, reorganization energy, or driving force do not appear to be possible. In addition, dependence of the internal reorganization energy on the presence of equatorial ligands in the inner coordination sphere varies unpredictably with ligand number and structure. Therefore, it is anticipated that the current experimental and modeling efforts will bring significant insight on the physical quantities that control the reduction rate. Contact Cal Ainsworth, project manager, for additional information. Assumptions - The DOE client (Office of Biological and Environmental Research) expects us to conduct fundamental research to further our knowledge in areas of importance to DOE's mission, and to disseminate this knowledge through publications in peer-reviewed journals and presentations of results at scientific meetings. Products/Deliverables: - Publications in scientific journals - Presentations at scientific meetings and symposia.
Project Details
Project type
Exploratory Research
Start Date
2003-12-03
End Date
2004-12-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Rosso KM, DM Smith, Z Wang, CC Ainsworth, and JK Fredrickson. 2004. "Self-Exchange Electron Transfer Kinetics and Reduction Potentials for Anthraquinone Disulfonate." Journal of Physical Chemistry A 108(16):3292-3303.