Participating Faculty
Sue Clark

Professor & Chair
Chemistry
Washington State University
Pullman, WA 99164-4630

Ph.D., 1989
Florida State University

Phone: 509-335-1411
Fax: 509-335-8867
Email: s_clark@wsu.edu

Research Interests: Biochemistry of U(VI)

Research Summary: The behavior of U in the environment is influenced by its oxidation state, the chemistry of the groundwater, and the presence or absence of microbes. U(VI) solids are more soluble than similar U(IV) solids. Under typical, oxidizing environmental conditions, the hexavalent form of U as the uranyl cation (UO22+) is the thermodynamically favored oxidation state. Remediation of U contaminated sites is a priority for the U.S. Department of Energy (DOE), and some proposed remediation strategies are focused on microbial reduction of U(VI) to U(IV). Although microbial reduction is known to occur, the long-term stability of the tetravalent U solids formed in such processes is questionable unless anoxia of the system can be guaranteed. On the other hand, U(VI)-phosphate solids such as meta-autunite are quite refractory and are known to persist under oxidizing conditions on a geologic time scale. The focus of the proposed research is to examine the ability of the surfaces of Gram positive microbe Bacillus sphaericus and Gram negative species Shewanella putrefaciens to influence the
transformation pathways of a typical U(VI) oxyhydroxide, metaschoepite, to meta-autunite. The abiotic alteration sequence has been studied previously, and is known to proceed by dissolution of the metaschoepite starting material and reprecipitation of the meta-autunite. Reprecipitation was
slow, possibly due to a kinetic barrier for nucleation. In addition, other solids, such as the calcium oxyhydroxide solid becquerelite also form via exchange of Ca2+ with water molecules in the
metaschoepite structure. We hypothesize that microbes may facilitate the reprecipitation of U(VI)-phosphate solids by providing nucleation surfaces. It is possible that phosphate functional
groups present on the surfaces of some bacteria will facilitate alteration to phosphate solids. Such information is necessary to design long-term bioremediation strategies for U contaminated
sites, and for understand the biogeochemistry of U.

Representative Publications:

Lee MH, Clark SB. (2005) Activities of Pu and Am isotopes and isotopic ratios in a soil contaminated by weapons-grade plutonium. Environ Sci Technol. 39(15):5512-6.

C. A. Delegard and S. B. Clark (in press), “Plutonium in Non-Ideal Systems”, a review paper in Advances in Plutonium Chemistry: 1967-2000, D. C. Hoffman (ed.), ANRCP Press.

R. C. Ewing, F. Chen, and S. B. Clark (2002), “An Empirical Method for Calculating Thermody-namic Parameters for U(VI) Phases: Applications to Performance Assessment Calculations”, The Use of Thermodynamic Databases in Performance Assessment, Nuclear Energy Agency, 93-103.

J. I. Friese * , B. Ritherdon † , S. B. Clark, Z. Zhang, L. Rao, and D. Rai (2002), “Chromatographic Separation and Characterization of Hydrolyzed Cr(III) Species”, Analytical Chemistry, 74, 2977- 2984.

L. Rao, Z. Zhang, J. I. Friese * , B. Ritherdon † , S. B. Clark, N. J. Hess, and D. Rai (2002), “Oli-gomerization of Chromium(III) and its Impact on the Oxidation of Chromium(III) by Hydrogen Per-oxide
in Alkaline Solutions”, Journal of the Chemical Society – Dalton Transactions, 2, 267-274.

D. Rai, N. J. Hess, L. Rao, Z. Zhang, A. R. Felmy, D. A. Moore, S. B. Clark, and G. J. Lumetta
(2002), “Thermodynamic model for the Solubility of Cr(OH)3(am) in Concentrated NaOH and NaOH-NaNO3 Solutions”, Journal of Solution Chemistry, 31(5), 343-367.

S. M. Loyland-Asbury * , S. P. Lamont † , and S. B. Clark (2001), “Plutonium Partitioning to Colloidal and Particulate Matter in an Acidic, Sandy Sediment: Implications for Remediation Alternatives and Plutonium Migration”, Environmental Science & Technology, 35(11), 2295-2300.