August 1, 2006
Journal Article

Kinetics of Triscarbonato Uranyl Reduction by Aqueous Ferrous Iron: A Theoretical Study

Abstract

Uranium is a pollutant whose mobility is tied to its oxidation state. While U(VI) in the form of the uranyl cation is capable of being reduced by a range of natural reductants, complexation by carbonate greatly reduces its reduction potential as well as imposing increased electron transfer (ET) distances. Very little is known about the elementary processes involved in uranium reduction from U(VI) to U(V) to U(IV) in general. In this study, we examine the theoretical kinetics of ET from ferrous iron to triscarbonato uranyl in aqueous solution. A combination of molecular dynamics (MD) simulations and density functional theory (DFT) electronic structure calculations are employed to compute the ET parameters that enter into Marcus’ model, including the thermodynamic driving force, reorganization energies, and electronic coupling matrix elements. MD simulations predict that two ferrous iron atoms will bind in an inner-sphere fashion to the three-membered carbonate ring of tricarbonato uranium, forming the charge-neutral Fe2UO2(CO3)3(H2O)8 complex. Through a sequential proton-coupled electron transfer mechanism, the first ET step converting U(VI) to U(V) is predicted by DFT to occur at a rate on the order of 1 s^-1. The second ET step converting U(V) to U(IV) is predicted to be significantly endergonic. Therefore, U(V) is a stabilized end-product in this ET system.

Revised: May 16, 2012 | Published: August 1, 2006

Citation

Wander M.C., S.N. Kerisit, K.M. Rosso, and M.A. Schoonen. 2006. Kinetics of Triscarbonato Uranyl Reduction by Aqueous Ferrous Iron: A Theoretical Study. Journal of Physical Chemistry A 110, no. 31:9691-9701. PNNL-SA-47151. doi:10.1021/jp062325t