The isotopic signature of UV during bacterial reduction

Abstract

The two step electron transfer during bacterial reduction of U VI to U IV is typically accompanied by mass-independent fractionation of the 238 U and 235 U isotopes, whereby the heavy isotope accumulates in the reduced product. However, the role of the U V intermediate in the fractionation mechanism is unresolved due to the challenges associated with its chemical stability. Here, we employed the U V stabilising ligand, dpaea 2-, to trap aqueous U V during U VI reduction by Shewanella oneidensis. Whilst the first reduction step from U VI to U V displayed negligible fractionation, reduction of U V to U IV revealed mass-dependent isotope fractionation (preferential reduction of the 235 U), contrary to most previous observations. This surprising behaviour highlights the control that the U-coordinating ligand exerts over the balance between reactant U supply, electron transfer rate, and U IV product sequestration, suggesting that U V speciation should be considered when using U isotope ratios to reconstruct environmental redox conditions.

Electron correlation effects on uranium isotope fractionation in U(vi)–U(vi) and U(iv)–U(vi) equilibrium isotopic exchange systems

“…In recent studies, the present authors conducted relativistic calculations of ln K nv utilizing the X2C Hamiltonian within the density functional theory (DFT) formalism, which can incorporate electron correlation to some extent while maintaining computational efficiency comparable to that of the HF technique. [43][44][45] However, the effect of employing DFT on the results was not examined in detail in this prior research. To the best of our knowledge, the effects of electron correlation on ln K nv have been examined only with regard to the Hg(0)-Hg(II) systems in work by Wiederhold et al and Schable.…”