Ab initio and steady-state models for uranium isotope fractionation in multi-step biotic and abiotic reduction

Hexavalent uranium (U(VI)) is reduced to tetravalent uranium (U(IV)) by microorganisms (e.g., Geobacter or Shewanella spp.) as well as by abiotic reductants such as sulfide or Fe(II) species. During reduction, the heavy isotope (U-238) is typically enriched in U(IV) but the extent of fractionation varies substantially. The use of U isotope signatures in rocks and sediments is an attractive tool for probing paleo-redox conditions and deconvoluting modern processes in the subsurface. However, these signatures are being used with little understanding of the mechanistic underpinnings of U isotopic fractionation. Here, we contribute a theoretical elucidation of U isotope fractionation during the biological reduction of U(VI) to U(IV) by introducing a steady-state model for the multi-step reduction reaction. This model was derived based on the requirement of the Rayleigh distillation model that the isotope fractionation coefficient sRayleigh is time-independent, and the final product is removed from the system. In this model, sRayleigh depends on the equilibrium isotope fractionation coefficient seq for each reaction step, and hence, we calculated seq using ab-initio methods. Our calculations revealed that seq is largest for redox steps (1.44-1.60 parts per thousand for U(VI) to U(V), 0.76-0.79 parts per thousand for U(V) to U(IV)) and for the binding of U(VI) to a cytochrome (0.42- 0.73 parts per thousand). Using experimentally-derived sRayleigh and the calculated seq, we determined that the U isotopic fractionation associated with the binding of U(VI) to a cytochrome or with the reduction from U(VI) to U(V) could not be achieved solely with equilibrium fractionation and must include a kinetic isotope fractionation component. We also interpreted previously reported sRayleigh for abiotic U reduction by FeS, which followed the Rayleigh model. The abiotic sRayleigh depended on the rate of removal of U(VI) from the solution and the amounts of neutrally charged species (i.e., Ca2UO2(CO3)(3)). These experimental trends can be explained consistently using the steady-state model. Hence, we propose that the present steadystate model can be used more generally for any U multi-step reaction for which the experimental data follow the Rayleigh model. (C) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

In this study, a steady-state model was introduced to elucidate the U isotopic fractionation during the biological reduction of U(VI) to U(IV), showing that the model can be used more generally for any U multi-step reaction following the Rayleigh model. Results revealed that the U isotopic fractionation associated with the binding of U(VI) to a cytochrome or with the reduction from U(VI) to U(V) could not be achieved solely with equilibrium fractionation and must include a kinetic isotope fractionation component.