Equilibrium fractionation | EPFL Graph Search

Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis of the most widely used isotopic paleothermometers (or climate proxies): D/H and 18O/16O records from ice cores, and 18O/16O records from calcium carbonate. It is thus important for the construction of geologic temperature records. Isotopic fractionations attributed to equilibrium processes have been observed in many elements, from hydrogen (D/H) to uranium (238U/235U). In general, the light elements (especially hydrogen, boron, carbon, nitrogen, oxygen and sulfur) are most susceptible to fractionation, and their isotopes tend to be separated to a greater degree than heavier elements. Definition Most equilibrium fractionations are thought to result from the reduction in vibrational energy (especially zero-point energy) when a more m

Uranium Isotope Fractionation during the Anoxic Mobilization of Noncrystalline U(IV) by Ligand Complexation

Rizlan Bernier-Latmani, Ashley Richards Brown

Uranium (U) isotopes are suggested as a tool to trace U reduction. However, noncrystalline U(IV), formed predominantly in near-surface environments, may be complexed and remobilized using ligands under anoxic conditions. This may cause additional U isotope fractionation and alter the signatures generated by U reduction. Here, we investigate the efficacy of noncrystalline U(IV) mobilization by ligand complexation and the associated U isotope fractionation. Noncrystalline U(IV) was produced via the reduction of U(VI) (400 μM) by Shewanella oneidensis MR-1 and was subsequently mobilized with EDTA (1 mM), citrate (1 mM), or bicarbonate (500 mM) in batch experiments. Complexation with all investigated ligands resulted in significant mobilization of U(IV) and led to an enrichment of 238U in the mobilized fraction (δ238U = 0.4–0.7 ‰ for EDTA; 0.3 ‰ for citrate; 0.2–0.3 ‰ for bicarbonate). For mobilization with bicarbonate, a Rayleigh approach was the most suitable isotope fractionation model, yielding a fractionation factor α of 1.00026–1.00036. Mobilization with EDTA could be modeled with equilibrium isotope fractionation (α: 1.00039–1.00049). The results show that U isotope fractionation associated with U(IV) mobilization under anoxic conditions is significant and needs to be considered when applying U isotopes in remediation monitoring or as a paleo-redox proxy.

2021

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

Rizlan Bernier-Latmani

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 (238U) is typically enriched in U(IV), while abiotic reduction exhibits variable isotope fractionation directions. 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 εRayleigh is time-independent, and the final product is removed from the system. In this model, ε Rayleigh depends on the equilibrium isotope fractionation coefficient ε eq for each reaction step, and hence, we calculated ε eq using ab-initio methods. Our calculations revealed that ε eq is largest for redox steps (1.44–1.60‰ for U(VI) to U(V), 0.76–0.79‰ for U(V) to U(IV)) and for the binding of U(VI) to a cytochrome (0.42–0.73‰). Using experimentally-derived ε Rayleigh and the calculated ε eq, we determined that 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 ε Rayleigh for abiotic U reduction by FeS, which followed the Rayleigh model. The abiotic ε Rayleigh 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 steady-state model can be used more generally for any U multi-step reaction for which the experimental data follow the Rayleigh model.

2021

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

Rizlan Bernier-Latmani

2021