Stable cerium isotopes as a potential new redox proxy: insight from first-principles calculations

Abstract

Redox fluctuation plays a key role in shaping the evolution of the atmosphere, biosphere, lithosphere and other planets in the solar system. Cerium (Ce) isotope fractionation has recently been suggested to be redox sensitive due to the high reduction potential of Ce. However, the mechanism of Ce isotope behavior is not yet clear. Therefore, we use first-principles calculations to investigate mass-dependent equilibrium Ce isotope fractionations among Ce(III) and Ce(IV) species in aqueous systems and adsorption process of surface complexes of Ce onto goethite. Our results show that the equilibrium isotope fractionation factors between Ce(III)-bearing species are relatively insignificant (mostly less than 0.14 ‰ at 25 °C). In contrast, the fractionations between Ce(III) and Ce(IV) species are significantly larger (e.g., 10³lnα_{Ce(IV)-Ce(III)} is −1.04 ‰ at 25°C), highlighting the importance of chemical bonding environment. During the adsorption onto goethite, our results show that the Ce isotope fractionation direction is opposite for Ce (III) and Ce (IV) species, respectively. Although the nuclear volume effect (NVE) induces negligible fractionation on the ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce ratios, it has a significant effect on the commonly used ¹⁴²Ce/¹⁴⁰Ce ratio which is opposite to that of mass dependent fractionation (MDF). The contrasting isotopic behaviors of MDF and NVE on the ¹⁴²Ce/¹⁴⁰Ce ratio result in minimum total isotopic fractionation at equilibrium. Given the lack of NVE on the ¹³⁶Ce/¹⁴⁰Ce ratio, it could be fractionated significantly under surface environments due to redox fluctuation, suggesting the strong potential of the Ce stable isotopes to serve as a valuable redox tracer.