Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 212-
It is known that the heavy uranium isotope enriches in tetravalent state in the reduction of hexavalent to tetravalent uranium catalyzed by enzymes in bacteria. In contrast, abiotic reductions do not show such fractionation. Using this fact, it is expected to understand the biogeochemical events of the era from the isotope composition of uranium in the ancient deposits. However, the mechanism of isotope fractionation has not been uncovered. In this study, we focus on the biotic reduction by bacteria and calculate the isotope fractionation coefficient ε using ab initio methods. We assume that all the elementary reactions of the reduction pathway are in thermal equilibrium in our calculations. We consider the reduction pathway that forms a uranium di-nuclear complex bound to an enzyme residue which is reduced by electron transfer from an enzyme and disproportionation. The ε value between initial substance (UO2(CO3)34-) and product (non-crystalline CaU(PO4)2) is about three times larger than the experimental result. Since our result is obtained assuming that all elementary reactions are in thermal equilibrium, the disagreement with the experiment suggests that non-equilibrium isotope effect may occur in any elementary reaction.
