The behavior of uranium (U) in the environment is of central importance to many branches of Earth Science. Decay of the radioactive uranium isotopes 238U and 235U forms the basis for the most accurate dating techniques suitable for studying events in deep time. The rarer, shorter-lived 234U isotope is also used for measuring the ages of events within the past million years. Uranium is also a significant environmental pollutant, and a useful trace element for studying chemical processes in the Earth. Recently, geochemists have begun to study the ways that natural chemical reactions (as opposed to radioactivity and other nuclear reactions) act to separate uranium isotopes from each other. These separations are subtle, changing 238U/235U ratios by roughly one part in a thousand, but they may provide a way to study the history of environmental oxygen, life, rock melting, and other natural processes through their effects on the uranium isotope compositions of ancient samples. The goal of the study is to use methods from theoretical chemistry to try to predict how chemical reactions in nature and in the laboratory might preferentially affect different uranium isotopes. Theoretical studies of this type have been important in the development of other isotopic techniques, such as the use of oxygen 18 to 16 ratio measurements in carbonate shells to infer ancient environmental temperatures. However, the very high atomic number of uranium makes its isotopes susceptible to a different type of chemical separation than most other elements, for which isotope separation is almost always driven by differences in mass between different isotopes. For uranium, isotope separations driven by differences in the physical size (i.e., the volume) of the uranium nuclei must also be accounted for, in order for theoretical calculations to be accurate. Including this nuclear volume effect will be a focus of the technical effort. The proposed research will support the training of a female doctoral student, undergraduate research, a scientific computing facility used for both research and classroom instruction, and outreach through the creation and improvement of Wikipedia articles on topics in geochemistry and environmental science.
The project will investigate equilibrium isotope fractionation of uranium in a variety of crystalline and aqueous species using electronic structure theory, taking advantage of recently developed methods for predicting mass-independent isotope fractionations driven by the volume component of the nuclear field shift effect in crystalline materials. The present theoretical understanding of uranium isotope fractionation is limited to a few simple gas-phase molecules and analogues of dissolved species, despite a rapidly growing interest in 238U/235U measurements, and researchers hope to greatly expand the set of studied compounds. -Uranium is a redox-sensitive metal in the surface environment, and the basis of the most precise geochronometers. Its isotopic behavior is therefore of broad interest to Earth Scientists, but is poorly understood. Uranium is a strong candidate for theoretical study of mass-independent fractionation because it has the highest atomic number of any element with more than one long-lived, nonradiogenic isotope. It is thus expected to show the largest nuclear field shift effect of any element. This hypothesis is supported by theoretical and empirical evidence that the field shift effect on isotope fractionation between oxidized U(VI) and reduced U(IV) species overwhelms a mass-dependent fractionation in the opposite direction, leading to high 238U/235U in chemically reduced phases such as black shales. Uranium is also a good target for investigation because its elemental neighbor neptunium (Np)has the best-studied Mössbauer isotope among the actinide elements, 237Np. Isomer shifts measured by Mössbauer spectroscopy directly probe the chemical and geochemical parameters that control nuclear field shift isotope fractionation, so study of analogous Np- and U-bearing species provides a robust check on the accuracy of the theoretical method. -This project aims to establish theoretical methods to predict isotope signatures in all elements suspected to display mass-independent nuclear field shift isotope fractionations, without being restricted to very time consuming calculations on small gas-phase molecules. The integration of electronic structure modeling with Mössbauer spectroscopy could allow investigators to leverage the existing literature, and may stimulate interdisciplinary studies via targeted Mössbauer or near-resonant synchotron X-ray studies of phases where measurements are presently lacking.
