Testing Hypotheses of Near-Equilibrium Kinetics For Silicate Minerals with an Innovative Silicon Isotope Tracer Method

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The assessment of the safety of storing nuclear wastes and carbon dioxide in geological repositories requires projections into a distant future. For example, would the interactions between these wastes and surrounding minerals and rocks cause the release of harmful materials into the environment in 10,000 years or 100,000 years? Reliable predictions over such a time span require a greater understanding of the fundamentals regarding the rates or speed of chemical reactions during these interactions. But these subjects have been a challenge to study. For example, 90% of Earth's crust is made of silicate minerals, for which silicon (Si) and oxygen are the main chemical elements, and their reactions are slow and difficult to detect. The proposed research uses an innovative experimental method, the isotope tracer method, to measure reaction rates in the laboratory under conditions not possible before. These new rates will be eventually incorporated into safety assessment models for society's general environmental projects. In addition to scientific innovations, this project also aims at human resources development. The underlying fundamental sciences are thermodynamics and kinetics, which are difficult learning subjects for students. Despite its significance, courses in thermodynamics and kinetics are disappearing from the geology curriculum and most experts on thermodynamics are either retired or near retirement age. To train the next generation of experts, this project will develop a webinar series on thermodynamics and kinetics and work with Indiana University's Center for Innovative Teaching and Learning for broadcasting and recording. Finally, the grant will leverage Indiana University's Minority-Serving Institution STEM initiative and the Center of Excellence for Women in Technology to recruit underrepresented faculty and potential doctoral students to summer research at Indiana University. The proposed research will focus on geochemical kinetics at near-equilibrium conditions and test various hypotheses using unidirectional dissolution rate data obtained from the isotope tracer experimental method. The key hypothesis is that the scatter and conflicts of near-equilibrium data are caused by unaccounted-for secondary phase precipitation. Eliminating or accounting for this possibility is a critical first step toward testing other hypotheses of near-equilibrium reaction mechanisms and interfacial processes. In the proposed experiments, a rare Si isotope (29Si or 30Si) is added to the experimental solutions, which reacts with natural silicates having mostly 28Si. The reaction rates are tracked by 29Si/28Si ratios of reacted solutions using High-Resolution Multi-Collector ICP-MS technology. In contrast, the conventional experimental method measures rates based on Si concentrations. Because isotope ratios are used instead of concentrations to calculate rates, the precipitation of Si-containing secondary phases, which consumes Si concentrations while having a negligible effect on Si isotope ratios, does not interfere with dissolution rate determination. The huge isotope disequilibrium introduced by overwhelming the system with 29Si makes Si isotope fractionation during dissolution and precipitation (2-4?) unimportant for rate determination. Currently, extrapolation from far-from-equilibrium to near-equilibrium conditions is necessary to translate laboratory experimental data to models of geological and environmental processes. But little confidence can be placed on these extrapolations because of the lack of near-equilibrium data and unsupported assumptions. The project will fill this significant knowledge gap. The isotope tracer method is general and novel and can be expanded to using Barium, Calcium, and Magnesium isotopes to study sulfate, carbonate, and other minerals, and applied to the fields of chemistry, chemical engineering, and materials science.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.