Thermodynamics and Redox Reactivity of Birnessite

With support from the Environmental Chemical Sciences Program in the Division of Chemistry, Christopher Gorski and Peter Heaney and their graduate students at Pennsylvania State University will study chemical reactions that occur at the interface between minerals and water. These reactions are important because they play a significant role in determining how environmental contaminants, such as toxic metals and hazardous organic compounds, behave in the environment and in water treatment systems. The project focuses on birnessite, which is a highly reactive manganese oxide commonly found in the environment. The rate at which birnessite transforms environmental contaminants present in water can depend on the properties of the birnessite and the water chemistry, but there is currently no quantitative method for determining why. To address this gap, the project will characterize the elemental composition, oxidation state, crystal structure, aggregation state, and surface charge of birnessite in various types of water and determine how quickly different environmental contaminants react with birnessite under the same conditions. To broaden the societal and scientific impact of the project, the researchers will also develop a workshop to teach other scientists and engineers how to use electrochemical methods to study minerals, and organize a demonstration called “Mysterious Manganese Minerals” at an outreach event for grade 6-12 young women interested in STEM (science, technology, engineering and mathematics) fields. This project will use a combination of mediated electrochemical techniques, spectroscopic analyses, and contaminant fate studies to thoroughly characterize birnessite and its reactivity. The project has three objectives with sequentially increasing complexities: (1) to determine how and why the oxidation state of Mn in sodium birnessite influences its thermodynamic properties and reactivity at pH 5.0; (2) to determine how and why the type and concentration of cations in solution and the pH influence birnessite’s thermodynamic properties and reactivity and (3) to determine if the relationships developed in first two objectives can predict contaminant oxidation rates in complex waters containing multiple cations and/or natural organic matter. If successful, this work will provide the environmental chemistry and geochemistry communities with a method for estimating the thermodynamic properties of birnessite under a range of solution chemistries. The work will also create a framework for characterizing the thermodynamic properties of other complex environmentally relevant minerals in the future. 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.