RUI: Where does all the molybdenum go? Evaluating the role of iron sulfides in molybdenum accumulation in marine sediments

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The project will study the role of sediment iron sulfides in removing dissolved molybdenum from the ocean. Molybdenum (Mo) is a trace metal found in ocean sediments. It is used as an indicator of low-oxygen and high-sulfide conditions in the past. Understanding the history of ocean oxygenation is important for monitoring low-oxygen “dead zones” in modern coastal waters and predicting the impact of climate change on the oceans. However, the processes by which Mo accumulates in sediments are not well understood. This project will use laboratory experiments to study the interactions between solution Mo and solid iron sulfides. The research will be carried out at an undergraduate institution. The study will provide a structured progression of experiences for students from historically underrepresented groups in STEM. It will include a cohort-mentor approach to support students before they arrive on campus and throughout their introductory science courses. Summer Research Students (rising juniors and seniors) will be engaged in every aspect of this project. These juniors and seniors will also serve as peer mentors for Research Cohort students (rising sophomores), forming an effective research team. Student exposure to graduate school opportunities will occur through meetings with research groups at the Pennsylvania State University. Finally, interest in geochemistry will be fostered by developing an environmentally-themed laboratory module within an introductory General Chemistry course. The goal of this work is to clarify the relative importance in marine sediments of: (1) sorption of Mo to iron sulfides, (2) retention of Mo during the formation of pyrite and/or exclusion of Mo upon growth of crystalline pyrite domains, and (3) possible formation of a distinct FeMoS product. Recent work established that Mo removal from sulfidic waters occurs with the initial formation of metastable iron sulfides and Mo is retained during the transformation of these precursor phases to the thermodynamically stable form of pyrite. With this groundwork, it should be possible to conclusively determine the role that iron sulfides play in the accumulation of Mo in marine sediments by: 1) synthesizing metastable iron sulfides and evaluating Mo adsorption to these species under various conditions to determine the impact of Mo thiolation, iron sulfide surface chemistry, and redox behavior; 2) monitoring the transformation of these materials to pyrite to determine how various initial conditions alter the retention or incorporation of Mo; and 3) developing a model that traces the fate of Mo through these processes and reconciles the disparate literature evidence. Understanding the role that iron sulfides and their phase transformations play in Mo accumulation will provide a strong foundation for work that employs Mo as a paleotracer.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.