This project will study the size-dependence of on critical biogeochemical processes in active environmental nanosystems. Active nanosystems comprised of iron oxide nanoparticles and iron-reducing microorganisms influence natural biogeochemical cycles and can provide the basis for improved environmental remediation technologies. It will include a quantitative kinetic analysis and modeling of surface coordination of adsorbed species. The project will focus on fundamental aspects of nanoparticle reactivity research will be conducted on three related topics: (1) adsorption of U(VI) and Fe(II) onto nano-hematite, (2) rates of U(VI) reduction by surface-bonded Fe(II), and (3) rates of biological and chemical reduction of hematite. The proposed research will contribute to a variety of fields, including chemistry, geochemistry, and material science. The PIs have conducted some preliminary experiments that indicate a high likelihood that the project will be successful. The three PIs have complementary areas of expertise and are well qualified in the area of surface chemistry. The project has a strong educational and outreach component, including participation of undergraduates through an existing REU program and outreach to high schools through the NSF-STARS program.