Interfacial Free Energy of Nanominerals in Solutions, Biofilms and Microbial Cells

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Interfacial phenomena are common in low temperature geochemical and biological environments. Interfacial free energy is a key parameter that controls the thermodynamic phase stability of nanominerals, impacts biomineralization, determines the form and reactivity of minerals precipitated in the environment (e.g., in soils, during bioremediation), and directly impacts surface reaction kinetics (e.g., sorption and dissolution). Knowledge about how this quantity varies with environment type is essential for understanding and controlling these processes. The purpose of the present research is to determine the interfacial free energy of goethite, an important natural nanomineral, in various surface environments, and to use the results to analyze nucleation, adsorption and reactivity. Goethite nanoparticles form by mineral weathering and neutralization of acid mine drainage and are often associated with microbial cells, sometimes as byproducts of their metabolism. Determination of interfacial free energy of goethite nanoparticles in various environments necessitates a technique that is non-destructive and does not modify the nanoparticles or their surroundings. This requirement is not fulfilled with conventional calorimetry and contact angle measurements. This challenge will be addressed using a diffraction-based method developed recently by the investigators, which shows that the size-dependent interfacial free energy can be derived from integration of surface stress data obtained from diffraction experiments. The investigators will use both conventional and synchrotron-based x-ray diffraction to measure the lattice parameters and hence the surface stress as a function of particle size for ?dry? (degassed) goethite nanoparticles and nanoparticles in water, in solution containing a small organic molecule, attached to bacterial cells (Geobacter sp.), and coated by lipid bilayers. Strong size- and environment-dependence of the interfacial free energy is expected. Goethite nucleation will be analyzed by modifying classical theory to incorporate the environment- and size-dependence of interfacial free energy. The relationship between binding strength and interfacial free energy will be examined and the results used to predict reactivity as a function of environment type. The proposed research will serve as a new template for study of interfacial phenomena involving nanoparticles in natural environments.Broader Impacts: Nanoparticle interfacial phenomena are key to many geochemical and biological processes near the Earth?s surface. Because the interfacial free energy of nanoparticles determines their phase stability, reactivity and transformation kinetics, new insights will be broadly relevant in geochemistry, environmental and medical sciences and engineering, and for development of environment-compatible nanotechnologies. The methodology to be developed should also be broadly applicable. Undergraduate research is a central theme of the project. The experimental determinations are straightforward, enabling full participation of students in state-of-the-art research. Through the research, students will develop critical and creative thinking skills in addition to obtaining practical research experience. Science concepts and their practical applications in bioremediation will be conveyed to high school students through interactions with a local teacher who will participate in summer fieldwork at the Rifle CO site. The knowledge acquired from the research will be disseminated to a broad audience through professional publications and presentations, seminars to interested groups, and posting to the Nanogeoscience web site and the open Wikipedia web site for public access.