Heavy metal & nanoparticle research
Professor John Lenhart received an NSF CAREER grant for research to improve computer models used to predict the movement of heavy metals in soils, as well as assess the potential of whether nanoparticle migration in soils could pose a risk to human health or the environment. For his project, "Natural Organic Matter Mediated Processes in the Subsurface? Heavy Metal Adsorption and Nanoparticle Migration," Professor Lenhart investigates how natural organic matter will affect the chemical behavior of lead, one of the most often measured contaminants in soils, and impact the transport of nanoparticles through soils. He selected this project for his CAREER application because, "the ideas contained within this proposal represent the essence of my research focus and educational philosophy. It captures what I am most interested in and the areas of research and education where I hope to make a significant impact."
Iron oxides are present in nearly all environmental compartments, where they play critical roles in regulating nutrient and contaminant mobility. Although many iron oxides exist, we are particularly interested in evaluating hematite (a-Fe2O3), with the goal of understanding how reactions at the solid/water interface of hematite influence contaminant fate. We apply many tools to this research; however, synchrotron radiation is particularly powerful due to its ability to provide a great deal of information about the mineral structure as well as that of the structure of adions at the mineral surface. For this research, we are using multiple synchrotron-based X-ray techniques, but primarily apply Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and X-ray reflectivity (XR), to investigate the adsorption of lead to hematite in the presence of naturally occurring organic acids. EXAFS is an element specific method we apply to nano-sized hematite wet pastes that provides element-specific information on coordination, distance and types of neighboring atoms. EXAFS is an averaging technique and thus it takes into account the presence of all the forms of the target element to all faces of the hematite particles. XR makes use of a single crystal cut to expose a specific plane and it provides electron density as a function of the distance from the surface.
Combined, EXAFS and XR provide complimentary information on the crystal structure as well as the structure of adsorbed lead and its proximity and coordination to the organic-acid coated surface. Using these methods, coupled with wet chemical and infrared spectroscopy, we determined that lead adsorption observed in systems with organic acids under slightly acidic conditions exhibits significant differences that reflect slight differences in the organic acid structure. We are probing how these differences systematically vary with organic acid properties or structure of the oxide surface in order to better constrain geochemical models needed to accurately depict lead speciation and transport.