RUI: Influence of Surface Interactions of Organic Ligand-Iron Oxide/Water Interfaces on Carbon and Iron Cycling Studied by Second Harmonic and Sum Frequency Generation Spectroscopy

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With support from the Environmental Chemical Sciences Program of the Division of Chemistry, Dr. Amanda Mifflin and undergraduate researchers at the University of Puget Sound are probing the interactions between organic ligands and the surfaces of iron oxide minerals using surface-specific laser techniques. Their experimental studies are complemented by computational modeling carried out in collaboration with scientists in the Molecular Science Computing group at Pacific Northwest National Laboratory (PNNL). The systems under study serve as models for geochemical systems commonly found in soils, and have important implications for understanding the process by which elements (or their chemical compounds) are transferred between living systems and the environment. This project specifically considers the transfer of iron and carbon from plants and animals and the soil and streams. The research provides undergraduate students with opportunities to develop critical technical and communication skills that prepare them for successful careers. Their time at PNNL shows them career paths to government laboratories. Their presentations in regional venues also help inform the general public about environmental issues.Direct surface measurements of metal-surface complexes at environmentally relevant concentrations, especially those involving the mechanisms of surface complexation with organic ligands, are difficult to study. The Mifflin group is probing mineral oxide/aqueous organic systems using second harmonic generation (SHG) and sum frequency generation (SFG) to gain molecular-level information of the surface properties and binding processes of metal complexes. The researchers seek a more complete understanding of the binding and redox chemistry of these systems under environmentally relevant conditions. Specifically, the adsorption, desorption and dissolution behavior of the aqueous siderophore desferrioxamine-B (DFO-B) at the hematite/water interface is being studied as a model for organic-mineral geochemical systems commonly found in soils. The computational studies aid in the interpretation of experimental results, targeting predictive capability for other mineral surface-organic ligand interactions. The experiments provide direct spectroscopic evidence of surface coverage, structure and kinetics of complexation and dissolution, and molecular orientation. This information about biogeochemical iron and carbon cycling can in turn lead to improved global models.