A new mixed-valent (ferric-ferrous) precursor to ferrihydrite, a floating iron film often misidentified as an oil film, has been identified in wetland soils and soil seeps by ourselves in Oregon coastal settings and by others in the Willamette Valley and the Lueneburger Heide (Germany). This mixed-valent iron oxide is similar to green rust and fougerite. Its high reductive capacity makes it attractive for use in environmental remediation, by reducing highly toxic contaminants such as hexavalent chromium. The film may also play an important part in the iron cycle. For example, by facilitating transport of Fe - a limiting nutrient - to the ocean. This precursor to ferrihydrite may also be an important nutrient source for microbial communities in wetlands and riparian zones. We propose to further characterize this phase in a number of natural environments (coastal wetlands, riparian areas, and alpine wetlands) and to synthesize it in the lab in order to determine its solubility and thermodynamic stability.
Objectives and Methods: Our objectives are to document the occurrence of this mixed-valent iron film in various natural environments and to determine the thermodynamics and solubility of the phase. In this pilot study, we will determine how this film forms in three settings: 1) in the natural field environment, 2) in the lab using only inorganic reagents and 3) in the lab using field-sampled soils that produce the film, including any associated microbes and organics. We will use XRD, FTIR, Moessbauer, SEM, HRTEM, ICPMS, AAS, IC, and UV-Vis to characterize the film and associated waters. Once we have established the knowledge base and techniques for working with this novel material, we anticipate proposing additional work to determine its stability and solubility over a range of environmental conditions (e.g., pH, redox, and temperature), to determine its reductive capacity and rate, and to apply this technique at a field site for environmental clean up.
Broader impact: Controlled synthesis of this mixed-valent Fe oxide will allow us to determine the environmental conditions under which the phase forms and persists. Such fundamental information would be useful to workers interested in remedial applications, such as inducing formation of this Fe oxide upstream from contaminated plumes. Solid ferrous Fe phases are valuable remedial agent because they reduce many contaminants to forms with lower toxicity or mobility. Such contaminant reduction does not occur in the absence of a solid iron phase. Zerovalent Fe, structural Fe(II) in smectites, and Fe(II) adsorbed to goethite have all been shown to reduce chlorinated hydrocarbons and hexavalent chromium. But currently there is no alternative if there is no Fe present in the aquifer. Synthesis of the mixed-valent Fe film could provide such an alternative. The proposed work will include student training and public education. Students typically misidentify the Fe film as an oil film. This makes it an excellent material for inquiry-based learning experiences. Because the Fe film lends itself to study of Fe reductions, it will be used in lab projects and field-based assignments in Clay Mineralogy and Chemical Hydrogeology classes. We also plan to create public displays at the Hatfield Marine Science Center.
