Nanoparticle Fe as a reactive constituent in air, water and soil

Iron (Fe) oxides and oxyhydroxides are two common constituents of the Earth’s crust and play an important role in iron cycling in air, water and soil. In natural environments, these iron minerals often display a wide distribution of particle sizes that range from ultra-fine aerosols to precipitates in soils and sediments. The occurrence of these particles in the nanoscale size regime has been previously established. Iron oxide and oxyhydroxide nanoparticles are of interest because they are present and produced in the environment and may display unique properties and higher reaction rates relative to larger-sized iron particles. Size-dependent properties and reactivity of iron oxides have been the focus of many laboratory investigations as a result of their prominence in nature as well as their use in industry. In general, nanoparticles are assumed to be more reactive than larger particles, and this higher reactivity is sometimes attributed to the large specific surface areas of these small particles. Size-dependent trends observed in recent studies of the dissolution of iron oxide nanoparticles, however, could not be explained by an increase in specific surface area alone. Rather reactivity trends were explained by either a greater density of reactive sites per unit surface area for nanoparticles relative to large particles or an increase in the inherent reactivity and higher surface free energy of smaller-sized particles.

Iron is an essential element for all biological organisms including those in marine environments. Fe-containing mineral dust aerosol (goethite, hematite, iron containing clay) and fly ash are major sources of iron to the open ocean with an annual deposition of ~450 Tg. Factors responsible for the dissolution rates of these iron containing particles include surface restructuring, surface curvature, and quantum confinement effects, all of which emerge as a function of decreasing particle size. Further, nanoparticle aggregation has been increasingly discussed as an important consideration in iron nanoparticle reactivity. In our experiments, we investigate nanoscale size-effects on the dissolution of iron nanoparticles under different environmentally relevant conditions.