| Iron oxides and oxyhydroxides are common and important materials in the environment, typically occur as nanoparticles in the 3--10 nm size range, that strongly impact the biogeochemical cycle of iron and other species at the Earth's surface. This dissertation presents quantitative results on the reactivity of natural and synthetic iron oxide nanoparticles. Reactivity was characterized using two types of reactions that play important roles in the fate and transport of pollutants and many naturally occurring chemical species: reductive dissolution by hydroquinone and adsorption of perfluorooctane sulfonate (PFOS). Results show that surface-area-normalized rates of reductive dissolution of ferrihydrite (Fe5HO8•4H2O) nanoparticles are size dependent. In addition, ferrihydrite is approximately two orders of magnitude more reactive than goethite (alpha-FeOOH). The rate and extent of the reductive dissolution of ferrihydrite particles are dependent on pH, buffer concentration, carboxylic acid buffer chain length, and temperature. However, the rate of dissolution is not dependent on ionic strength, suggesting the reduction takes place through an inner sphere mechanism. The chemical reactivity of ferrihydrite evolves with time and depends on the conditions in which the particles are stored. Furthermore, reproducibility depends on storage conditions. For example, storing the particles as aqueous suspensions results in superior reproducibility but steadily decreasing reactivity as compared to storing the particles as dry powders Reductive dissolution experiments using several natural sediments demonstrate that reactive ferric oxides can be characterized for relative reactivity using the hydroquinone assay. Finally, adsorption experiments using an important pollutant, PFOS, and several synthetic and natural nanomaterials yielded surprising results. Adsorption, normalized to surface area, was highest for Ottawa sand standard, followed by iron-coated sands, kaolinite nanoparticles, and was lowest for synthetic goethite particles. Adsorption of PFOS onto goethite and kaolinite decreased with increasing pH, which supports an electrostatic mechanism of adsorption. However, the observation that the highest adsorption occurred on the most negatively charged mineral surface clearly contradicts that hypothesis, showing that adsorption is most likely not dominated by electrostatics. |
