Reactivity of Iron Oxide Nanoparticle Surfaces: Understanding the Role of Crystallite Size and Morphology on Contaminant Behavior

Iron oxide nanoparticles, such as hematite (a-Fe2O3) and magnetite (Fe3O4), are found in the natural environment. Factors such as primary crystallite size and morphology can impact iron oxide nanoparticle reactivity and their interactions with contaminants. Currently, there are knowledge gaps related to how iron oxide nanoparticles behave in the environment in the presence of contaminants. To fill these knowledge gaps, the work presented here describes the reactivity of iron oxide nanoparticle surfaces as a function of crystallite size and morphology. It explores three themes relevant to understanding the reactivity of iron oxide nanoparticle surfaces: (i) understanding the effects of vacuum and oxygen annealing on nanoparticle surfaces as a function of crystallite size, (ii) interrogating the role of crystallite size on sorption and retention of Pu(V/VI), and (iii) investigating the impacts of nanoscale morphology on Pu(V/VI) sorption. The results of the first area of research show that carbon-mediated iron reduction increases with decreasing crystallite size after vacuum annealing, whereas an oxygen annealing + vacuum annealing step changes the extent of iron reduction caused by carbon. In the second area of research, the constant mass experimental results show there were changes in plutonium sorption and retention based on crystallite size and mineral type. Meanwhile, constant surface area experimental results show that there are no changes in plutonium sorption and retention regardless of crystallite size. Finally, the results of the third area of research show that morphology impacts the sorption of Pu(V/VI). It was found that (i) surface mediated reduction of Pu(V/VI) was observed on all morphologies and (ii) Pu sorbed species and Pu oxide nanoparticles form on platelet and rhombohedral morphologies. The result of this work provides insight into the impacts of crystallite size and morphology on contaminant behavior.