Nanoscience Meets Geochemistry: Size-Dependent Reactivity of Hematite
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The rate of Mn2+(aq) oxidation on hematite with average diameters of 7.3 nm and 37 nm was measured in the presence of O2(aq). In the pH range of 7-8, the surface area normalized rate was one to two orders of magnitude greater on the 7.3 nm average diameter particles. Based on the application of electron transfer theory, it is hypothesized that the particles with diameters less than approximately 10 nm have surface crystal chemical environments which distort the symmetry of the MnMn2+ surface complex, reducing the energy required to reorganize the coordinated ligands after oxidation to Mn3+.
Cu2+, an analog for Mn3+, was used to probe for the presence and nature of the proposed changes in surface structure. Cu2+ and Mn3+ show similar electronic structure changes in response to the surrounding crystal field due to their d-electron configurations and Jahn-Teller coordinative distortions. Batch sorption experiments on hematite nanoparticles revealed a shift in the pH-dependent adsorption of Cu2+(aq). Specifically, an affinity sequence of 7 nm > 25 nm = 88 nm was determined based on the shift of the 7 nm sorption edge to approximately 0.8 pH units lower than that for the 25 nm and 88 nm samples. These data support the hypothesis that unique binding sites exist on the 7 nm nanoparticles that are not significantly present on the larger particles.
The National Nanotechnology Initiative stresses the need to address the broader societal impacts of nanoscale research. This dissertation embraces this viewpoint through the development and inclusion of "Nano2Earth: Introducing Nanotechnology Through Investigations of Groundwater," a curriculum which combines nanoscience with the Earth sciences for high school students.
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