| The redox reactivity and ubiquitous presence of iron oxide minerals cause them to be influential in environmental and biological systems. Iron oxides can be formed through a variety of redox or transformation processes, and nanoscale iron oxide particles are commonly found in soil and water systems. Aluminum, which is commonly found with iron, can easily be incorporated into iron oxide species; natural samples of iron oxides have been found to contain up to 32% aluminum substitution. The substitution of a redox inactive cation is expected to affect the chemical and physical properties of iron oxide materials. Here, the kinetic behavior of iron oxide nanoparticles, including ferrihydrite, magnetite, and goethite, is studied as a function of aluminum substitution.;Synthetic samples of iron oxides with aluminum substitutions of 0-18 mol% were prepared and characterized. Inductively-coupled plasma mass spectrometry (ICP-MS) was used to determine the extent of aluminum substitution, while powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to identify the composition of the materials, as well as to obtain information about the relative crystallinity, size, and morphology of the nanoparticles. In general, increasing aluminum substitution caused a decrease in particle size as well as crystallinity.;The kinetic behavior of the aluminum-doped samples was examined via redox reaction with one of two quinone species: hydroquinone as a reducing agent or benzoquinone as an oxidizing agent. The rate of each reaction was determined at several temperatures to obtain activation energy and frequency factor information as a function of aluminum substitution. In the cases of ferrihydrite and magnetite, small amounts of aluminum substitution caused a decrease in activation energy and frequency factor; this trend then reversed, as both subsequently increased as a function of aluminum content at higher substitution levels. Overall, aluminum substitution caused a decrease in reactivity for magnetite, and an increase in reactivity for ferrihydrite and goethite. Evidence of reductive dissolution was observed on the goethite rods examined using TEM as pitting in the edges of the goethite rods, indicating that dissolution occurred preferentially at misorientation boundaries and other defect sites. |
