Iron oxide minerals are of great interest because of their high abundance in the environment and the increasing role of hematite (alpha-Fe2O3) in technological applications. There is little understanding, however, how impurities with different symmetry requirements and charge density are accommodated in the structure of these minerals. Zn, for example, is an essential micronutrient that is often sequestered in nominally refractory iron oxide phases, such as goethite and hematite, which can undergo reductive recrystallization to release metal impurities into aqueous solutions. Previous studies indicate that this process is driven by Fe vacancies. However, Zn incorporation into hematite and goethite, as opposed to adsorption, has not been studied at a fundamental level. To determine the structural environment of Zn in synthetic hematites, this study focuses on the incorporation of Zn into hydrothermally synthesized hematite at elevated pH values. Analysis of Extended X-ray Absorption Fine-Structure (EXAFS) by 21 ab initio Molecular Dynamics (AIMD) models shows that the Zn K-edge of doped hematite is best matched by models that preserve the hematite symmetry. Most significant configurations also suggest that the hematite structure accommodates the larger size of Zn2+ cations by formation of protonated iron vacancies and coupled iron-oxygen vacancies. Analysis of annealed hematite shows both types of vacancies persist after heating samples to 500 °C for 12 hours, which supports the idea that these defects are likely stable conformations. Because iron vacancies have been shown to correlate with the extent of reductive recrystallization, it is likely that the bioavailability of Zn is sensitive to local redox potentials.

Defects in zinc-bearing hematite: Insights from AIMD-guided EXAFS and implications for iron oxide reactivity

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.