Crystal Morphology Controls Redox-Catalyzed Mass Transfer at Mineral–Water Interfaces

The Science

The growth, dissolution, and recrystallization of iron-based oxides are central processes to the cycling of iron and other nutrients in the environment. Understanding how the surface structure of hematite, an iron oxide mineral, influences these processes can help the development of a more accurate picture of mineral behavior. Careful examination of single metal oxide crystallites exposed to redox disequilibrium clearly shows that dissolution and growth processes are sensitive to crystal morphology. This finding demonstrates the importance of understanding the detailed structure at mineral–water interfaces, including defects and roughness, to accurately predicting reaction mechanisms.

The Impact

Mineral interfacial reactions are important for a wide range of applications, such as improving groundwater quality, managing soil chemistry, and developing advanced energy devices. By identifying and detailing the differences in hematite surface reactivity at redox disequilibrium, this work will help lead to more accurate models of mineral reaction kinetics and environmental iron cycling.

Summary

Understanding the reactivity of iron oxide minerals is relevant to environmental and industrial applications. In particular, interactions between reductants and chelating ligands with iron oxide surfaces can lead to competitive mineral dissolution and growth. Researchers used hematite crystals with known, well-defined morphologies to probe facet and defect-specific growth and dissolution behavior with iron ions and oxalic acid in solution. To accurately differentiate between iron in solution and iron on the mineral surface, the researchers used isotopically labeled iron in combination with mass-sensitive imaging techniques. These techniques, including nanoscale secondary ion mass spectrometry and three-dimensional atom probe tomography, enabled the team to track the flux of iron on the various surfaces. They found that while dissolution prevailed overall under the studied conditions, the actual rate of dissolution depended on the specific surface, with the oxalate in solution binding differently on individual facet types. These results have implications for broader iron oxide reactivity and may be incorporated into models of mineral behavior to improve their accuracy.

Contact

Sandra Taylor, Pacific Northwest National Laboratory, sandra.taylor@pnnl.gov

Kevin Rosso, Pacific Northwest National Laboratory, kevin.rosso@pnnl.gov

Funding

This material is based upon work supported by the Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through its Geosciences Program at Pacific Northwest National Laboratory (FWP 56674). A portion of the work was performed in the Environmental and Molecular Sciences Laboratory, a national scientific user facility at Pacific Northwest National Laboratory that is sponsored by the Department of Energy’s Office of Biological and Environmental Research, under user proposals 10.46936/lser.proj.2020.51382/60000186 and 10.46936/lser.proj.2021.51922/60000373. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle Memorial Institute under contract number DE-AC05-76RL01830 for the Department of Energy.