Nanosized iron oxide minerals are widespread in natural environment and play an important role in a variety of biogeochemical processes. Studying dissolution processes of nanosized iron oxide minerals is essential to understand their bioavailability, transformation, reactivity, and fate, as well as biogeochemical cycle of iron in the Earth's surface environment. The conventional methods for dissolution studies, such as batch dissolution experiments, cannot distinguish the effects of different factors, such as particle size distribution, aggregation state, and surface defects, on apparent dissolution rates. Whether nanosized minerals can exhibit special dissolution behavior or have distinct thermodynamic properties controlling dissolution processes are still unclear. We studied dissolution processes of hematite (α-Fe2O3) nanoparticles and goethite (α-FeOOH) nanorods using in situ liquid cell transmission electron microscopy (LCTEM) that allows us to observe dissolution behavior of each nanocrystal with well-defined properties (including aggregation state, shape, size, and defects) at high spatial and temporal resolution. The results of this study present that the initial surface-area normalized dissolution rates (RSA,Int) of isolated hematite nanoparticles exponentially increase as particle size linearly decreases, indicating a linear relationship of RSA,Int and degree of chemical non-equilibrium. Interface free energy of hematite particle was derived from the correlation between the measured RSA,Int and the primary sizes, suggesting a new method to measure interfacial surface energy of nanocrystals. The effects of crystal defects and aggregation state on the dynamic dissolution processes were revealed at nanoscale by LCTEM. Anisotropic dissolution of goethite nanorods were also presented using LCTEM, which quantitatively described the distinct surface properties of different goethite facets. More generally, this study illustrates an integrated method, involving in situ LCTEM, conventional batch experiments, and computational simulation, for probing interfacial reactions of iron oxide minerals at nanoscale.

Dissolution of iron oxide at the nanoscale revealed using in situ liquid cell transmission electron microscopy

ACS Fall 2021

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