Naturally occurring nanominerals provide unimaginably large surface areas for Earth-influencing interfacial reactions and exchange of electrons, elements, and energy. It is well known in materials science communities that synthetic nanoparticles are often difficult to disperse in a bulk substance, or by themselves, commonly resulting in nanoparticle aggregates. Just so, in nature, nanominerals are often found to be aggregated. As a result, it is often thought that such nanomineral aggregates will not show unusual properties associated with well dispersed nanominerals. Even advanced, yet existing ultrafine grinding technique cannot easily crush these aggregates into nanoscale single particles. On the other hand, transmission electron microscopy (TEM) suggests that meso-porosity and permeability, defined as pores and channels in the dimensional range of 2 to 50 nm, can exist in natural nanocrystalline aggregates, perhaps allowing reactive contributions from the surfaces of nanominerals in aggregates. The present study was undertaken to directly demonstrate this phenomenon for a manganese oxide because such minerals typically form nanocrystalline aggregates in nature, and they are well known as one of the most reactive mineral components in soils, rocks, and natural waters. Specifically, we have found that the manganese oxide mineral cryptomelane and aqueous reactants can chemically interact through mesopores that exist in dense, natural cryptomelane aggregates, even though the aggregates are not ground or dispersed into their constituent nanocrystals. As a result, these mesopores give aggregates exceptionally small scale porosity and permeability and provide an abundance of reactive sites on internal nanomineral surfaces. Our experiments show that natural, densely packed, nanocyrstalline mineral aggregates can display highly efficient catalytic activity with reactants in aqueous solutions. These aggregates most likely play an important role, yet to date still a largely undetermined one, in the cycling of Earth materials, in environmental behavior, and in remediation efforts.

Reactivity on nanomineral surfaces within natural dense aggregates of manganese oxide minerals

247th National Meeting (2014)

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