Ions can have complex behavior at mineral-water interfaces, with wide-ranging technological and environmental implications. This is particularly true of aluminum ions at surface of phyllosilicate minerals such as mica, which can have important impacts on diverse phenomena such as particle aggregation and flocculation, crystal nucleation, mineral dissolution, and aluminum toxicity. However, investigations on aluminum adsorption at mica-water interfaces are made difficult by the presence of aluminum within the mica structure, which interferes with many common analytical approaches for studying surface adsorption. We overcome this challenge and develop a self-consistent understanding of aluminum adsorption by using a combination of advanced surface-specific methods that include true molecular-resolution in-situ atomic force microscopy (AFM), streaming-potential measurements, and dynamic force spectroscopy (DFS), along with ab-initio molecular dynamics (AIMD). Our results enable the development of a triple-layer model (TLM) for aluminum adsorption at the mica-water interface, and indicate that the mica-water interface strongly favors the adsorption of hydrolyzed species such as AlOH2+ and Al(OH)2+ over Al3+. We hypothesize that this ability of the mica surface to concentrate hydrolyzed aluminum ions is critical to its ability to promote the nucleation of aluminum-hydroxide films. Complementary dynamic-force-spectroscopy measurements on the binding-strength between mica surfaces and a silicon-nitride AFM probe shows a strong correlation with the surface-concentration of Al(OH)2+ species, suggesting that these species can play an important role in binding together mineral particles, with potential consequences for the aggregation of mineral particles, particle-based contaminant transport, and soil-mechanics.

Aluminum hydrolysis at the mica interface and consequences for crystal nucleation and aggregation

2019 Fall National Meeting

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