Direct Visualization of Aggregate Morphology and Dynamics in a Model Soil Organic–Mineral System

Interactions between mineral surfaces and organic matter are ubiquitous in soils and the environment. Through both physical and chemical mechanisms, organic mineral assemblages prevent decomposition of soil organic matter by limiting accessibility or reducing efficacy of enzymes and microbes. To understand the mechanisms underlying organic mineral interactions, researchers have begun to interrogate these systems at micro and nanometer length scales. Current techniques that maintain a hydrated state and allow researchers to characterize nanometer length scales are limited. Here we chose a model organic mineral system and performed complementary imaging techniques that allowed direct nanoscale observations in environmentally relevant conditions: cryogenic transmission electron microscopy (cryo-TEM) and in situ liquid cell transmission electron microscopy (TEM). We observed a 3-fold increase in the aggregate size of goethite nanoparticles upon addition of a model organic phosphate ligand and a preference for side-to-side interactions independent of the addition of the organic ligand. Additionally, in situ liquid cell TEM experiments provided a dynamic view of the interactions allowing us to report velocities of mineral assemblages during aggregation and disaggregation, which could potentially provide binding energetics and kinetic parameters for organic-mineral and mineral-mineral systems.