Evaluating molecular associations of key organic compounds with clay size minerals.

Soils sequester organic matter by binding organic C and N through strong organo-mineral associations. The capacity of soils to sequester organic matter is typically estimated based on the strong correlation between organic C and the soil's clay content and specific surface area. It is predicted that organic molecules in contact with soil will bind to reactive sites of clay-sized minerals. However, a recent study has proposed that the predominant pathway is the attachment of organic C to rough surfaces of mineral-organic submicron clusters.

To gather more insights into organo-mineral associations at reactive sites and mineral-organic cluster accumulation, we evaluated the binding mechanisms of organic functional groups at a micron scale using direct probing techniques. Molecular scale computer simulations further elucidated the dynamic relationships between specific mineral surfaces, organic compound structures, and their preferential surface accumulation. We conducted experiments using two common clay-sized minerals, illite and nanosized synthetic calcite, as model systems. These minerals were reacted with a combination of four organic molecules chosen to represent different classes of organic matter: lignin, proteins, lipids, and sugars.

Nuclear magnetic resonance solution analyses at two weeks reaction revealed preferential adsorption of organic moieties (lignin) by the clay size minerals. A suite of microscopy, spectroscopy, and tomography techniques investigated the surface of reacted minerals, from atomic level to 100 nm below the interface. We detected organic molecules or their fragments accumulated on the mineral surfaces and identified zones where organic compounds appeared to correlate with mineral specific locations. Experimental data were supported by molecular simulations, which elucidated the dynamics of the organic molecules, alone or in conjunction with each-other, as they approached specific mineral facets. This characterization of model mineral-organic systems at an atomic level can serve as a foundation towards mechanistic understandings of natural complex systems.