Quantifying molecular scale stabilization mechanisms at the organic-mineral interface

Understanding the dynamics of the carbon pool in the carbon cycle is crucial to both predicting climate and sustaining ecosystem services. However, a detailed description of the stabilization mechanisms of soil organic carbon (SOC) is currently lacking. Interactions between organic molecules and minerals (both biotic and abiotic) are thought to play a significant role in carbon stabilization. A molecular-scale view of organic-mineral interactions could provide insight into carbon sequestration and help parameterize models of dynamic environmental systems. In this study, we probe the organic-mineral interface at the molecular scale using dynamic force spectroscopy. This allows us to directly measure the rupture forces required to remove organic molecules from the mineral surface, thus gaining an understanding of (1) the energetic landscape for molecular binding. By combining this with in-situ Fourier transform infrared spectroscopy (FTIR), we are also able to characterize the kinetics of molecular desorption while monitoring the chemistry of the ligand. Our force spectroscopy data demonstrate that the binding free energies between organic functional groups and model mineral surfaces are dependent on the chemistry of the organic ligand, mineralogy, and solution chemistry. To correlate these results with the chemistry at the surface, we investigated the details of the interaction between an organic phosphate ligand and goethite. Force spectroscopy enabled us to quantify the binding free energies of organic phosphate ligands binding to goethite surfaces. Correlative FTIR experiments of methyl-phosphate desorption from goethite suggest that the phosphate group has two predominant species, which show dynamic behavior as a function of protonated sites on the mineral surface. Ultimately, the combination of DFS measurements with FTIR based kinetics will help use to develop a mechanistic understanding of abiotic sorption processes at the mineral interface, and their role in carbon stabilization.