PNNL

Abstract

Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to isolation and chemical stabilization at the organic-mineral interface. Due to the chemical, physical, and hydrological heterogeneities in soils, researchers describe the organic-mineral binding by using operational proxies, which lack the specificity required to elucidate binding mechanisms. Additionally, instrumentation for directly probing the organic-mineral interface is limited; as a result, much of the micron- and molecular-scale knowledge about organic-mineral interactions remains largely qualitative. Here, we use force spectroscopy to directly quantify the binding energies between organic ligands with known chemical functionalities to soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that the chemistry of both the organic ligand and mineral contribute to values of binding free energy. However, the most significant changes in binding strength were observed for the same model compounds when the local environmental conditions of pH and ionic strength were altered. These direct molecular scale measurements of organic-mineral binding establish quantitative constraints on the energy landscape, ultimately providing mechanistic insight into factors that will contribute to models for predicting the response of soil organic matter in microscale conditions in the presence of a changing climate.