Recent Developments on the Three-Dimensional Structure of Dissolved Organic Matter: Toward a Unified Description

Advancing a common understanding about the chemical composition, size, and three-dimensional (3D) structure of dissolved organic matter (DOM) is paramount to deciphering its impact on and involvement in environmental processes, such as the fate and transport of contaminants and carbon cycling. Traditionally, DOM has been described as a collection of solvent-separated molecules or macromolecules. More recently, DOM has been depicted as a "supramolecular assembly", a collection of individual molecules and associations of molecules held together by non-covalent interactions. The supramolecular assembly model has been broadly invoked to rationalize certain behaviors and properties of DOM, yet the complexity of DOM has made it difficult to fully unravel the nature and contributions of its intermolecular interactions. Discussed in this perspective is evidence regarding thermodynamic drivers of intermolecular associations, DOM molecular size, sorption of organic contaminants to DOM, and optical properties of DOM. While single observations may be rationalized by former structural models, such as the supramolecular assembly model, combined evidence shows that the 3D structure of DOM is best described by a mixed dynamic assembly model (MDAM). The MDAM depicts DOM as a collection of solvent-separated molecules and small, tightly knit assemblies held together by strong hydrogen bonds, which may form large assemblies through weak intermolecular interactions only at specific pH values, high ionic strength, or high DOM concentration.