The mechanisms governing the adsorption of a commercial humic acid (HA) on a kaolinitic clay were determined. The amount of HA fixed increased with the ionic strength of the medium and the presence of divalent ions (Ca2+). The effect of Ca2+ is due to its ability to establish (1) intramolecular bridges causing condensation of the HA, and (2) intermolecular bridges between the clay and the HA. A stable clay-humic complex containing only the fraction of HA that withstands washing with water was prepared and characterized by high-resolution Ar adsorption volumetry. The overall results show that (1) HA interacts strongly with the basal surfaces of the clay, (2) Ca serves to establish Ca2+ bridges between HA and clay, and (3) HA and clay interact via at least two distinct adsorption mechanisms.

Natural organic matter (NOM) generically refers to organic substances found in soils, waters, and sediments. It is the brown-to-black, heterogeneous organic material produced through the diagenetic alteration of plant tissue and microbial biomass via a myriad of biotic and abiotic reactions. Since NOM is the primary source of organic carbon in the earth’s surficial environment, understanding the processes by which NOM is produced is integral to understanding carbon sequestration, contaminant fate and transport, and other earth surface processes. NOM samples (HA0) consist of nonamphiphilic (HA1), lipid-like (L0 and L1), and strongly amphiphilic (HA2) components. Here we present the structure and morphology of self-assembled NOM components based on scanning electron microscopy (SEM), atomic force microscopy (AFM), and electrostatic force microscopy (EFM) characterizations. Effects of surface charge and hydrophobicity/hydrophilicity of the amphiphile on the interaction and resulting structures were investigated using SEM, AFM, and EFM. Data shows that the component’s amphiphilic nature plays a key role in the formation of NOM. SEM data show that aggregates form while AFM/EFM analysis verifies the existence of hydrophobic/hydrophilic moieties in different fractions of HA0. Subsequently, the amphiphilic nature of HA2 will have a substantial effect on interfacial interactions and subsequent self-assembly of HA0’s components.