Assessment of the sorptive fractionation of organic matter at the goethite-water interface

The carboxylic (COOH) and phenolic (PhOH) functional groups of natural organic matter (NOM) are the major binding sites for environmental compounds such as minerals and contaminants. This study focusses on the sorptive fractionation of a humic acid onto goethite (α-FeOOH) at various pH, [NOM], [NaCl], and under static (batch) and dynamic flow (column) conditions. During batch experiments at low pH and [NOM], adsorption behaviors of COOH- and PhOH-rich compounds were similar. At low [NaCl] and high pH or [NOM], the adsorption of NOM decreased, with a preferential adsorption of PhOH over COOH-rich compounds. This can be ascribed to the surface site saturation and competition between COOH, PhOH and OH^- for surface binding, combined with electrostatic effects. Higher [NaCl], which was previously shown to favor NOM-NOM interactions at mineral surface, led to increased adsorption of COOH, while the adsorption of PhOH was not affected. Under flow-through conditions, multiple-binding-site mechanism was shown to control the fractionation of PhOH- and COOH-rich compounds. These results provide insights into the molecular interactions between the NOM and soil components, which control the fate and behavior of contaminants and nutrients in the environment.