Model-Assisted Interpretation of 4,4'-Methylene Dianiline Adsorption on Soils at Micromolar Concentrations

Adsorption kinetics and isotherms were determined for 4,4'-methylene dianiline (MDA) on five diverse soils at nominal concentrations of 0.01-1.0 mg L^-1 (nominal soil loading 0.1-40 μg g_s ^-1 ). The data were used to model the adsorption process based on the two-step mechanism that is characteristic of the adsorption of aromatic amines, consisting of a physical equilibrium between the aqueous phase and the soil organic matter and a chemical reaction between the adsorbed MDA and reactive sites in the soil organic matter. Generic parameters were determined that enabled application of the model to other soils, which was checked against previously published data for MDA adsorption. At the low concentrations evaluated, the adsorption process took place almost exclusively in the organic matter without the need to account for a separate ion exchange process with the soil mineral fraction. Physical adsorption was found to be mainly dependent on the protonation state of MDA and increased with decreasing pH of the soils. Because of the chemical reaction taking place, adsorption equilibrium constants (organic-carbon partition coefficient [K_OC ]) normalized to the organic carbon content in the soil gradually increased with time; and it was demonstrated that, at steady-state conditions, values of log K_OC  > 3.5 can be expected for most any soil at conservatively estimated potential environmental MDA concentrations. Environ Toxicol Chem 2023;42:2580-2588. © 2023 SETAC.