KEYNOTE Modeling of herbicide adsorption by goethite: 4-chloro-2-methylphenoxyacetic acid (MCPA)

Interactions between goethite (FeOOH) and the ionic herbicide agent MCPA were studied using molecular dynamics (MD) calculations at temperature 300 K in the canonical (NVT) ensemble applying a Nosé-Hoover thermostat.... [ view full abstract ]

Interactions between goethite (FeOOH) and the ionic herbicide agent MCPA were studied using molecular dynamics (MD) calculations at temperature 300 K in the canonical (NVT) ensemble applying a Nosé-Hoover thermostat. Newton’s equations of motion were integrated using the Verlet velocity algorithm with a time step of 1 fs for up to 25 ps (slow-motion movies will be shown). Modeling both a solvated and protonated mineral surface provided a major breakthrough showing that there were energetically optimized hydrogen bonded MCPA surface complexes. However, such strong inner-sphere complexation is predominating only at acidic pH < pKa = 3.1, whereby the innersphere complex shares one of the oxygens between the MCPA carboxylate group and a singly coordinated surface hydroxyl group of the goethite surface, releasing an H2O molecule. At ambient soil pH values, MCPA is only weakly sorbed by outer-sphere complexation. The molecular modeling results were used in turn to constrain charge distribution parameters of an advanced CD-MUSIC surface complexation model. The adsorption constants were fitted to experimental batch equilibrium data [1]. The model was then extended by adding Cu which was found to trigger strong MCPA sorption even at circumneutral pH values forming a ternary innersphere surface complex. These informations enabled to parameterize in turn a reactive transport model to predict MCPA mobility in Cu-contaminated soil at any pH conditions. Positron emission tomography using Cu-64 tracer was used to verify the model.

Reference:
[1] Kersten et al. (2014): Adsorption of the Herbicide 4‑Chloro-2-methylphenoxyacetic Acid (MCPA) by Goethite. Environ. Sci. Technol. 48, 11803-11810.