Adsorption of the Herbicide 4-Chloro-2-methylphenoxyacetic Acid (MCPA) by Goethite

Interaction between the goethite surface and 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide was studied using density functional theory (DFT) calculations combined with molecular dynamics (MD). The important step made here lies in the use of a periodic DFT method enabling the study of a mineral surface of different protonation states, in strong contrast with previous molecular modeling studies limited to single protonation state corresponding to the point of zero charge. Different surface OH groups and MCPA proton states were used to mimic the strong effects of pH on the outer- and inner-sphere surface complexes that are theoretically possible, together with their binding energies, and their bond lengths. Modeling both a solvated and a protonated (110) goethite surface provided a major breakthrough in the acidic adsorption regime. An outer-sphere complex and a monodentate inner-sphere complex with the neutral MCPA molecule were found to be the most energetically stable adsorbate forms. MD modeling p...