Molecular modeling of MCPA herbicide adsorption by goethite (110) surface in dependence of pH

Abstract

The sorption mechanism between 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide and the dominating (110) surface of the mineral goethite was studied by molecular modeling of the full set of possible surface complexes using density functional theory with periodic boundary conditions for the structural surface models. The most stable arrangements of the MCPA species were predicted taking into account the type and topology of the surface OH groups, protonation states (pH effect), the structure of carboxyl/carboxylate group of MCPA, and the binding type (outer- or inner-sphere complexes). Acid–base properties of MCPA and the goethite surface OH groups led to creation of several pH ranges (3–4, 4–9, 9) for combining neutral/deprotonated MCPA with neutral/protonated goethite surface. The predicted strongest adsorption (physisorption) for the complexes in the pH 4–9 range was followed by largest solvent destabilization of the outer-sphere complexes due to the high solvent energy of the MCPA and surface hydration of the hydroxylated goethite surface. In line with experimental data, the adsorption of MCPA should increase with decreasing pH owing to the presence of neutral MCPA molecule (pKa ~ 3) and its lower solvation energy that can produce more stable complexes in solution than that of anionic MCPA in pH 4–9 range. The formation of the inner-sphere chemisorbed surface complex contributes significantly to the overall adsorption of MCPA at acidic pH range. In the chemisorbed inner-sphere complexes, monodentate binding was revealed through the formation of a Fe–O–C bridge.