Unravelling the nature of glyphosate binding to goethite surfaces byab initiomolecular dynamics simulations

Abstract

Investigation of the interaction between glyphosate (GLP) and soil minerals is essential for understanding GLP's fate in the environment. Whereas GLP-goethite binding has been discussed extensively, the impact of water as well as of different goethite surface planes has not been studied yet. In this contribution, periodic density functional theory-based molecular dynamics simulations are applied to explore possible binding mechanisms for GLP with three goethite surface planes (010, 001, and 100) in the presence of water. The investigation included several binding motifs of monodentate (M) and bidentate (B) type. It was found that the binding stability increases in the order M@001 < M@010 < (2O + 2Fe) B@100 < M@100 < (1O + 2Fe) B@001 < (2O + 1Fe) B@010. This behavior has been traced to the presence of intramolecular H-bonds (HBs) in GLP as well as intermolecular HBs between GLP and water, GLP and goethite, and water and goethite. These interactions are accompanied by proton transfer from GLP to water and to goethite, and from water to goethite as well as water dissociation at the goethite surface. Further, it was observed that the OH species can replace the adsorbed GLP at the goethite surface, which could explain the well-known drastic drop in GLP adsorption at high pH. The present results highlight the role of water in the GLP-goethite interaction and provide a molecular level perspective on available experimental data.

Glyphosate Transport in Two Louisiana Agricultural Soils: Miscible Displacement Studies and Numerical Modeling

“…In accordance with the above discussion involving sorption by the Commerce soil, the relatively high amounts of crystalline Fe and Al oxides in this soil may also largely contribute to the high retentive behavior of the Sharkey soil [39]. The results of several recent studies have emphasized the importance of organic compounds in the sorption of solvated GPS, which may be of importance here due to the higher amount of organic matter in this soil (as indicated by TOC).…”

“…Since electrostatic repulsion between the negatively-charged clay surfaces and the herbicide molecule will likely prevent the formation of direct surface complexes, GPS sorption onto clay minerals within this soil likely takes place via ligand exchange with surface coordinated hydroxyl groups on the edge sites of layer silicates [44]. In accordance with the above discussion involving sorption by the Commerce soil, the relatively high amounts of crystalline Fe and Al oxides in this soil may also largely contribute to the high retentive behavior of the Sharkey soil [39]. The results of several recent studies have emphasized the importance of organic compounds in the sorption of solvated GPS, which may be of importance here due to the higher amount of organic matter in this soil (as indicated by TOC).…”

“…Additionally, the content of crystalline Fe and Al oxides may also lend to the high retentive capacity of this soil. It has been reported widely in the literature that these materials display a high affinity for solvated GPS, where sorption likely occurs via mono or bidentate inner-sphere complexation [38,39].…”

Ab Initio Molecular Dynamics Simulations of the Interaction between Organic Phosphates and Goethite

“…Molecular Dynamics (MD) simulations are employed to sample the equilibrium dynamics including reactive events. As the phosphate interaction at goethite-water interface involve proton transfer events and covalent bond changes [46][47][48], using the QM/MM MD technique is mandatory. Specifically, the electron density cut-off for the auxiliary plane wave basis set is chosen to be 500 Ry with SCF convergence threshold of 10 −4 Hartree.…”

“…In fact, the P colloids often exist in a solvated state in both arable and forest soils. The study by Ahmed et al [46] showed the importance of water in surface complexation reactions and highlighted how water maneuvers the phosphate interaction with mineral by forming strong to moderately strong HBs with phosphates [47,48].…”