The Effect of pH on Pesticide-Soil-Mineral Interactions: A Molecular Modelling Study of Glyphosate-Clay Interfaces Jessica N. S. Leung, Valentina Erastova, Thomas Underwood, and H. Christopher Greenwell Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE Glyphosate, the active ingredient in the widely used herbicide Round-up(R), has been recently identified as a potential cause of numerous health problems. The behaviour of glyphosate in the soil has been widely studied at the macroscopic level, but a molecular-level explanation is currently lacking. In this study, the effect of pH on glyphosate–clay interactions is considered using molecular dynamics simulations. The systems studied include sodium-saturated (Na-MMT) and calcium-saturated (Ca-MMT) montmorillonite solvated with both water and with 0.1 M CaCl2 solution. In the Na-MMT systems, glyphosate adsorption decreased with increasing pH. In the CaMMT system, glyphosate molecules were always adsorbed to the clay mineral, however, the associated adsorption mechanisms were pH dependent. In general, at very low pH levels (< 2.2) glyphosate was directly adsorbed to the clay surface via hydrogen bonding. As pH increased, glyphosate molecules were increasingly adsorbed through electrostatic interactions with surface adsorbed calcium ions. It was also found that the 0.1M CaCl2 solution enhanced glyphosate adsorption on Na-MMT as Ca2+ ions replaced the Na+ ions initially bound to the clay surface. Since the Ca-MMT surface is already saturated with Ca2+, the addition of CaCl2 solution did not have a significant effect on glyphosate adsorption. These results have important implications for the development of the retention time models and thus a more accurate transport model for the prediction of the fate of glyphosate in soil.