TITLE: Phosphonate Herbicide Interactions with Quartz, Montmorillonite, and Quartz-Enriched Agricultural Soil: Molecular Modeling Simulations and Adsorption-Desorption Experiments
–ABSTRACT: Phosphonate-based herbicides are extensively used in agriculture. Glyphosate was long considered immobile in soils due to strong complexation with metal oxides, but detection in agricultural effluents and surface waters points to glyphosate mobilization and transport. In particular, mechanisms of glyphosate adsorption and subsequent mobility from runoff-prone soils enriched in quartz have not been fully elucidated. Here we employed theoretical and experimental approaches to gain new insights on the soil mobility of glyphosate and two other phosphonate herbicides, glufosinate and fosamine. Molecular modeling simulations of optimized adsorbate complexes of the phosphonate herbicides with quartz and montmorillonite revealed Ca-enhanced interactions for all three herbicides at both mineral interfaces, weak binding of Na-complexed phosphonate herbicides on quartz, and more favorable interactions for glufosinate and fosamine with Na-montmorillonite than for glyphosate. We conducted adsorption and desorption experiments using glyphosate and quartz-enriched agricultural soils to evaluate the mobility of adsorbed glyphosate. Consistent with our theoretical findings, adsorption experiments conducted with CaCl2 resulted in greater than a two-fold increase in glyphosate retention compared to experiments with NaCl. The desorption experiments revealed that about 30-90% of the initially bound glyphosate could be desorbed, whereby Ca in the desorption solution enhanced glyphosate mobility compared to Na. Furthermore, in all conditions, the relative initial wetness of the agricultural soil was correlated positively with the promoted glyphosate mobilization, consistent with reported field-scale data that herbicide applied to soils following rain events were prone to runoff. [Link to Article]