Glyphosate retention dynamics at organo-mineral interfaces: Formation of multiple hydrogen-bonds and inner-sphere configurations at pectin-goethite interfaces

Abstract

The retention of glyphosate is strongly governed by adsorption and desorption processes at environmental interfaces. In particular, glyphosate can react with organo-mineral associations (OMAs) in soils, sediments, and aquatic environments. Here, we use in-situ time-resolved attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy to provide a mechanistic understanding of glyphosate’s adsorption and desorption processes at an organo-mineral interface. Pectin-goethite complexes prepared with a range of pectin concentrations (0.25 ‒ 4.0 mg mL-1) were used as model OMAs with varying organic surface loading. Adsorption and desorption kinetic studies were conducted by sequentially introducing a 4.0 mM glyphosate solution and a 10 mM KCl solution, respectively, over pectin-goethite films until equilibrium was reached. All experiments were conducted at pH = 5.0. ATR-FTIR spectra reveal adsorbed-pectin significantly alters the rate, quantity and binding mechanisms of glyphosate in pectin-goethite compared to goethite films. Our results illustrate, for the first time, that multiple hydrogen bonds between oxygen- and nitrogen- containing residues of adsorbed-pectin and glyphosate enhanced the relative retention of glyphosate at pectin-goethite interfaces and decreased desorption. It is also observed that glyphosate’s phosphonate moiety preferentially forms inner-sphere mononuclear complexes with goethite in pectin-goethite films whereas binuclear configurations are more favorable in goethite films. Since the proportion of organo-mineral to mineral interfaces decreases with soil depth, this work provides insights as to the extent, kinetics and mechanisms that might be involved during glyphosate downward transport in soils. Overall, our findings have valuable implications for characterizing and predicting the fate and behavior of glyphosate at heterogeneous interfaces present in natural systems.