Hydrogen-bonding interactions between the herbicide Glyphosate and water-soluble humic substances

In this study, infrared spectroscopy (IR) was used to investigate the mechanism of interaction between the widely used herbicide Glyphosate [N-(phosphonomethyl) glycine] and a water-soluble humic acid (HA) obtained from a purified Leonardite Aliquots of the purified humic solution were set to pH 4, 5, and 6, and added with Glyphosate volumes corresponding to increasing percentages of total acidity of the water soluble HA The IR spectra of the HA-Glyphosate complexes showed two bands at 1,168 and 1,090 cm−1 for the stretchings of the P=O and P—O bonds, respectively, as they appear in the spectrum of Glyphosate alone where the phosphono group is involved in hydrogen bondings occurring in Glyphosate dimers Conversely, when the complex solutions were brought back to the starting pH levels (4, 5, and 6), the two IR bands of the phosphono group were shifted to higher frequencies (1,195 and 1,134 cm−1, respectively), suggesting that the P—O bonds were no longer involved in hydrogen bondings This behaviour supports the occurrence of hydrogen bondings in the Glyphosate-HA complex In fact, the IR band shift in the herbicide-HA complex indicates that, because the Glyphosate molecules are distributed along the humic macromolecule, they are too far apart to form the Glyphosate dimers again when the initial conditions are restored The hydrogen-bonding interaction between HA and Glyphosate evidenced in this study may well be responsible for the transport of Glyphosate in lower soil depths through the adsorption of Glyphosate on water soluble humic substances