Adsorption of glyphosate by humic substances

Four humic substances were extracted from a peat (HA1) and a volcanic soil (HA2), an oxidized coal (HA3), and a lignite (HA4). The four humic materials presented distinct differences in their chemical and physical-chemical characteristics as assessed by chemical methods, C-13-NMR spectroscopy, and high-performance size exclusion chromatography. The interactions between these well-characterized and purified humic substances and the widely used glyphosate [N-(phosphonomethyl)glycine] herbicide were studied by means of adsorption isotherms. Adsorption of glyphosate was found to be surprisingly high and followed the order KA1 > HA2 > KA3 greater than or equal to HA4. Humic extracts from soil adsorbed glyphosate even more than clay minerals, thereby indicating that the interactions with humic substances, in either a solid or dissolved form, are far more important than previously believed. Adsorption is explained by the multiple hydrogen bondings which can occur among the various acidic and oxygen-containing groups of both molecules. However, the order of adsorption did not simply follow the order of acidity but rather that of increasing aliphaticity and molecular size of humic substances. In fact, the least adsorbing humic materials from nonsoil sources showed a high content of aromatic structures and small molecular dimensions. These results reveal that the extent of glyphosate adsorption on humic substances varies considerably with their macromolecular structure and dimension and that is favored by a high degree of stereochemical flexibility combined with a large molecular size.