Steric Hindrance of Glyphosate Adsorption to Metal (Hydr)oxides: A Novel Model Approach for Organic Matter-Mineral Interactions.

The environmental fate of the herbicide glyphosate (PMG) is determined by its favorable binding to metal (hydr)oxides, which is affected by environmental factors and the presence of competitors. A major competitor binding to metal (hydr)oxides is natural organic matter (NOM). This study investigated the competitive binding between humic acids (HA) and PMG on goethite with batch adsorption experiments, varying pH, ionic strength, and HA surface loading. HA strongly decreases PMG adsorption, increasing its solution concentration by multiple orders of magnitude. Interpretation of the competitive adsorption data with the NOM-CD model revealed that site and electrostatic competition insufficiently explain the competition. The model can be greatly improved by introducing steric hindrance as an additional mechanism, requiring only a single adjustable parameter. At low pH, HA maximizes its interaction with the surface while at high pH, the ligands tend to move outward. Our model reveals that steric hindrance is most significant in acidic conditions, while in alkaline conditions, competition is primarily controlled by electrostatics. The steric NOM-CD model provides excellent predictions of the behavior of PMG competing with HA and provides a tool to describe the key role of NOM in assessing the availability, mobility, and risk of PMG in the environment.