A multi-surface approach to organic chemical binding: The role of mineral phases in polar and charged pesticide sorption

The mobility and availability of pesticides in soils is controlled by their sorption to soil constituents. The sorption of pesticides and other organic molecules is to date poorly understood. Based on observations with strongly hydrophobic organic molecules, it is assumed that organic chemicals mainly sorb to soil organic matter, which led to the ubiquitous use of the organic carbon normalized distribution coefficient Koc to describe and predict sorption of these molecules. This parameter and the underlying assumption are, however, not valid for polar and charged organic molecules, where other soil constituents may play a significant role in controlling the sorption. In this thesis, a multi-surface approach is used to study the sorption of polar and charged organic chemicals to soil constituents comprising metal (hydr)oxides, clay minerals, and soil organic matter. Glyphosate, a highly polar and multiply charged phosphonate herbicide, imidacloprid, a polar neutral insecticide, and fomesafen, an acidic herbicide, were selected as model pesticides. Adsorption experiments with these pesticides were conducted on the three separate soil constituents, while varying environmental conditions of pH and ionic strength, and including relevant competitors where appropriate. The main adsorbates in soils for glyphosate are metal (hydr)oxides. In this thesis, a mechanistic charge distribution-surface complexation model was developed for glyphosate sorption to goethite and ferrihydrite, the most common crystalline and poorly crystalline metal (hydr)oxides, respectively. Glyphosate adsorbs to metal (hydr)oxides with its phosphonate group via both monodentate and binuclear bidentate modes, while the protonation state of its other functional groups is dependent on pH. In natural environments, glyphosate experiences strong competition of phosphate and soil organic matter. For the latter, we identified steric hindrance as a competition mechanism in addition to site and electrostatic competition. Steric hindrance is strongest at low pH when soil organic matter is condensed near the surface. A novel model approach was developed to include this conformation-dependent competition mechanism. Imidacloprid shows strong adsorption to clay minerals and soil organic matter. Hydrogen bonds are proposed as the likely dominant adsorption mechanisms. For clay minerals, hydrogen bonds are likely to occur between imidacloprid and the hydration shell water molecules of multi-valent saturating cations. Fomesafen sorption is strongly impacted by electrostatics. Binding to the negatively charged clay minerals and soil organic matter only occurs at low pH when a significant fraction of fomesafen is in neutral form. Sorption to positively charged metal (hydr)oxides appears likewise to be governed by electrostatic attraction without observable intrinsic chemical binding affinity. The mutual interaction between soil organic matter and metal (hydr)oxides is an important factor in controlling the adsorption of fomesafen, as the acidic soil organic matter functional groups are (partially) neutralized, allowing the interaction with negatively charged fomesafen. This thesis highlights the need to improve simplistic sorption parameters such as Koc and to include all relevant soil constituents in sorption models, as well as relevant environmental parameters.