Sorption of pesticides in the sediment of the Teufelsweiher pond (Southern Germany). I: Equilibrium assessments, effect of organic carbon content and pH

Sediment adsorption–desorption batch technique studies of seven pesticides and metabolites with different physiochemical properties, that have been recently applied in agriculture in Germany were performed with sediment samples from the Teufelsweiher pond (area: 3155 m2, depth: 0.3–1.3 m) in Southern Germany. Sorption kinetics exhibited two phenomena: an immediate rapid sorption (<4 h) followed by a slow sorption process over a long period of time (two to four days). Sorption isotherms could be well described by the Freundlich equation in a nonlinear form (1/n<1) for all seven compounds. The Freundlich coefficient (Kf) and Freundlich constant (1/n) appeared to be closely related to the physiochemical properties of the compounds. With increasing pesticide hydrophobicity (higher Kow, lower solubility) their adsorption was stronger and faster, their desorption much less effective and incomplete even after a long equilibration time. This hysteresis phenomenon was attributed to both sediment–particle size and organic matter content, which also controlled the pesticide distribution to the particle-size fractions. This distribution exhibited a bimodal behavior: both, the smaller particles (clay, fine silt, <6.3 μm) as well as the larger particles (fine sand, 63–200 μm) showed the most effective sorption of pesticides. In addition, the pH also exhibited a considerable effect on the sorption behavior of the pesticides.