Sorption of pesticides in the sediment of the Teufelsweiher pond (Southern Germany). II: Competitive adsorption, desorption of aged residues and effect of dissolved organic carbon

The adsorption-desorption behaviour of seven pesticides and metabolites with different physiochemical properties in sediments of the Teufelsweiher pond (area: 3155 m(2), depth: 0.3-1.3 m) in Southern Germany has been studied employing a batch technique. Atrazine and bifenox, which have been detected in the pond sediments, showed different competitive effects: the adsorption of the relatively weak sorbing atrazine was slightly reduced in the presence of the stronger adsorbing bifenox, whereas the sorption of bifenox was less affected by competition of atrazine at a. concentration range of 1-5 mg l(-1) The desorbed amounts of aged atrazine and bifenox residues were two or three orders of magnitude smaller than in other experiments with the same sediment recently incubated with these pesticides. in addition, the apparent desorption coefficients(K-fapp) of the aged residues were approximately 3 and 16 times higher than those of recent incubations with atrazine and bifenox, respectively. This time-dependent desorption strongly suggests that the concentrations of atrazine and bifenox in natural waters are likely to be substantially lower than these predicted from laboratory-determined desorption coefficients. The dissolved organic carbon (DOC) also exhibited a significant effect on pesticide adsorption-desorption behaviour. The pesticides can be bound to UOC resulting in higher total concentrations of pesticides in the pore-water and a decreased K-d. This "solubilization effect" may be important for enhancing the desorption and facilitating the transport of contaminants in the aquatic environment. (C) 1998 Elsevier Science Ltd. All rights reserved.