Effect of liquid sewage sludge addition on atrazine sorption and desorption by soil

Liquid sewage sludge (LSS) addition to soil incorporates both insoluble suspended organic material and large amounts of dissolved organic matter (LSSDOM) which can influence the sorption-desorption behaviour of pesticides by soil constituents. Batch sorption isotherm techniques were used to determine the relative effect of the insoluble and dissolved organic matter from a LSS on the sorption and desorption of the herbicide atrazine by soil. Atrazine sorption and desorption isotherms were obtained on soil, LSS, LSS-amended soil, and LSSDOM-pretreated soil and described by the Freundlich equation. The overall effect of LSS addition to soil (insoluble and dissolved organic matter) was to increase atrazine sorption, due to the high sorption capacity of the added insoluble organic matter. In contrast, LSSDOM, which was mainly constituted by low molecular weight molecules, decreased atrazine sorption by the soil. No evidences for stable interactions in solution between LSSDOM and atrazine were found, thus suggesting processes taking place at the soil/solution interface, such as competition for sorption sites on the soil surface, are the main responsible for the observed decrease in atrazine sorption by LSSDOM. Desorption of atrazine from soil was also enhanced by LSSDOM, but this effect was highly reduced when the soil had been pretreated with LSS (insoluble and soluble organic matter) or with LSSDOM alone. In these cases, interactions of LSSDOM with the soil surface have already taken place before the desorption experiment and the LSSDOM effects during desorption are less evident. The results of this work suggest that LSSDOM applied to soils may enhance the risk of groundwater contamination by promoting atrazine desorption from soil, especially when the soil surfaces are free of LSSDOM. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.