EFFECT OF LIQUID SEWAGE-SLUDGE ADDITION ON ATRAZINE SORPTION AND DESORPTION BY SOIL

Liquid sewage sludge (LSS) addition to soil incorporates both insolubl e suspended organic material and large amounts of dissolved organic ma tter (LSSDOM) which can influence the sorption-desorption behaviour of pesticides by soil constituents. Batch sorption isotherm techniques w ere used to determine the relative effect of the insoluble and dissolv ed organic matter from a LSS on the sorption and desorption of the her bicide atrazine by soil. Atrazine sorption and desorption isotherms we re obtained on soil, LSS, LSS-amended soil, and LSSDOM-pretreated soil and described by the Freundlich equation. The overall effect of LSS a ddition to soil (insoluble and dissolved organic matter) was to increa se atrazine sorption, due to the high sorption capacity of the added i nsoluble organic matter. In contrast, LSSDOM, which was mainly constit uted by low molecular weight molecules, decreased atrazine sorption by the soil. No evidences for stable interactions in solution between LS SDOM and atrazine were found, thus suggesting processes taking place a t the soil/solution interface, such as competition for sorption sites on the soil surface, are the main responsible for the observed decreas e 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 matte r) or with LSSDOM alone. In these cases, interactions of LSSDOM with t he soil surface have already taken place before the desorption experim ent and the LSSDOM effects during desorption are less evident. The res ults of this work suggest that LSSDOM applied to soils may enhance the risk of groundwater contamination by promoting atrazine desorption fr om soil, especially when the soil surfaces are free of LSSDOM. (C) 199 8 Published by Elsevier Science Ltd. All rights reserved.