Study of the fast competitive adsorption of pesticides in soils by simultaneous filtration and solid phase extraction with subsequent GC-MS

A simultaneous filtration-plus-solid phase extraction (SPE) with subsequent GC-MS method previously validated for the study of the adsorption equilibr ia of pesticides in an organic soil has been used to study samples with dif ferent physicochemical characteristics, i.e., a sandy soil and a sediment. The results of this study are consistent with those found for the organic s oil. The online filtration-SPE procedure allows the determination of the am ount of pesticide adsorbed in the insoluble soil (or sediment) particles an d of that remaining in the aqueous phase by a single injection of the total slurry in the system. The standard deviations for the whole procedure were less than 6.5% (adsorbed) and 10% (solution) for all sorbents and for all test compounds. Comparison of the results found for the various samples sho wed the organic matter content as a key factor governing the partition equi librium. The relative merits of different adsorption isotherms, including l inear, Langmuir and Freundlich, for analyzing the organic sail fast adsorpt ion data were examined. Results showed that fast adsorption of relatively p olar compounds, with log K-ow values ranging from 1.51 to 3.74, on this soi l was less ideal than simple partition and revealed the Freundlich coeffici ents n and K-f to be time-dependent. Interactions between the pesticides an d specific sites on the soil are proposed as a plausible cause for this non linear behavior.