Adsorption-desorption behavior of selected pesticides in some Western Australian soils

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Abstract
The adsorption-desorption behaviour of four pesticides, in four Western Australian soils differing in their physico-chemical properties, has been studied using a batch technique. Sorption data for Fenamiphos, Linuron and Simazine could be fitted to a Freundlich-type equation, while that for Diquat more closely fitted the Langmuir equation. The value of the exponent in the fitted Freundlich isotherms varied from 0.70 to 1.00 for the various pesticide and soil combinations. Adsorption was found to be better correlated with organic matter than other soil properties, and decreased with increasing pH for all pesticides except Diquat. This study supports the observation of B. T. Bowman that the Freundlich equation in mole fraction form is more useful for comparing relative adsorption. Comparison of the present adsorption data with previous studies showed that the organic matter in these soils behaved differently in terms of pesticide adsorption. In particular, the octanol-water partition approach, for predicting the adsorption of non-ionic organic compounds, gave unsatisfactory predictions of Linuron and Fenamiphos adsorption in these soils. Desorption by a consecutive method showed hysteresis with all pesticide-soil combinations examined. The magnitude of the hysteresis was unaffected by changes in the soil : solution ratio. Using a dilution method, experimental procedures such as centrifugation and shaking have been shown to only slightly affect desorption hysteresis. The suggestion that hysteresis results from organic matter effectively 'locking in' the adsorbed pesticides is supported. The presence of methanol in the desorption solution results in swelling of organic polymers and facilitates the release of the adsorbed pesticides. Consequently, hysteresis was found to be essentially absent from the Linuron isotherm on removal of organic matter from the soil.