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item Wauchope, Robert - Don

Submitted to: American Chemical Society Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 7/20/2002
Publication Date: 8/21/2002
Citation: Wauchope, R.D., Russell, M. 2002. Freundlich isotherm: some limitations in its use for pesticide environmental fate modeling. American Chemical Society National Meeting, Boston, Mass., August 21, 2002.

Interpretive Summary: Regulatory agencies and environmental scientists all over the world rely on a simple laboratory test, the "soil absorption coefficient" test, to predict a pesticide's mobility in the environment. In this test a small amount of pesticide is added to a wet soil and mixed and the degree to which the pesticide concentrates in the soil from the water is measured. A chemical with low absorption has a high likelihood of becoming a water and air pollutant. When this test is done over a range of pesticide concentrations the result fits an equation called the "Freundlich isotherm", an equation that is very good at describing a huge variety of adsorption data from every area of science. Some scientists (especially in Europe) are proposing that this equation should be used in pesticide pollution simulation models that simulate pesticide mobility; this presentation is being made to highlight some potential pitfalls in that approach.

Technical Abstract: The Freundlich isotherm robustly fits most experimental adsorption-desorption isotherm curves, and is especially good for fitting data from highly heterogeneous sorbent systems, including soils. In a "batch" aqueous pesticide soil adsorption experiment, the data may be fitted to the Freundlich equation, using a linear regression on the log-log transformation of the data. A Freundlich isotherm with an exponent in the range 0.6-1 typically results, indicating that there is an increase in the partitioning coefficient Cs/Cw as concentrations go down. Thus, the Freundlich equation has been proposed as a more realistic description than the usual linear isotherm (constant Cs/Cw ) for environmental simulation models such as PRZM or RZWQM. But there are problems with using this isotherm that may lead to serious errors when the equation is used without an awareness of its limitations. Since the model may simulate concentrations far outside the range of the defining batch experiment, extrapolating equation 2 into such concentration regions may give results of unknown, but potentially very large error. For example, it is likely that there is a limiting value to the partition coefficient at lower concentrations, possibly reflecting sorption on only the most active sites as concentrations approach zero.