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Partitioning Behavior of Insecticides in Soil-Water Systems: II. Desorption Hysteresis Effects¹

ABSTRACT

This investigation was undertaken to determine whether desorption hysteresis effects were dependent upon methods used in obtaining desorption data. Adsorption-desorption isotherms were obtained for two organophosphorus insecticides in aqueous suspensions of a clay, a sandy loam, and an organic soil. Desorption isotherms were obtained using both the consecutive desorption method and the dilution method. In the consecutive method, the same sample (after adsorption step) was put through a series of equilibration-centrifugation steps in which, following equilibration, the adsorbent was thrown down by centrifugation, and part of the supernatant pesticide solution was replaced by distilled water before reequilibration. In the dilution method a series of replicate samples (same adsorbent weight), after the adsorption step, were diluted to different volumes with distilled water, reequilibrated, then centrifuged to separate the phases. With the exception of one insecticide-soil system (where both methods produced minimal hysteresis), hysteresis effects were considerably reduced by using the dilution method. Repeated centrifugation appeared to be associated with the appearance of hysteresis effects observed using the consecutive desorption method. A short discussion is included on the improper use of desorption data to construct "single point desorption isotherms," which has created confusion in the literature.

Key Words: irreversibility • non-singularity • isotherm • organic matter

¹ Contribution from the Research Centre, Canada Agriculture, University Sub Post Office, London, Ontario, N6A 5B7.

² Research scientist and research assistant, respectively.

Received for publication January 27, 1984.