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DIVISION S-2-SOIL CHEMISTRY

Adsorption Effects on Kinetics of Aldicarb Degradation

Equilibrium Model and Application to Incubation and Transport Experiments

^a Dep. of Soil, Crop and Atmospheric Sciences, Cornell Univ., Ithaca, NY 14853 USA
^b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521 USA

leiguo{at}mail.ucr.edu

The assumption of equilibrium adsorption was applied to both batch incubation and soil column leaching experiments to estimate degradation rate constants of the pesticide aldicarb [2-methyl-2-(methylthio) propionaldehyde o-(methylcarbamoyl) oxim] in the liquid (µ_l) and sorbed (µ_s) phases. The estimation was based on the observed K_d–µ relationship yielded in a soil amended with various amounts of activated C (AC), where K_d is the adsorption coefficient, and µ is the composed degradation rate constant from both phases. The inverse dependence of aldicarb degradation on K_d reveals that µ_l is faster than µ_s. For batch incubation experiments, the calculated µ_l and µ_s were 0.1228 and 0.0019 d^-1, respectively, differing by a factor of 65. In continuous-flow columns, µ_l and µ_s were both increased, with an estimated value of 0.2063 and 0.0055 d^-1, respectively, resulting in an accelerated overall degradation rate of aldicarb by 181% compared with the batch reactors. The results of our study indicate that, although degradation of aldicarb occurred primarily in the soil solution, it did not cease completely on the sorbed chemicals. The relative contributions of the two phases to the total degradation were therefore dependent on both the adsorption coefficient and the relative degradation rate constants for the dissolved and sorbed chemicals.

Abbreviations: AC, activated C • BTC, breakthrough curve • HPLC, high pressure liquid chromatography

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