Adsorption Effects on Kinetics of Aldicarb Degradation Equilibrium Model and Application to Incubation and Transport Experiments

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.