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 incubati on and soil column leaching experiments to estimate degradation rate consta nts of the pesticide aldicarb [2-methgl-2-(methylthio) propionaldehyde o-(m ethylrarbamoyl) oxim] in the liquid (CLI) and sorbed (CLI) phases, The esti mation was based on the observed K-d-mu relationship yielded in a soil amen ded with various amounts of activated C (AC), where K-d is the adsorption c oefficient, and mu(l) is the composed degradation rate constant from both p hases. The inverse dependence of aldicarb degradation on K-d reveals that m u(l) is faster than rho(s) For batch incubation experiments, the calculated mu(l) and mu(s) Were 0.1228 and 0.0019 d(-1,) respectively, differing by a factor of 65. In continuous-flow columns, mu(l) and mu(s) were both increa sed, with an estimated value of 0.2063 and 0.0055 d(-1), respectively, resu lting in an accelerated overall degradation rate of aldicarb by 181% compar ed with the batch reactors. The results of our study indicate that, althoug h degradation of aldicarb occurred primarily in the soil solution, it did n ot cease completely on the sorbed chemicals. The relative contributions of the two phases to the total degradation were therefore dependent on both th e adsorption coefficient and the relative degradation rate constants for th e dissolved and sorbed chemicals.