The adsorption behavior of three organophosphorus pesticides in peat and soil samples and their degradation in aqueous solutions at different temperatures and pH values

The sorption behavior of three organophosphorus pesticides, methyl parathion, parathion and phoxim, was studied with batch equilibrium experiments in three different soil and peat samples. The results showed that their sorption behavior could be best described by the Freundlich adsorption isotherm. The soil organic matter content plays a controlling role in the adsorption of these pesticides on the soils. The normalized carbon adsorption constants (K oc) for the three pesticides, ranging from 657 to 1976, were relatively high and thus the mobilities of these pesticides are expected to be moderate for methyl parathion and very low for parathion and phoxim. The hydrolysis of these pesticides was investigated in double distilled water incubated under different temperature and pH conditions. Temperature showed a significant effect on the rates of hydrolysis. Methyl parathion hydrolyzed in the dark with a half-life of 2.25 days at 45°C compared to 68 days at 8°C. The three pesticides are unstable in alkaline conditions, especially phoxim. Hydrolysis rather than photodegradation was found to be the main degradation path for these three pesticides in aqueous conditions. The results showed that degradation of the three organophosphorus pesticides in water follows first-order kinetics. The half-lives of these pesticides were low, hence they are not expected to persist in the environment.