Parathion, hydrolysis kaolinites

Parathion hydrolysis on clay surfaces also is affected by environmental factors, such as temperature and water content. A rise in temperature generally enhances parathion hydrolysis on kaolinite, but the effect is greater in Na+-kaolinite than Ca +-kaolinite. These differences are due to the different hydrolysis pathways in the presence of Na - and Ca +- saturating cations. In the limit of sorbed water, the addition... 

Moisture content was also found to have a significant effect on the hydrolysis kinetics of parathion and methyl parathion on kaolinite. For example, the hydrolysis rate constant for parathion and methyl parathion on kaolinite increased by 2 orders of magnitude when the moisture content was increased to the limit of sorbed water (11% moisture content) (Saltzman et al. 1976). At moisture contents above the limit of sorbed water, however, a significant decrease in the hydrolysis rate constant was observed. Mingelgrin et al. (1977) concluded that the hydrolysis of parathion. 

Saltzman, S., B. Yaron and U. Mingelgrin. 1976. The surface catalyzed hydrolysis of parathion on kaolinite, Soil Sci. Soc. Amer. Proc., 38 231-234. 

A qualitatively similar relationship was observed for the hydrolysis of parathion on Ca- and Al-kaolinite (Figure 4). In order to explain the lower hydrolysis rate of parathion at Al-kaolinite than at Na-kaolinite surfaces, it was suggested (70) that steric hindrance may force the parathion molecule into a position or conformation less favorable to hydrolysis. On the other hand, the hydrolysis rate for methyl parathion on the Al-clay was higher than the one on the Na-clay (71), probably due to the smaller size of the methoxy group as compared to that of the ethoxy group of parathion... 

The conversion of parathion is affected by soil constituents in the order kaolinite > smectite > organic matter and is related inversely to the adsorption affinity of these materials for this contaminant (Mingelgrin and Saltzman 1977). Although the extent of hydrolysis differs among soils, it is apparent that soil-surface degradation of parathion is caused by hydrolysis of the phosphate ester bond. The presence of water in the soil-parathion system led to a decrease in the surface-induced degradation of the organic molecule and diminished the effect of natural soil properties on its persistence. 

Saltzman, S., U. Mingelgrin, and B. Yaron. 1976. Role of water in the hydrolysis of parathion and methylparathion on kaolinite, J. Agric. Food Chem., 24(4) 739-743. 

---