The persistence, degradation, movement, and absorption of phorate and demeton in clay and soil systems

The purpose of this investigation was to study several factors which influence the fate of phorate and demeton when applied to soil. Radioactive P³²-labeled phorate and P³²-labeled thiol isomer of demeton were used in leaching and adsorption studies. In the investigation on persistence, thin-layer chromatography was employed for phorate; and paper chromatography was used for demeton. The degradation was determined by infrared analysis of phorate and phosphorus analysis of demeton. The organic matter content of soils was found to be related to the number of metabolites found over a six-week period and the relative concentration of the metabolites. The half-life of phorate was 62.4 hours at pH 4.5 and 70.0 hours at pH 7.0. The half-life of demeton was determined to be 1,525 hours at both pH 4.5 and 7.0 if calculated for the rate of decomposition for the period of three to seven days. Either a change in order of kinetic decomposition or a catalytic effect resulting from a drop in the pH caused the decomposition rate to increase several fold in seven to fourteen days' treatment time. In movement studies the quantity of phorate remaining in the leaching column was influenced by the moisture condition of the column at the time of application of the insecticide and thereafter before leaching. The dry columns retained more insecticide than did the moist ones. The moisture status of the clay columns had little or no effect on the amount of demeton remaining after leaching. The charge on the phorate molecule was predominantly positive, and the charge on the demeton molecule was predominantly negative. Both insecticides were adsorbed by Southern Bentonite from aqueous suspensions. Tako Kaolin adsorbed both phorate and demeton when calcium saturated. Phorate was adsorbed by Tako Kaolin when aluminum saturated, but demeton was repelled. Phosphorus decreased the amount of phorate adsorbed by calcium saturated Southern Bentonite. Nitrogen and potassium reduced the amount of demeton adsorbed by the same clay. Phorate and demeton were found to expand Southern Bentonite in all cases. The insecticide which caused an expansion above 15.1 and 14.5 angstroms for calcium- and aluminum-saturated clay was very weakly adsorbed and could be removed by dialysis.