Electroanalytical procedure for the determination of methylparathion in soil suspensions and its application for sorption studies with Brazilian soils

The differential pulse polarography technique was used to establish an electroanalytical procedure for the determination of the organophosphorous insecticide methyl parathion (O,O-dimethyl O-(4-nitrophenyl) phosphorothioate) in soil samples. Three reduction peaks were observed in mercury electrodes as a function of the solution pH. The more cathodic peak (E-p= - 0.55V), only observed for pH values higher than 5.0, was chosen for the analytical determinations. The limit of detection was 1.93x10(-8) mol L-1 for pure water and about 8x10(-8) mol L-1 for soil suspensions with a scan rate of 2 mV s(-1) and a pH of 6.75. The electroanalytical procedure developed was applied for the determination of sorption isotherms of methylparathion on 3 soils from Sao Paulo State, Brazil, at different pH and diverse amounts of clay and organic matter. The experimental data were fitted using the Freundlich isotherm model and the Freundlich coefficients (K-F) obtained for each soil varied from 7 to 29 L kg(-1), representing a low to medium sorption capacity, according to the IBAMA ( Brazilian Environmental Protection Agency) standards. The amounts of organic matter and clay were the most important soil parameters controling the sorption of methylparathion by these soils.