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Effect of Fulvic Acid on Adsorption of Methyl Parathion and Parathion by Ca²⁺ – and Fe³⁺-Montmorillonite Suspensions.¹

ABSTRACT

A study was conducted to examine the effect of fulvic acid (FA) on the adsorption of methyl parathion and parathion by Ca²⁺ – and Fe³⁺-montmorillonite suspensions, and to determine whether FA in solution could alter the water solubility of these insecticides. FA, in the 10- to 100-mg/liter range, did not affect the water solubility of the two insecticides. In Ca²⁺-montmorillonite suspensions, increasing FA concentrations first enhanced methyl parathion adsorption, followed by adsorption decrease at higher FA levels. Methyl parathion adsorption in Fe³⁺-montmorillonite suspensions decreased, with increasing FA levels. Parathion adsorption was not significantly altered in either clay suspension. The order in which the FA, clay, and insecticide were mixed affected methyl parathion adsorption. The greatest effects were observed where FA was adsorbed by the clay before the insecticide solutions were added. FA adsorption by Ca²⁺-montmorillonite fitted the Freundlich isotherm, whereas the much greater adsorption by Fe³⁺-montmorillonite fitted the Langmuir isotherm. Although both CaCl₂ and FeCl₃ solutions greatly enhanced FA removal by their respective clay suspensions (even at low concentrations), parathion adsorption was not affected. The presence of FA in the FeCl₃-Fe³⁺-montmorillonite suspensions appeared to prevent parathion degradation.

¹ Contribution no. 705, Research Institute, Agric. Canada, Univ. Sub Post Office, London, Ontario, Canada, N6A 5B7.

² Research Scientist.

Received for publication October 17, 1977. Accepted for publication February 15, 1978.