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Adsorption of Parathion, Fenitrothion, Methyl Parathion, Aminoparathion and Paraoxon by Na⁺, Ca²⁺, and Fe³⁺ Montmorillonite Suspensions¹

ABSTRACT

This study was conducted to investigate the influence of the saturating cation on the adsorption of parathion, methyl parathion, fenitrothion, aminoparathion, and paraoxon by montmorillonite suspensions. In all cases the saturating cation distinctly influenced the Freundlich-type adsorption, with adsorption decreasing in the following sequence: Fe³⁺ – >Ca²⁺- > Na⁺-montmorillonite. Adsorption of parathion, methyl parathion, fenitrothion and aminoparathion at low concentrations varied inversely with their water solubilities in Na⁺ – and Ca²⁺-montmorillonite suspensions, i.e. parathion > fenitrothion > methyl parathion > aminoparathion > fenitrothion > methyl parathion > aminoparathion adsorption. Paraoxon adsorption was slightly greater than the compound with the next lower solubility, aminoparathion. The water solubility of parathion at 20°C was 12.9 µg/ml. Aminoparathion was more than 99.9% adsorbed from solution by Fe³⁺-montmorillonite, suggesting the possibility of protonation of the -NH₂ group by the acidic clay surfaces. In Na⁺ – and Ca²⁺-montmorillonite suspensions, there was some conversion of paraoxon to p-nitrophenol. Parathion adsorption-desorption exhibited greater hysteresis effects in Fe³⁺-montmorillonite than in Ca²⁺-montmorillonite. The desorption pathway was dependent only on the initial concentration. The volume of solution removed in each cycle did not alter the desorption pathway, but only the rate at which the desorption proceeded down the desorption isotherm.

¹ Contribution no. 669, Research Instit., Agric. Canada, Univ. Sub Post Office, London, Ontario, Canada N6A 5B7.

² Research Scientist and Research Assistant, respectively, Research Institute, Agric. Canada, London, Ontario, Canada N6A 5B7.

Received for publication August 16, 1976. Accepted for publication December 20, 1976.