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

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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.

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