Adsorption of Parathion, Fenitrothion, Methyl Parathion, Aminoparathion and Paraoxon by Na+, Ca2+, and Fe 3+ Montmorillonite Suspensions. 1977

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: Fe—> Ca— > Na-montmorillonite. Adsorption of parathion, methyl parathion, fenitrothion and aminoparathion at low concentrations varied inversely with their water solubilities in Na+and Camontmorillonite 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 20oC was 12.9 μg/ml. Aminoparathion was more than 99.9% adsorbed from solution by Fe-montmorillonite, suggesting the possibility of protonation of the -NH2 group by the acidic clay surfaces. In Na and Ca-montmorillonite suspensions, there was some conversion of paraoxon to p-nitrophenol. Parathion adsorption-desorption exhibited greater hysteresis effects in Fe-montmorillonite than in Ca-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. Additional Index Words: desorption, isotherm, hysteresis, water solubility, water/methanol injection-GLC(AFID). TWO PREVIOUS PAPERS (2,9) have extensively reviewed cation interactions with organic compounds, with specific reference to sulfoxide-containing compounds. There have been few reports in the literature of cation effects in soil on the adsorption of parathion or related compounds. Chopra et al. (4) found that parathion adsorption by homoionic soils followed the cationic sequence: H—> Ca— > Mg— > K— > Na soil. They also found that the percentage of cation saturation in their soils followed the same decreasing sequence and concluded that parathion adsorption on homoionic soils depended directly on the saturation percentage of the various cations. Leenheer and Ahlrichs (10) have shown that H-saturated organic matter and a H-saturated cation exchange resin adsorbed more parathion than the respective Ca-saturated systems. They believed that the H-saturated adsorbents exhibited greater hydrophobic characteristics than the more hydrophilic Ca-adsorbents, thereby accounting for higher insecticide adsorption capacities. Certain cations can complex with various organophosphorus insecticides. Anhydrous ZnCl2 forms a 1:1 complex with the P —> S group of parathion (3). Bidleman and Frei (1) demonstrated that palladium forms 1:1 complexes with the thiono sulfur of the phosphorodithioate insecticides azinphosmethyl, azinphosethyl, phosmet and dimephenthoate, and a 2:1 complex with the phosphorothioate insecticide, fenthion. They based their conclusions on frequency and intensity __________________ 1 Contribution no. 669, Research Inst., Agric. Canada, Univ. Sub Post Office, London, Ontario, Canada N6A 5B7. Received 16 Aug. 1976. Approved 20 Dec. 1976. 2 Research Scientist and Research Assistant, respectively; Research Institute, Agric. Canada, London, Ontario. Canada N6A 5B7. 3 M. Yang. 1974. Processes of adsorption, desorption, degradation, volatilization and movement of O,O-diethyl O-p-nitrophenol phosphorothioate (parathion) in soils. Ph.D. Thesis. Univ. of California, Davis.