Effect of Fe (III) on acid degradation of methylparathion
A study was undertaken to determine the transformation kinetic of methylparathion (O, O, -dimethyl O-4 nitrophenylphosphorotioate) in the presence of Fe(III) between pH 2 and 7. The Fe(III) was not electroactive under the conditions used in this study, and polarographic signals were exhibited by methylparathion and main degradation product only. Data suggest that hydrolysis of methylparathion in an acid medium is catalyzed by Fe(III) and the pesticide did not degrade in this medium without this cation. Methylparathion degradation was observed at all the pHs studied and was independent of the predominant chemical form of Fe(III) in the aqueous medium. The reaction was first-order with pH-dependent rate constant (k) values ranging from 3.3 × 10− 3 h− 1 to 7.0 × 10− 3 h− 1. The k values increased as pH decreased, suggesting that Fe(III) acted as an electrophile in the reaction mechanism.
CINÉTICA DE LA HIDRÓLISIS DEL METILPARATIÓN EN MEDIO ACUOSO
The kinetics of alkaline hydrolysis of methylparathion was studied at different temperatures (O-50%) in the pH range of 8-12 by ultraviolet-visible absorption spectroscopy. Optimum pH and wavelenght conditions were defined to carry out the simultaneous determination of methylparathion and one of its hydrolysis product, paranitrophenol, in buffered aqueous madium. Based on the experimental data and the mathematical equation of the kinetics, a rate constant (k) of firsts-order and an activation energy (Ea) of 9.2 kcal/mol, were estimated.
Degradation Kinetics of Organophosphorus and Organonitrogen Pesticides in Different Waters under Various Environmental Conditions.
The evolution of a mixture containing 19 organophosphorus (OP) and organonitrogen (ON) pesticides at ppb level was studied over a 6-month period in differentwatertypes (ultrapure water, natural seawater, river water, filtered river water) and under various conditions. The degradation kinetics were monitored in closed bottles in darkness at two temperatures (T=6 and 22 o C) and in a system exposed to natural sunlight (variable temperature). The mixture was analyzed by gas chromatography coupled with a nitrogen phosphorus detector (GC/NPD). Energy activation (E a ) and half-lifetimes (t 1/2 ) were determined. Very different degradation behavior with respect to physicochemical conditions and molecular structures of the pesticides was observed. These experiments confirm that half-lives of OP pesticides can be more than several months and consequently lead to lasting environmental pollution