Cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some other organophosphates

Cerium dioxide was used for the first time as reactive sorbent for the degradation of the organophosphate pesticides parathion methyl, chlorpyrifos, dichlofenthion, fenchlorphos, and prothiofos, as well as of some chemical warfare agents-nerve gases soman and O-ethyl S-2-(diisopropylamino) ethyl] methylphosphonothioate (VX). CeO₂ specimens were prepared by calcination of basic cerous carbonate obtained by precipitation from an aqueous solution. The CeO₂ samples containing certain amounts (1 wt.%-5 wt.%) of the neighboring lanthanides (La, Pr, Nd) were prepared in a similar way from pure lanthanide salts. It was shown that ceria accelerated markedly the decomposition of parathion methyl causing the cleavage of the P-O-aryl bond in the pesticide molecule. A similar reaction mechanism was proposed for the degradation of other organophosphate pesticides and nerve agents. The degradation times (reaction half-times) were in an order of minutes in the presence of CeO₂, compared to hours or days under common environmental conditions. The reaction in suitable organic solvents allowed conversions of about 90% for parathion methyl loading of 20 mg pesticide/g CeO₂ within 2 h with a reactant half-life in the order of 0.1 min. The key parameter governing the degradation efficiency of CeO₂ was the temperature during calcination. At optimum calcination temperature (about 773.15 K), the produced ceria retained a sufficiently high surface area, and attained an optimum degree of crystallinity (related to a number of crystal defects, and thus potential reactive sites). The presence of other lanthanides somewhat decreased the reaction rate, but this effect was not detrimental and permitted the possible use of chemically impure ceria as a reactive sorbent. A fast organophosphate degradation was demonstrated not only in non-polar solvents (such as heptane), but also in polar aprotic solvents (acetonitrile, acetone) that are miscible with water. This opens new possibilities for designing more versatile decontamination strategies. The cleavage of phosphate ester bonds is of a great importance not only for the degradation of dangerous chemicals (chemical weapons, pesticides), but also for interactions of ceria (especially the nano-sized one) in biologically relevant systems.