Treatment of Aqueous Solutions Containing Four Commercial Pesticides by Means of $TiO2$ Solar Photocatalysis

Metasystox, Ultracid, Sevnol, and Laition are commercial pesticides, whose active ingredients are, respectively, oxydemethon-methyl, methidathion, carbaryl, and dimethoate. Laboratory scale experiments were carried out to treat solutions of the pure active ingredient and the commercial formulation employing a solar simulator as irradiation source. Degradation of the active ingredient followed in all cases a pseudo-first-order kinetic and rate constants, $k$⁠, indicated that reaction was faster when the thiophosphate moiety was present. Additives existing in the commercial formulations resulted in an important decrease in the reaction rate, as $k$ values were at least 50% lower than those obtained with the corresponding pure active principle (⁠$0.013min−1$ for Ultracid and $0.033min−1$ for methidathion, $0.011min−1$ for Laition and $0.026min−1$ for dimethoate, and $0.007min−1$ for Sevnol and $0.016min−1$ for carbaryl). Important increase in the surface tension was measured in all cases, indicating that the photocatalytic treatment is able to remove the surface active species present in the commercial formulation. Finally, assays based on the inhibition of the respiration of activated sludge indicated a decrease in the toxicity of all four commercial mixtures from initial values in the range 40–80% to achieve complete detoxification when the elimination of the active ingredient is reached.