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Dimethoate, a systemic insecticide, has been used extensively in vegetable production. Insecticide residues in treated vegetables, however, pose a potential risk to consumers. Photocatalytic degradation is a new alternative to managing pesticide residues. In this study, the degradation of dimethoate in Bok choy was investigated under the field conditions using cerium-doped nano titanium dioxide (TiO2/Ce) hydrosol as a photocatalyst. The results show that TiO2/Ce hydrosol can accelerate the degradation of dimethoate in Bok choy. Specifically, the application of TiO2/Ce hydrosol significantly increased the reactive oxygen species (ROS) contents in the treated Bok choy, which speeds up the degradation of dimethoate. Ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS) analysis detected three major degradation products, including omethoate, O,O,S-trimethyl thiophosphorothioate, and 1,2-Bis (acetyl-N-methyl-) methane disulfide. Two potential photodegradation pathways have been proposed based on the intermediate products. To understand the relationship between photodegradation and the molecular structure of target insecticides, we investigated the bond length, Mulliken atomic charge and frontier electron density of dimethoate using ab initio quantum analysis. These results suggest the P = S, P-S and S-C of dimethoate are the initiation sites for the photocatalytic reaction in Bok choy, which is consistent with our empirical data. [ABSTRACT FROM AUTHOR]

The photocatalytic degradation of acephate was investigated using Ce-doped TiO2(TiO2/Ce) hydrosol. In contrast to previous research conducted under artificial light in the laboratory, this study investigated the decomposition of acephate in a field trial. The results show that acephate can be efficiently degraded by the TiO2/Ce system under natural field conditions; the degradation efficiency was affected by the dosage of the photocatalyst and acephate. The optimum dosage of TiO2/Ce was 2400 g a.i.ha−1, and the photodegradation efficiency of acephate reached 93.5% after 20 h at an acephate dosage of 675 g a.i.ha−1. Ultra-performance liquid chromatography/mass spectrometry (UPLC/MS) analysis detected and identified four degradation products—methamidophos, phosphorothioic acid O,O,S-trimethyl ester, S-methyl methanethiosulfonate and phosphorous acid—that were formed during the TiO2/Ce photodegradation of acephate. Based on the structural identification of the degradation products, a probable photodegradation pathway was proposed, and the first decomposition step may be the cleavage of the C‒N bond of acephate. Subsequently, the P‒S and P‒O bonds may be oxidized gradually or simultaneously to complete the mineralization. [ABSTRACT FROM AUTHOR]

Pesticides have revolutionized the modern day of agriculture and substantially reduced crop losses. Synthetic pesticides pose a potential risk to the ecosystem and to the non-target organisms due to their persistency and bioaccumulation in the environment. In recent years, a light-mediated advanced oxidation processes (AOPs) has been adopted to resolve pesticide residue issues in the field. Among the current available semiconductors, titanium dioxide (TiO2) is one of the most promising photocatalysts. In this study, we investigated the photocatalytic degradation of profenofos and triazophos residues in Chinese cabbage, Brassica chinensis, using a Cerium-doped nano semiconductor TiO2 (TiO2/Ce) under the field conditions. The results showed that the degradation efficiency of these organophosphate pesticides in B. chinensis was significantly enhanced in the presence of TiO2/Ce. Specifically, the reactive oxygen species (ROS) contents were significantly increased in B. chinensis with TiO2/Ce treatment, accelerating the degradation of profenofos and triazophos. Ultra-performance liquid chromatography–mass spectroscopy (UPLC-MS) analysis detected 4-bromo-2-chlorophenol and 1-phenyl-3-hydroxy-1,2,4-triazole, the major photodegradation byproducts of profenofos and triazophos, respectively. To better understand the relationship between photodegradation and the molecular structure of these organophosphate pesticides, we investigated the spatial configuration, the bond length and Mulliken atomic charge using quantum chemistry. Ab initio analysis suggests that the bonds connected by P atom of profenofos/triazophos are the initiation cleavage site for photocatalytic degradation in B. chinensis. [ABSTRACT FROM AUTHOR]