ABSTRACT
In surface waters, the illumination of photoactive engineered nanomaterials (ENMs) with ultraviolet (UV) light triggers the formation of reactive intermediates, consequently altering the ecotoxicological potential of co-occurring organic micropollutants including pesticides due to catalytic degradation. Simultaneously, omnipresent natural organic matter (NOM) adsorbs onto ENM surfaces, altering the ENM surface properties. Also, NOM absorbs light, reducing the photo(cata)lytic transformation of pesticides. Interactions between these environmental factors impact 1) directly the ecotoxicity of photoactive ENMs, and 2) indirectly the degradation of pesticides. We assessed the impact of field-relevant UV radiation (up to 2.6 W UVA/m²), NOM (4 mg TOC/L), and photoactive ENM (nTiO2, 50 µg/L) on the acute toxicity of 6 pesticides in Daphnia magna. We selected azoxystrobin, dimethoate, malathion, parathion, permethrin, and pirimicarb because of their varying photo- and hydrolytic stabilities. Increasing UVA alone partially reduced pesticide toxicity, seemingly due to enhanced degradation. Even at 50 µg/L, nano-sized titanium dioxide (nTiO2) reduced but also increased pesticide toxicity (depending on the applied pesticide), which is attributable to 1) more efficient degradation and potentially 2) photocatalytically induced formation of toxic by-products. Natural organic matter 1) partially reduced pesticide toxicity, not evidently accompanied by enhanced pesticide degradation, but also 2) inhibited pesticide degradation, effectively increasing the pesticide toxicity. Predicting the ecotoxicological potential of pesticides based on their interaction with UV light or interaction with NOM was hardly possible, which was even more difficult in the presence of nTiO2. Environ Toxicol Chem 2020;39:2237-2246. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.
Subject(s)
Nanoparticles/chemistry , Organic Chemicals/chemistry , Pesticides/chemistry , Titanium/chemistry , Ultraviolet Rays , Animals , Catalysis , Daphnia/drug effects , Dimethoate/chemistry , Dimethoate/radiation effects , Dimethoate/toxicity , Malathion/chemistry , Malathion/radiation effects , Malathion/toxicity , Pesticides/radiation effects , Pesticides/toxicity , Water Pollutants, Chemical/chemistry , Water Pollutants, Chemical/radiation effects , Water Pollutants, Chemical/toxicityABSTRACT
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.
Subject(s)
Brassica rapa/chemistry , Cerium/pharmacology , Dimethoate/radiation effects , Nanoparticles/radiation effects , Pesticide Residues/radiation effects , Photolysis , Titanium/radiation effects , Dimethoate/chemistry , Food Contamination , Molecular Structure , Oxidation-Reduction , Pesticide Residues/chemistry , Reactive Oxygen Species , Semiconductors , Spectrometry, Mass, Electrospray IonizationABSTRACT
Different methods have been used to measure changes in biodegradability/toxicity of aqueous solutions of the pesticide Laition (a commercial formula of methidathion) when it is treated by means of TiO(2) photocatalysis: short time biological oxygen demand (BOD(st)) was used to determine the instantaneous biodegradability of the sample; BOD(5) was also chosen to determine biodegradability, employing in this case the manometric method; the BOD(5)/COD ratio was also calculated. Finally, the Zahn-Wellens test was employed to evaluate the long-term biodegradation of the effluents. The inhibition of the respiration of activated sludge in the presence of toxic pollutants was used to test the toxicity of the treated sample. An alternative method based on the decrease of BOD(5) of a very biodegradable mixture (glucose+glutamic acid) upon addition of the toxic solution was also employed. Similar trends were obtained with all methods and allowed us to distinguish between two periods: At the beginning of the reaction, there is a decrease in the concentration of dimethoate to reach complete abatement of this pesticide; this produces a nearly complete detoxification of the solution and a very significant increase of biodegradability (BOD(5)/COD ratio reached values close to 0.5 and important increase of BOD(5) and BOD(st) were observed). Beyond this point, slow mineralization is detected, but further improvement of the biodegradability cannot be achieved.
