Mechanistic study on thiacloprid transformation: Free radical reactions

Detection of Neonicotinoids in agriculture soil and degradation of thiacloprid through photo degradation, biodegradation and photo-biodegradation

The social and ecological influence of Neonicotinoids (NEOs) usage in agriculture sector is progressively higher. There are seven NEOs insecticides widely used for the insects control. Among the NEOs, thiacloprid (THD) was extensively used for insect control during crop cultivation. This study targets to analyse the contamination levels of NEOs in agricultural soil and identify photo-biodegradation of THD degradation using pure isolates and mixed consortium. The photo degradation (PD), biodegradation (BD) and photo-biodegradation (PBD) of THD were compared. The corn field agricultural soils were polluted by four NEOs, among them THD had greater contamination level (surface soil: 3901.2 ± 0.04 μg/g) and (sub-surface soil: 3988.6 ± 0.05 μg/g). Three soil free enriched bacterial strains following Bacillus atrophaeus (PB-2), Priestia megaterium (PB-3) (formerly known as Bacillus megaterium), and Peribacillus simplex (PB-4) (formerly known as Bacillus simplex) were identified by microbiological and molecular 16s rRNA gene sequencing. The PD, BD and PBD of THD were conducted and degradation rate was detected by instrument UPLC-MS-MS. The PBD process with blue-LEDs showed better THD degradation efficiency than PD and BD, where the specific THD degradation rate was 85 ± 0.2%, 87 ± 0.5%, and 89 ± 0.3%, respectively for PB-2, PB-3 and PB-4. Then, the photo-biodegradation performance is greater at 150, 175, 200 rpm, pH 7.0-9.0, and temperature 30-35 °C. After the PBD system deliver four intermediate metabolites, the THD degradation process maybe through nitro reduction, hydroxylation and oxidative cleavage pathway.

Assessment of the validity of the quenching method for evaluating the role of reactive species in pollutant abatement during the persulfate-based process

Reactive species such as sulfate radicals (SO₄^•-), hydroxyl radicals (^•OH), and/or singlet oxygen (¹O₂) have often been proposed as the main reactive species for pollutant abatement during the persulfate-based process, and their relative importance is conventionally assessed by the quenching method based on an implicit fundamental assumption that the added high-concentration quenchers (e.g., tert-butanol and methanol) only scavenge their target reactive species, but do not considerably affect the other reaction mechanism of the system. To examine the validity of this assumption, this study evaluated the effects of several commonly used quenchers (tert-butanol, methanol, ethanol, isopropanol, furfuryl alcohol, and L-histidine) on the mechanism of a cobalt mediated peroxymonosulfate (Co(II)/PMS) process. The results demonstrate that besides quenching target reactive species, the added high-concentration quenchers can cause many confounding effects on the Co(II)/PMS process, e.g., accelerating PMS decomposition, interfering reactive species production, and quenching of non-target reactive species. Because of these confounding effects, the quenching method can actually lead to serious misinterpretation of the role of reactive species in pollutant abatement during the persulfate-based process. The findings of this study highlight that the underlying assumption of the quenching method is usually invalid for the persulfate-based process. Therefore, it should be cautious to apply the quenching method to investigate the mechanism of the persulfate-based process, and some debatable conclusions of prior studies obtained with the quenching method may require further verification.

Impact of Reaction Parameters and Water Matrices on the Removal of Organic Pollutants by TiO2/LED and ZnO/LED Heterogeneous Photocatalysis Using 365 and 398 nm Radiation

In this work, the application of high-power LED_365nm and commercial, low-price LED_398nm for heterogeneous photocatalysis with TiO₂ and ZnO photocatalysts are studied and compared, focusing on the effect of light intensity, photon energy, quantum yield, electrical energy consumption, and effect of matrices and inorganic components on radical formation. Coumarin (COU) and its hydroxylated product (7-HC) were used to investigate operating parameters on the ^•OH formation rate. In addition to COU, two neonicotinoids, imidacloprid and thiacloprid, were also used to study the effect of various LEDs, matrices, and inorganic ions. The transformation of COU was slower for LED_398nm than for LED_365nm, but r₀^7-HC/r₀^COU ratio was significantly higher for LED_398nm. The COU mineralization rate was the same for both photocatalysts using LED_365nm, but a significant difference was observed using LED_398nm. The impact of matrices and their main inorganic components Cl^- and HCO₃^- were significantly different for ZnO and TiO₂. The negative effect of HCO₃^- was evident, however, in the case of high-power LED_365nm and TiO₂, and the formation of CO₃^•- almost doubled the r₀^7-HC and contributes to the conversion of neonicotinoids by altering the product distribution and mineralization rate.