Aqueous picloram degradation by hydroxyl radicals: Unveiling mechanism, kinetics, and ecotoxicity through experimental and theoretical approaches

Effect of pH on the ozonolysis degradation of p-nitrophenol in aquatic environment and the synergistic effect of hydroxy radical

Density functional theory (DFT) was employed to elucidate the mechanisms for ozonolysis reaction of p-nitrophenol (PNP) and its anion form aPNP. Thermodynamic data, coupled with Average Local Ionization Energies (ALIE) analysis, reveal that the ortho-positions of the OH/O- groups are the most favorable reaction sites. Moreover, rate constant calculations demonstrate that the O3 attack on the C2-C3 bond is the predominant process in the reaction between neutral PNP and O3. For the aPNP + O3 reaction, the most favorable pathways involve O3 attacking the C1-C2 and C6-C1 bonds. The rate constant for PNP ozonolysis positively correlates with pH, ranging from 5.47 × 108 to 2.86 × 109 M-1 s-1 in the natural aquatic environment. In addition, the formation of hydroxyl radicals in the ozonation process of PNP and the mechanisms of its synergistic reaction of PNP with ozone were investigated. Furthermore, the ozonation and hydroxylation processes involving the intermediate OH-derivatives were both thermodynamically and kinetic analyzed, which illustrate that OH radicals could promote the elimination of PNP. Finally, the toxic of PNP and the main products for fish, daphnia, green algae and rat were assessed. The findings reveal that certain intermediates possess greater toxicity than the original reactant. Consequently, the potential health risks these compounds pose to organisms warrant serious consideration.

Plasmonic photocatalytic degradation of tebuconazole and 2,4-dichlorophenoxyacetic acid by Ag nanoparticles-decorated TiO2 tracked by SERS analysis

Chemical oxidation technologies have been notably used for the mineralization of organic pollutants from aqueous effluents, been especially relevant for the degradation of pesticides. In this context, both tebuconazole (TEB) and 2,4-dichlorophenoxyacetic acid (2,4-D) pesticides were photodegraded by a combined catalyst of TiO2 and silver nanoparticles irradiated by UV-A light (λmax = 368 nm), and the experiments were tracked by surface-enhanced Raman scattering (SERS) spectroscopy. For 2,4-D, the degradation of about 70% was observed after almost 200 min, while for TEB, a decrease of 80% of the initial concentration was observed after approximately 100 min. The SERS monitoring allowed the proposal of some by-products, such as oxidized aliphatic chain and triazole from TEB besides glycolic, glyoxylic and dihydroxyacetic acids from 2,4-D. Their toxicities were predicted through ECOSAR software, verifying that most of them were not harmful to populations of fish, Daphnia and green algae. Thus, the performed oxidative process was efficient in the photodecomposition of TEB and 2,4-D pesticides, inclusive in terms of the decreasing of the toxicity of contaminated effluents.

Aqueous degradation of 6-APA by hydroxyl radical: a theoretical study

CONTEXT

Degradation reactions of micropollutants such as antibiotics with OH radicals are very important in terms of environmental pollution. Therefore, in this study, the degradation kinetic mechanism of 6-aminopenicillanic acid (6-APA) with OH radical was investigated by density functional theory (DFT) methods.

METHODS

For the calculations, different functionals such as B3LYP, MPW1PW91, and M06-2X were used with a 6-31 g(d,p) basis set. The aquatic effect on the reaction mechanism was investigated by conductor-like polarizable continuum model (CPCM). For the degradation kinetics in aqueous media, the addition of explicit water molecules was also calculated. Subsequent reaction mechanism for the most probable reaction product was briefly discussed.

RESULTS

Among the functionals used, B3LYP results were consistent with the experimental results. Calculated kinetic parameters indicated that the OH-addition path was more dominant than the H-abstraction paths. With the increase of explicit water molecules in the models, the energy required for the formation of transition state complexes decreased. The overall rate constant is calculated as 2.28 × 10¹¹ M^-1 s^-1 at 298 K for the titled reaction.

Kinetics, mechanism, and tautomerism in ametryn acid hydrolysis: From molecular structure to environmental impacts

The excessive use of pesticides and the demand for environmentally friendly compounds have driven the focus to detailed studies of the environmental destination of these compounds. Degradation by hydrolysis of pesticides, when released into the soil, can result in the formation of metabolites with potentially adverse effects on the environment. Moving in this direction, we investigated the mechanism of acid hydrolysis of the herbicide ametryn (AMT) and predicted the toxicities of metabolites through experimental and theoretical approaches. The formation of ionized hydroxyatrazine (HA) occurs with the release of the SCH₃- group and the addition of H₃O⁺ to the triazine ring. The tautomerization reactions privileged the conversion of AMT into HA. Furthermore, the ionized HA is stabilized by an intramolecular reaction that provides the molecule in two tautomeric states. Experimentally, the hydrolysis of AMT was obtained under acidic conditions and at room temperature with HA as the main product. HA was isolated in a solid state through its crystallization as organic counterions. The mechanism of conversion of AMT to HA and the experimental investigation of the reaction kinetics allowed us to determine the dissociation of CH₃SH as the rate-controlling step in the degradation process that culminates in a half-life between 7 and 24 months under typical acid soil conditions of the Brazilian Midwest - region with strong agricultural and livestock vocation. The keto and hydroxy metabolites showed substantial thermodynamic stability and a decrease in toxicity compared to AMT. We hope that this comprehensive study will support the understanding of the degradation of s-triazine-based pesticides.

Photolysis of the novel meta-diamide insecticide broflanilide in solutions: Kinetics, degradation pathway, DFT calculation and ecotoxicity assessment

Broflanilide, as a novel meta-diamide insecticide, presents high bioactivity against agricultural pests. However, there was limited report regarding the photolysis fate of broflanilide. In this study, the photodegradation kinetics and influence factors of broflanilide, including different solvents, pH, iron, S₂O₈^2- and SO₃^2- were investigated under UV condition, and the reaction mechanism and transformation pathway were explored. The reaction rates (k) showed solvent-specificity in ultrapure water (0.015 min^-1), ethyl acetate (0.051 min^-1), methanol (0.084 min^-1) and acetonitrile (0.193 min^-1), correspondingly. The photolysis of broflanilide was slowest in the acid condition (pH = 4.0) compared with that in the neutral (pH = 7.0) and alkaline (pH = 9.0) conditions. The iron (Fe²⁺ and Fe³⁺) presented significant inhibition on the photodegradation due to the light shielding effect. Additionally, the UV/peroxydisulfate (S₂O₈^2-) and UV/sulfite (SO₃^2-) technologies could effectively accelerate the photodegradation of broflanilide, which has the potential for rapid treatment of pesticides in the aqueous environment. Six transformation products (TPs) were detected in water, peroxydisulfate and sulfite solutions, and the possible transformation pathways, including dehalogenation, cyclization, N-dealkylation, oxidation, reduction and hydrolysis, were proposed. Importantly, the reaction mechanism was explained through the analysis of molecular electrostatic potential and molecular orbitals. The predicted toxicity of the TPs indicated that several highly toxic TPs need to pay more attention in future risk assessments. This study provides a new perspective for evaluating the ecological fate and risks of pesticides.

Tailoring the mechanistic pathways and kinetics of OH-addition reaction of sulfoxaflor and its ecotoxicity assessment

Sulfoxaflor is one of the widely used insecticides in agricultural lands to protect crops from insects. Due to its persistent nature, sulfoxaflor is identified as an environmental pollutant. In the present work, the mechanism and kinetics of sulfoxaflor degradation initiated by OH radical addition reaction are studied by using quantum chemical calculations. In the gas phase, the OH addition reaction at the C4 position of sulfoxaflor is found to be the favorable reaction pathway. The rate constant for the initial OH-addition reaction has been studied using canonical variational transition state theory (CVT) over the temperature range of 200-350 K. The initially formed sulfoxaflor-OH adduct intermediate transforms by reacting with O₂, H₂O, HO₂, and NO_x (x = 1-2) radicals. The excited-state calculation performed for the stationary points shows that the intermediates formed along the reaction pathway are easily photolyzed in normal sunlight. The toxicity assessment result shows that sulfoxaflor and few of its degradation products are harmful and toxic. The acidification potential of sulfoxaflor was found to be one, which shows its contribution to acid rain. This study gives an in-depth understanding of the mechanism, kinetics, and risk assessment of sulfoxaflor in the environment and aquatic system.

Insights into chalcone analogues with potential as antioxidant additives in diesel-biodiesel blends

Biodiesel production is one of the promising strategies to reduce diesel consumption and an important contribution to climate change. However, biodiesel stability remains a challenging problem in biofuel use in the global energy matrix. In this context, organic additives have been investigated to minimize these problems and reduce harmful emissions to comply with fuel requirement standards. In this study, we discuss a comprehensive structural description, a behavior of B15 [85% volume of diesel and 15% volume of biodiesel (B100)] stability in the presence of antioxidants (chalcone analogues), and a theoretical calculation to pave the way for clarifying and expanding the potential of title compounds as an antioxidant additive for diesel-biodiesel blends. Finally, a systematic description of the oxidation stability was undertaken using a specialized machine learning computational pySIRC platform.

"pySiRC": Machine Learning Combined with Molecular Fingerprints to Predict the Reaction Rate Constant of the Radical-Based Oxidation Processes of Aqueous Organic Contaminants

We developed a web application structured in a machine learning and molecular fingerprint algorithm for the automatic calculation of the reaction rate constant of the oxidative processes of organic pollutants by ^•OH and SO₄^•- radicals in the aqueous phase-the pySiRC platform. The model development followed the OECD principles: internal and external validation, applicability domain, and mechanistic interpretation. Three machine learning algorithms combined with molecular fingerprints were evaluated, and all the models resulted in high goodness-of-fit for the training set with R² > 0.931 for the ^•OH radical and R² > 0.916 for the SO₄^•- radical and good predictive capacity for the test set with R_ext² = Q_ext² values in the range of 0.639-0.823 and 0.767-0.824 for the ^•OH and SO₄^•- radicals. The model was interpreted using the SHAP (SHapley Additive exPlanations) method: the results showed that the model developed made the prediction based on a reasonable understanding of how electron-withdrawing and -donating groups interfere with the reactivity of the ^•OH and SO₄^•- radicals. We hope that our models and web interface can stimulate and expand the application and interpretation of kinetic research on contaminants in water treatment units based on advanced oxidative technologies.