Indirect photodegradation of fludioxonil by hydroxyl radical and singlet oxygen in aquatic environment: Mechanism, photoproducts formation and eco-toxicity assessment

Magnetic MoS2/Fe3O4 composite as an effective activator of persulfate for the degradation of tetracycline: performance, activation mechanisms and degradation pathways

The activated persulfate (PS) process could produce sulfate radical (SO4·-) and rapidly degrade organic pollutants. The application of Fe3O4 as a promising PS activator was limited due to the rapid conversion of Fe2+ to Fe3+ on its surface. Mo4+ on MoS2 surface could be used as a reducing site to convert Fe3+ to Fe2+, but the separation and recovery of MoS2 was complex. In this study, MoS2/Fe3O4 was prepared to accelerate the Fe3+/Fe2+ cycle on Fe3O4 surface and achieved efficient separation of MoS2. The results showed that MoS2/Fe3O4 was more effective for PS activation compared to Fe3O4 or MoS2, with a removal efficiency of 91.8% for 20 mg·L-1 tetracycline (TC) solution under the optimal conditions. Fe2+ and Mo4+ on MoS2/Fe3O4 surface acted as active sites for PS activation with the generation of SO4•-, •OH, •O2-, and 1O2. Mo4+ acted as an electron donor to promote the Fe3+/Fe2+ cycling and thus improved the PS activation capability of MoS2/Fe3O4. The degradation pathways of TC were inferred as hydroxylation, ketylation of dimethylamino group and C-N bond breaking. This study provided a promising activated persulfate-based advanced oxidation process for the efficient degradation of TC by employing MoS2/Fe3O4 as an effective activator.

A precise method to monitor hydroxyl radical in natural waters based on a fluoride-containing fluorescence probe

In natural waters, hydroxyl radical (OH) can initiate many free radical-induced reactions, oxidizing various inorganic and organic compounds through electron transfer reactions, dehydrogenation reactions, addition reactions, and self-quenching reactions. However, due to its extremely low concentration and short lifetime in natural waters, studies on the quantitative measurement of OH levels are insufficient. In this work, we developed the first quinolinium-based fluorescence probe containing fluoride substituted donor that could detect hydroxyl radicals in the water system. This probe exhibits excellent selectivity towards OH with a large Stokes shift (114 nm) and 23-fold enhancement in fluorescence. Additionally, this probe has been proven to be low toxicity and applied to detect OH in living cells, zebrafish, and natural water samples with good recovery (over 92 %).

New theoretical investigation of mechanism, kinetics, and toxicity in the degradation of dimetridazole and ornidazole by hydroxyl radicals in aqueous phase

Dimetridazole (DMZ) and ornidazole (ONZ) have been widely used to treat anaerobic and protozoal infections. The residues of DMZ/ONZ persist in the water environment. The mechanisms and kinetics of hydroxyl-initiated oxidation, the primary DMZ/ONZ degradation method, were evaluated by quantum chemical methods.·OH-induced degradation of DMZ and ONZ shared many mechanistic and kinetic characteristics. The most feasible degradation pathway involved forming OH-imidazole adducts and NO₂. The OH-imidazole adducts were subsequently degraded into double·OH imidazole intermediates. The rate coefficients for·OH degradation of DMZ and ONZ were 4.32 × 10⁹ M^-1 s^-1 and 4.42 × 10⁹ M^-1 s^-1 at 298 K, respectively. The lifetimes of DMZ and ONZ treated with·OH at concentrations of 10^-9-10^-18 mol L^-1 at 298 K were τ_DMZ = 0.231-2.31 × 10⁸ s and τ_ONZ = 0.226-2.26 × 10⁸ s, respectively. Toxicity assessment showed that the first degradation products of DMZ and ONZ exhibited enhanced aquatic toxicity, whereas most of the secondary degradation products were not harmful to aquatic organisms. Some of transformation products were still developmental toxicant or mutagenicity positive.

"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.

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

Pesticides are chemical compounds widely used to combat pests in crops, and they thus play a key role in agricultural production. However, due to their persistence in aquatic environments, even at low concentrations, their use has been considered an environmental problem and caused concern regarding the adverse effects on human health. This paper reports, for the first time, the mechanisms, kinetics, and an evaluation of the toxicity of picloram degradation initiated by OH radicals in the aqueous environment using quantum chemistry and computational toxicology calculations. The rate constants are calculated using a combination of formulations derived from the Transition State Theory in a realistic temperature range (250-310 K). The results indicate that the two favorable pathways (R1 and R5) of OH -based reactions occur by addition to the pyridine ring. The calculated rate constant at 298 K is compared with the overall second-order reaction rate constant, quantified herein experimentally via the competition kinetics method and data available in the literature showing an excellent agreement. The toxicity assessment and a photolysis study provide important information: i) picloram and the majority of degradation products are estimated as harmful; however, ii) these compounds can suffer photolysis in sunlight. The results of the present study can help understand the mechanism of picloram, also providing important clues regarding risk assessment in aquatic environments as well as novel experimental information.

Mechanisms, kinetics and eco-toxicity assessment of singlet oxygen, sulfate and hydroxyl radicals-initiated degradation of fenpiclonil in aquatic environments

Fenpiclonil is an agricultural phenylpyrrole fungicide, which raise the concern about its ecotoxicological effects. In this paper, we investigate the indirect photochemical transformation mechanisms, environmental persistence and eco-toxicity of fenpiclonil initiated by various active oxidants (¹O₂, •OH and SO₄•‾) in aquatic environments. The results shown that ¹O₂ can react with pyrrole ring by cycloaddition pathways to form the endo-peroxides. In addition, •OH and SO₄•‾ initial mechanisms are calculated, suggesting that •OH-initiated mechanisms play a dominant role in the degradation process of fenpiclonil at high rate constants (2.26 ×10⁹ M^-1 s^-1, at 298 K). The kinetic calculation results indicate that high temperature is more favorable for the degradation of fenpiclonil. To better understand the adverse effects of the transformation products formed during the subsequent reaction of •OH-adduct IM10, the computational toxicology has been used for the toxicity estimation. The results show that aquatic toxicity of these products decrease with degradation process, especially the decomposition products (TP3 and TP4). However, TP1 and TP2 are still toxic and developmental toxicant. The study provides guidance for further experimental research and industrial application of fungicide degradation from the perspective of theoretical calculation.

Characterization of the Field Fludioxonil Resistance and Its Molecular Basis in Botrytis cinerea from Shanghai Province in China

Botrytis cinerea is a destructive necrotrophic pathogen that can infect many plant species. The control of gray mold mainly relies on the application of fungicides, and the fungicide fludioxonil is widely used in China. However, the field fungicide resistance of B. cinerea to this compound is largely unknown. In this study, B. cinerea isolates were collected from different districts of Shanghai province in 2015–2017, and their sensitivity to fludioxonil was determined. A total of 65 out of 187 field isolates (34.76%) were found to be resistant to fludioxonil, with 36 (19.25%) showing high resistance and 29 (15.51%) showing moderate resistance. Most of these resistant isolates also showed resistance to iprodione, and some developed resistance to fungicides of other modes of action. AtrB gene expression, an indicator of MDR1 and MDR1h phenotypes, was not dramatically increased in the tested resistant isolates. Biological characteristics and osmotic sensitivity investigations showed that the fitness of resistant isolates was lower than that of sensitive ones. To investigate the molecular resistance mechanisms of B. cinerea to fludioxonil, the Bos1 amino acid sequences were compared between resistant and sensitive isolates. Resistant isolates revealed either no amino acid variations or the mutations I365S, I365N, Q369P/N373S, and N373S.

Effects of natural organic matter on the photolysis of tetracycline in aquatic environment: Kinetics and mechanism

The residues of tetracycline in environment have raised increasing concern for the deleterious impact on ecological and human health. Natural organic matter (NOM), ubiquitous in natural waters, is unavoidable to encounter tetracycline, which might affect the fate of tetracycline in aquatic environment. In this study, we investigated the effect of natural organic matter (NOM) on the photolytic fate of tetracycline (TC). The photolysis kinetics of TC were evaluated with two representative NOM, tannic acid (TA) and gallic acid (GA). The presence of TA and GA obviously inhibited the removal of TC under UV irradiation with photolysis rate constant at 0.067 h^-1 and 0.071 h^-1, respectively, which were 32.3% and 28.3% less than that without TA and GA (0.099 h^-1). Furthermore, NOM exhibited different impacts on both indirect photolysis and direct photolysis. NOM promoted the formation of hydroxyl radical, induced the generation of triplet-excited state NOM and thus greatly enhanced the indirect photolysis of TC. However, direct photolysis was almost completely inhibited by NOM via inner filter effect and interacting with TC to form ground-state complex with low photoreactive. Moreover, similar intermediates were detected in the presence and absence of NOM, indicating that NOM exhibited limited influence on the degradation pathways of TC. This study reveals the multiple roles of NOM on tetracycline photolysis, contributing to better understand the photolytic fate of antibiotics in natural waters.