Experimental and theoretical insights into the photochemical decomposition of environmentally persistent perfluorocarboxylic acids

Fe2+-NTA synergized UV254 photolytic defluorination of perfluorooctane sulfonate (PFOS): Enhancing through intramolecular electron density perturbation via electron acquisition

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant posing a risk in environmental persistence, bioaccumulation and biotoxicity. This study was to reach a comprehensive and deeper understanding of PFOS elimination in a UV254 photolytic treatment with the co-presence of Fe2+ and nitrilotriacetic acid trisodium salt (NTA). PFOS defluorination was noticeably enhanced in the UV/Fe2+-NTA treatment compared with UV/NTA, UV/Fe2+ and our previously studied UV/Fe3+ treatments. UV-vis, FTIR, and UPLC/MS-MS results indicated the formation of PFOS-Fe2+-NTA complex in PFOS, Fe2+ and NTA mixture. The transition energy gap of PFOS-Fe2+-NTA decreased below the excitation energy supplied by UV254 irradiation, corresponding with red shift appearing in UV-vis scanning spectrum. This favored intramolecular electron transfer from Fe2+-NTA to PFOS under UV254 irradiation to form electron-accepting PFOS. Molecular electrostatic potential and atom charge distribution analyses suggested electron density rearrangement and perturbation in the perfluorinated carbon chain of electron-accepting PFOS, leading to the decrease in bond dissociation energies. Intermediate products detection suggested the parallel defluorination pathways of PFOS desulfonation, middle carbon chain scission and direct C-F cleavage. NTA exhibited crucial functions in the UV/Fe2+-NTA treatment by holding Fe2+/Fe3+ in soluble form as a chelant and favoring water activation to generate hydrated electrons (eaq-) under UV irradiation as a photosensitizer. Fe2+ acting as the conduit for electron transfer and the bridge of PFOS anion and NTA was thought functioning best at 200 µM in this study. The degree of UV/Fe2+-NTA -synergized PFOS defluorination also depended on eaq- yield and UV254 photon flux. The structure dependence on the electron transfer process of PFOS and PFOA was explored incorporating molecular structure descriptors. Because of possessing greater potential to acquire electrons or less likeliness to donate its electrons than PFOA, PFOS exhibited faster defluorination kinetics in the published "reduction treatments" than "oxidation" ones. Whereas, PFOA defluorination kinetics were at similar level in both "reduction" and "oxidation" treatments.

Occurrence, behaviors, and fate of per- and polyfluoroalkyl substances (PFASs) in typical municipal solid waste disposal sites

Per- and polyfluoroalkyl substances (PFASs) have become a crucial environmental concern owing to their exceptional persistence, ability to bioaccumulate within ecosystems, and potential to adversely affect biota. Products and materials containing PFASs are usually discarded into municipal solid waste (MSW) at the end of their life cycle, and the fate of PFASs may differ when different disposal methods of MSWs are employed. To date, limited research has focus on the occurrence, behaviors, and fate of PFASs emitted from various MSW disposal sites. This knowledge gap may lead to an underestimation of the contribution of MSW disposal sites as a source of PFASs in the environment. In this review, we collated publications concerning PFASs from typical MSW disposal sites (i.e., landfills, incineration plants, and composting facilities) and explored the occurrence patterns and behaviors of PFASs across various media (e.g., landfill leachate/ambient air, incineration plant leachate/ash, and compost products) in these typical MSW disposal sites. In particular, this review highlighted ultrashort-chain perfluoroalkyl acids and "unknown"/emerging PFASs. Additionally, it meticulously elucidated the use of non-specific techniques and non-target analysis for screening and identifying these overlooked PFASs. Furthermore, the composition profiles, mass loads, and ecological risks of PFASs were compared across the three typical disposal methods. To the best of our knowledge, this is the first review regarding the occurrence, behaviors, and fate of PFASs in typical MSW disposal sites on a global scale, which can help shed light on the potential environmental impacts of PFASs harbored in MSWs and guide future waste management practices.

Multiscale computational simulation of pollutant behavior at water interfaces

The investigation of pollutant behavior at water interfaces is critical to understand pollution in aquatic systems. Computational methods allow us to overcome the limitations of experimental analysis, delivering valuable insights into the chemical mechanisms and structural characteristics of pollutant behavior at interfaces across a range of scales, from microscopic to mesoscopic. Quantum mechanics, all-atom molecular dynamics simulations, coarse-grained molecular dynamics simulations, and dissipative particle dynamics simulations represent diverse molecular interaction calculation methods that can effectively model pollutant behavior at environmental interfaces from atomic to mesoscopic scales. These methods provide a rich variety of information on pollutant interactions with water surfaces. This review synthesizes the advancements in applying typical computational methods to the formation, adsorption, binding, and catalytic conversion of pollutants at water interfaces. By drawing on recent advancements, we critically examine the current challenges and offer our perspective on future directions. This review seeks to advance our understanding of computational techniques for elucidating pollutant behavior at water interfaces, a critical aspect of water research.

Thermo-activated periodate oxidation process for tetracycline degradation: Kinetics and byproducts transformation pathways

Periodate-based advanced oxidation processes have been diffusely practiced for pollutant decontamination. However, the thermo-activation of periodate process (heat/PI), an effective water pollution removal process, has been rarely discussed, and the degradation pathway of this heat/PI system requires investigation. In this work, tetracycline antibiotics were selected as the model micropollutant for the comprehensive evaluation of the heat/PI system. The heat/PI system exhibited good performance for tetracycline (TC) remediation with temperature increases. The principal reactive oxidative species in the heat/PI system was confirmed using quenching experiments and electron paramagnetic resonance experiments. Further, the potential reactive sites in the TC were identified based on the density functional theory calculation. Based on the detection results of intermediates, there was no significant difference in byproducts generated during TC degradation under various temperatures in the heat/PI system. The Toxicity Estimation Software Tool (T.E.S.T.) method was applied to calculate the individual toxicity of the byproducts. This study contributes to a comprehensive explanation of the process of the thermal activation of periodate, and in particular, it explains the source of oxidation power, the transformation of byproducts, and the toxicity of reaction systems.

First insights into 6PPD-quinone formation from 6PPD photodegradation in water environment

p-Phenylenediamines (PPDs), an important type of rubber antioxidants, have received little study on their environmental fate, particularly for their vital photodegradation process in water environment. Accordingly, N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine (6PPD), as a representative of PPDs, was investigated experimentally and theoretically for its photodegradation in water. Rapid photodegradation occurred when 6PPD was exposed to illumination especially UV region irradiation. Under acidic conditions, the photodegradation of 6PPD accelerated mainly due to the increased absorption of long wavelength irradiation by ionized 6PPD. Nine photodegradation products (e.g., 6PPD-quinone (6PPDQ)) of 6PPD were identified by an ultra-performance liquid chromatography QTOF mass spectrometry. Molar yields of photoproducts such as 6PPDQ, aniline, 4-aminodiphenylamine, and 4-hydroxydiphenylamine were 0.03 ± 0.00, 0.10 ± 0.01, 0.03 ± 0.02, and 0.08 ± 0.01, respectively. Mechanisms involved in 6PPD photodegradation include photoexcitation, direct photolysis, self-sensitized photodegradation, and 1O2 oxidation, as demonstrated by electron paramagnetic resonance (EPR) analysis, scavenging experiments, and the time-dependent density functional theory (TD-DFT). Notably, the toxicity of the reaction solution formed during the photodegradation of 6PPD was increased by the formation of highly toxic products (e.g., 6PPDQ). This study provides the first explanation for photodegradation mechanisms of 6PPD and confirms the pathway of 6PPDQ produced by the photoreaction in water environment.

Computational Study of Photodegradation Process and Conversion Products of the Antidepressant Citalopram in Water

Citalopram (CIT) is a commonly prescribed medication for depression. However, the photodegradation mechanism of CIT has not yet been fully analyzed. Therefore, the photodegradation process of CIT in water is studied by density functional theory and time-dependent density functional theory. The calculated results show that during the indirect photodegradation process, the indirect photodegradation of CIT with ·OH occurs via OH-addition and F-substitution. The minimum activation energy of C10 site was 0.4 kcal/mol. All OH-addition and F-substitution reactions are exothermic. The reaction of ¹O₂ with CIT includes the substitution of ¹O₂ for F and an addition reaction at the C14 site. The E_a value of this process is 1.7 kcal/mol, which is the lowest activation energy required for the reaction of ¹O₂ with CIT. C-C/C-N/C-F cleavage is involved in the direct photodegradation process. In the direct photodegradation of CIT, the activation energy of the C7-C16 cleavage reaction was the lowest, which was 12.5 kcal/mol. Analysis of the E_a values found that OH-addition and F-substitution, the substitution of ¹O₂ for F and addition at the C14 site, as well as the cleavage reactions of C6-F/C7-C16/C17-C18/C18-N/C19-N/C20-N are the main pathways of photodegradation of CIT.

The critical impacts of pyrochar during 2,4,6-trichlorophenol photochemical remediation process: Cooperation between persistent free radicals and oxygenated functional groups

The widespread use of polychlorophenols poses enormous environmental challenges. Biochar has the potential to accelerate the transformation of polychlorophenols. But the biochar-triggered photochemical decomposition mechanism of polychlorophenols still remains unclear. Herein, the photochemical behavior of pyrochar was comprehensively investigated in 2,4,6-trichlorophenol (TCP) remediation. Researches revealed that persistent free radicals (PFRs) and oxygenated functional groups (OFGs) on the surface of pyrochar cooperatively promoted ROS generation for TCP degradation. PFRs performed a key role of electron-donating and energy transfer in ROS conversion, especially in the activation of H₂O₂ into •OH. The hydroxyl groups of photosensitive components of pyrochar were photo-excited and provided electrons for enhanced ROS formation as well. With photogenerated ROS involved, more TCP was decomposed through dechlorination under light irradiation than that in the dark, in which ¹O₂, •OH, and •O₂^- were the dominant active species. During this process, stronger light intensities (3 W/m²) and shorter light wavelengths (400 nm) can provide more energy for the activation of PFRs and OFGs, promoting the decomposition of TCP. This work casts a new light on the environmental roles of pyrochar in the photochemical removal of polychlorophenol pollutants.

Mechanistic insight into the degradation of 1H-benzotriazole and 4-methyl-1H-benzotriazole by •OH-based advanced oxidation process and toxicity assessment

Benzotriazoles (BTs) are highly produced chemicals that are commonly used in the manufacture of aircraft de-icing/antifreeze fluids (ADAFs), coolants, etc. BTs have been detected in a variety of water environments, causing health hazards to aquatic species and humans. In this study, 1H-benzotriazole (BTri) and 4-methyl-1H-benzotriazole (4-TTri) were selected to investigate their degradation mechanisms in the aqueous phase initiated by ·OH using a theoretical calculation method. Addition reactions are the main type of reactions of ·OH with BTri and 4-TTri. The total rate constants for the reactions of BTri and 4-TTri with ·OH at 298 K are 8.26 × 10⁹ M^-1 s^-1 and 1.81 × 10¹⁰ M^-1 s^-1, respectively. The reaction rate constants increase as the temperature rises, indicating that rising temperatures promote the degradation of BTri and 4-TTri. 7-hydroxy-1H-benzotriazole (1-P1) and 4-hydroxy-benzotriazoles (1-P2) produced via multiple reaction pathways are important transformation products of BTri. After successive reactions with ·OH, 1-P1 and 1-P2 can be successively converted to 4,7-dihydroxy-1H-benzotriazole (1-P7), 4,7-dione-1H-benzotriazole (1-P8), and 1,2,3-triazole-4,5-dicarboxylic acid (1-P9), which is consistent with the product compositions detected in the experiments. The toxicity assessment indicated that the acute toxicity and chronic toxicity of the resulting transformation products are significantly reduced compared to BTri as the degradation process progressed, and ultimately showed no harm to all three aquatic organisms (fish, daphnia, and green algae). Hence, advanced oxidation processes (AOPs) can not only effectively remove BTs from water, but also reduce their toxic effects on aquatic organisms.

Photodegradation of hydroxyfluorenes in ice and water: A comparison of kinetics, effects of water constituents, and phototransformation by-products

The photochemical behavior of organic pollutants in ice is poorly studied in comparison to aqueous photochemistry. Here we report a detailed comparison of ice and aqueous photodegradation of two representative OH-PAHs, 2-hydroxyfluorene (2-OHFL) and 9-hydroxyfluorene (9-OHFL), which are newly recognized contaminants in the wider environment including colder regions. Interestingly, their photodegradation kinetics were clearly influenced by whether they reside in ice or water. Under the same simulated solar irradiation (λ > 290 nm), OHFLs photodegraded faster in ice than in equivalent aqueous solutions and this was attributed to the specific concentration effect caused by freezing. Furthermore, the presence of dissolved constituents in ice also influenced photodegradation with 2-OHFL phototransforming the fastest in 'seawater' ice (k = (11.4 ± 1.0) × 10^-2 min^-1) followed by 'pure-water' ice ((8.7 ± 0.4) × 10^-2 min^-1) and 'freshwater' ice ((8.0 ± 0.7) × 10^-2 min^-1). The presence of dissolved constituents (specifically Cl^-, NO₃^-, Fe(III) and humic acid) influences the phototransformation kinetics, either enhancing or suppressing phototransformation, but this is based on the quantity of the constituent present in the matrixes, the specific OHFL isomer and the matrix type (e.g., ice or aqueous solution). Careful derivation of key photointermediates was undertaken in both ice and water samples using tandem mass spectrometry. Ice phototransformation exhibited fewer by-products and 'simpler' pathways giving rise to a range of hydroxylated fluorenes and hydroxylated fluorenones in ice. These results are of importance when considering the fate of PAHs and OH-PAHs in cold regions and their persistence in sunlit ice.

OH-initiated degradation of 1,2,3-trimethylbenzene in the atmosphere and wastewater: Mechanisms, kinetics, and ecotoxicity

1,2,3-Trimethylbenzene (1,2,3-TMB) is an important volatile organic compound (VOC) present in petroleum wastewater and the atmosphere. This compound can be degraded by OH radicals via abstraction, addition and substitution mechanisms. Results show that the addition mechanism is dominant and H-abstraction is subdominant, while methyl abstraction and substitution mechanisms are negligible in the gas and aqueous phases. Moreover, H-abstraction products undergo further reactions with O₂, NO, NO₂, H₂O, and OH radicals in the atmosphere. Time-dependent density functional theory (TDDFT) calculations show that the degraded products, including 2,3,4-trimethylphenyl-nitroperoxoite, 1,2,3-trimethyl-4-nitrobenzene, 1,2,3-trimethyl-5-nitrobenzene, 2,6-dimethylbenzyl nitroperoxoite, 2,3-dimethylphenyl nitroperoxoite, 2,6-dimethylbenzaldehyde, and 2,3-dimethylbenzaldehyde, can photolyze under the sunlight. Kinetically, the calculated total rate constant is 5.57 × 10^-11 cm³ molecule^-1·s^-1 at 1 atm and 298 K, which is consistent with available experimental values measured in the atmosphere. In addition, the calculated total reaction rate constant in water is close to that in the gas phase. In terms of ecotoxicity, all degradation products are less toxic than the initial reactant to fish, green algae and daphnia. For mammals represented by rats, 1,2,3-TMB and its products are moderately toxic, except for 2,3-dimethylphenol and 2,6-dimethylphenol, which are slightly toxic.

Thermal decomposition mechanism and kinetics of perfluorooctanoic acid (PFOA) and other perfluorinated carboxylic acids: a theoretical study

Perfluorinated carboxylic acids (PFCAs), particularly perfluorooctanoic acid (PFOA), are broadly used for chemical synthesis and as surfactants, but they pose a serious threat to humans and wildlife because of toxicity concerns, environmental stability, and tendency to bioaccumulate. PFCA waste is commercially treated in incinerators, however, their exact degradation mechanisms are still unknown. In the present work, we report the decomposition mechanism and kinetics of straight-chain PFCAs using quantum chemistry and reaction rate theory calculations. Degradation mechanisms and associated kinetic parameters are determined for the complete series of straight-chain PFCAs from perfluorononanoic acid (C₈F₁₇COOH, C9) to fluoroformic acid (FCOOH, C1). Our results show that PFCA decomposition follows an analogous mechanism to perfluorinated sulfonic acids, where HF elimination from the acid head group produces a three membered ring intermediate, in this case a perfluorinated α-lactone. These perfluorinated α-lactones are short-lived intermediates that readily degrade into perfluorinated acyl fluorides and CO, thus shortening the perfluorinated chain by one C atom. Because perfluorinated acyl fluorides are known to hydrolyse to PFCAs, repeated cycles of carboxylic acid decomposition followed by acyl fluoride hydrolysis provides a mechanism for the complete mineralization of PFCAs to HF, CO, CO₂, COF₂, and CF₂ during thermal decomposition in the presence of water vapor. These results provide a theoretical basis for future detailed chemical kinetic studies of incineration reactors and will assist in their design and optimisation so as to more efficiently decompose PFCAs and related waste.

Investigations on mechanisms, kinetics, and ecotoxicity in OH-initiated degradation of 1,2,4,5-tetramethylbenzene in the environment

As one of the volatile organic compounds (VOCs) in the environment, 1,2,4,5-tetramethylbenzene (1,2,4,5-TeMB) present in oily wastewater, and it can occur substitution, abstraction, and addition reactions with OH radicals in the atmosphere and wastewater. Electrostatic potential (ESP) and average local ionization energy (ALIE) prediction indicate that H atoms from CH₃ group and the benzene ring are the most active sites in 1,2,4,5-TeMB. The result shows that potential energy surfaces (PESs) in the gas and aqueous phase are similar, and the relevant barriers in the latter one are higher. The dominant channel is H abstraction from the benzene ring, and the subdominant one is OH radical addition to the benzene ring. Furthermore, subsequent reactions of dominant products with O₂, NO₂, NO, and OH radicals in the atmosphere are studied, as well. The total reaction rate constant is calculated to be 2.36×10^-10 cm³ molecule^-1 s^-1 at 1 atm and 298 K in the atmosphere, which agrees well with the experimental data. While the total rate constant in the aqueous phase is much lower than that in the gas phase. Ecologic toxicity analysis shows that 1,2,4,5-TeMB is very toxic to fish, daphnia, and green algae; and OH-initiated degradation in the environment will reduce its toxicity.

Ultraviolet oxidative degradation of typical antidepressants: Pathway, product toxicity, and DFT theoretical calculation

The ubiquity of antidepressants in the environment has posed a potential threat to eco-systematic safety. In this study, six kinds of antidepressants including fluoxetine (FLU), paroxetine (PAR), sertraline (SER), fluvoxamine (FLX), citalopram (CTP), and venlafaxine (VEN) were selected to explore their degrading kinetics, transformation pathways, and the acute toxicity of the reaction solution during UV oxidation. The results showed that the order of the photodegradation rate was FLU > PAR > SER > CTP > FLX > VEN. The calculation results of density functional theory (DFT) and molecular orbital theory showed that it was positively correlated with the frontier electron density of drugs and negatively correlated with the HOMO-LUMO gap, respectively. Intermediates were identified with UHPLC-Q-TOF/MS/MS to propose the possible degradation pathways of the drugs and the most likely directions of the reactions were determined by the single point energy calculation. The results of toxicity tests indicated that the acute toxicity of the reaction solution of PAR did not change significantly. The photolysates toxicity of FLU, SER, and FLX decreased at the end of the reaction, while that of CTP and VEN was increased by 1.5 and 1.3 times compared with the parent compound, respectively. Toxicity predictions by the quantitative structure activity relationship (QSAR) model showed that except FLU-162, FLX-174, and VEN-230, other degradation products have developmental toxicity. The results revealed the transformation pathways of these drugs under the UV disinfection process in wastewater treatment plants, especially the formation of toxic by-products during the disinfection process.

Electrochemical oxidation combined with UV irradiation for synergistic removal of perfluorooctane sulfonate (PFOS) in water

The effect of electrochemical degradation on Magnéli phase Ti₄O₇ anode combined with UV irradiation on the removal of PFOS was systematically evaluated in the present study. A synergistic effect of electrolysis and UV irradiation rather than a simple additive effect for PFOS degradation was demonstrated experimentally and theoretically. The short wavelength irradiation within 400 nm is the main contribution to enhance the electrochemical degradation of PFOS, while the initial pH of the solution has little effect on the PFOS degradation. The increase of current density accelerates the removal of PFOS either by electrolysis treatment or the joint process. The time-dependent density functional theory (TD-DFT) calculation indicates that the synergistic effect of the electrolysis and UV irradiation is most likely due to the involvement of the excited PFOS induced under UV irradiation in the electrochemical reaction. This study provides the first mechanistic explanation for the electrochemical degradation of PFOS enhanced by UV irradiation.

Theoretical evidence for the formation of perfluorocarboxylic acids form atmospheric oxidation degradation of fluorotelomer acrylates

The atmospheric oxidation degradation of fluorotelomer acrylates (FTAcs) has been proposed as a potential source of perfluorocarboxylic acids (PFCAs) in remote locations. In this paper, detailed reactions of the main oxidant OH radicals with 4:2 FTAc in the atmosphere have been investigated by using density functional theory (DFT) calculation. All possible pathways involved in the oxidation process were presented and discussed. Based on the mechanism, transition state theory (TST) was used to predict the rate constants of the key elementary steps including the initial reactions of OH radical with n:2 FTAcs and the subsequent reactions of the main intermediates. Studies show that the reaction processes of OH radical addition to C = C bond are dominant and the fluorotelomer glyoxylate and formaldehyde are the major products. At 296 K, the calculated overall rate constant of 4:2 FTAc with OH radical is 1.19 × 10^-11 cm³ molecule^-1 s^-1 with an atmospheric lifetime of 23.3 h. In the atmosphere, fluorotelomer glyoxylate will continue to be oxidized, which will lead to the formation of PFCAs ultimately. In addition, atmospheric reactions of more carbons FTAc (C_nF_2n+1CH₂CH₂OC(O)CH = CH₂, n = 6, 8, 10) are also discussed in the presence of O₂/NO_x.

The transformation of Benzophenone-3 in natural waters and AOPs: The roles of reactive oxygen species and potential environmental risks of products

Benzophenone-3 (BP-3) is a widespread emerging organic pollutant. However, little is known about the synergistic effect of various reactive oxygen species (ROS) in natural waters and wastewater treatment plants on its transformation. In this study, the indirect photochemical behavior of BP-3 in the natural aquatic environments and the degradation process in the AOPs system were investigated by theoretical chemistry calculations. Besides the potential eco-toxicity effects, health effects, and bioaccumulation of the transformation products were assessed by computational toxicology. Results of transformation mechanism and kinetics showed that OH· and ¹O₂ are the keys to the transformation of BP-3, whereas the role of HO₂· and O₃ can be ignored. AOPs based on OH· and ¹O₂ could lead to the rapid transformation of BP-3, while the transformation of BP-3 in natural waters is slow, and even environmental persistence can be observed. However, dissolved organic matter (DOM) promotes the indirect phototransformation of BP-3 in natural waters. A variety of transformation products are generated under the synergistic effects of ROS, H₂O, and ³O₂. Assessments of environmental risks indicated that the potential eco-toxicity and health effects of the main products are significantly lower than that of the parent BP-3. More importantly, low bioaccumulation of transformation products would not enlarge their eco-toxicity and health effects. This study not only gives valuable insights into the indirect phototransformation of BP-3 in natural waters but also provides theoretical support for the feasibility of BP-3 degradation in industrial wastewater by AOPs based on OH· and ¹O₂.

Theoretical study of the reaction mechanism between Criegee intermediates and hydroxyl radicals in the presence of ammonia and amine

Criegee intermediates (CIs), formed in the ozonolysis process of unsaturated hydrocarbons, play an important role in the formation of OH radicals, sulfuric acid, and aerosols. In this study, quantum chemical calculations were carried out to investigate the mechanism for the reaction of Criegee intermediates [involving CH₂OO, CH₃CHOO and (CH₃)₂COO] with OH radicals at the level of CCSD(T)/jun-cc-pVTZ//M06-2X/6-311 + G(2d,2p). A third component, such as water, ammonia, or amines, was introduced to the reaction of CIs with OH to evaluate their catalytic effect. The results show that the OH addition is the favorable channel among four channels involving cis-H abstraction, trans-H abstraction and O abstraction. The third component has a positively catalytic effect on the trans-H abstraction and O abstraction pathways. Moreover, for the trans-H abstraction of CH₃CHOO and (CH₃)₂COO with OH, ammonia and amine exhibit more effectively catalytic ability than water. Furthermore, Born-Oppenheimer molecular dynamic simulation results show that the addition of third component to CIs and hydrogen abstraction from the third component by OH occur simultaneously.

Insight into degradation mechanism of PCBs from thermal desorption off-gas over iron-based catalysts

Iron-based catalysts were developed to achieve the hydrodechlorination (HDC)/oxidation of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, and Fe₃O₄/γ-Al₂O₃ showed higher dechlorination efficiency than Fe₂O₃/γ-Al₂O₃. The optimal Fe loading resulted in 95.5% degradation efficiency and 76.9% toxicity reduction of gaseous PCBs, and the optimal Fe₃O₄/γ-Al₂O₃ exhibited excellent stability during a 60-h test. The gas chromatography-mass spectrometry analysis of intermediate products indicated the presence of two competitive degradation pathways, namely, hydrodechlorination and oxidation with Fe₃O₄/γ-Al₂O₃ as catalyst. During the first stage (reductive dechlorination), the reductive activity of iron-based catalysts was effectively enhanced in the presence of water, which was confirmed by density functional theory (DFT) calculations. The removal of chlorine atoms was found in the order of meta > para > ortho. During the second stage (oxidation), hydroxyl and superoxide anion radicals were found to attack PCBs on the surface of Fe₃O₄/γ-Al₂O₃. This study provides an insight into the HDC and oxidation mechanism of gaseous PCBs over iron-based catalysts.

Dependency of the photocatalytic and photochemical decomposition of per- and polyfluoroalkyl substances (PFAS) on their chain lengths, functional groups, and structural properties

This study reports the removal of per- and polyfluoroalkyl substances (PFAS) in water using various photocatalytic and photochemical processes. PFAS were chosen, based on chain lengths, functional groups, and structural properties: four perfluorocarboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), three perfluorosulfonic acids (PFSAs), including perfluorooctanesulfonic acid (PFOS), hexafluoropropylene oxide dimer (GenX), and 6:2 fluorotelomer sulfonate (6:2 FTS), and dependency of the photocatalytic decomposition of PFAS on their properties was investigated. Oxidants and reductants were introduced to study the photochemical decomposition of PFAS, and reactive species and reaction byproducts were identified to elucidate the decomposition mechanism of PFAS. Some notable findings include: long chain PFCAs (95% in 48 h) and 6:2 FTS (100%) were removed via chemical decomposition in TiO₂/UVC while GenX (37%), long chain PFSAs (60%), short chain PFSAs (0-10%) and short chain PFCAs (5-18%) were removed via physical adsorption. Sulfate radicals generated with persulfate (PS) played an important role in decomposing PFCAs (60-90%). Sulfite activated by UVC worked for defluorination of PFOA (75%) and PFOS (80%). PFOA was removed faster by UVC/sulfite > UVC/TiO₂/sulfite ≈ UVC/TiO₂/PS ≥ UVC/PS > UVC/TiO₂ while PFOS was removed faster by UVC/sulfite ≫ UVC/TiO₂/sulfite ≈ UVC/TiO₂/PS ≈ UVC/TiO₂ ≫ UVC/PS. Susceptibility of PFAS to the chemical reactions could be explained by their properties and the reactive species produced in each system.

Enhanced abatement of pharmaceuticals by permanganate via the addition of Co3O4 nanoparticles

Pharmaceuticals may pose serious potential risks, such as biological responses and chronic health effects, due to their ubiquitous in natural aquatic water bodies. In this study, we proposed an effective, feasible, and low-cost strategy for the abatement of pharmaceuticals (i.e., phenylbutazone (PBZ) and sulfinpyrazone (SPZ)) via Co₃O₄ nanoparticles (NPs) as heterogeneous catalyst in permanganate (Mn(VII)) oxidation for the first time. The performance of the Co₃O₄ NPs in permanganate oxidation is highly dependent on pH and its dosage. Co₃O₄ NPs play as electron shuttles in the catalytic permanganate oxidation process involving one-electron transfer with the oxidation of ≡Co^II to ≡Co^III by permanganate and the formation of colloidal manganese dioxide (MnO₂), as well as the reduction of the newly formed ≡Co^III to ≡Co^II by organics and the production of oxidized organic byproducts. The degradation pathways of PBZ and SPZ in catalytic permanganate oxidation were proposed based on the liquid chromatography-tandem mass spectrometry (LC-MS/MS) results and Gaussian calculation, and the toxicity decay of pharmaceuticals during oxidation was observed. Considering the stability, reusability, and cost, Co₃O₄ coupled with Mn(VII) is suitable for water pretreatment and is potentially feasible for industrial application, which is not only effective for decomposing PBZ and SPZ, but also for eliminating their toxicity.

Gas-phase and aqueous-surface reaction mechanism of Criegee radicals with serine and nucleation of products: A theoretical study

Criegee intermediates (CIs) are short-lived carbonyl oxides, which can affect the budget of OH radicals, ozone, ammonia, organic/inorganic acids in the troposphere. This study investigated the reaction of CIs with serine (Ser) in the gas phase by using density functional theory (DFT) calculations and at the gas-liquid interface by using Born-Oppenheimer molecular dynamics (BOMD). The results reveal that the reactivity of the three functional groups of Ser can be ordered as follows: COOH > NH₂ > OH. Water-mediated reactions of CIs with NH₂ and OH groups of Ser on the droplet follow the proton exchange mechanism. The products, sulfuric acids, ammonia, and water molecules form stable clusters within 20 ns. This study shows that hydroperoxide products can contribute to new particle formation (NPF). The result deepens the understanding of the reaction of CIs with multifunctional pollutants and atmospheric behavior of CIs in polluted areas.

Mechanisms and kinetics studies of the atmospheric oxidation of eugenol by hydroxyl radicals and ozone molecules

Eugenol is a representative methoxyphenol derived from the pyrolysis of lignin containing a branched alkene group. Its concentration in the atmosphere is equivalent to guaiacol and syringol. In this present paper, the gas phase reaction mechanisms and kinetic parameters of eugenol with hydroxyl radicals (OH) and ozone molecules (O₃) were calculated at the M06-2×/6-311+G(3df,2p)//M06-2×/6-311+G(d,p) level. There are two distinct reaction types between eugenol and OH. In particular, Path2 is most favorable in the OH additions, whereas IM16 is most advantageous in H atom abstraction pathways. OH additions have more advantages than H abstraction reactions. Thus, the comprehensive and detailed reaction schemes for the further reactions of IM2 were presented. The main products generated by IM2 are methyl (Z)-3-(2-formylpenta-1,4-dien-1-yl)-2-hydroxyoxirane-2-carboxylate (P2B-4), 2-methoxy-2-oxoacetic acid (P2B-10), 2-allylmalealdehyde (P2B-11) and other carbonyl or carboxyl compounds. As for the reaction of eugenol with O₃, the cycloaddition reactions and subsequent oxidative degradation processes were also explored, which yielded the most dominant product 2-(4-hydroxy-3-methoxyphenyl) acetaldehyde (P8-1). The reaction constants of the primary reactions for eugenol with OH and O₃ under the temperature range of 225- 375 K were successively calculated by POLYRATE and MESMER program. At 298 K and 1 atm, the respective rate coefficients are 5.91 × 10^-11 and 5.48 × 10^-16 cm³ molecule^-1 s^-1 and the corresponding atmospheric lifetimes are 4.70 h and 0.72 h. The short lifetimes suggest that once eugenol enters the atmosphere, it is likely to be rapidly degraded. This work aims to provide theoretical guidance for the photochemical reaction mechanisms of eugenol with OH and O₃, and present a reference for more experimental researches.

Role of hydrogenated moiety in redox treatability of 6:2 fluorotelomer sulfonic acid in chrome mist suppressant solution

6:2 Fluorotelomer sulfonic acid (6:2 FTS) is used as alternative to perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) for different purposes such as chrome mist suppressant (CMS) and active ingredient in fire-fighting foams. In this study, degradability of 6:2 FTS under ultraviolet/persulfate (UV/PS) and ultraviolet/sulfite (UV/SF), which are typical technologies for advanced oxidation and reduction, were investigated respectively. Due to the hydrogenated moiety, 6:2 FTS was decomposed completely by UV/PS within 10 min, forming a mixture of short-chain perfluoroalkyl carboxylic acids with variable chain length (2-7 carbon atoms). Such oxidation products account for > 50% organofluorine of 6:2 FTS unmineralized portion. 6:2 FTS degradability under reductive UV/SF system was dramatically slowed down by the hydrogenated moiety, which lowered electron affinity and, consequently, reactivity with aqueous electron (e_aq‾) produced by UV/SF. Fluorine mass balance showed that degradation intermediates were almost negligible: most of decomposed 6:2 FTS fluorine was converted to fluoride. A real 6:2 FTS-based CMS solution prepared from a commercial product was also tested. Both types of treatment were effective and in good agreement with the trends observed for tests with sole 6:2 FTS. Moreover, experimental results highlighted a remarkable amount of identifiable (like 4:2 FTS, 8:2 FTS and other per-/polyfluoroalkyl substances) and unidentifiable components in the CMS mixture. Indeed, fluoride concentration under UV/SF (73.8 mg/L) and UV/PS (44.9 mg/L) treatment were both higher than the estimated total concentration (<23 mg/L, according to 6:2 FTS concentration). Results strongly suggest that an oxidation pretreatment followed by reduction might be a better way to degrade and defluorinate 6:2 FTS and other precursors with non-fluorinated moieties, rather than employing single reduction or oxidation technology.

Application of a 2D-QSAR with a sine normalization method for the biodegradation of fluoroquinolones to poison cyanobacteria

Cyanobacteria are photosynthetic autotrophic aquatic prokaryotes. One of the methods for controlling cyanobacterial blooms is to destroy the phycobiliproteins required for photosynthesis. In this study, to improve the biodegradation of the fluoroquinolones through inhibit cyanobacteria, the molecular docking scores of 32 fluoroquinolones (FQs) with four categories of phycobiliproteins from cyanobacteria were calculated after sine normalization to characterize the binding ability between them. A two-dimensional quantitative structure-activity relationship (2D-QSAR) model was constructed based on the comprehensive scores. Danofloxacin (DAN) with the highest comprehensive score was chosen for molecular modification. When docking with four categories of phycobiliproteins from cyanobacteria, the docking values of DAN-11 and DAN-16 were increased up to 35.75%. Moreover, their functional characteristics and environmentally friendly predictive values were improved. When the DAN-11 and DAN-16 molecules docked with the other cyanobacterial phycobiliproteins, indicating that the designed DAN derivatives had general applicability to poison cyanobacteria, the weak interaction forces might increase the binding ability between the DAN derivatives and the receptor phycobiliprotein compared with the target molecule.

Enhanced decomposition of long-chain perfluorocarboxylic acids (C9-C10) by electrochemical activation of peroxymonosulfate in aqueous solution

The decomposition of long-chain perfluorocarboxylic acids (PFCAs), including perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA), were investigated by electrochemical activation of peroxymonosulfate (PMS) on porous Ti/SnO₂-Sb membrane anode. The results indicated that PMS activation could efficiently promote PFNA/PFDA decomposition, with pseudo-first-order rate constants about 3.12/2.06 times as compared with that of direct electro-oxidations. The energy consumptions of PFNA and PFDA decomposition were 36.31 and 37.46 kWh·m^-3·order^-1, respectively. The quantitative detection results of •OH with electron paramagnetic resonance (EPR) demonstrated that PMS activation promoted •OH formation. The inhibited performance in radical scavengers indicated both •OH and SO₄^•- might be mainly involved in PFNA decomposition, while SO₄^•- might be mainly involved in PFDA decomposition during PMS activation process. The mineralization mechanism for long-chain PFCAs decomposition which was mainly by repeating CF₂-unzipping cycle via radical reaction based on the intermediates verification and mass balance of C and F, was proposed. These results suggested that electrochemical activation of PMS on porous Ti/SnO₂-Sb membrane anode exhibited high efficiency in mineralizing PFNA and PFDA under mild conditions. This work might provide an efficient way for persistent organic pollutants, including, but not limited to long-chain PFCAs elimination from wastewater.

Profound implication of histological alterations, haematological responses and biocidal assessment of cationic amphiphiles unified with their molecular architecture

The interfacial properties depicting the micellization behaviour of the cationic amphiphiles (surfactants) belonging to the class of quaternary ammonium salts varying in degree of hydrophobicity were evaluated using tensiometry, conductivity and fluorescence spectrophotometric methods at 303.15 K. The impact of the amphiphilic nature of these amphiphiles as a function of their concentration is accounted against the selective microbial strains using the well-diffusion approach. Also, its influence on the histological (shrinkage/curling of lamellae, necrosis, haemorrhage, hyperplasia of villi in gills and intestine) alterations and haematological (blood parameters) changes in fingerling of Cirrhinus mrigala (C. mrigala) offers an insight into the stern damages reported as aquatic toxicity. The lesions exhibited moderate to severe alterations that are further correlated with the semi-quantitative mean alteration value (MAV). The in vitro and in vivo findings are explained significantly in terms of amphiphilic hydrophobicity which followed the order: C₁₆TAB > C₁₂TAB. All the observed outcomes are rationalized by the structural assessment of the selected amphiphiles as specified by the computational simulation approach using density functional theory (DFT) with B3LYP method and 3-21G basis source set. This work also portrays the biodegradability of these cationic amphiphiles and their fate on the environment. Graphical abstract Molecular architecture of cationic amphiphiles integrated with their in vitro and in vivo rejoinders.

New insight into photodegradation mechanisms, kinetics and health effects of p-nitrophenol by ozonation in polluted water

P-nitrophenol (p-NP) is a recalcitrant organic compound attracted great environmental attention, but its degradation mechanism is indeterminacy, which challenges its treatment, migration, transformation and ecological impact in the environment. In the present study, the aqueous-phase decomposition process of p-NP initiated by O₃ has been investigated by a theoretical calculation method. The detailed possible reaction pathways for the oxidative degradation of p-NP by ozone have been proposed. The chemical reaction thermodynamics results show that the reaction barriers of all ozone-initiated pathways are below 15 kcal·mol^-1, indicating that ozone can completely initiate the oxidation of p-NP under natural conditions. However, the kinetic results show that the initiation reaction of p-NP by ozone alone is relatively slow compared to the reaction by OH. Interestingly, under ultraviolet (UV) radiation, the dissolved ozone interacts with water and produces two active radicals: OH and HO₂. The reaction rate of p-NP initiated with OH is much higher than that with ozone, implying that the OH produced in the photochemical process can improve the removal efficiency of p-NP. The intermediates generated in the ozone-initiated reaction have been found to decompose into small molecule organic acids, aldehydes and ketones. The potential carcinogenicities and teratogenicities of the transformation products have also been studied, and some of them still have carcinogenic activity, which deserve further attention. In addition, to our knowledge, this may be the first computational chemistry study on the degradation of p-NP initiated by HO₂. All the results provide a new fundamental understanding for the migration and transformation of p-NP in water environment, and indicate that further assessment is needed for the impact of p-NP and especially its transformation products on the ecological environment in a significant way.

Decomposition of highly persistent perfluorooctanoic acid by hollow Bi/BiOI1-xFx: Synergistic effects of surface plasmon resonance and modified band structures

Perfluorooctanoic acid (PFOA) is highly stable due to the strong CF bond and extremely difficult to be removed by conventional photocatalysts. In this study, Bi doped BiOI_1-xF_x solid solutions with hollow microsphere structure were prepared through a facile one-step hydrothermal method. Compared with pure BiOI and BiOF, the band gap of the Bi/BiOI_1-xF_x solid solutions was significantly reduced, thus promoting the visible light absorbance. The cavity structure of the BiOI_1-xF_x solid solutions enhanced the surface areas and active sites for reaction. The local electromagnetic field dominated by surface plasmon resonance (SPR) effect of Bi metal on the surface favored the separation of the photoinduced charge pairs. As a consequence, Bi/BiOI_0.8F_0.2 (x = 0.20, the doping amount of fluorine was 20 %) composite displayed the best photocatalytic performance for decomposing PFOA, and 40 mg/L PFOA could be removed within 2 h illumination. The degradation rate constant (k = 0.0375 min^-1) of PFOA by Bi/BiOI_0.8F_0.2 was about tenfold of that by pure BiOI and BiOF. Superoxide radical (·O₂-) predominated in the degradation of PFOA by Bi/BiOI_0.8F_0.2, and the possible degradation pathway of PFOA by Bi/BiOI_0.8F_0.2 was proposed. This work provides a highly efficient catalyst for the practical application in removal of highly persistent PFOA.

Experimental and theoretical study of kinetic and mechanism of hydroxyl radical-mediated degradation of sulfamethazine

Hydroxyl radical (^•OH)-based advanced oxidation technologies (AOTs) is an effective and clean way to remove sulfonamide antibiotics in water at ambient temperature and pressure. In this study, we systematically investigated the degradation kinetics of sulfamethazine (SMT) by ^•OH with a combination of experimental and theoretical approaches. The second-order rate constant (k) of SMT with ^•OH was experimentally determined to be 5.27 ± 0.06 × 10⁹ M^-1 s^-1 at pH 4.5. We also calculated the thermodynamic and kinetic behaviors for the reactions by density functional theory (DFT) using the B3LYP/6-31G*. The results revealed that ^•OH addition pathways at the methylene (C4) site on the pyridine ring and the ortho sites (C12 and C14) of the amino group on the benzene ring dominate the reaction, especially C14 site on the benzene ring accounted for 43.95% of SMT degradation kinetics. The theoretical k value which was calculated by conventional transition state theory is 3.96 × 10⁹ M^-1 s^-1, indicating that experimental observation (5.27 ± 0.06 × 10⁹) is correct. These results could further help AOTs design in treating sulfonamide during wastewater treatment processes.

Fluoroacetate dehalogenase catalyzed dehalogenation of halogenated carboxylic acids: A QM/MM approach

Dehalogenation is one of the most important reactions in environmental pollution control, for instance, the degradation of persistent organic pollutants (POPs). Recently, fluoroacetate dehalogenase (FAcD) has been reported to catalyze the dehalogenation reactions, which shows great potential in treating halogenated pollutants. Here the dehalogenation mechanism catalyzed by FAcD was fully deciphered with the aid of quantum mechanics/molecular mechanics method. The results show that FAcD catalyzed dehalogenation efficiency follows the order of defluorination > dechlorination > debromination. The corresponding Boltzmann-weighted average barriers are 10.1, 19.7, and 20.9 kcal mol^-1. Positive/negative correlations between activation barriers and structural parameters (e.g. distance and angle) for FAcD catalyzed dechlorination and debromination were established. Based on the structure-energy relationship, we propose that mutation of the binding pocket amino acids (e.g. His155, Trp156, Tyr219) to smaller proton donor amino acids (e.g. Serine, Threonine, Cysteine, Asparagine) may increase the efficiency for dechlorination and debromination. The results may of practical value for the efficient degradation of chlorined and bromined pollutants by harnessing FAcD.

Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment

Ultraviolet photochemical reaction of sulfite (SO₃^2-) photosensitizer generates strongly reducing hydrated electrons (e_aq^-; NHE = -2.9 V) that have been shown to effectively degrade individual per- and polyfluoroalkyl substances (PFASs), including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). However, treatment of complex PFAS mixtures in aqueous film-forming foam (AFFF) remains largely unknown. Here, UV-sulfite was applied to a diluted AFFF to characterize e_aq^- reactions with 15 PFASs identified by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) targeted analysis. Results show that reactivity varies widely among PFASs, but reaction rates observed for individual PFASs in AFFF are similar to rates observed in single-solute experiments. While some structures, including long-chain perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs) were readily degraded, other structures, most notably short-chain PFSAs and fluorotelomer sulfonic acids (FTSs), were more recalcitrant. This finding is consistent with results showing incomplete fluoride ion release (up to 53% of the F content in AFFF) during reactions. Furthermore, results show that selected PFSAs, PFCAs, and FTSs can form as transient intermediates or unreactive end-products via e_aq^- reactions with precursor structures in AFFF. These results indicate that while UV-sulfite treatment can be effective for treating PFOS and PFOA to meet health advisory levels, remediation of the wider range of PFASs in AFFF will prove more challenging.

Rapid photochemical decomposition of perfluorooctanoic acid mediated by a comprehensive effect of nitrogen dioxide radicals and Fe3+/Fe2+ redox cycle

Developing efficient methods to degrade perfluorochemicals (PFCs), an emerging class of highly recalcitrant contaminants, are urgently needed in recent years, due to their persistence, high toxicity, and resistance to most regular treatment procedures. Here, a UV-photolysis system is reported for efficient mineralization of perfluorooctanoic acid (PFOA) via irradiation of ferric nitrate aqueous solution, where in-situ generating •NO₂ and the effective Fe³⁺/Fe²⁺ redox cycle synergistically play great roles on rapidly mediating the mineralization of PFOA. A fast PFOA removal kinetics with first-order kinetic constants of 2.262 h^-1 is observed at initial PFOA concentration of 5 ppm (50 mL volume), reaching ∼ 92 % removal efficiency within only 0.5-h irradiation. Near-stoichiometric fluoride ions liberation and high total organic carbon (TOC) removal efficiency (∼100 %) further validated the capability for completely destructive removal of PFOA. A tentative pathway for PFOA destruction is proposed. This work, by UV photolysis of abundant existing iron/nitrate-based systems in natural environment, provides an economical, sustainable and highly efficient approach for complete mineralization of perfluorinated chemicals.

TiO2 quantum dots loaded sulfonated graphene aerogel for effective adsorption-photocatalysis of PFOA

With the pollution of perfluoroalkyl substances (PFASs) became increasingly serious, the researches focused on removal of PFASs by adsorption-photocatalysis method has attracted considerable attention. To make the catalyst TiO₂ disperse uniformly as quantum dots onto hydrophobic surface which was liable to attract perfluorooctanoic acid (PFOA), the surfactant sodium dodecyl sulfate (SDS) were used in this work, which not only connected the hydrophilic TiCl₃ to the hydrophobic sulfonated graphene (SG) nanosheets, but also behaved as the molecular template for controlled nucleation and growth of the nanostructured TiO₂. After 3D SG-TiO₂ QD nanosheets were fabricated, a series of 3D SG-TiO₂ QD aerogels were self-assembled by ice-template. TiO₂ uniformly distributed on the surface of SG aerogel at QD size level (2-3 nm) and the size of TiO₂ could be effectively regulated by concentration of SDS. Compared with aggregated TiO₂ material, 3D SG-TiO₂ QD aerogels owned higher adsorption and photocatalytic performance. Benefiting from the hydrophobic surface of 3D SG as well as dispersed TiO₂ QDs, 3D SG-TiO₂ QD could enrich PFOA instantaneously (0.0381/s) and photocatalytic decomposed them effectively (1.898 E^-4/s). PFOA degradation by hole and hydroxyl radicals proceeded via a stepwise mechanism. The column made of 3D SG-TiO₂ QD could remove PFOA persistently in cycles of permeation. 3D SG-TiO₂ QD possessed powerful adsorption-photocatalytic decomposition capability of PFOA and steady reusability performance. The present work highlights the individual roles and synergistic effect of TiO₂ QD and 3D SG for effectively removing PFOA.

Photochemical formation of hydroxylated polychlorinated biphenyls (OH-PCBs) from decachlorobiphenyl (PCB-209) on solids/air interface

In this work, the photochemical transformation of decachlorobiphenyl (PCB-209) on the surface of several solid particles were systematically evaluated under simulated solar irradiation. The degradation kinetics of PCB-209 were first investigated using silica as a model aerosol particulate. It was found that PCB-209 photodegradation was enhanced at small silica particle size, low surface coverage and low humidity. Electron paramagnetic resonance (EPR) analysis and radicals quenching experiments demonstrated that hydroxyl radicals contributed to PCB-209 degradation. Stepwise hydrodechlorination, hydroxyl addition and cleavage of the CC bridge bond were mainly observed in the reaction process, leading to the formation of lower chlorinated PCBs, hydroxylated PCBs (OH-PCBs) and chlorophenols. Based on density functional theory (DFT) calculation, the dissociation energy of the CCl bond requires 354.81-359.79 kJ/mol energy that corresponds to a wavelength of less than 322 nm. And the minimum activation energy of OH radicals attack on PCB-209 is only 18.12 kJ/mol. Photochemical transformation of PCB-209 can also occur on the surface of natural particles, but the rates were inhibited as compared to silica. The hydroxylation and hydrodechlorination products of PCB-209 were detected in all natural particles. This study would make significant contribution to understanding the fate of PCBs in solids/air interface.

Harnessing fluoroacetate dehalogenase for defluorination of fluorocarboxylic acids: in silico and in vitro approach

Widely distributed fluorocarboxylic acids have aroused worldwide environmental concerns due to its toxicity, persistence, and bioaccumulation. Enzyme-based eco-friendly biodegradation techniques have become increasingly important in treating fluorocarboxylic acids. Here we utilized in silico and in vitro approaches to investigate the defluorination mechanism of fluoroacetate dehalogenase (FAcD) toward monofluoropropionic acids at atomic-level. The experimentally determined k_cat and k_M for defluorination of 2-fluoropropionic acid are 330 ± 60 min^-1 and 6.12 ± 0.13 mM. The in silico results demonstrated positive/negative correlations between activation barriers and structural parameters (e.g. distance and angle) under different enzymatic conformations. We also screened computationally and tested in vitro (enzyme assay and kinetic study) the catalytic proficiency of FAcD toward polyfluoropropionic acids and perfluoropropionic acids which are known to be challenging for enzymatic degradation. The results revealed potential degradation activity of FAcD enzyme toward 2,3,3,3-tetrafluoropropionic acids. Our work will initiate the development of a new "integrated approach" for enzyme engineering to degrade environmentally persistent fluorocarboxylic acids.

Natural sunlight-driven aquatic toxicity enhancement of 2,6-di-tert-butylphenol toward Photobacterium phosphoreum

The tert-butylphenols (TBPs) are one group of alkylated phenolic compounds with wide applications in UV absorbers and antioxidants. They are becoming contaminants of emerging concern with residues frequently detected in natural surface water or drinking water. The direct sunlight may photolyze TBPs in waters and affect their aquatic toxicities; however, such data are very limited. In the present study, we investigate the photodegradation of 2,6-DTBP by direct sunlight in water and compare the aquatic toxicities of 2,6-DTBP with that of its product toward Photobacterium phosphoreum. 2,6-DTBP is photodegraded by 71.31 ± 2.64% under simulated sunlight following a pseudo-first-order kinetics with rate constant (k) of 0.061 h^-1. Density functional theory simulations at M06-2X/def2-SVP level reveal that the photodegradation occurred sequentially through oxidation, photo-isomerization and hydrogenation. The degradation product 2,5-DTBP is toxic to P. phosphoreum (EC₅₀ 3.389 × 10^-5 mol/L) whereas 2,6-DTBP is not harmful (EC₅₀ 3.917 × 10^-3 mol/L) as designated by the European Union Standard, indicating the enhanced toxicities driven by the direct sunlight photodegradation. We demonstrate the enhanced toxicities of 2,6-DTBP by natural sunlight, suggesting that negligence of photodegradation of TBPs-related contaminants will underestimate the comprehensive risk of these emerging contaminant in natural waters.

Photochemical behavior of benzophenone sunscreens induced by nitrate in aquatic environments

Benzophenones (BPs), which are widely used UV filters, have aroused considerable public concern owing to their potential endocrine-disrupting activities. Herein, we systematically investigated their photochemical behavior and fate, which is mediated by nitrate in aquatic environments. The results showed that 10 μM of 3 BPs can be completely degraded within 4 h of simulated sunlight irradiation in a 10 mM nitrate solution at pH 8.0, and 2,4-dihydroxybenzophenone (BP-1) has a 31.6% mineralization rate after 12 h irradiation. Their photolytic rates (k_obs) presented a significant linear correlation with the logarithmic values of the nitrate concentration for 0.1-10 mM (R² > 0.98), and in three actual waters, the rates of BP-1 were also positively related to the intrinsic nitrate concentration. Furthermore, higher transformation rates under alkaline condition were observed, especially for BP-1, with its k_obs at pH 10 being 8.3-fold higher than that at pH 6.0. Moreover, dissolved oxygen (DO) also has an impact on the reaction kinetics to some degree. According to the quenching experiments, we found that three reactive oxygen species (ROS), namely, •OH, •NO, and •NO₂, participated in this photolysis of BPs, and the contribution of •OH accounted for 32.1%. Furthermore, we selected BP-1 as the model molecule to study the transformation pathways and toxicity changes in this system. Four main transformation pathways including hydroxylation, nitrosylation, nitration, and dimerization were proposed, based on liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) analysis and density functional theory (DFT). According to the toxicity test, the formed intermediates were more toxic to Photobacterium phosphoreum than the parent BP-1. Therefore, these results can help reveal primary phototransformation mechanisms and evaluate the potential ecological risks of BPs in aquatic environments.

Formation of perfluorocarboxylic acids from photodegradation of tetrahydroperfluorocarboxylic acids in water

Tetrahydroperfluorocarboxylic acids (2H,2H,3H,3H-PFCAs) have aroused the interest of scholars worldwide due to their potential to generate perfluorinated compounds. In this work, we systematically examined the photodegradation kinetics and mechanisms of typical 2H,2H,3H,3H-PFCAs (C_nF_2n+1C₂H₄COOH, n = 6, 7, 8) in aqueous solution by a 500 W Hg lamp. The photodecomposition of 2H,2H,3H,3H-PFCAs all followed pseudo-first-order kinetics, and the photolysis rate coefficients increased with the increasing carbon chain length. Under the same reaction condition, 2H,2H,3H,3H-PFCAs degraded much faster than the corresponding PFCAs. The photodecomposition rate coefficient of C₈F₁₇CH₂CH₂COOH was accelerated by low pH and Fe³⁺ addition, but decreased by the existence of humic acid, carbonate and bicarbonate. Compared with ultrapure water, a decreased removal of 2H,2H,3H,3H-PFCAs was observed in four types of natural waters, i.e., tap water, Jiuxiang river water, primary effluent and secondary effluent. According to mass analysis, C₈F₁₇CH₂CH₂COOH was mainly decomposed into 8:2 fluorotelomer acid (C₈F₁₇CH₂COOH), shorter-chain perfluorocarboxylic acids (PFCAs), perfluoro-1-enes (C_nF_2n) and perfluoroketenes (C_nF_2n+1CF = C = O). Thus, α-oxidation, decarboxylation and elimination reaction were proposed as reaction pathways. ECOSAR predictions showed that photolysis generally decreased the aquatic toxicity of C₈F₁₇CH₂CH₂COOH.

Rethinking sulfate radical-based oxidation of nitrophenols: Formation of toxic polynitrophenols, nitrated biphenyls and diphenyl ethers

Sulfate radical (SO₄^-)-based oxidation of nitrophenols (NPs) have been widely studied; however, formation of potentially more toxic polynitroaromatic intermediates has been overlooked. In this contribution, we systematically investigated the degradation of four NPs by a SO₄^--based oxidation process. Degradation efficiency of NPs followed the order: 2-nitrophenol (2-NP) > 4-nitrophenol (4-NP) > 2,4-dinitrophenol (2,4-DNP) > 2,6-dinitrophenol (2,6-DNP). HPLC and LC-MS/MS analysis confirmed the formation of 2,4-DNP, 2,6-DNP and 2,4,6-trinitrophenol (2,4,6-TNP) during NPs transformation by SO₄^-, suggesting that both denitration and renitration processes occurred. Nitrogen dioxide radicals (NO₂) and phenoxy radicals are responsible for the formation of polynitrophenols. Coupling products including nitrated biphenyls and diphenyl ethers were also detected, which were proposed to be formed by combinations of resonance-stabilized radicals. Electron spin density and charge density calculation showed that ortho C-ortho C and ortho C-phenolic O were the most likely combination ways responsible for coupling products formation. ECOSAR program predicted that polynitrated diphenyl ethers and biphenyls had higher ecotoxicological effects on aquatic species such as fish and daphnia. Therefore, the formation of toxic polynitroaromatic intermediates in SO₄^--based advanced oxidation processes should be scrutinized before this technology can be safely utilized for water and wastewater treatment.

Fe(VI)-Mediated Single-Electron Coupling Processes for the Removal of Chlorophene: A Combined Experimental and Computational Study

Potassium ferrate [Fe(VI)] is a promising oxidant widely used in water treatment for the elimination of organic pollutants. In this work, the reaction kinetics, products, and mechanisms of the antimicrobial agent chlorophene (CP) undergoing Fe(VI) oxidation in aqueous solutions were investigated. CP is very readily degraded by Fe(VI), with the apparent second-order rate constant, k, being 423.2 M^-1 s^-1 at pH 8.0. A total of 22 oxidation products were identified using liquid chromatography-quadrupole time-of-flight-mass spectrometry , and their structures were further elucidated using tandem mass spectrometry. According to the extracted peak areas in mass spectra, the main reaction products were the coupling products (dimers, trimers, and tetramers) that formed via single-electron coupling. Theoretical calculations demonstrated that hydrogen abstraction should easily occur at the hydroxyl group to produce reactive CP· radicals for subsequent polymerization. Cleavage of the C-C bridge bond, electrophilic substitution, hydroxylation, ring opening, and decarboxylation were also observed during the Fe(VI) oxidation process. In addition, the degradation of CP by Fe(VI) was also effective in real waters, which provides a basis for potential applications.

Photodegradation of 17β-estradiol on silica gel and natural soil by UV treatment

This paper evaluates the UV photodegradation of 17β-estradiol (E2) on silica gel and in natural soil with different soil components. Silica gel was chosen as a stable and pure support to simulate the photochemical behavior of E2 on the surface of natural soil. Ultraviolet light, rather than visible light, was confirmed to play a decisive role in the photodegradation of E2 on silica gel. The effect of three soil components, including humic acid (HA), inorganic salts, and relative humidity (RH), on the photochemical behavior of E2 on silica gel or soil under UV irradiation was then evaluated. Two HA concentrations (10 and 20 mg g^-1) and three salts (ferric sulfate, copper sulfate and sodium carbonate) were observed to obviously inhibit the degradation of E2 on silica gel. Interestingly, nitrate was found to obviously improve the removal efficiency of E2. Both too-dry and too-wet conditions obviously reduced the removal rate of E2, and the optimum relative humidity (RH) value was found to be approximately about 35% (30 °C). Furthermore, twenty intermediate products and two major pathways were proposed to describe the transformation processes of E2 treated by UV irradiation, among which oligomers were found to be the major intermediate products before complete mineralization. The efficient UV removal of E2 on silica gel and natural soil suggested a feasible strategy to remediate E2 contaminated soil.

Degradation of the UV-filter benzophenone-3 in aqueous solution using persulfate activated by heat, metal ions and light

The goals of this study were to bring forward new data and insights into the effect of activation methods, operational variables and reaction pathways during sulfate radicals-based oxidation of benzophenone-3 (BP-3) in aqueous solution. Heat, transition metal ions (Fe²⁺, Cu²⁺, Co²⁺), UV and visible light irradiation were used to activate persulfate (PS) to degrade BP-3. The results showed that these three activation methods can remarkably enhance BP-3 removal efficiency. Under the conditions of [BP-3]₀: [PS]₀ = 1: 500, pH = 7.0, and 40 °C, complete removal of BP-3 (1.31 μM) was observed in 3 h. In the pH range of 3.0-9.0, the degradation of BP-3 decreased with increasing pH. Increasing the PS dosage accelerated the reaction, while the presence of humic acid (HA) significantly inhibited the efficiency of BP-3 removal. Based on electron paramagnetic resonance (EPR) and radical quenching studies, sulfate and hydroxyl radicals contributed to the oxidation process. According to the evolution of BP-3 and its 7 by-products, as well as frontier electron densities (FED) calculation, two routes were proposed involving hydroxylation, demethylation and direct oxidation. On the whole, this work is a unique contribution to the systematic elucidation of BP-3 removal by PS.

Photocatalytic degradation and mineralization of perfluorooctanoic acid (PFOA) using a composite TiO2 -rGO catalyst

The inherent resistance of perfluoroalkyl substances (PFASs) to biological degradation makes necessary to develop advanced technologies for the abatement of this group of hazardous substances. The present work investigated the photocatalytic decomposition of perfluorooctanoic acid (PFOA) using a composite catalyst based on TiO₂ and reduced graphene oxide (95% TiO₂/5% rGO) that was synthesized using a facile hydrothermal method. The efficient photoactivity of the TiO₂-rGO (0.1gL^-1) composite was confirmed for PFOA (0.24mmolL^-1) degradation that reached 93±7% after 12h of UV-vis irradiation using a medium pressure mercury lamp, a great improvement compared to the TiO₂ photocatalysis (24±11% PFOA removal) and direct photolysis (58±9%). These findings indicate that rGO provided the suited properties of TiO₂-rGO, possibly as a result of acting as electron acceptor and avoiding the high recombination electron/hole pairs. The release of fluoride and the formation of shorter-chain perfluorocarboxilyc acids, that were progressively eliminated in a good match with the analysed reduction of total organic carbon, is consistent with a step-by-step PFOA decomposition via photogenerated hydroxyl radicals. Finally, the apparent first order rate constants of the TiO₂-rGO UV-vis PFOA decompositions, and the intermediate perfluorcarboxylic acids were found to increase as the length of the carbon chain was shorter.