Electrochemical decomposition of environmentally persistent perfluorooctanoic acid (PFOA) in aqueous solution was investigated over Ti/SnO2–Sb, Ti/SnO2–Sb/PbO2, and Ti/SnO2–Sb/MnO2 anodes. The ...degradation of PFOA followed pseudo-first-order kinetics. The degradation ratios on Ti/SnO2–Sb, Ti/SnO2–Sb/PbO2, and Ti/SnO2–Sb/MnO2 anodes achieved 90.3%, 91.1%, and 31.7%, respectively, after 90 min electrolysis at an initial 100 mg/L PFOA concentration at a constant current density of 10 mA/cm2 with a 10 mmol/L NaClO4 supporting electrolyte solution. The defluorination rates of PFOA on these three anodes were 72.9%, 77.4%, 45.6%, respectively. The main influencing factors on electrochemical decomposition of PFOA over Ti/SnO2–Sb anode were evaluated, including current density (5–40 mA/cm2), initial pH value (3–11), plate distance (0.5–2.0 cm), and initial concentration (5–500 mg/L). The results indicated that PFOA (100 mL of 100 mg/L) degradation ratio and defluorination ratio achieved 98.8% and 73.9%, respectively, at the optimal conditions after 90 min electrolysis. Under this optimal condition, the degradation rate constant and the degradation half-life were 0.064 min−1 and 10.8 min, respectively. The intermediate products including short-chain perfluorinated carboxylic acids (PFCAs, C2∼C6) and perfluorocarbons (C2∼C7) were detected by electrospray ionization (ESI) mass spectrum. A possible electrochemical degradation mechanism of PFOA including electron transfer, Kolbe decarboxylation, radical reaction, decomposition, and hydrolysis was proposed. The electrochemical technique could be employed to degrade PFOA from contaminated wastewater as well as to reduce the toxicity of PFOA.
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► PFOA electrochemical degradation by using three typical anodes. ► Performance can be significantly affected by applied current, initial pH, and plate distance. ► A PFOA electrochemical degradation mechanism was proposed.
Abstract
Single atom catalysts have been found to exhibit superior selectivity over nanoparticulate catalysts for catalytic reactions such as hydrogenation due to their single-site nature. However, ...improved selectively is often accompanied by loss of activity and slow kinetics. Here we demonstrate that neighboring Pd single atom catalysts retain the high selectivity merit of sparsely isolated single atom catalysts, while the cooperative interactions between neighboring atoms greatly enhance the activity for hydrogenation of carbon-halogen bonds. Experimental results and computational calculations suggest that neighboring Pd atoms work in synergy to lower the energy of key meta-stable reactions steps, i.e., initial water desorption and final hydrogenated product desorption. The placement of neighboring Pd atoms also contribute to nearly exclusive hydrogenation of carbon-chlorine bond without altering any other bonds in organohalogens. The promising hydrogenation performance achieved by neighboring single atoms sheds light on a new approach for manipulating the activity and selectivity of single atom catalysts that are increasingly studied in multiple applications.
Perfluorinated compounds (PFCs) are persistent and refractory organic pollutants that have been detected in various environmental matrices and municipal wastewater. Electrochemical oxidation (EO) is ...a promising remediation technique for wastewater contaminated with PFCs. A number of recent studies have demonstrated that the “non-active” anodes, including boron-doped diamond, tin oxide, and lead dioxide, are effective in PFCs elimination in wastewater due to their high oxygen evolution potential. Many researchers have conducted experiments to investigate the optimal conditions (i.e., potential, current density, pH value, plate distance, initial PFCs concentration, electrolyte, and other factors) for PFCs elimination to obtain the maximal elimination efficiency and current efficiency. The EO mechanism and pathways of PFCs have been clearly elucidated, which undergo electron transfer, Kolbe decarboxylation or desulfonation, hydrolysis, and radical reaction. In addition, the safety evaluation and energy consumption evaluation of the EO technology have also been summarized to decrease toxic ion release from electrode and reduce the cost of this technique. Although the ultrasonication and hydrothermal techniques combined with the EO process can improve the removal efficiency and current efficiency significantly, these coupled techniques have not been commercialized and applied in industrial wastewater treatment. Finally, key challenges facing EO technology are listed and the directions for further research are pointed out (such as combination with other techniques, treatment for natural waters contaminated by low levels of PFCs, and reactor design).
•Electrochemical oxidation (EO) is promising in PFCs elimination from wastewater.•Influencing factors of PFCAs electrochemical oxidation are critically reviewed.•EO pathways of PFCs undergo electron transfer, hydrolysis, and radical reaction.•Safety and energy consumption evaluation of EO technology are summarized.•Existing problem and direction for future research in EO technology are suggested.
The electrochemical mineralization mechanism of environmentally persistent perfluorooctanoic acid (PFOA) at a Ce-doped modified porous nanocrystalline PbO2 film anode was investigated using density ...functional theory (DFT) simulation and further validated experimentally. The potential energy surface was mapped out for all possible reactions during electrochemical mineralization reaction of PFOA. The hydroxyl radical (·OH), O2 and H2O took part in the mineralization process and played different roles. The ·OH-initiated process was found to be the main degradation pathway, and the existence of O2 obviously accelerated the degradation process of PFOA in aqueous solution. On the basis of the DFT calculations, an optimal electrochemical mineralization mechanism of PFOA was proposed, which involved the electronic migration, decarboxylation, radical reaction, hydrogen abstraction reaction, and radical fragmentation reaction. The proposed mechanism was verified by the dynamics and intermediate determination experiments. Furthermore, the observed ·OH concentration showed that the electrolysis system could produce enough ·OH for PFOA mineralization process, indicating that the proposed ·OH-initiated process derived from DFT calculations was feasible. These insightful findings are instrumental for a comprehensive understanding of the mineralization of PFOA in the electrolysis system.
Electrochemical degradation of trace antiretroviral drug stavudine was investigated by using a reactive electrochemical membrane (REM) with Ti/SnO2-Sb anode. From the results it was evident that the ...stavudine degradation followed pseudo-first-order kinetics, with the values of the degradation rate constant and half-life being 0.24 min−1 and 2.9 min, respectively, at a current density of 8 mA cm−2. The degradation rate was obviously decreased under alkaline condition (pH = 11.0) and the degradation was also inhibited in the presence of NO3− and Cl−. Five intermediates were identified in the electrochemical degradation of stavudine, and the degradation pathways were proposed. Density functional theory calculation revealed that the double bond carbon atom nearby hydroxymethyl group was the site attacked by OH and the cleavage of CN bond was the rate-determining step in the electrochemical degradation of stavudine. The nitrogen in stavudine was mainly converted to nitrate and ammonium. Quantitative structure-activity relationship model indicated that the toxicity of some intermediates was higher than the parent compound stavudine. The electric energy consumption for 90% stavudine degradation ranged from 0.87 to 2.29 Wh L−1 at the experimental conditions, indicating that stavudine can be degraded efficiently by the REM with Ti/SnO2-Sb anode.
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•Effects of different factors on degradation of trace stavudine were investigated.•Reaction site between stavudine and OH was carbon-carbon double bond.•The nitrogen in stavudine was mainly converted to nitrate and ammonium.•Some harmful intermediates were produced during degradation of stavudine.
Cleavage of the strong carbon–fluorine bonds is critical for elimination of perfluorooctanoic acid (PFOA) from the environment. In this work, we investigated the decomposition of PFOA with the ...SiC/graphene catalyst under UV light irradiation. The decomposition rate constant (k) with SiC/graphene was 0.096 h–1, 2.2 times higher than that with commercial nano-TiO2. Surface fluorination on SiC/graphene was analyzed by X-ray photoelectron spectroscopy (XPS), revealing the conversions of Si–H bonds into Si–F bonds. A different route was found to generate the reactive Si–H bonds on SiC/graphene, substituting for silylium (R3Si+) to activate C–F bonds. During the activation process, photogenerated electrons on SiC transfer rapidly to perfluoroalkyl groups by the medium of graphene, further reducing the electron cloud density of C–F bonds to promote the activation. The hydrogen-containing hydrodefluorination intermediates including (CF3(CF2)2CFH, CF3(CF2)3CH2, CF3(CF2)4CH2, and CF3(CF2)4CFHCOOH) were detected to verify the hydrodefluorination process. The photoinduced hydrodefluorination mechanisms of PFOA can be consequently inferred as follows: (1) fluorine atoms in perfluoroalkyl groups were replaced by hydrogen atoms due to the nucleophilic substitution reaction via the Si–H/C–F redistribution, and (2) generation of CH2 carbene from the hydrogen-containing perfluoroalkyl groups and the C–C bonds scission by the Photo-Kolbe decarboxylation reaction under UV light excitation. This photoinduced hydrodefluorination provides insight into the photocatalytic decomposition of perfluorocarboxylic acids (PFCAs) in an aqueous environment.
Oxidative stress induced by reactive oxygen species (ROS) is one of the major toxicity mechanisms of engineered nanoparticles (NPs). To advance our knowledge of the photogeneration of ROS on NPs, ...this Letter reports the ROS generation kinetics of uncoated silver (AgNPs), gold (AuNPs), nickel (NiNPs), and silicon (SiNPs) NPs in aqueous suspension under UV irradiation (365 nm) and analyzes the potential ROS photogeneration mechanisms as well as the associated antibacterial effects. The results showed that AgNPs generated superoxide and hydroxyl radicals, whereas AuNPs, NiNPs, and SiNPs generated only singlet oxygen. The electronic structure and redox potentials of SiNPs were shown to mediate ROS generation. By contrast, ROS generation on AuNPs, AgNPs, and NiNPs was primarily due to surface plasmon resonance. The antibacterial activities of these NPs toward E. coli cells under UV irradiation were AgNPs (strongest) > SiNPs > NiNPs > AuNPs. ROS generation and metal ion release significantly enhanced the NPs’ antibacterial activity.
Plants are exposed to various environmental stresses. The sensing of environmental cues and the transduction of stress signals into intracellular signaling are initial events in the cellular ...signaling network. As a second messenger, Ca
2+
links environmental stimuli to different biological processes, such as growth, physiology, and sensing of and response to stress. An increase in intracellular calcium concentrations (Ca
2+
i
) is a common event in most stress-induced signal transduction pathways. In recent years, significant progress has been made in research related to the early events of stress signaling in plants, particularly in the identification of primary stress sensors. This review highlights current advances that are beginning to elucidate the mechanisms by which abiotic environmental cues are sensed
via
Ca
2+
signals. Additionally, this review discusses important questions about the integration of the sensing of multiple stress conditions and subsequent signaling responses that need to be addressed in the future.
The Ce-doped modified porous nanocrystalline PbO2 film electrode prepared by electrodeposition technology was used for electrochemical mineralization of environmentally persistent perfluorinated ...carboxylic acids (PFCAs) (∼C4–C8), i.e., perfluorobutanoic acid (PFBA), perfluopentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluoheptanoic acid (PFHpA), and perfluorooctanoic acid (PFOA) in aqueous solution (100 mL of 100 mg L–1). The degradation of PFCAs follows pseudo-first-order kinetics, and the values of the relative rate constant (k) depend upon chain length k PFHpA (4.1 × 10–2 min–1; corresponding half-life 16.8 min) ≈ 1.1k PFOA ≈ 2.5k PFHxA≈ 6.9k PFPeA ≈ 9.7k PFBA. The carbon mineralization indices i.e., 1 – (TOCinsolution/TOCinPFCA,degraded) were 0.49, 0.70, 0.84, 0.91, and 0.95 for PFBA, PFPeA, PFHxA, PFHpA, and PFOA, respectively, after 90 min electrolysis. The major mineralization product, F–, as well as low amount of intermediate PFCAs with shortened chain lengths were detected in aqueous solution. By observing the intermediates and tracking the concentration change, a possible pathway of electrochemical mineralization is proposed as follows: Kolbe decarboxylation reaction occurs first at the anode to form the perfluoroalkyl radical, followed by reaction with hydroxyl radicals to form the perfluoroalkyl alcohol which then undergoes intramolecular rearrangement to form the perfluoroalkyl fluoride. After this, the perfluoroalkyl fluoride reforms perfluorinated carboxylic with shorter chain length than its origin by hydrolysis. This electrochemical technique could be employed to treat PFCAs (∼C4–C8) in contaminated wastewater.
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•Sulfamethoxazole removed by peroxymonosulfate without catalyst.•Carbonate radicals generated and promote the sulfamethoxazole removal.•Two carbonyldioxy degradation intermediate ...products are detected.•Singlet oxygen is also an active species for sulfamethoxazole degradation.
Peroxymonosulfate (PMS) can be activated by various catalysts to degrade organic contaminants in wastewater treatment processes. In this research, the co-activation of PMS by sulfamethoxazole (SMX) and carbonate is investigated. The results show that SMX can be degraded in situ, and the main reactive oxygen species are singlet oxygen and carbonate radicals. Only singlet oxygen is detected when SMX is degraded by PMS without carbonate. However, both carbonate radicals and hydroxyl radicals are measured in the presence of carbonate. Among which, hydroxyl radicals are identified by electron paramagnetic resonance spectroscopy method, and carbonate radicals are confirmed by radical quenching experiments, as well as the variation of SMX degradation kinetic and the appreance of carbonyldioxy derivatives by-products in the presence of carbonate anions. Based on this phenomenon, it is proposed that carbonate can enhance the decomposition of PMS and the generation of secondary free radicals (carbonate radicals). Thus carbonate radicals can enhance the oxidizability for sulfonamide antibiotics in the PMS-based advanced oxidation systems. These results suggest that the addition of carbonate is an important enhancement method for the treatment of sulfonamide antibiotics in water.
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