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The investigation into the degradation of metronidazole (MNZ), a frequently employed antibiotic, through the electrochemical activation of peroxymonosulfate (PMS) utilizing either boron-doped diamond (BDD) or dimensional stable anode (DSA) as the anode, was conducted in a systematic manner. The enhancement of MNZ removal was observed with increasing current density, PMS dosage, and initial pH. Response surface methodology (RSM), based on a Box–Benken design, was utilized to evaluate the efficiency of MNZ elimination concerning current density (ranging from 11.1 to 33.3 mA/cm2), initial pH (ranging from 3 to 9), PMS dosage (ranging from 1 to 5 mmol·L−1), and reaction time (ranging from 25 to 45 min). The optimal operational conditions for MNZ removal were determined as follows: a current density of 13.3 mA/cm2, a pH of 3.7, a PMS dosage of 2.4 mmol·L−1, and a reaction time of 40 min. Electron paramagnetic resonance (EPR), quenching experiments, and chemical probe experiments confirmed the involvement of •OH, SO4•− and 1O2 radicals as the primary reactive species in MNZ degradation. The presence of HCO3− and H2PO4− hindered MNZ removal, whereas the presence of Cl− accelerated it. The degradation pathways of MNZ were elucidated by identifying intermediates and assessing their toxicity. Additionally, the removal efficiencies of other organic pollutants, such as sulfamethoxazole (SMX), carbamazepine (CBZ), and nitrobenzene (NB), were compared. This study contributes to a comprehensive understanding of MNZ degradation efficiency, mechanisms, and pathways through electrochemical activation of PMS employing BDD or DSA anodes, thereby offering valuable insights for the selection of wastewater treatment systems. [ABSTRACT FROM AUTHOR]

The ethylenediamine-N,N′-disuccinic acid (EDDS) was utilized to form Fe-EDDS complex to activate peroxymonosulfate (PMS) in the electrochemical (EC) co-catalytic system for effective oxidation of naphthenic acids (NAs) under neutral pH conditions. 1-adamantanecarboxylic acid (ACA) was used as a model compound to represent NAs, which are persistent pollutants that are abundantly present in oil and gas field wastewater. The ACA degradation rate was significantly enhanced in the EC/PMS/Fe(III)-EDDS system (96.6%) compared to that of the EC/PMS/Fe(III) system (65.4%). The addition of EDDS led to the formation of a stable complex of Fe-EDDS under neutral pH conditions, which effectively promoted the redox cycle of Fe(III)-EDDS/Fe(II)-EDDS to activate PMS to generate oxidative species for ACA degradation. The results of quenching and chemical probe experiments, as well as electron paramagnetic resonance (EPR) analysis, identified significant contributions of •OH, 1O2, and SO4•− in the removal of ACA. The ACA degradation pathways were revealed based on the results of high resolution mass spectrometry analysis and calculation of the Fukui index. The presence of anions, such as NO3−, Cl−, and HCO3−, as well as humic acids, induced nonsignificant influence on the ACA degradation, indicating the robustness of the current system for applications in authentic scenarios. Overall results indicated the EC/PMS/Fe(III)-EDDS system is a promising strategy for the practical treatment of NAs in oil and gas field wastewater. [ABSTRACT FROM AUTHOR]

In this study, molybdenum disulfide (MoS 2) has been investigated as a catalyst to boost the redox cycle of Fe(III)/Fe(II) to activate peroxymonosulfate (PMS) to effectively break down cyclohexanecarboxylic acid (CHA), which is a model naphthenic acid (NA) pollutant. The optimum conditions of pH 3, MoS 2 concentration of 0.4 g/L, Fe(III) concentration of 0.3 mmol/L, and PMS concentration of 2 mmol/L has been applied for complete degradation of 0.1 mmol/L CHA in 10 minutes reaction time. Based on results of electron paramagnetic resonance analysis, chemical probe and quenching experiment, it was found that all of •OH, SO 4 •−, 1O 2 , and Fe(IV) contributed to the MoS 2 /Fe(III)/PMS oxidation system. Eighteen CHA byproducts and three primary CHA degradation pathways were revealed according to the results of UHPLC-QTOF-MS analysis. By using XPS, SEM, and XRD to characterize the MoS 2 powders before and after the reaction, MoS 2 was found with high stability and the oxidation of Mo(IV) on the surface was negligible. The impacts of background anions on CHA degradation were found to be nonimportant, and commercial NAs mixture were verified to be efficiently degraded in current system. Overall results indicate the capacity and potential of MoS 2 /Fe(III)/PMS system to be utilized for future treatment of authentic oil and gas field wastewater. [Display omitted] • Both Mo(IV) and S2- of MoS 2 can activate PMS to generate ROS to degrade CHA. • The main reactive species are 1O 2 , •OH, SO 4 •−and Fe(IV) in the system. • Chain scission, hydrogen extraction and decarboxylation are main reaction pathways. • No Mo(V) oxidation was found on the surface of MoS 2 powder after the reaction. • The commercial NAs mixture was examined for efficient degradation. [ABSTRACT FROM AUTHOR]

[Display omitted] • BDD/PMS/Fe(III) process is dominated by radical oxidation. • DSA/PMS/Fe(III) process is dominated by nonradical oxidation. • BDD/PMS/Fe(III) system is inclined to oxidize electron-rich contaminants. • DSA/PMS/Fe(III) system is apt to oxidize electron-deficient contaminants. • Degradation pathways of sulfamethoxazole were systematically revealed. Herein, peroxymonosulfate (PMS) activation via electro-cocatalytic system with circulated Fe(III)/Fe(II) was investigated with boron-doped diamond (BDD) anode and dimensionally stable anode (DSA) for degradation of sulfamethoxazole (SMX). Radicals of OH and SO 4 − as well as non-radicals of 1O 2 and Fe(IV) were all present in BDD/PMS/Fe(III) and DSA/PMS/Fe(III) systems, according to the results of scavenging experiment and electron paramagnetic resonance (EPR) analysis. Interestingly based on the results of chemical probe experiments, the ratio of OH, SO 4 −, and 1O 2 were 9% (7.19 μmol/L), 76% (58.39 μmol/L), and 15% (11.45 μmol/L) in BDD/PMS/Fe(III) system, and were 28% (5.81 μmol/L), 18% (3.76 μmol/L), and 54% (10.93 μmol/L) in DSA/PMS/Fe(III) system. The yield of Fe(IV) in DSA/PMS/Fe(III) process was relatively higher than that in BDD/PMS/Fe(III) process. These results indicated that BDD/PMS/Fe(III) is a radical oxidation system while DSA/PMS/Fe(III) system is mainly non-radical system. Due to the high electrochemical activity of BDD anode, the one-electron transfer reaction might occur to produce more SO 4 −. The co-existing ions including Cl−, H 2 PO 4 −, and NO 3 − showed significant effects on the degradation efficiency of BDD/PMS/Fe(III) and nonsignificant impacts on DSA/PMS/Fe(III) process, indicating that DSA/PMS/Fe(III) process could be less affected by complex water matrix. BDD/PMS/Fe(III) is inclined to oxidize electron-rich contaminants, while DSA/PMS/Fe(III) is apt to oxidize electron-deficient contaminants. The degradation routes of SMX and its toxicity evolution were systematically studied with the analysis of byproducts. Overall results compared the oxidation mechanisms and primary oxidation pathways of electro-cocatalytic systems with BDD versus DSA anodes, and revealed the merits of each system for future selection and system optimization of treatment processes regarding targeted contaminants. [ABSTRACT FROM AUTHOR]

This study investigated the application of a natural plant polyphenol, gallic acid (GA) to form complex with iron to promote the redox cycle of Fe(III)/Fe(II) under neutral initial pH conditions in the electrochemical (EC) system for activation of peroxymonosulfate (PMS) to efficiently degrade carbamazepine (CBZ). Results demonstrated that the synergistic effects of GA and EC significantly improved the removal efficiency, and the EC/GA/Fe(III)/PMS system effectively removed 100% of CBZ within a wide initial pH range of 3.0–7.0. The optimum stoichiometric ratio of GA to Fe(III) was found as 2:1. Investigations including quenching experiment, chemical probe analysis, and electron paramagnetic resonance (EPR) analysis were conducted to identify the primary reaction radicals as •OH, SO 4 •−, along with the 1O 2 and Fe(IV). In the EC/GA/Fe(III)/PMS system, the synergistic effect of GA and electrochemistry led to a remarkable enhancement in the generation of •OH. Furthermore, the complexation reduction mechanism of GA and Fe(III) was proposed based on experimental and instrumental analyses, which demonstrated that the semi-quinone products of GA were the main substances promoting the Fe(III)/Fe(II) cycle. Mass spectrometry results showed that CBZ generated 27 byproducts during degradation, with formic acid as the main product of GA. The degradation efficiency of the EC/GA/Fe(III)/PMS system remained stable and excellent, exhibiting remarkable performance in the presence of various inorganic anions, including Cl− and NO 3 −, as well as naturally occurring organic compounds such as fulvic acid (FA). Overall results indicated that the EC/GA/Fe(III)/PMS system can be applied to effectively treat practical wastewater treatment without requirement of pH adjustment. [Display omitted] • The EC/GA/Fe(III)/PMS system could efficiently remove CBZ at neutral pH condition. • Electro-synergistic GA significantly enhance the cycling of Fe(III)/Fe(III). • Electro-synergistic GA promoted the formation of •OH. • Chelation-reduction between GA and Fe(III) was examined under electric field. • The degradation pathways and byproducts of CBZ and GA were analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The main active species in EC/PI system are •OH, IO 3 •, and 1O 2. • The carbon felt cathode shows excellent capacity for PI activation in EC/PI system. • The EC/PI system is robust with complex water background and wide pH range. • Degradation byproducts and pathways of sulfamethoxazole were revealed. Herein, this work investigated a novel strategy utilizing electrochemical activation of periodate (EC/PI) for effective degradation of sulfamethoxazole (SMX) with wide appliable pH range. The EC/PI system achieved 100% degradation efficiency of SMX under optimum conditions of 7.78 mA/cm2 current density, 1 mmol/L IO 4 − dosage, and 20 mmol/L Na 2 SO 4 as the supporting electrolyte. Strong synergistic effect between applied electric current and IO 4 − activation was observed, indicating the robustness of EC/PI system. Through a series of chemical probe experiments and electron paramagnetic resonance analysis, it was revealed that EC/PI is a composite oxidation system with multiple active species, including hydroxyl radical (•OH), iodate radical (IO 3 •), and singlet oxygen (1O 2). The production mechanisms of these active species were comprised of two routes: direct production via the electrode supplying electrons and indirect production via hydrogen peroxide (H 2 O 2) and superoxide radical (O 2 •−) as intermediates. The EC/PI process was slightly affected by the coexisting substances in the water background, and cycle experiments verified its degradation stability, indicating that this EC/PI system exhibits excellent capacity for treatment of authentic wastewater. Finally, the SMX degradation intermediates and pathways were obtained using mass spectrometry analysis and Gauss theory calculations. These results indicated the establishment of EC/PI system provides an efficient and environmental-friendly strategy for treatment of emerging organic pollutants. [ABSTRACT FROM AUTHOR]

Periodate (PI) has recently been studied as an excellent oxidant in advanced oxidation processes, and its reported mechanism is mainly the formation of reactive oxygen species (ROS). This work presents an efficient approach using N-doped iron-based porous carbon (Fe@N-C) to activate periodate for the degradation of sulfisoxazole (SIZ). Characterization results indicated the catalyst has high catalytic activity, stable structure, and high electron transfer activity. In terms of degradation mechanism, it is pointed out that the non-radical pathway is the dominant mechanism. In order to prove this mechanism, we have carried out scavenging experiments, electron paramagnetic resonance (EPR) analysis, salt bridge experiments and electrochemical experiments, which demonstrate the occurrence of mediated electron transfer mechanism. Fe@N-C could mediate the electron transfer from organic contaminant molecules to PI, thus improving the efficiency of PI utilization, rather than simply inducing the activation of PI through Fe@N-C. The overall results of this study provided a new understanding into the application of Fe@N-C activated PI in wastewater treatment. [Display omitted] • The activation of PI was successfully promoted with Fe@N-C. • The reaction mechanism of Fe@N-C/PI system is electron transfer mechanism. • The degradation pathway of pollutants in Fe@N-C/PI system was proposed. • No iodate and other harmful byproducts were produced during the reaction. [ABSTRACT FROM AUTHOR]

Herein, electro-catalysis (EC) as the electron donor to accelerate the continuable Fe(III)/Fe(II) cycles in different inorganic peroxides (i.e., peroxymonosulfate (PMS), peroxydisulfate (PDS) and hydrogen peroxide (HP)) activation systems were established. These electro-cocatalytic Fenton-like systems exhibited an excellent degradation efficiency of sulfamethoxazole (SMX). A series of analytical and characterization methods including quenching experiments, probe experiments, and electron paramagnetic resonance spectrometry (EPR) were implemented to systematically sort out the source and yield of reactive oxygen species (ROS). A wide kind of ROS including hydroxyl radical (•OH), singlet oxygen (1O 2), and sulfate radical (SO 4 •−), which contributed 38%, 37%, and 24% were produced in EC/Fe(III)/PMS system, respectively. •OH was the dominant ROS in both EC/Fe(III)/PDS and EC/Fe(III)/HP processes. According to the analysis of SMX degradation routes and biotoxicity, abundant degradation pathways were identified in EC/Fe(III)/PMS process and lower environmental impact was achieved in EC/Fe(III)/HP process. The diversiform ROS of EC/Fe(III)/PMS system makes it exhibit greater environmental adaptability in complex water matrixes and excellent low-energy consumption performance in many organic pollutants degradation. Continuous flow treatment experiments proved that the three systems have great sustainability and practical application prospect. This work provides a strong basis for constructing suitable systems to achieve different treatment requirements. [Display omitted] • EC/Fe(III)/oxidant processes exhibited distinguished degradation efficiency of SMX. • Oxidation mechanisms in various EC/Fe(III)/oxidant processes was lucubrated. • Different pollutant degradation routes and biotoxicity were identified. • Sustainability of processes was testified via continuous flow treatment experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Efficient degradation of NAs was achieved by EC/PMS/Fe(III) system. • The redox cycle of Fe(III)/Fe(II) was triggered on the cathode. • The chemical structures of NA molecules affect their reactivity. • •OH, SO 4 •− and 1O 2 are the main reactive species in EC/PMS/Fe(III) system. • CHA degradation pathways and byproducts were revealed. In this study, an electrochemical/peroxymonosulfate/Fe(III) (EC/PMS/Fe(III)) system was established for efficient degradation of four naphthenic acids (NAs) compounds including cyclohexanecarboxylic (CHA), heptanoic (HPA), benzoic (BA) and 2-methylhexanoic acids (2-MHA). The introduction of electric field enhances the redox cycle of Fe(III)/Fe(II) to activate PMS for degredation of NAs. Complete degradation of NAs compounds (5 mg/L) has been achieved within 30 min in the EC/PMS/Fe(III) system. The results of electron paramagnetic resonance (EPR) analysis, scavenging experiment, and chemical probe experiments indicated hydroxyl radical (•OH), sulfate radical (SO 4 •−) and singlet oxygen (1O 2) are primary reactive oxygen species (ROS) in the EC/PMS/Fe(III) system. NAs compounds with different structures exhibit inconsistent degradation efficiency in current system (k CHA > k BA > k HPA > k 2-MHA), depending on the selectivity of the different ROS and stability of respective intermediate organic radicals. Besides, multiple degradation pathways of CHA, mainly including hydroxylation and carbonylation, were proposed by combining the results of mass spectrometry analysis with the calculated Fukui index based on density functional theory (DFT) calculation. In addition, the EC/PMS/Fe(III) system exhibited robust performance with the addition of co-existing ions. This work provides a novel strategy for efficient degradation of NAs for future remediation of petroleum wastewater. [ABSTRACT FROM AUTHOR]

• The EC/PMS/Fe(III)-EDTA system was tested under a pH range of 3–9. • Fe(II)-EDTA complex maintains high reaction rates to degrade ACA at neutral pH. • •OH, 1O 2 , SO 4 •− and Fe(IV) are the dominant reactive species in the system. • The addition of EDTA established a new pathway for producing 1O 2 in the system. • ACA degradation pathways and byproducts were revealed based on mass spectra. In this work, ethylene diamine tetraacetic acid (EDTA) was applied as a chelating agent to stabilize Fe(III)/Fe(II) cycle to effectively activate peroxymonosulfate (PMS) in an electrochemical (EC) co-catalytic system for elimination of naphthenic acids (NAs) under neutral pH condition. 1-Adamantanecarboxylic acid (ACA) was applied as the model compound for NAs that are persistent contaminants in petroleum industrial wastewater. In the PMS/EC/Fe(III)-EDTA system, complete removal of ACA has been achieved in 20-min reaction under neutral pH condition. Degradation mechanisms were investigated via electron paramagnetic resonance (EPR) analysis, probe experiments and quenching experiments to reveal the reactive species •OH, SO 4 •− and 1O 2 for ACA removal, the production of which have been significantly facilitated by the introduction of EDTA complex. A new pathway has been found for the production of 1O 2 via several intermediates including hydrogen peroxide and superoxide radicals. Density functional theory (DFT) calculations and mass spectrometry analysis were applied to predict ACA degradation pathways and byproducts. Decarboxylation, hydroxyl substitution and carbonyl substitution byproducts of ACA were detected. The PMS/EC/Fe(III)-EDTA system suffered nonimportant influence at the presence of humic acid, H 2 PO 4 -, HCO 3 –, NO 3 – and Cl- for ACA removal. Overall results demonstrate the effectiveness and reliability of the PMS/EC/Fe(III)-EDTA system for efficient removal of NAs in petroleum wastewater in natural environment under neutral pH condition. [ABSTRACT FROM AUTHOR]

This work investigated the activation of peroxymonosulfate by electrochemical (EC) system assisted with Fe(III)-nitrilotriacetic acid (NTA) complex for degradation of persistent naphthenic acids (NAs) under neutral initial pH conditions. As NAs are a complicated mixture, 1-adamantanecarboxylic acid (ACA) was selected as the model NA compound for degradation experiment. The addition of NTA is to chelate with Fe(III), gaining stability under neutral pH condition to facilitate the circulation of Fe(II)/Fe(III) by the electrochemical process to activate PMS. The EC/Fe(III)-NTA/PMS system was explored with applicable pH range of 3–9 and an optimized molar ratio 1: 2 for Fe: NTA. Results of quenching and chemical probe experiment together with results of electron paramagnetic resonance (EPR) analysis revealed the main reactive species of the system, including •OH, SO 4 •−, 1O 2 and possibly Fe(IV). With the addition of NTA, the yields of •OH, SO 4 •−, 1O 2 were enhanced. Results of mass spectrometry analysis and DFT calculations indicated the formation of 9 degradation byproducts of ACA via three primary degradation pathways such as hydroxyl substitution, carbonyl substitution, and decarboxylation. Furthermore, the EC/Fe(III)-NTA/PMS system could achieve excellent removal efficiency of ACA with different anions such as Cl-, HCO 3 -, NO 3 - and H 2 PO 4 - in the background. The practical applicability of the system was also verified with the high removal of commercial NAs mixture standard. Overall results have indicated the EC/Fe(III)-NTA/PMS system could be utilized for efficient reclamation of authentic oil and gas industrial wastewater under natural pH conditions. [Display omitted] • The EC/Fe(III)-NTA/PMS system was explored with applicable pH range 3–9. • The redox cycle of Fe(III)-NTA/Fe(II)-NTA was boosted to activate PMS. • Fe(II)-NTA complex maintains high reaction rates to activate PMS at neutral pH. • •OH, SO 4 •−, 1O 2 and Fe(IV) are the main reactive species in the system. • ACA degradation pathways and byproducts were revealed based on mass spectra. [ABSTRACT FROM AUTHOR]