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Due to the fast and high oxidation efficiency for organic pollutants, it is extremely challengeable to develop efficient catalysts for peroxymonosulfate (PMS) to enhance the removal efficiency of pollutants. Herein, we fabricated a bimetallic MOF (CuCo-ABDC) by a facile hydrothermal process, which displayed 2D nanosheets structure and rich active sites. The characterization results showed that CuCo-ABDC-4 with Cu:Co ratio of 1:1 synthesized via temperature gradient ((100 ℃, 24 h) + (150 ℃, 12 h)) had low-crystalline but high catalytic performance towards PMS. With orange G (OG) as the model organic pollutant, CuCo-ABDC-4 displayed the highest catalytic activity for activation of PMS, where 96.4% of OG removal efficiency was achieved within 60 min via a rate constant of 0.052 min−1. The excellent catalytic activity can be attributed to the high density and uniformly distributed unsaturated Co(II) which was considered to be the main active site, and the interaction between Co and Cu species in the bimetallic MOF. The quenching experiments indicated the production of SO 4 •−, · OH, and 1O 2 in CuCo-ABDC-4/PMS system and 1O 2 acted as the predominant reactive species for degradation of OG. Moreover, the presence of NO 2 −, H 2 PO 4 −, SO 4 2−, HCO 3 − and HA exhibited evident inhibition on the OG degradation rate. On the contrary, the existence of CO 3 2− could promote the OG degradation due to the production of carbonate radical. This work introduces a novel and facile fabrication strategy to attain 2D nanosheets structure bimetallic catalysts with high catalytic performance for activation of PMS in wastewater treatment. [Display omitted] • MOFs with rich active sites was prepared by temperature gradient method for the first time. • CuCo-ABDC-4 displayed high catalytic activity for activation of PMS. • 1O2 was the major species coexistence with SO4•− and · OH in CuCo-ABDC-4/PMS system. • Uniformly distributed unsaturated Co(II) and Cu(II), and their interaction effect contributed to catalytic activity. • The existence of CO32− could promote the OG degradation rate [ABSTRACT FROM AUTHOR]

• Synthesis of the nZVI-Ni@BC composites to activate persulfate for TCE degradation. • Excess oxygen functional groups and abundant active sites of nZVI-Ni@BC efficiently activated persulfate. • SO 4 ˉ and OH have dominant roles for complete removal of TCE in the nZVI-Ni@BC-persulfate system. • In the nZVI-Ni@BC-persulfate system acidic solution was favorable for the removal of TCE. • High level of CO 3 2¬, HCO 3 ˉ, and NOM inhibited the overall persulfate oxidation of TCE. In Fenton-like oxidation processes, the use of biochar (BC) as support material for the nanoscale zero valent iron-nickel (nZVI-Ni) bimetallic particles attained much attention to activate persulfate (PS) for TCE degradation in aqueous medium. In present work, nZVI-Ni@BC particles with nZVI-Ni to BC mass ratio of 1:5 exhibited excellent results (> 99 % ± 0.24) for TCE degradation. The physico-chemical characteristics, surface morphologies, and elemental mapping of the synthesized nZVI-Ni@BC particles investigated through SEM, EDX, TEM, XPS, XRD, BET and FTIR spectroscopy. For the nZVI-Ni@BC-persulfate system, the effects of PS concentration, initial pH, inorganic ions and natural organic matter (NOM) on TCE degradation were evaluated. Batch experiments revealed that complete removal of TCE (> 99 % ± 0.25) was attained at 250 mg L−1 of nZVI-Ni@BC and 4.0 mM PS dosages at pH 3.49 ± 0.55. Scavenging and electron paramagnetic resonance (EPR) verified the dominant role of both SO 4 ˉ and OH radicals in acidic environment for degradation of TCE. Moreover, high level of carbonate, bicarbonates and NOM inhibited the overall TCE oxidation reaction. In summary, these results suggested that nZVI-NI@BC composite was an alternative, economic and promising catalyst for the activation of PS to degrade chlorinated contaminants in aqueous medium. [ABSTRACT FROM AUTHOR]

In this study, nano‐CaO2 (nCaO2) was successfully synthesized and constituted the nCaO2/Fe(II) system applying to remediate BTEX, which are typical mixed pollutants in contaminated groundwater. The particle size of the synthesized nCaO2 was 108.91 nm, and it displayed better BTEX remediation performance than that of commercial CaO2. The innovative generation pattern of hydroxyl radicals (HO·) in the nCaO2/Fe(II) system has been investigated using benzoic acid as the HO· probe, and the proper molar ratio of nCaO2/Fe(II) was optimized as 1/1. Over 90% of BTEX was removed in 180 min with the nCaO2/Fe(II)/BTEX molar ratio of 40/40/1. Further experiments evaluated the influence of co‐existence of mixed pollutants chlorinated hydrocarbon compounds (CHCs) or surfactant constituents on BTEX remediation performance. The experimental results suggested that CHCs have limited influence on BTEX removal rate and surfactants have negative effects on BTEX remediation performance in the experimental conditions. In conclusion, the findings in this study could give some inspirations to apply the nCaO2/Fe(II) process in remediating co‐existing pollutants in contaminated groundwater. Practitioner points: nCaO2/Fe(II) system applied to remediate mixed contaminants.HO· generation pattern of the nCaO2/Fe(II) system has been investigated.The influence of chloride hydrocarbon compounds have been studied.The effects of surfactants were evaluated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe 3 S 4 was synthesized and used for the enhancement of advanced oxidation systems. • The enhanced mechanisms on the surface of Fe 3 S 4 were elucidated. • The existing reactive species and their contributions were evaluated. • The degradation pathways and the toxicities of intermediates were investigated. In this research, greigite (Fe 3 S 4) was innovatively applied for the enhancement of PDS/Fe(II) system. From XRD and SEM analyses, Fe 3 S 4 was successfully synthesized and appeared uniform sphere-like particles with a petal-like structure. Trichloroethylene (TCE), as the target, was significantly degraded from 28.5 % to 92.8 % in PDS/Fe(II)/Fe 3 S 4 system. The enhanced mechanisms, differences, and superiority of Fe 3 S 4 against the other three iron sulfides were elucidated as follows: (1) on account of the abundant reduction sites on its surface, Fe 3 S 4 could not only directly release Fe(II) and accelerate Fe(II) regeneration by sulfur substances, but also participate in the process of O 2 −• generation, and (2) the reaction mainly took place on Fe 3 S 4 surface, in which the surface-bound reactive species (RSs) were of great importance for TCE degradation. Through EPR, quantification, and scavenging tests of RSs, HO•, SO 4 −•, and O 2 −• were all the dominant RSs responsible for TCE removal. Besides, these enhanced systems had high tolerance on complex water conditions and could remediate various contaminants in actual groundwater. Three TCE degradation pathways were deduced in four iron sulfide-enhanced systems and the toxicities of intermediates were evaluated, recommending that Fe 3 S 4 -enhanced system was of the least toxicity and most favorable for application in practice. [ABSTRACT FROM AUTHOR]

[Display omitted] • ZVI and B-assisted nCP/Fe(III) systems could achieve efficient decontamination. • The dominant active species in the enhanced nCP/Fe(III) systems were identified. • Fe(II) was generated simultaneously by Fe(III) reduction and ZVI corrosion. • B continuously supplied electrons to Fe(III) to ensure fast Fe(II) generation. • Efficient NAP degradation in various water matrixes was achieved. Nano calcium peroxide (nCP) has attracted attention as an alternative for liquid H 2 O 2 -mediated advanced oxidation processes. However, the slow reduction kinetics of Fe(III) still significantly limit its application. Here, zero valent iron (ZVI) and amorphous boron (B) could remarkably promote the Fe(III)/Fe(II) cycle in nCP/Fe(III) system to generate various active species (HO•, O 2 −•, and 1O 2) with HO• acting as the predominant role in naphthalene (NAP) degradation. nCP/Fe(III)/ZVI process could achieve 98.2% NAP degradation and 70.6% TOC removal, compared with 97.5% NAP degradation and 56.3% TOC removal in nCP/Fe(III)/B process. ZVI could donate electrons to Fe(III) to generate Fe(II) and itself could also be corroded to release Fe(II). B was stepwise oxidized (B0 to B(I)/B(II) to B 2 O 3) and continuously supplied electrons to Fe(III) to ensure fast and long-lasting Fe(II) generation. Moreover, B 2 O 3 was dissolved as H 3 BO 3 in aqueous solution, which kept the B surface reactive for the continuous reduction of Fe(III). Although the two co-activators showed different promotion mechanisms in nCP/Fe(III) system, both systems exhibited excellent NAP degradation performance in different experimental conditions and various water matrixes. Overall, this study provided the strategies to overcome the limitation of nCP/Fe(III) system and extended its application potential in contaminants remediation. [ABSTRACT FROM AUTHOR]

In this study, ferrous sulfide (FeS) was introduced to nano calcium peroxide (nCP)/Fe(III) system to facilitate the generation of Fe(II), more than 90% of naphthalene (NAP) could be removed at a wide pH range of 3–9. As a heterogeneous reductant, FeS could mitigate competitive reactions with reactive oxygen species (ROS), which favored the NAP degradation. As evidenced by scavenging experiments, HO• was the major ROS contributing to NAP degradation. The role of sulfur species (S2−, SO 3 2−, and S 2 O 3 2−) in nCP/Fe(III) system was investigated with S 2 O 3 2− showing the preferable reactivity in Fe(III) reduction. In addition, the surface-bound HO• and surface Fe(II) were detected and the role of them on NAP degradation was revealed and concluded that both dissolved and surface Fe(II) contributed to NAP degradation, whereas surface-bound HO• was not superior to solution HO• in degrading NAP. Furthermore, nCP/Fe(III)/FeS system showed high feasibility to different solution matrixes and various types of water as well as the broad-spectrum reactivity to other toxic organic pollutants, exhibiting promise for practical application to remediate complex contaminants. [Display omitted] • FeS played a vital role in accelerating Fe(II)/Fe(III) circulation. • The competitive reactions in nCP/Fe(III)/FeS system could be minimized. • Mechanisms of NAP degradation in nCP/Fe(III)/FeS system were revealed. • NAP degradation performances in various water matrixes were investigated. • Excellent reactivity for other organic contaminants was achieved. [ABSTRACT FROM AUTHOR]

[Display omitted] • CT removal could be effectively achieved with surfactant presence in Fenton system. • Radical scavenger and EPR tests revealed that surfactant radicals (R•CHOH) with TW-80 presence, and O 2 −• and CO 2 −• with SDS presence were responsible for CT degradation. • Surfactant solutions could be recycled for CT solubilization and degradation. • Significant CT removal in actual groundwater containing TW-80 or SDS was achieved. This study investigates the mechanisms of surfactants in surfactant-in situ chemical oxidation (S-ISCO) for enhanced degradation of perchlorinated organic contaminants. The addition of Tween-80 (TW-80) or sodium dodecyl sulfate (SDS) in Fenton system resulted in a significant improvement in carbon tetrachloride (CT) removal from 6.2 % to 97.2 % and from 8.2 % to 91.8 %, respectively. Upon further analysis of free radical quenching studies and electron paramagnetic resonance (EPR) tests, it was observed that surfactant radicals were responsible for CT removal with TW-80 presence, while superoxide radicals (O 2 −•) and carbon dioxide radicals (CO 2 −•) were the primary radicals with SDS presence. The dechlorination of CT reached 82.1 % and 75.3 % with TW-80 or SDS presence, respectively, indicating that most of the degraded CT could be completely mineralized. The possible degradation pathways of CT were proposed based on the detected intermediates. Additionally, degradation of various single or mixed chlorinated hydrocarbon contaminants with TW-80 or SDS presence in Fenton system was performed. The study also assessed the influence of different solution matrixes on CT degradation and the impact of the recovered surfactant solution after reaction on CT solubilization and removal. In summary, this study provides theoretical support for groundwater remediation with perchlorinated contaminants when containing surfactants. [ABSTRACT FROM AUTHOR]

The application of S(IV) for the regeneration of Fe(II) has been widely investigated. As the common S(IV) sources, sodium sulfite (Na 2 SO 3) and sodium bisulfite (NaHSO 3) are soluble in the solution, resulting in excessive SO 3 2− concentration and redundant radical scavenging problems. In this research, calcium sulfite (CaSO 3) was applied as the substitution for the enhancement of different oxidant/Fe(II) systems. The advantages of CaSO 3 could be summarized as follows: (1) it could sustainedly supplement SO 3 2− for Fe(II) regeneration, preventing radical scavenging and unnecessary reagent waste; (2) the cost and toxicity of CaSO 3 were extremely lower than that of other S(IV) sources; (3) the concentration of reactive species increased in the presence of CaSO 3 ; and (4) after the reaction, SO 4 2− would form CaSO 4 precipitate, which would not increase the burden of SO 4 2− in the solution. In the participation of CaSO 3 , the removal of trichloroethylene (TCE) and other organic contaminants were significantly promoted and different enhanced systems had high tolerance on complex solution conditions. The major reactive species in different systems were determined through qualitative and quantitative analyses. Eventually, the dechlorination and mineralization of TCE were measured and the differentiated degradation pathways in different CaSO 3 -enhanced oxidants/Fe(II) systems were elucidated. [Display omitted] • CaSO 3 was introduced in different oxidants/Fe(II) systems for the removal of various organic contaminants. • The advantages and enhanced mechanisms in CaSO 3 over other S(IV)-based chemicals were compared. • The major reactive species and their quantities were determined. • The differentiated degradation pathways of TCE in various CaSO 3 -enhanced oxidants/Fe(II) systems were elucidated. • The effects of complex solution conditions on contaminant removal were demonstrated. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The oxidation capabilities of CaO 2 /Fe(II) and CaO 2 /Fe(III) systems were compared. • Fe(II)/Fe(III), H 2 O 2 , HO, and intermediate products are evaluated in both systems. • An optimal implementation strategy for the CaO 2 application was proposed. • The strategy derived from benzene degradation works for TCE degradation. Abstract This study conducted a comparison of the CaO 2 -based Fenton (CaO 2 /Fe(II)) and Fenton-like (CaO 2 /Fe(III)) systems on their benzene degradation performance. The H 2 O 2 , Fe(II), Fe(III), and HO variations were investigated during the benzene degradation. Although benzene has been totally removed in the two systems, the variation patterns of the investigated parameters were different, leading to different benzene degradation patterns. In terms of the Fe(II)/Fe(III) conversion, the CaO 2 /Fe(II) and CaO 2 /Fe(III) systems were actually inseparable and had the inherent mechanism relationships. For the CaO 2 /Fe(III) system, the initial Fe(III) must be converted to Fe(II), and then the consequent Fenton reaction could be later developed with the regenerated Fe(II). Moreover, some benzene degradation intermediates could have the ability to facilitate the transformation of the Fe(III) to Fe(II) without the classic H 2 O 2 -associated propagation reactions. By varying the Fe(II) dosing method, an effective degradation strategy has been developed to take advantage of the two CaO 2 -based oxidation systems. The proposed strategy was further successfully tested in TCE degradation, therefore extending the potential for the application of this technique. [ABSTRACT FROM AUTHOR]

• CaSO 3 , slow-released SO 3 2− source, was introduced in S(IV)-based AOPs. • Fe(III)/CaSO 3 system was established for the removal of various organic contaminants. • The major ROS, contaminants degradation mechanism, and pathways were revealed. • The degradation performance of contaminants in complex solution matrixes was investigated. • Extensive applications and broad-spectrum of Fe(III)/CaSO 3 system were demonstrated. S(IV)-based advanced oxidation process has been applied for contaminants remediation. However, as a traditional source of sulfite (SO 3 2−), Na 2 SO 3 is extremely soluble in water, resulting in a high concentration of SO 3 2− to quench the generated reactive oxygen species (ROS). In this work, CaSO 3 was introduced instead of Na 2 SO 3 for its slow-released SO 3 2− ability and Fe(III)/CaSO 3 system was established for the removal of trichloroethylene (TCE) and other organic contaminants. The degradation efficiency of TCE reached 94.0% and TCE could be completely dechlorinated and mineralized, while the removal of other contaminants was all over 85.0% at the optimal tested conditions. Through EPR detection, ROS scavenging and probe tests, and quantification of ROS amounts, it was concluded that the dominant ROS in Fe(III)/CaSO 3 system were SO 4 −· and 1O 2 , of which the transformation mechanism of SO 4 −· to 1O 2 was revealed and demonstrated comprehensively. The synergistic contaminants degradation performance in different sulfur-iron-containing systems and in the presence of oxidants was evaluated. The effects of various solution conditions were assessed and Fe(III)/CaSO 3 system was of higher resistance on complex solution matrixes, suggesting the broad-spectrum and application perspective for the remediation of complex contaminants in actual water. [ABSTRACT FROM AUTHOR]

• HSO 3 – significantly enhanced HO• generation and TCE degradation. • HO• was the dominant ROSs in nCaO 2 /Fe(III)/HSO 3 – system. • The double effect of CaO 2 as both oxidant and O 2 source was clarified. • The enhancement mechanism caused by HSO 3 – was revealed. The effect of bisulfite (HSO 3 –) in nCaO 2 /Fe(III)/HSO 3 – system on improving HO• generation and trichloroethene (TCE) removal was innovatively reported. The enhancement mechanism of HSO 3 – for TCE removal in nCaO 2 /Fe(III)/HSO 3 – system was caused not only by the complexing and reducing effects on promoting the conversion of Fe(III) to Fe(II), but also due to the reaction with O 2 in water to produce SO 4 –• for accelerating TCE degradation. A double effect of nCaO 2 as an oxidant source to generate HO• and as a O 2 source to promote SO 4 –• generation was revealed. A pseudo-second-order kinetic model of TCE removal was determined and 94.6% TCE degradation was achieved within 60 min at the nCaO 2 /Fe(III)/HSO 3 –/TCE molar ratio of 6/12/9/1, in which 82.8% TCE was dechlorinated. The scavenger test and EPR detection found that various kinds of reactive oxygen species (ROSs) including HO•, SO 4 –•, O 2 –• and 1O 2 were generated, all contributed to TCE removal and HO• was the dominant one in nCaO 2 /Fe(III)/HSO 3 – system. The effects of surfactant presence and the complex matrix in the actual groundwater were evaluated. The experiments of high efficiency for chlorinated olefins and benzene series removals but low applicability for chlorinated alkanes provided an alternative technique to remedy groundwater based on the various kinds of contaminants performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel Fe(II)/CYS/SPC process for the degradation of TCA with extensive pH range. • CYS addition promoted the oxidative degradation of contaminant. • CYS presented a dual and stable role in the enhanced remediation of TCA in Fe(II)/CYS/SPC system. • Fe(II)/CYS/SPC process showed good performance in real water system. The removal of toxic contaminants such as trichloroethane (TCA) from groundwater had a huge impact due to associated health hazards. Advanced oxidation technology has an outstanding potential to remove these toxins. However, the redox cycling of Fe(II)/Fe(III) is mostly a barrier for the efficient removal of these contaminants. In this study, we extracted two in one nature of cysteine (CYS) to enhance TCA degradation in Fe(II)/sodium percarbonate (SPC) system. The chelating and reducing capability of CYS, concentration impact of Fe(II), SPC, and pH role were all thoroughly investigated. The existence of hydroxyl (OH●) and superoxide (O 2 –●) radicals were confirmed through quencher tests. Furthermore, the electron paramagnetic resonance (EPR) spectra evaluated the enhancement of OH● radicals in the Fe(II)/CYS/SPC system due to the electron transfer capability of CYS. The Fe(II)/CYS/SPC showed excellent recyclability and stability characteristics up to 68% TCA degradation in the fourth round. The degradation products and dechlorination analysis confirmed the existence of non-toxic final products. Similarly, the system demonstrated admirable performance in a real groundwater system, bolstering its real-time industrial applications. Conclusively, Fe(II)/CYS/SPC can be a viable and effective method for degrading chlorinated organic hydrocarbons in groundwater. [ABSTRACT FROM AUTHOR]

[Display omitted] • The presence of synergistic activated catalysts greatly enhanced HO generation and TCE degradation. • The synergistic activation mechanism was elucidated. • The mechanism of S(-II) enhancement was described in detail. • HO produced from SO 4 − was of greater significance for TCE removal. • The broad-spectrum reactivity of different systems was investigated for other contaminants removal. In this study, trichloroethene (TCE) removal was remarkably enhanced with the addition of Fe(II)/CA (citric acid), Fe(II)/nZVI (nano zero-valent iron) and Fe(II)/S-nZVI (sulfidated nano zero-valent iron), respectively, as synergistic activated catalysts in persulfate environment. Among them, the better performance of TCE removal followed the order of PS/Fe(II)/S-nZVI > PS/Fe(II)/nZVI > PS/Fe(II)/CA ≫ PS/Fe(II), and the majority of TCE was declorinated to Cl−. The scavenger and EPR tests indicated that HO and SO 4 − were responsible for TCE degradation. Further study was conducted through the calculation of reaction rate constants, demonstrating HO produced from SO 4 − was of greater significance than SO 4 − itself. With the presence of CA, the concentration of Fe(II) was maintained at a desired level to proceed the catalytic activity. Meanwhile, S-nZVI and nZVI had ability to convert Fe(III) to Fe(II), and sulfur species, especially S(-II) in S-nZVI acted as an reductant to a certain extent. Sufficient Fe(II) in solution activated PS to produce adequate HO for TCE degradation. Furthermore, the broad-spectrum reactivity in different systems were investigated, and the results indicated that most of the chloroalkenes and benzene series frequently detected in contaminated sites could also be removed at the desired chemicals ratio. In conclusion, this research revealed the superiority of synergistic activated catalysts and provided theoretical bases for the remediation of TCE and other pollutants in actual contaminated groundwater. [ABSTRACT FROM AUTHOR]