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• nCaO 2 performed better for TCE degradation than conventional CaO 2. • Citric acid significantly enhanced TCE degradation. • HO was the primary ROSs in CaO 2 /Fe(II) and CaO 2 /Fe(II)–CA systems. • Cl− and HCO 3 − inhibited TCE degradation at different levels. • Effective degradation of TCE in actual groundwater was achieved. In this study, nCaO 2 was synthesized successfully and applied in the Fe(II)-based catalytic environments in investigating trichloroethylene (TCE) removal performance. nCaO 2 with the particle sizes in the range of 50–200 nm was prepared, and it performed better for TCE removal when compared to the conventional CaO 2. Further experimental results showed that 70.4% of TCE could be removed in 180 min at the nCaO 2 /Fe(II)/TCE molar ratio of 1/2/1, while this data was elevated to 86.1% in the presence of citric acid (CA) at the nCaO 2 /Fe(II)/CA/TCE molar ratio of 1/2/2/1 in the same test period. Probe compound tests, specifically designed for free radicals confirmation, demonstrated the presence of HO and O 2 − . Moreover, scavenging tests indicated that HO was the major radical responsible for TCE degradation but O 2 − promoted the removal of TCE in both nCaO 2 /Fe(II) and nCaO 2 /Fe(II)-CA system. In addition, the effects of initial solution pH and anions (Cl−, HCO 3 −) were also evaluated. The performance of TCE degradation in actual groundwater demonstrated that both nCaO 2 /Fe(II) and nCaO 2 /Fe(II)-CA systems can be applicable for TCE removal in ISCO practice and the nCaO 2 /Fe(II)-CA system is much promising technique. These fundamental data strongly confirmed the feasibility and potential of nCaO 2 based technique in the remediation of TCE contaminated groundwater. [ABSTRACT FROM AUTHOR]

The degradation of carbon tetrachloride (CT) by Fe(II) activated calcium peroxide (CaO 2 ) in the presence of ethanol (CH 3 CH 2 OH, EtOH) was investigated. The results showed that CT could be completely degraded in 20 min with 3.90 mmol L −1 CaO 2 , 5.20 mmol L −1 Fe(II) and 1.30 mmol L −1 EtOH in CaO 2 /Fe(II)/EtOH system. Superoxide radical anion (O 2 •− ) and hydroxyethyl radical ( • CHCH 3 OH, HER) were demonstrated as the main radicals for CT degradation which were confirmed by scavenging tests and electron paramagnetic resonance (EPR) detection. The mechanism of CT degradation was mainly due to the split of C–Cl and CT degradation pathway was proposed accordingly. Further investigation suggested that the solution pH in range of 3–5 were favorable to CT degradation. Both Cl − and HCO 3 − had inhibitive effect on CT removal, while the influence of NO 3 − and SO 4 2− could be negligible. Moreover, the results of experiments performed using actual groundwater demonstrated that an adverse impact occurred on CT degradation, but this drawback could be overcome by increasing reagent dosages. In summary, CT degradation in Fe(II) activated CaO 2 system can be significantly promoted by EtOH addition, and this finding, for the first time, provides an innovative technology for degradation of the highly oxidized organic contaminants. [ABSTRACT FROM AUTHOR]

[Display omitted] • The presence of TW-80 improved the reductive degradation of CT in PS oxidation system. • The strengthening effect of CA or nZVI on CT removal in PS/Fe(II) system was confirmed. • Both the secondary surfactant radicals (R•CHOH) and O 2 −• were responsible for CT removal. • Efficient degradation of mixed TCE, PCE, and CT was achieved with TW-80 presence in both PS/Fe(II)/CA and PS/Fe(II)/nZVI systems. Despite the wide range of applications for persulfate-based advanced oxidation processes (PS-based AOPs) in groundwater remediation, the elimination of perchlorinated hydrocarbon contaminants such as carbon tetrachloride (CT) remains a challenge. In this research, we discovered that CT degradation in PS/Fe(II) system was greatly promoted with the presence of surfactant Tween-80 (TW-80), whereas it was hardly removed without TW-80 presence. The introduction of citric acid (CA) or nano zero-valent iron (nZVI) could both enhance the efficacy of PS/Fe(II) system. Through scavenging experiments and electron paramagnetic resonance analysis, the generated secondary surfactant radicals (R•CHOH) from TW-80 and superoxide radicals (O 2 −•) were identified as the driving factors behind CT degradation. CT dechlorination reached 70.6% and 71.8% in PS/Fe(II)/CA and PS/Fe(II)/nZVI systems, respectively, and the potential degradation routes of CT were offered. Additionally, the effect of TW-80 concentration and solution matrixes (solution pH and inorganic ions) on CT degradation was assessed. Furthermore, the removal of mixed chlorinated hydrocarbon contaminants and actual groundwater tests were also performed. Given that surfactants are commonly used in soil and groundwater remediation, this research provides a new prospect and theoretical support for the treatment of perchlorinated contaminants in groundwater with the presence of surfactants. [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] • Over 90% NAP could be removed in PDS/Fe(II)/CaSO 3 and PMS/Fe(II)/CaSO 3 systems. • The enhancement mechanism of CaSO 3 on the removal of NAP was illuminated. • SO 4 − was the dominant ROS for NAP degradation in CaSO 3 -enhanced systems. • NAP degradation intermediates and pathways were elucidated. • The applicability of CaSO 3 -enhanced systems in various water matrixes was assessed. Na 2 SO 3 and NaHSO 3 have been widely used as the source of SO 3 2− to participate in the reproduction of Fe(II) in iron-based advanced oxidation processes. In this work, CaSO 3 with low solubility was innovatively introduced and compared with Na 2 SO 3 and NaHSO 3 to investigate their effects on naphthalene (NAP) degradation in Fe(II)-activated peroxydisulfate (PDS) and peroxymonosulfate (PMS) processes. The results showed that CaSO 3 displayed a better performance due to its sustained-release of SO 3 2−, and NAP removal increased from 57.6% and 77.7% to 91.4% and 98.5% in PDS/Fe(II) and PMS/Fe(II) processes, respectively, with the addition of CaSO 3. The enhancement mechanisms of CaSO 3 were illustrated by measuring the variation of iron and SO 3 2− concentrations and by quantitatively determining the production of reactive oxygen species (ROS). The dominant ROS generated in CaSO 3 -enhanced systems was confirmed by scavenging tests. Moreover, NAP degradation intermediates and pathways, as well as the toxicological properties of intermediates, were accordingly elucidated. Finally, CaSO 3 -enhanced systems had a wide application range of pH, and exhibited a great performance on the tolerance of various water matrixes. The significant removal of various contaminants in CaSO 3 -enhanced processes confirmed that these techniques are suitable for the remediation of organic contaminated groundwater. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanisms of FLT degradation by FeS 2 /SPC process were revealed. • HO•, CO 3 –•, O 2 –•, and 1O 2 were participated in FLT degradation. • Three degradation pathways of FLT were proposed. • The effects of various solution matrixes on FLT removal were explored. • The effects of surfactants and organic acids on FLT solubilization from soil were investigated. Sodium percarbonate (SPC) induced advanced oxidation processes have been used for the remediation of polycyclic aromatic hydrocarbons (PAHs) contamination, however, the feasibility studies of SPC and pyrite (FeS 2) process are scarce. In this work, FeS 2 served as a catalyst and reductant to activate SPC for fluoranthene (FLT) removal. 100% FLT removal was obtained at an initial solution pH of 5.90 with FeS 2 and SPC dosages of 0.5 g/L and 0.02 mM, respectively, and the influences of initial aqueous solution pH, dissolved oxygen, and various aqueous solution matrixes on FLT degradation were explored. Based on the results of electron paramagnetic resonance detection and scavenging experiments, it was demonstrated that HO•, 1O 2 , and O 2 −• played dominant roles in the removal of FLT, whereas CO 3 −• could only be involved in the ROS proliferative reaction in FeS 2 /SPC process due to the weak oxidizing capacity. Additionally, three FLT degradation pathways were proposed based on the detected degradation intermediates 1-phenylnaphthalene, indane, and phthalic acid. Finally, the desorption of FLT in soil was investigated by employing surfactants and organic acids, with organic acids being less effective than surfactants. Since the majority of organic acids can be utilized as advanced oxidation process promoters, their practical application is advantageous. [ABSTRACT FROM AUTHOR]

• Glutamate (Glu) significantly enhanced carbon tetrachloride (CT) degradation. • O 2 − was one of the primary reactive species responsible to degrade CT. • The introduction of Glu benefited the Fe(III)/Fe(II) recycle and OH generation. • Simultaneous removal of CT and trichloroethene was attained. In this study, we examined the effects of chelators on reductive degradation of carbon tetrachloride (CT) in Fe(II) activated CaO 2 system in the presence of methanol (MeOH). The results suggested that glutamate (Glu) could significantly enhance CT degradation percent with around 25.9% increase in 20 min at the initial CaO 2 /Fe(II)/MeOH/Glu/CT molar ratios of 20/20/10/10/1. In the system, MeOH and Glu acted through different mechanisms, in which MeOH was used to directly stimulate the reductants formation and its reactivity, whereas Glu was acted to keep relatively high soluble iron and thus promote Fenton reaction. The results of scavenging tests suggested that O 2 − was one of the reductive radicals for CT depletion and the mechanism of CT degradation was significantly distinguished from trichloroethene (TCE) degradation which was primarily through oxidation by OH. CH 2 Cl 2 , C 2 Cl 4 , and C 2 Cl 6 were detected as the intermediates accompanied by CT destruction, and a reduction mechanism for CT degradation by the CaO 2 /Fe(II)/MeOH/Glu process was proposed accordingly. Finally, the simultaneous removal of CT and TCE was achieved in the CaO 2 /Fe(II)/MeOH/Glu system, indicating a potential technique for complex contaminated sites remediation. [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]

[Display omitted] • The homogeneous reaction played a dominant role in H 2 O 2 /FeS 2 /NAP system. • FeS 2 performed best compared with S-nZVI and FeS. • Sulfur intermediates from FeS 2 facilitated the iron redox cycle. • FeS 2 performed well in complex environment conditions. Fe-containing natural minerals, such as pyrite, have been conducted an extensive study in Fenton-like processes, however, answers for the role of homogeneous and heterogeneous reactions and their application potentials need to be theoretically demonstrated. In this study, the performance and mechanism of Fenton-like process catalyzed by pyrite were investigated by surface characterizations and degradation experiments using naphthalene as a target contaminant. First, the major role of homogeneous reaction in the naphthalene degradation in H 2 O 2 /pyrite system was confirmed by iron determination and iron quenching tests, where 2,2′-bipyridine was used as the iron quencher. Second, it was clarified that sulfur species including polysulfides, elemental sulfur, and thiosulfate facilitated the Iron redox cycle and the yield of reactive oxygen species, thus enhanced the degradation of naphthalene. Moreover, compared with sulfidated nanoscale zero valent iron and ferrous sulfide, pyrite exhibited superiority in naphthalene degradation due to higher reaction stoichiometry efficiency (0.31) and lower running cost (77 RMB (kg naphthalene)−1). Finally, pyrite showed excellent performance for various pollutants removal and in multiple water environments. This work provides the underlying mechanisms of H 2 O 2 /pyrite process and a solid step forward to pyrite application in the remediation of naphthalene contaminated 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] • 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]

[Display omitted] • The influence of surfactants TW-80 and SDS on TCE degradation was investigated. • The strengthening effect of nZVI on TCE degradation in PS/Fe(II) system was proved. • HO and SO 4 − were the primary ROSs in PS/Fe(II)/nZVI system in the presence of TW-80 or SDS. • Significant TCE degradation in actual groundwater containing TW-80 or SDS was achieved. Trichloroethylene (TCE) removal performance in non-ionic surfactant Tween-80 (TW-80) and anionic surfactant sodium dodecyl sulfate (SDS) involved solution by Fe(II) activated persulfate (PS) was investigated through the strengthening of nanoscale zero valent iron (nZVI). 99.5% removal of TCE could be obtained at the PS/Fe(II)/nZVI/TCE molar ratio of 8/4/4/1 with 1.0 critical micelle concentration (CMC) of TW-80 presence (13 mg L−1), while this value was slightly decreased to 88.3% at the molar ratio of 40/20/20/1 with 1.0 CMC of SDS presence (2.3 g L−1) in 180 min. The addition of nZVI significantly promoted TCE degradation, while TW-80 or SDS definitely inhibited TCE removal. Further, TCE removal declined with TW-80 or SDS concentration increasing from 0 to 10 CMC, and continuously decreased at 20 CMC of TW-80 but elevated TCE removal at 20 CMC of SDS. Tests of radical scavengers revealed that HO and SO 4 − contributed a major part while O 2 − had less contribution to TCE degradation in PS/Fe(II)/nZVI system. The dichlorination of TCE in PS/Fe(II)/nZVI system was 94.5% with TW-80 and 97.2% with SDS. In addition, TCE (with TW-80) removal was decreased with the initial pH increase from 3.0 to 11.0, while TCE (with SDS) degradation was always beyond 83% in the initial pH change from 3.0 to 11.0. Moreover, the effects of inorganic anions (Cl−, HCO 3 −) in the solution were evaluated. Experimental results using the actual groundwater revealed the excellent availability of PS/Fe(II)/nZVI system in remediating TCE contaminated field involved with TW-80 or SDS surfactants. In summary, these findings provide a new direction in remediating groundwater from contaminated sites polluted by chlorinated organic solvents after pretreatment by surfactant solubilization. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SPC/Fe(II) system in the environment of SDS presence applied to remediate CT under acidic condition. • The role of SDS in the SPC/Fe(II) system in S-ISCO process was investigated. • HO•, O 2 −•, CO 2 −• and 1O 2 were the main ROSs in SPC/Fe(II) system in the environment of SDS. • The mechanisms of the specific redox degradation pathway of CT were revealed. • The SPC/Fe(II) system with SDS presence possessed a great potential for the remediation of CT-contaminated groundwater. This work innovatively found that Fe(II) activated sodium percarbonate (SPC) process could successfully degrade carbon tetrachloride (CT) in the environment of sodium dodecyl sulfate (SDS). CT degradation was remarkably accelerated to 87.2% at the optimum SPC/Fe(II)/CT molar ratio of 5/10/1 with the environment of 0.2 critical micelle concentration (CMC) SDS concentration. Fe(II) could effectively catalyze the decomposition of SPC to generate reactive oxygen species (ROSs). The results demonstrated that SDS enhanced the Fenton system and developed ROSs genertion. Hydroxyl radical (HO•), superoxide radical (O 2 −•), carbon dioxide radical (CO 2 −•), and singlet oxygen (1O 2) were mainly generated in SPC/Fe(II) system under the condition of 0.2 CMC SDS, while CO 2 −• and 1O 2 were the dominant active substances at 2.0 CMC SDS presence. The SPC/Fe(II) process performed much effective for the efficient degradation of CT at the acidic pH condition. The specific redox degradation pathway of CT started from the reduction of CT to •CCl 3 through CO 2 −• and O 2 −• to form intermediates that were further dechlorinated by ROSs. In conclusion, the application of Fe(II) catalyzed SPC in the environment of SDS is a promising technology in converting the oxidation process into the simultaneous existence of oxidation and reduction process. [ABSTRACT FROM AUTHOR]

[Display omitted] • 98.7% removal of NAP was achieved in actual groundwater. • The strengthening effect of CA on NAP degradation was investigated. • HO• was the predominant ROSs in PS/Fe(II)/CA system for NAP degradation. • The effects of TW-80 and SDS on NAP degradation were investigated. • The possible degradation pathways of NAP were proposed. More and more attention has been paid to polycyclic aromatic hydrocarbons (PAHs) contaminated sites caused by coking industries and chemical manufactures. In this study, the degradation performance of naphthalene (NAP), one of the typical pollutants in PAHs contaminated sites, in persulfate (PS) oxidation system activated by Fe(II) coupled with citric acid (CA) was investigated. The effects of PS, CA, and Fe(II) dosages on NAP were evaluated. 97.5% removal of NAP was obtained at the molar ratio of 15/5/1/1 of PS/Fe(II)/CA/NAP within 120 min, demonstrating that the addition of CA could significantly strengthen NAP degradation. The results of probe tests, EPR detection and scavenging experiments suggested that HO• played a predominant role in the degradation of NAP. The effect of surfactants including tween-80 (TW-80) and sodium dodecyl sulfate (SDS) on NAP degradation was explored. Moreover, the major degradation intermediates of NAP were determined by GC–MS and the possible degradation pathways of NAP were proposed. Finally, the effectiveness of PS/Fe(II)/CA process for NAP degradation in the actual groundwater demonstrated that this technique has a remarkable preponderance and prospect in NAP contaminated groundwater remediation. [ABSTRACT FROM AUTHOR]