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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 insight mechanism of Fe(II)/nFeS activated oxidant (H 2 O 2 , CP & PVA@CP) process for trichloroethylene degradation with prolonged benefits of PVA@CP. • TCE degradation was assessed in various oxic environments by Fe(II)/nFeS. • PVA@nCP system showed a lower rate constant of 0.031 M−1 min−1 than other systems. • Surface-bound radicals contributed to TCE degradation via a diffusion mechanism. • Sulfur species of nFeS converted 10 % of surface Fe(III) to Fe(II). • Comparing to Fe(III), Fe(II) significantly activated PVA@nCP with 91 % TCE removal. This study demonstrated the prolonged benefits of polyvinyl-coated calcium peroxide (PVA@CP) in Fe(II)/nFeS mediated Fenton-like reaction. PVA@CP was prepared by a coating of PVA on calcium peroxide (CP) and synthesized nano-sized iron sulfide (nFeS) was characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) with energy dispersive spectroscopy (EDS) elemental mapping, X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectroscopy (ICP-OES) and Fourier transmission infrared (FTIR) techniques. Trichloroethylene (TCE) degradation in various oxic environments (H 2 O 2 , CP, or PVA@CP) was investigated using Fe(II)/nFeS, and the slowest rate constant of 0.031 M−1 min−1 with 91 % TCE removal highlighted the long-lasting effects of PVA@CP. The effect of initial solution pH revealed that the acidic condition was most favorable. PVA@CP system accumulated the least amount (6.34 mg/L) of Fe(III) than H 2 O 2 and CP systems. Electron paramagnetic resonance (EPR) and scavenging tests highlighted that free hydroxyl radicals (HO free) contributed dominantly in H 2 O 2 system while both free and surface-bound species participated well in CP and PVA@CP systems. Moreover, the insight mechanism of regenerated Fe(II) on the nFeS surface was explored by XPS. Both oxidation and reduction of sulfur were discussed and declared that S(-II) was responsible for regenerating Fe(II). The recycling sample checked the reusability of nFeS, and 98.8 % TCE removal in actual groundwater emphasized that PVA@CP/Fe(II)/nFeS is a viable technique. In conclusion, the PVA@CP/Fe(II)/nFeS system is highly recommended for long-lasting remediation of TCE contaminated groundwater. [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]

[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]

• 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]

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]

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]

• 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]

Naphthalene (NAP) has received increasing concern due to frequent detection in groundwater and harm to humans. In this study, FeS 2 was selected as a novel catalyst to activate nano calcium peroxide (nCP) for NAP degradation. Batch experiments were conducted in a 250 mL glass reactor containing 0.1 mM NAP solution to investigate the effect of reagents dosage, pH, air conditions (with or without N 2 purge), and different solution matrixes on NAP degradation. Scavenging tests, electron paramagnetic resonance (EPR) spectrum, and radical probe tests were conducted to identify the main radicals. Results indicated that over 96% NAP was removed in a wide pH range (3.0–9.0) within 180 min at optimal dosage of nCP = 1.0 mM and FeS 2 = 5.0 g L−1 in nCP/FeS 2 system. Aerobic condition was more beneficial to NAP degradation and the system could tolerate complex solution conditions. Moreover, HO• was determined to be responsible for NAP degradation. NAP degradation intermediates were detected by gas chromatography-mass spectrometry (GC-MS) and the possible degradation pathways were revealed. Finally, the efficient degradation of other organic pollutants confirmed the broad-spectrum reactivity of the nCP/FeS 2 system. Overall, these findings strongly demonstrated the potential applicability of nCP/FeS 2 system in remediating organic contaminated groundwater. [Display omitted] • nCP/FeS 2 system could degrade NAP efficiently at a wide pH range of 3.0–9.0. • FeS 2 played a significant role in Fe(II)/Fe(III) cycle. • Mechanisms of NAP degradation in nCP/FeS 2 system were revealed. • Performance of NAP degradation in different solution matrixes was investigated. • Excellent reactivity for other organic pollutants in nCP/FeS 2 system was achieved. [ABSTRACT FROM AUTHOR]

• PVA@nCP was proposed as a slow-release H 2 O 2 oxidant. • nFeS significantly enhanced TCE degradation in the presence of Fe(II) or Fe(III) • Regenerated Fe(II) enhanced TCE removal in PVA@nCP/Fe(III)/nFeS system. • XPS, ESR and quenching tests confirmed the synergistic mechanism. • The proposed techniques exhibited potential for groundwater remediation. This study demonstrated a novel approach for prolonged benefits in groundwater remediation schemes utilizing PVA coated nano calcium peroxide (PVA@nCP) activated by Fe(II)/nFeS or Fe(III)/nFeS. Various techniques (SEM, TEM, XRD, EDS, FTIR, and BET) were used to characterize the synthesized nFeS and the prepared PVA@nCP. Trichloroethylene (TCE) was chosen as the target pollutant and its degradation performance was assessed in both Fe(II)/nFeS and Fe(III)/nFeS activated PVA@nCP systems, and the second-order reaction kinetics were determined. The effect of nFeS dose revealed its dominant role in Fe(III)/nFeS system by providing a surface platform, and at the same time, it also enhanced TCE removal in Fe(II)/nFeS system·H 2 O 2 release behavior, the participation of Fe(II) in both solution and surface, and the generated (free and surface-bound) active species (HO , O 2 − &1O 2) determined the insights of synergistic mechanisms. Electron spin resonance (ESR) identified the active species initially, which correspondingly confirmed by quenching tests. Moreover, X-ray photoelectron spectroscopy (XPS) investigated the surface reaction mechanism, showing the dominant role of S(-II) as an efficient converter of iron species as Fe(II) increased from 35% to 45% and 45.5% and Fe(III) decreased from 65% to 55% and 54.5% in Fe(II)/nFeS and Fe(III)/nFeS systems respectively. The effects of initial solution pH and common anions (Cl−, HCO 3 −, NO 3 −, & SO 4 2−) on TCE removal were further examined, and recycling use of nFeS was investigated for its reusability in both systems. The findings of this work suggest that both systems, i.e., PVA@nCP/Fe(II)/nFeS and PVA@nCP/Fe(III)/nFeS, are novel processes and possessing potential applicability in TCE contaminated groundwater remediation. [ABSTRACT FROM AUTHOR]