Degradation Of Carbon Tetrachloride Research Articles

Enhanced Fe(OH)2-driven reductive Dechlorination via shortened Fe-O bonds and colloidal medium

Fe2+ is usually adsorbed to the surface of iron-bearing clay, and iron (hydr)oxide in groundwater. However, the reductive activity of Fe(OH)2, a prevalent intermediate during the transformation of Fe2+, remains unclear. In this study, high-purity Fe(OH)2 was synthesized and tested for its activity in the degradation of carbon tetrachloride (CT). XRD data confirm that the synthesized material is a pure Fe(OH)2 crystal, exhibiting sharp peaks of (001) and (100) facets. Zeta potential analysis confirms that the off-white Fe(OH)2 is a colloidal suspension with a positive charge of ∼+35-50 mV. FTIR spectra reveal the formation of a coordination compound Fe2+ with OH−/OD−, derived from NaOH/OD. SEM and HRTEM results demonstrate that the Fe(OH)2 crystal has a regular octahedral structure with a size of ∼30-70 nm and average lattice spacings of 2.58 Å. Mössbauer spectrum verifies that the Fe2+ in Fe(OH)2/Fe(OD)2 is hexacoordinated with six Fe-O bonds. XAFS data demonstrate that the Fe-O bonds become shorter as the OH−:Fe(II) ratios increase. DFT results indicate that the (100) crystal face of Fe(OH)2 more readily transfers electrons to CT. In addition to being adsorbed to iron compounds, structural Fe2+ compounds such as Fe(OH)2 could also accelerate the electron transfer from Fe2+ to CT through shortened Fe-O bonds. The rate constant of CT reduction by Fe(OH)2 is as high as 0.794 min−1 when the OH−:Fe(II) ratio is 2.5 in water. This study aims to enhance our understanding of the structure-reactivity relationship of Fe2+ compounds in groundwater, particularly in relation to electron transfer mechanisms.

Reductive carbon tetrachloride degradation by surfactant derived radicals in persulfate activation system

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 (O2−•) 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.

Nitrate enhanced the carbon tetrachloride degradation in the UV/HCOOH reductive system

Catalytic hydrodechlorination is one of the most effective methods for remediating chlorinated organic wastewater. The influence of nitrate on the production capacity of the carbon dioxide radical anion (CO2·-) and the reduction efficiency of carbon tetrachloride (CT) in an ultraviolet/formic acid (UV/HCOOH) system was evaluated in this study. In the water distribution system, nitrate enhanced the degradation efficiency of CT (5 mg L−1) from 19.1 % to 95.2 % using formic acid (20 mM) under UV radiation, and the degradation rate was positively related to the nitrate concentration. Chloroform was the main by-product during the CT reduction and conversion and could be further dechlorinated to dichloromethane. Moreover, the environmental adaptability of UV/HCOOH/NO3 was better than that of UV/HCOOH over a wide pH range, and the degradation efficiency of CT in the UV/HCOOH/NO3 system was 83.4 % at pH 12. Except for the high concentrations of bicarbonate ions and humic acid, chloride and sulfate had little effect on the CT reduction efficiency in the UV/HCOOH/NO3 system. Experiments by electron paramagnetic resonance and quenching experiment showed that CO2·- radicals were primarily responsible for the CT reduction. In the water distribution system, the addition of nitrate enhances the rate of CO2·- radical generation in the UV/HCOOH system. The novel insights in this study highlight the significant potential of nitrate solutions to repair chlorinated solvents by enhancing the generation of CO2·- radicals in UV/HCOOH systems.

Mechanistic insights into surfactant in enhanced reductive degradation of perchlorinated organic contaminants in Fenton system

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 (O2−•) and carbon dioxide radicals (CO2−•) 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.

A microbial consortium led by a novel Pseudomonas species enables degradation of carbon tetrachloride under aerobic conditions

Carbon tetrachloride (CT) is a recalcitrant and high priority pollutant known for its toxicity, environmental prevalence, and inhibitory activities. Although much is known about anaerobic CT biodegradation, microbial degradation of CT under aerobic conditions has not yet been reported. This study reports for the first time the enrichment of a stable aerobic CT-degrading bacterial consortium, from a CT-contaminated groundwater sample, capable of co-metabolically degrading 30 μM of CT within a week. A Pseudomonas strain (designated as Stari2) that is the predominant bacterium in this consortium was isolated, and further characterization showed that this bacterium can tolerate and co-metabolically degrade up to 5 mM of CT under aerobic conditions in the presence of different carbon/energy sources. The CT biodegradation profiles of strain Stari2 and the consortium were found to be identical, while no significant positive correlation between strain Stari2 and other bacteria was observed in the consortium during the period of higher CT biodegradation. These results confirmed that the isolated Pseudomonas strain Stari2 is the key player in the consortium catalyzing the biodegradation of CT. No chloroform (CF) or other chlorinated compound was detected during the cometabolism of CT. The whole genome sequencing of strain Stari2 showed that it is a novel Pseudomonas species. The findings demonstrated that biodegradation of CT under aerobic conditions is feasible, and the isolated CT-degrader Pseudomonas sp. strain Stari2 has a great potential for in-situ bioremediation of CT-contaminated environments.

Toward rapid reduction of carbon tetrachloride in water by zero-valent aluminum/persulfate system.

The oxidation performance of the zero-valent aluminum (ZVAl)/persulfate (PS) combined system had been studied by researchers in the past, which relied on the activation of PS by ZVAl to generate potent oxidizing radicals (•OH and SO4•-) to degrade pollutants. However, ZVAl is a strong reductant and its reduction effect cannot be ignored. The reductive performance of the ZVAl/PS combined system is still unknown. Therefore, carbon tetrachloride (CT), an antioxidant organic pollutant, was selected as the target pollutant to test the reductive performance of the ZVAl/PS system in this study. We found a significant synergistic effect between ZVAl and PS, and the ZVAl/PS combined system could rapidly degrade CT in a wide pH range of 3-11 after an induction period. By SEM-EDS, TEM, XPS, and XRD analysis, it was found that PS could promote the corrosion of the oxide film on the ZVAl surface. The quenching experiment proved that PS could accept the electrons released from ZVAl to produce superoxide radical anion (O2•-), which led to the degradation of CT rather than the oxidative process by •OH and SO4•-. The hydrogen evolution experiment indicated that electronic reduction might play a secondary role in CT degradation. In conclusion, our study further explored the reductive performance of the ZVAl/PS combined system and expanded the pathway of CT degradation without any organic solvent addition, which provides a new strategy for the efficient degradation of refractory halogenated organic pollutants.

Effect of the degradation performance on carbon tetrachloride by anaerobic co-metabolism under different external energy sources

In this research, a comprehensive study was carried out on the removal of carbon tetrachloride (CT) in the anaerobic co-metabolism (ACM) reactor. The experiments showed that when the hydraulic retention time (HRT) was 36 h, pH was 7, and influent CT was 2.5mg/L, the average removal efficiency reached 82.45 ± 2.56% in the glucose co-metabolism substrate reactor, exhibiting a dramatic excellent difference in reaction performance from the other two reactors (p < 0.05) and a favorable tolerance on the CT shock loading. The content of extracellular polymeric substances (EPS) and volatile fatty acids (VFA) demonstrated that glucose could supply more energy to protect the microorganisms, which was the appropriate external energy source. Moreover, microbial community structure and biostatistics analysis demonstrated that Pseudomonas was the most important dechlorination bacteria in ACM reactors, which might via dehalogenation process mediate the transformation of CT. The succession of methanogenic bacteria further demonstrated that CT degradation using co-digestion require to destroy hydrogenotrophic methane generation pathway and the external energy substances could make up the lack of hydrogen in the treatment of CT. The change of intermediate products hinted that anaerobic dechlorination process of CT in an ACM reactor was a sequential dechlorination process, and major transformation products measured were CF. Overall, this study has improved our understanding of the roles of CT degradation process in ACM reactors.

Reductive degradation of carbon tetrachloride with guava leaf extract

Guava leaves contain polyphenols, which exhibit antioxidant activities and act as electron suppliers. It was postulated that guava leaf extract (GLE) may have the potential to be used as a natural reducing agent for in situ chemical reduction of chlorinated solvents, such as carbon tetrachloride (CT). Extraction of GLE was conducted by boiling guava leaves, and filtering the liquid to produce GLE-Filtrate. GLE-Powder was produced by drying the GLE-Filtrate at 50 °C. Characterization analysis showed a yield of 16.6 g GLE-Powder/100 g of leaves, with a total polyphenolic content of 300 mg g−1, and with gallocatechin as a major constituent. Both GLE-Powder and GLE-Filtrate exhibited the capability to degrade CT in aqueous phase. The CT degradation rate increased with increasing alkaline conditions, at pH higher than the pKa of polyphenols. GLE-Filtrate reductive degradation of CT can be enhanced with the presence of iron salts, and various minerals, or soils containing iron, at pH 10. The specific surface area rate constant (kSA) of CT degradation for the GLE-Filtrate/Fe2O3 aqueous system was as high as 5.52 × 10−3 L d−1 m−2. The analysis of byproducts formation found trichloromethane to be a major intermediate, which was further mineralized to generate Cl−.

Transformation mechanism of carbon tetrachloride and the associated micro-ecology in landfill cover, a typical functional layer zone

Landfill is one of the important sources of carbon tetrachloride (CT) pollution, and it is important to understand the degradation mechanism of CT in landfill cover for better control. In this study, a simulated landfill cover system was set up, and the biotransformation mechanism of CT and the associated micro-ecology were investigated. The results showed that three stable functional zones along the depth, i.e., aerobic zone (0-15 cm), anoxic zone (15-45 cm) and anaerobic zone (> 45 cm), were generated because of long-term biological oxidation in landfill cover. There were significant differences in redox condition and microbial community structure in each zone, which provided microbial resources and favorable conditions for CT degradation. The results of biodegradation indicated that dechlorination of CT produced chloroform (CF), dichloromethane (DCM) and Cl- in anaerobic and anoxic zones. The highest concentration of dechlorination products occurred at 30 cm, which were degraded rapidly in aerobic zone. In addition, CT degradation rate was 13.2-103.6 μg/(m2·d), which decreased with the increase of landfill gas flux. The analysis of diversity sequencing revealed that Mesorhizobium, Thiobacillus and Intrasporangium were potential CT-degraders in aerobic, anaerobic and anoxic zone, respectively. Moreover, six species of dechlorination bacteria and eighteen species of methanotrophs were also responsible for anaerobic transformation of CT and aerobic degradation of CF and DCM, respectively. Interestingly, anaerobic dechlorination and aerobic transformation occurred simultaneously in the anoxic zone in landfill cover. Furthermore, analysis of degradation mechanism suggested that generation of stable anaerobic-anoxic-aerobic zone by regulation was very important for the harmless removal of full halogenated hydrocarbon in vadose zone, and the increase of anoxic zone scale enhanced their removal. These results provide theoretical guidance for the removal of chlorinated pollutants in landfills.

Insight into the action of magnetite loaded with nano Ni/Fe bimetallic particles (Ni4/Fe@Fe3O4) toward carbon tetrachloride degradation in aqueous solution

Nanometer zero-valent iron (nZVI), Fe2+ and H2 were potential electron donors for CT removal by nZVI. However, the action of these electron donors in CT reduction was often ambiguous. In this study, the actual catalytic activity of Ni4/Fe@Fe3O4 under common ions and humic acid (HA) was investigated. The results showed that Na+ and K+ had no effect on CT removal, Cl‐, SO42‐ and low concentration of Mg2+ was beneficial to CT removal, while NO3‐, HCO3‐, Ca2+ and HA exhibited the negative effects. According to the analysis results of gas chromatography (GC) and ion chromatography (IC), a possible dechlorination pathway of CT was proposed. Furthermore, the action mechanism of nZVI, Ni0 and magnetite (Fe3O4) on CT removal were explored. Among the three possible nZVI reducing mechanism, the use of H2 by Ni0 for the hydrodechlorination was dominant, while the direct reduction of nZVI was secondary. Fe2+, the product of nZVI corrosion, contributed little to CT degradation and required longer reaction time. Ni0 could not only utilize H2 but also form galvanic cells with nZVI to accelerate CT dechlorination. In addition, the support Fe3O4 significantly improved the dispersion of Ni/Fe nanoparticles and its corrosion product Fe2+ also promoted the CT removal. These findings were significant for comprehending the mechanisms of bimetallic nanocomposites in reducing CT, and might guide to the development of nZVI based technology for repairing polluted water or soil with organochlorine pollutants.

Enhanced dechlorination of carbon tetrachloride by Ni-doped zero-valent iron nanoparticles @ magnetic Fe3O4 (Ni4/Fe@Fe3O4) nanocomposites

A bimetallic nanocomposite, named as Ni4/Fe@Fe3O4, which was prepared via loading Fe/Ni bimetallic onto magnetite (Fe3O4), was applied to the reductive dechlorination of carbon tetrachloride (CT) for the first time and was identified by scanning electron microscopy (SEM), X‐ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) to explore its structure and properties. The effects of the Ni loading (weight ratio of Fe/Ni), CT concentration, initial solution pH, temperature and Ni4/Fe@Fe3O4 dosage on the reaction system were studied. The experimental results showed that the nanoscale Ni/Fe@Fe3O4 bimetallic particles with a mass ratio of 4% of Ni/Fe exhibited excellent performance for CT degradation. The removal efficiency of CT can reach 96.3% within 35 min with only 0.2 g/L of Ni4/Fe@Fe3O4. The degradation kinetics of CT by Ni4/Fe@Fe3O4 nanocomposites was conformed to the pseudo first-order kinetics and the activation energies (Ea) was 25.87 kJ/mol. Moreover, Ni4/Fe@Fe3O4 presented good reusability and stability after four cycles. Based on the detection results of CT degradation products by gas chromatography (GC), a possible degradation mechanism was discussed, which can be divided into the following ways: (1) Migration and adsorption of CT molecules. (2) Direct reductive dechlorination of CT molecules by nano zero-valent iron (nZVI). (3) Reduction dechlorination of CT by Fe2+ released by Ni4/Fe@Fe3O4. (4) Ni0 might induce the conversion of H2 to active atomic hydrogen (·H) to degrade chloromethane. These results exhibited that Ni4/Fe@Fe3O4 will be a nanocomposite with great application potential for CT degradation from aqueous solutions.

Citrate ligand-enhanced microscale zero-valent aluminum corrosion for carbon tetrachloride degradation with high electron utilization efficiency

Carbon tetrachloride (CT) is highly toxic and recalcitrant in groundwater. In recent years, zero-valent aluminum (ZVAl) is highly reductive but limited by its surface passivation film. One of the effective ways to overcome this bottleneck is to add ligands. In this paper, compared with several other ligands, sodium citrate (SC), a natural organic ligand, was introduced to enhance microscale ZVAl (mZVAl) reactivity for the reductive degradation of CT. The results showed that the SC system could effectively reduce but not completely dechlorinate CT and electron utilization efficiency was as high as 94%. However, without ligands, mZVAl is chemically inert for CT degradation. Through SEM-EDS, BET, XRD, and XPS characterizations and H2 evolution experiments, enhanced mZVAl surface corrosion at the solid-liquid interface of mZVAl/SC system was verified. SC participated in the complexation corrosion reaction with surface inert film to form Al[Cit] complex, which made internal Al0 active sites exposed and then promoted mZVAl corrosion. In the five consecutive reuse experiments of mZVAl, CT can be completely degraded, which indicates that mZVAl, with the help of SC, has excellent sustainable utilization efficiency.

Mechanism of carbon tetrachloride reduction in Fe(II) activated percarbonate system in the environment of sodium dodecyl sulfate

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 (O2−•), carbon dioxide radical (CO2−•), and singlet oxygen (1O2) were mainly generated in SPC/Fe(II) system under the condition of 0.2 CMC SDS, while CO2−• and 1O2 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 •CCl3 through CO2−• and O2−• 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.

Rapid Degradation of Carbon Tetrachloride by Microscale Ag/Fe Bimetallic Particles.

Cost-effective zero valent iron (ZVI)-based bimetallic particles are a novel and promising technology for contaminant removal. The objective of this study was to evaluate the effectiveness of CCl4 removal from aqueous solution using microscale Ag/Fe bimetallic particles which were prepared by depositing Ag on millimeter-scale sponge ZVI particles. Kinetics of CCl4 degradation, the effect of Ag loading, the Ag/Fe dosage, initial solution pH, and humic acid on degradation efficiency were investigated. Ag deposited on ZVI promoted the CCl4 degradation efficiency and rate. The CCl4 degradation resulted from the indirect catalytic reduction of absorbed atomic hydrogen and the direct reduction on the ZVI surface. The CCl4 degradation by Ag/Fe particles was divided into slow reaction stage and accelerated reaction stage, and both stages were in accordance with the pseudo-first-order reaction kinetics. The degradation rate of CCl4 in the accelerated reaction stage was 2.29–5.57-fold faster than that in the slow reaction stage. The maximum degradation efficiency was obtained for 0.2 wt.% Ag loading. The degradation efficiency increased with increasing Ag/Fe dosage. The optimal pH for CCl4 degradation by Ag/Fe was about 6. The presence of humic acid had an adverse effect on CCl4 removal.

Mechanistic insight into superoxide radical-mediated degradation of carbon tetrachloride in aqueous solution: An in situ spectroscopic and computational study

Contemporary studies emphasize that superoxide radical (O2∙-) exhibits the potential to degrade organic contaminants, but practical application of this radical in engineered waters require an in-depth understanding of its kinetic profiles in a quantitative way. Here, we developed, for the first time, a convenient and reliable approach to generate micromolar level O2∙- in aqueous solution by photolysis of formate and H2O2. The presence of O2∙- was confirmed by comparing the UV spectra under pulse radiolysis and chromogenic reaction. We then constructed an in situ long-path spectroscopy to investigate the kinetics and mechanisms of O2∙--mediated degradation of carbon tetrachloride (CCl4), a halogenated model contaminant. The rate constant for the reaction of O2∙- and CCl4 was determined to be 478 M−1 s−1. In addition, we employed the transition state theory to model the reaction rate constants. Both results show that O2∙- exhibited low reactivity towards CCl4 with bimolecular rate constant lower by at least one order of magnitude than those radicals generated in typical advanced oxidation processes such as hydroxyl and sulfate radicals. Our results also indicate that nucleophilic substitution is the major pathway, and the solvation effect plays an important role in the reaction. The complementary experimental and theoretical approaches provide a mechanistic basis for better understanding aqueous–phase O2∙- chemistry and a holistic evaluation on the application of O2∙- for the degradation of organic contaminants of emerging concern.

Iron coral: Novel fungal biomineralization of nanoscale zerovalent iron composites for treatment of chlorinated pollutants

In this research, a facile fungal biomineralization method was developed for the synthesis of nanoscale zerovalent iron (nZVI) with a unique N-doped branching structure, which showed excellent stability and mediated high degradation of carbon tetrachloride (CCl4) in aqueous solution. The ureolytic fungus Neurospora crassa was cultured in medium containing Fe2+ and urea which resulted in iron carbonate biomineral precipitation. Following carbonization at 900 °C, the fungal-carbonate composite became highly porous and granular nanoparticles (~50 nm diameter) were distributed evenly around the carbonized hyphae in a coralline manner. This ‘iron coral’ composite was identified as a mixture of zerovalent iron (Fe0), carbon iron (Fe1.91C0.09) and iron oxide (Fe3O4). The porous branching hyphal framework improved the capture efficiency of CCl4, and the N-doped sites may accelerate the electron transfer between CCl4 and nZVI. Geochemical simulation was applied to verify the formation of the biominerals, and chemical analyses confirmed its significant degradation ability for CCl4. These findings have therefore demonstrated that ureolytic fungi can provide a promising environmental-friendly system for the novel preparation of nZVI through biomineralization with the resulting ‘iron coral’ capable of significant removal of a chlorinated compound and therefore indicating new bioremediation applications.

Reductive degradation of CCl4 by sulfidized Fe and Pd-Fe nanoparticles: Kinetics, longevity, and morphology aspects

In this study a systematic comparison in morphology, long-term degradation, regeneration and reuse were conducted between palladized and sulfidized nanoscale zero-valent iron (Pd-Fe and S-Fe). Pd-Fe and S-Fe were prepared, after the synthesis of precursor Fe0 nanoparticles (spherical, ~35 nm radius) for carbon tetrachloride (CTC) treatment. With HAADF-TEM-EDS characterization, dispersive Pd islets were found on the Fe core of Pd-Fe. However, the Fe core was covered by the FeSx shell of S-Fe (FeS/FeS2 = 0.47). With an excessive Pd dose (10 mol%), the Pd-Fe were dramatically deformed to dendritic structures which significantly decreased reactivity. For CTC degradation, Pd-Fe (0.3 atomic% Pd) increased the degradation rate by 20-fold (ksa = 0.580 Lm−2 min−1) while S-Fe presented a greater life time. The major intermediate chloroform (CF) was further degraded and less than 5% CF was observed after 24 h using Pd-Fe or S-Fe while above 50% CF remained using Fe. During aging, the Fe core was converted to FeOOH and Fe3O4/γ-Fe2O3. The restoration of Fe0 was achieved using NaBH4, which regenerated Fe and Pd-Fe. However, the formed FeSx shell on S-Fe was disappeared. The results suggest that S-Fe extends longevity of Fe, but the loss of FeSx after aging makes S-Fe eventually perform like Fe in terms of CTC degradation.

Enhanced carbon tetrachloride degradation by hydroxylamine in ferrous ion activated calcium peroxide in the presence of formic acid

Hydroxyl radicals (HO•) show low reactivity with perchlorinated hydrocarbons, such as carbon tetrachloride (CT), in conventional Fenton reactions, therefore, the generation of reductive radicals has attracted increasing attention. This study investigated the enhancement of CT degradation by the synergistic effects of hydroxylamine (HA) and formic acid (FA) (initial [CT] = 0.13 mmol/L) in a Fe (II) activated calcium peroxide (CP) Fenton process. CT degradation increased from 56.6% to 99.9% with the addition of 0.78 mmol/L HA to the CP/Fe(II)/FA/CT process in a molar ratio of 12/6/12/1. The results also showed that the presence of HA enhanced the regeneration of Fe(II) from Fe(III), and the production of HO• increased one-fold when employing benzoic acid as the HO• probe. Additionally, FA slightly improves the production of HO•. A study of the mechanism confirmed that the carbon dioxide radical (CO2•−), a strong reductant generated by the reaction between FA and HO•, was the dominant radical responsible for CT degradation. Almost complete CT dechlorination was achieved in the process. The presence of humic acid and chloride ion slightly decreased CT removal, while high doses of bicarbonate and high pH inhibited CT degradation. This study helps us to better understand the synergistic roles of FA and HA for HO• and CO2•− generation and the removal of perchlorinated hydrocarbons in modified Fenton systems.

Application of glutamate to enhance carbon tetrachloride (CT) degradation by Fe(II) activated calcium peroxide in the presence of methanol: CT removal performance and mechanism

In this study, we examined the effects of chelators on reductive degradation of carbon tetrachloride (CT) in Fe(II) activated CaO2 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 CaO2/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 O2− 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. CH2Cl2, C2Cl4, and C2Cl6 were detected as the intermediates accompanied by CT destruction, and a reduction mechanism for CT degradation by the CaO2/Fe(II)/MeOH/Glu process was proposed accordingly. Finally, the simultaneous removal of CT and TCE was achieved in the CaO2/Fe(II)/MeOH/Glu system, indicating a potential technique for complex contaminated sites remediation.

Utilization of formic acid in nanoscale zero valent iron-catalyzed Fenton system for carbon tetrachloride degradation

Although the utilization of formic acid (FA) as a target compound for modeling the kinetic and mechanism in several Fenton processes has been previously reported, this study explored the potential of FA as an efficient hydroxyl radical (HO) scavenger to generate reductive carbon dioxide radical (CO2−) for carbon tetrachloride (CT) degradation in nanoscale zero valent iron (nZVI) suspension in the presence and absence of oxidant. The results showed that the presence of 3.12 mM FA enhanced CT removal from 11.4% to 85.0% within 60 min in oxygenated 0.5 g/L nZVI suspension. Meanwhile, the presence of 3.12 mM FA improved Fe2+ elution by 5 folds under pH = 3. The generation of H2O2 in oxygenated nZVI suspension was also monitored. Hence, it was hypothesized that the rapid CT degradation in nZVI/FA system was initiated by reductive CO2− radical. In anaerobic nZVI/oxidant/FA system, CT degradation of 69.3% and 75.1% was achieved when using H2O2 or calcium peroxide (CP) as the oxidant, respectively. The addition of oxidant at 10 min reaction performed the best for CT removal as enough Fe2+ has been released after the dissolution of nZVI. These results demonstrated that a suitable dose of released Fe2+ was the key factor to enhance HO and CO2− radical generation in nZVI/oxidant/FA system. This study provided new information about effective usage of nZVI and FA for enhancing CT degradation in heterogeneous Fenton system.