Current progress in remediation of chlorinated volatile organic compounds: A review

Metal-Organic Frameworks for the Capture of Chlorinated Volatile Organic Compounds: A Case Study on Perchloroethylene

Indoor air pollution, particularly the presence of chlorinated volatile organic compounds like perchloroethylene (PCE), poses significant risks to human health and comfort in residential environments and industrial settings. While conventional abatement methods such as catalytic oxidation are energy-intensive and produce toxic byproducts, adsorption-based techniques offer an eco-friendlier alternative to capture PCE. In this study, a range of metal-organic frameworks (MOFs) with different pore sizes and fluorine-based functional groups as PCE sorbents, focusing on their performance at trace concentrations typical of indoor environments are systematically explored. The experimental findings evidence that CAU-11(Al) is highly effective at capturing traces of PCE, while DUT-4(Al) and Cu-NH2-TPTC excel at higher concentrations found in dry-cleaning applications. Density functional theory and Monte Carlo simulations deliver microscopic insight into the performance of these MOFs. This work demonstrates that MOFs, with their tunable structures and chemical properties, present a promising solution for improving indoor air quality by effectively removing PCE from indoor environments.

Study on the Photocatalytic Reaction of Trichloroethylene (TCE) over TiO2 Using Synchrotron Radiation Photoionization Mass Spectrometry

Trichloroethylene (TCE) is a widely used solvent in industrial processes, which is harmful to human health and the environment. Photocatalysis is a promising method for the degradation of TCE. In this work, the photocatalytic reaction of TCE over TiO2 was studied using synchrotron radiation photoionization mass spectrometry (SR-PIMS) under 360 nm UV light irradiation. First, more than 12 kinds of gas phase degradation intermediates and products were detected, including newly identified products such as formaldehyde (HCHO), formic acid (HCOOH) and hypochlorous acid (HOCl) and tetrachloroethane (C2H2Cl4). Second, the effects of water and oxygen on the photocatalysis of TCE over TiO2 were investigated. It was found that water vapor showed a negligible effect on the photocatalytic degradation efficiency TCE, but could enhance the generation of oxygen-containing species, like hypochlorous acid (HOCl), Phosgene (COCl2), and dichloroacetyl chloride (C2HOCl3). The presence of oxygen in the gas phase significantly enhanced the photocatalytic degradation of TCE, due to its role as an electron acceptor, preventing the recombination of photogenerated electron-hole pairs on the TiO2 surface, thereby enhancing the generation of reactive species like superoxide radicals (O2•-), which are essential for the effective degradation of TCE. Finally, the photocatalytic degradation network of TCE over TiO2 was proposed.

Unveiling microbial community structure and metabolic pathway over carbon cloth-titanium nitride-polyaniline biocathode for effective dichloromethane transformation

Chlorinated volatile organic compounds (Cl-VOCs) have dramatically biotoxicity and environmental persistence due to the presence of chlorine atoms, seriously jeopardizing ecological security and human health. Dichloromethane (DCM) as a model pollutant, is widely applied in solvents, extractants and cleaning agents in the pharmaceutical, chemical and food industries. In this study, highly biocompatible and conductive carbon cloth-titanium nitride-polyaniline (CC-TiN-PANI) bioelectrodes were obtained for DCM degradation in microbial electrolysis cell (MEC). The good adhesion of TiN and PANI on the electrode surface was demonstrated. The degradation kinetics were fitted by the Haldane model, compared to the CC bioelectrode (0.8 h-1), the proportion of maximum degradation rates to half-saturation concentration (Vmax/Km) of CC-TiN (1.4 h-1) and CC-TiN-PANI (2.2 h-1) bioelectrodes were enhanced by 1.8 and 2.8 times, respectively. Microbial community structure analysis illuminated that the dominant genera on the biofilm were Alicycliphilus and Hyphomicrobium, and the abundance was enhanced significantly with the modification of TiN and PANI. The dechlorination of DCM to formaldehyde could be catalyzed by DCM dehalogenase (DcmA) or by haloalkane dehalogenase (DhlA). And further oxidized to formate: 1) direct catalyzed by formaldehyde dehydrogenase (FdhA); 2) conjugated with glutathione by S-(hydroxymethyl)-glutathione synthase (Gfa), S-(hydroxymethyl)-glutathione dehydrogenase (FrmA) and S-formyl-glutathione hydrolase (FrmB); 3) conjugation with tetrahydrofolate (H4F) and/or tetrahydromethanopterin.

Separation processes for the treatment of industrial flue gases - Effective methods for global industrial air pollution control

The treatment of flue gases has become a crucial area of interest with the increasing air emissions into the atmosphere from industries involved in combustion of fossil fuels in their operations. In essence, there is a critical need for effective methods of treatment more than ever. Treatment and separation are now a demand for the overall industrial operations to control the rate of flue gas emissions. The major culprit in this wise is power generating industry. The major associated air pollutants are carbon dioxide, sulfur oxides, trace metals, volatile organic compounds, particulate matters, and nitrogen oxides. However, the choice of technologies to be utilized requires more than just knowledge of the separation process, but also a good understanding of the properties of the pollutants. This review explored and evaluated the various separation processes and technologies for the treatment of industrial flue gases for the control of the associated air pollutants. It also analyzed the performance with references to cost and efficiency, the advantages and disadvantages, principles for selection, research direction, and/or potential opportunities in existing separation processes and technologies.

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

Homogeneous and heterogeneous atmospheric ozonolysis of chlorobenzene:Mechanism, kinetics and ecotoxicity assessment

The reactions between chlorobenzene(CB) and ozone have been studied comprehensively in this paper. Chlorobenzene is a commonly found chlorinated aromatic volatile organic compound(VOC), and its emission into the atmosphere can cause harm to the ecosystem and human health. The frequent occurrence of mineral particles from sandstorms exerts a significant influence on the atmospheric chemistry of the troposphere. Mineral particles are abundant in SiO2 and Al2O3 content. Therefore, we investigated the homogeneous and heterogeneous reaction processes of CB and ozone in the atmosphere by using density functional theory (DFT) method at the M06-2X/6-311++g(3df,2p)//M06-2X/6-31+g(d,p) level. The atmospheric fate, reaction rate and toxicity evaluation of CB ozonation were studied in the gas-phase section. Toxicity evaluation results showed that ozonation of CB could effectively reduce its toxicity. For the heterogeneous process, we simulated three types of SiO2 clusters and nine types of (Al2O3)n clusters, and studied the configurations of CB adsorbed on the cluster surfaces. We found that adsorption of CB on the SiO2 clusters was achieved through hydrogen bonding, while adsorption of CB on the Al2O3 clusters was achieved through both hydrogen bonding and metal bonding. The energy for CB adsorption on the (Al2O3)n cluster surface was higher than that for the SixOy(OH)z cluster surface, and both types of clusters exhibited efficient adsorption of CB. As the SixOy(OH)z clusters grew larger, the rates for the reactions between O3 and CB increased. CB travelled long distances along the Al2O3 clusters, leading to an extended influence range.

Condensable Gases Capture with Ionic Liquids

Condensable gases are the sum of condensable and volatile steam or organic compounds, including water vapor, which are discharged into the atmosphere in gaseous form at atmospheric pressure and room temperature. Condensable toxic and harmful gases emitted from petrochemical, chemical, packaging and printing, industrial coatings, and mineral mining activities seriously pollute the atmospheric environment and endanger human health. Meanwhile, these gases are necessary chemical raw materials; therefore, developing green and efficient capture technology is significant for efficiently utilizing condensed gas resources. To overcome the problems of pollution and corrosion existing in traditional organic solvent and alkali absorption methods, ionic liquids (ILs), known as "liquid molecular sieves", have received unprecedented attention thanks to their excellent separation and regeneration performance and have gradually become green solvents used by scholars to replace traditional absorbents. This work reviews the research progress of ILs in separating condensate gas. As the basis of chemical engineering, this review first provides a detailed discussion of the origin of predictive molecular thermodynamics and its broad application in theory and industry. Afterward, this review focuses on the latest research results of ILs in the capture of several important typical condensable gases, including water vapor, aromatic VOCs (i.e., BTEX), chlorinated VOC, fluorinated refrigerant gas, low-carbon alcohols, ketones, ethers, ester vapors, etc. Using pure IL, mixed ILs, and IL + organic solvent mixtures as absorbents also briefly expanded the related reports of porous materials loaded with an IL as adsorbents. Finally, future development and research directions in this exciting field are remarked.

Weak Metal-Support Interaction over CuO/TiO2 Catalyst Governed Low-Temperature Toluene Oxidation

Regulating the metal-support interaction is essential for obtaining highly efficient catalysts for the catalytic oxidation of volatile organic compounds (VOCs). In this work, CuO-TiO₂(coll) and CuO/TiO₂(imp) with different metal-support interactions were prepared via colloidal and impregnation methods, respectively. The results demonstrated that CuO/TiO₂(imp) has higher low-temperature catalytic activity, with a 50% removal of toluene at 170 °C compared to CuO-TiO₂(coll). Additionally, the normalized reaction rate (6.4 × 10^-6 mol·g^-1·s^-1) at 160 °C over CuO/TiO₂(imp) was almost four-fold higher than that over CuO-TiO₂(coll) (1.5 × 10^-6 mol·g^-1·s^-1), and the apparent activation energy value (27.9 ± 2.9 kJ·mol^-1) was lower. Systematic structure and surface analysis results disclosed that abundant Cu²⁺ active species and numerous small CuO particles were presented over CuO/TiO₂(imp). Owing to the weak interaction of CuO and TiO₂ in this optimized catalyst, the concentration of reducible oxygen species associated with the superior redox property could be enhanced, thus significantly contributing to its low-temperature catalytic activity for toluene oxidation. This work is helpful in exploring the influence of metal-support interaction on the catalytic oxidation of VOCs and developing low-temperature catalysts for VOCs catalytic oxidation.

Catalytic destruction of chlorobenzene over K-OMS-2: Inhibition of high toxic byproducts via phosphate modification

In the process of catalytic destruction of chlorinated volatile organic compounds (CVOCs), the catalyst is prone to chlorine poisoning and produce polychlorinated byproducts with high toxicity and persistence, bringing great risk to atmospheric environment and human health. To solve these problems, this work applied phosphate to modify K-OMS-2 catalysts. The physicochemical properties of catalysts were determined by using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), hydrogen temperature programmed reduction (H₂-TPR), pyridine adsorption Fourier-transform infrared (Py-IR) and water temperature programmed desorption (H₂O-TPD), and chlorobenzene was selected as a model pollutant to explore the catalytic performance and byproduct inhibition function of phosphating. Experimental results revealed that 1 wt.% phosphate modification yielded the best catalytic activity for chlorobenzene destruction, with the 90% conversion (T₉₀) at approximately 247°C. The phosphating significantly decreased the types and yields of polychlorinated byproducts in effluent. After phosphating, we observed significant hydroxyl groups on catalyst surface, and the active center was transformed into Mn(IV)-O…H, which promoted the formation of HCl, and enhanced the dechlorination process. Furthermore, the enriched Lewis acid sites by phosphating profoundly enhanced the deep oxidation ability of the catalyst, which promoted a rapid oxidation of reaction intermediates, so as to reduce byproducts generation. This study provided an effective strategy for inhibiting the toxic byproducts for the catalytic destruction of chlorinated organics.

Progress of catalytic oxidation of typical chlorined volatile organic compounds (CVOCs): A review

Chlorinated volatile organic compounds (CVOCs) are still a part of the current atmospheric environmental problems that cannot be ignored, but unlike conventional VOCs, the presence of Cl causes various catalyst deactivations in the catalytic process. In this paper, we focus on six common CVOCs and discuss various behavioral mechanisms of the whole catalytic process from six aspects: catalyst selection, factors affecting the catalytic effect, changes in catalytic behavior in the presence of different gases, catalyst poisoning deactivation behavior, degradation products and degradation mechanisms to provide guidance for further development of low-temperature and efficient CVOCs catalysts.

Removal of gaseous volatile organic compounds via vacuum ultraviolet photodegradation: Review and prospect

Volatile organic compounds (VOCs) have attracted much attention for decades as they are the precursors of photochemical smog and are harmful to the environment and human health. Vacuum ultraviolet (VUV) photodegradation is a simple and effective method to decompose VOCs (ranging from tens to hundreds of ppmV) without additional oxidants or catalysts in the air at atmospheric pressure. In this paper, we review the research progress of VOCs removal via VUV photodegradation. The fundamentals are outlined and the key operation factors for VOCs degradation, such as humidity, oxygen content, VOCs initial concentration, light intensity, and flow rate, are discussed. VUV photodegradation of VOCs mixture is elucidated. The application of VUV photodegradation in combination with ozone-assisted catalytic oxidation (OZCO) and photocatalytic oxidation (PCO) systems, and as the pre-treatment technique for biological purification are illustrated. Based on the summary, we propose the challenges of VUV photodegradation and perspectives for its future development.

Structure-property-performance relationship of transition metal doped WO3 mixed oxides for catalytic degradation of organic pollutants

Transition metal doped WO₃ mixed oxides (named as W-M-O, M = Nb, Fe, Cr, Cu, Ti or Sn, respectively) with high structure stability were synthesized by modified sol-gel method using citric acid as organic crosslinking agent, and were evaluated for catalytic elimination of low-concentration toluene, monochlorobenzene and 1,2-dichloroethance with high toxicity and relatively stable molecule structure, as the typical examples for the pollutants of various volatile organic compounds (VOCs). Results of the structure-property-performance relationship research showed that mesoporous structure and nanocrystalline/amorphous state were formed, and binary metal components were dispersed into each other, which contributed to promoting the metal/metal electron interaction and adjusting the physicochemical properties of mixed metal oxides. The sequence of apparent catalytic activity for toluene degradation was: W-Nb-O＞W-Fe-O＞W-Cr-O, W-Cu-O＞W-Ti-O＞W-Sn-O＞WO₃, and the sequence for monochlorobenzene degradation was: W-Nb-O＞W-Fe-O＞W-Cr-O, W-Ti-O＞W-Cu-O＞W-Sn-O＞WO₃. There existed cooperative catalytic effect: mesopore and surface acid sites of catalysts facilitated adsorption, activation and breakage of the C-X bond, and then redox sites of catalysts promoted deep oxidation of a series of reaction intermediates to transform into CO₂ and H₂O. Especially, the optimized W-Nb-O catalyst deserved more attention, since it represented remarkable catalytic activity, selectivity and durability for three typical VOCs degradation along with good resistance to water vapor and corrosion of HCl.

Volatile organic compounds degradation by nonthermal plasma: a review

Volatile organic compounds (VOCs) have posed a severe threat on both ecosystem and human health which thus have gained much attention in recent years. Nonthermal plasma (NTP) as an alternative to traditional methods has been employed to degrade VOC in the atmosphere and wastewater for its high removal efficiency (up to 100%), mild operating conditions, and environmental friendliness. This review outlined the principles of NTP production and the applications on VOC removal in different kinds of reactors, like single/double dielectric barrier discharge, surface discharge, and gliding arc discharge reactors. The combination of NTP with catalysts/oxidants was also applied for VOC degradation to further promote the energy efficiency. Further, detailed explanations were given of the effect of various important factors including input/reactor/external conditions on VOC degradation performance. The reactive species (e.g., high-energy electrons, HO·, O·, N₂⁺, Ar⁺, O₃, H₂O₂) generated in NTP discharge process have played crucial roles in decomposing VOC molecules; therefore, their variation under different parameter conditions along with the reaction mechanisms involved in these NTP technologies was emphatically explained. Finally, a conclusion of the NTP technologies was presented, and special attention was paid to future challenges for NTP technologies in VOC treatment to stimulate the advances in this topic.

Regulating Pt-based noble metal catalysts for the catalytic oxidation of volatile organic compounds: a mini review

Volatile organic compounds (VOCs) are an important class of environmental pollutants, and there is much interest in China to eliminate such pollutants. Noble metal catalysts have long been a family of catalysts with high efficiency and good low-temperature catalytic activity. As a representative of the noble metals, Pt has been widely used. This paper reviews the research trend of Pt-based catalysts for the catalytic oxidation of VOCs, and it compares several important components of Pt-based catalysts. The size of Pt particles, supported carriers, and reaction mechanism are reviewed. Toluene in VOCs is the main research subject. The activity, stability, water resistance, and selectivity of a series of Pt-based catalysts are summarized.

Selective dechlorination degradation of chlorobenzenes by dual single-atomic Fe/Ni catalyst with M-N/M-O active sites synergistic

Removal and detoxification of chlorobenzenes have attracted public concern, multiple active sites single-atom Fe and single-atom Ni composite nitrogen-doped graphene (Fe_SA/CN/Ni_SA) cathode catalyst supplied generation and adsorption capacity of hydrogen and hydroxyl active species. M-O active sites coupled with M-N improved activity and stability of the catalyst, and decreased bond breaking energy barrier of C-Cl, Fe_SA/CN/Ni_SA-NiF cathode showed superior removal performance of chlorinated aromatic hydrocarbons (monochlorobenzene: 98.9%, dichlorobenzene: over 90.4%, trichlorobenzene: over 85.7%) and selectivity. Chlorobenzenes were dechlorinated under low stepwise voltage on the Fe_SA/CN/Ni_SA-NiF cathode. The efficiencies of stepwise dechlorination reactions of chlorobenzenes were all above 76%, Faradaic efficiencies were above 71.8%. The Fe_SA/CN/Ni_SA-NiF cathode was not sensitive to the molecular structure and has overcome the high energy barrier of chlorobenzenes molecular structure. The electrophilic attack of H*_ads formed hyperconjugation bond weakened the possibility of the Cl atom forming a bond with the benzene ring, and was favorable for the Cl position to achieve single-electron transfer dechlorination. The selective stepwise dechlorination degradation of chlorobenzenes by Fe_SA/CN/Ni_SA-NiF cathode with multiple active sites demonstrated the advantaged performance of M-O and M-N active sites coupled synergistic in electrochemical reduction and degradation, providing a strategy for product-selective degradation of chlorinated aromatic hydrocarbons.

Unveiling the positive effect of mineral induced natural organic matter (NOM) on catalyst properties and catalytic dechlorination performance: An experiment and DFT study

Herein, we report the significant effects of natural organic matter contained in natural zeolite (Z-NOM) on the physicochemical characteristics of a Ni/Fe@natural zeolite (NF@NZ) catalyst and its decontamination performance toward the dechlorination of trichloroethylene (TCE). Z-NOM predominantly consists of humic-like substances and has demonstrable utility in the synthesis of bimetallic catalysts. Compared to NF@NZ_600C (devoid of Z-NOM), NF@NZ had increased dispersibility and mobility and showed significant enhancement in the catalytic dechlorination of TCE owing to the encapsulation of Ni⁰/Fe⁰ nanoparticles by Z-NOM. The results of corrosion experiments, spectroscopic analyses, and H₂ production experiments confirmed that Ni⁰ acted as an efficient cocatalyst with Fe⁰ to enhance the dechlorination of TCE to ethane, and Z-NOM-capped Ni⁰ showed improved adsorption of TCE and atomic hydrogen on their reactive sites and oxidation resistance. The density functional theory (DFT) studies have substantiated the improved adsorption of TCE due to the presence of NOM (especially by COOH structure) and the enhanced charge density at the Ni site in the Ni/Fe bimetal alloy for the stronger adsorption of hydrogen atoms that ultimately enhanced the TCE reduction reaction. These findings illustrate the efficiency of NOM containing natural minerals toward the synthesis of bimetallic catalysts for practical applications.

Decomposition of gaseous chlorobenzene using a DBD combined CuO/α-Fe2O3 catalysis system

Copper oxide and hematite (CuO/α-Fe₂O₃) composite catalysts were prepared by using goethite as precursor adopted impregnation way and applied to the dielectric barrier discharge (DBD) catalytic decomposition of gaseous chlorobenzene. The CuO/α-Fe₂O₃ composite was characterised by X-ray diffraction, Brunauer-Emmett-Teller method, scanning electron microscopy and X-ray photoelectron spectrometer technique. The decomposition efficiency and energy yield of gaseous chlorobenzene in DBD catalysis system were studied by a function of gas flow rate, initial concentration and input voltage. The results showed that the CuO/α-Fe₂O₃ composite catalyst exhibited remarkable performance on chlorobenzene decomposition when the molar ratio was 0.4 and calcination temperature was 450°C. When the chlorobenzene initial concentration was 230 mg m^-3, the chlorobenzene decomposition efficiency and mineralisation rate on the DBD catalysis system reached 73.33% and 63.37%, respectively, its decomposition and mineralisation efficiency were enhanced about 20.5% and 16.61%, respectively, compared with the bare DBD system, and it also benefited to significantly reduce the ozone and NO₂ by-products. The possible pathway of chlorobenzene decomposition in the DBD catalytic hybrid system was proposed based on the products analysis.

Recent advances in the chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase

The chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase may possess great advantages in its high removal efficiency, mild conditions, good reliability, wide applicability, and little potential secondary pollution, which has aroused extensive research interests in the past decade. This Overview Article summarizes the latest achievements to eliminate VOCs by chemical oxidation in liquid phase including gas-liquid mass transfer, homogeneous/heterogeneous oxidation, electrochemical oxidation, and coupling technologies. Important research contributions are highlighted in terms of mass transfer, catalytic materials, removal/mineralization efficiency, and reaction mechanism to evaluate their potential industrial applications. The current challenges and future strategies are discussed from the viewpoint of the deep degradation of refractory VOC substrates and their intermediates. It is anticipated that this review will attract more attention toward the development and application of chemical oxidation methods to clear complex industrial organic exhaust gas.

The Spectroscopic Characterization of Halogenated Pollutants through the Interplay between Theory and Experiment: Application to R1122

In the last decade, halogenated ethenes have seen an increasing interest for different applications; in particular, in refrigeration, air-conditioning and heat pumping. At the same time, their adverse effects as atmospheric pollutants require environmental monitoring, especially by remote sensing spectroscopic techniques. For this purpose, an accurate characterization of the spectroscopic fingerprint—in particular, those of relevance for rotational–vibrational spectroscopy—of the target molecules is strongly needed. This work provides an integrated computational–theoretical investigation on R1122 (2-Chloro-1,1-difluoro-ethylene, ClHC=CF2), a compound widely employed as a key intermediate in different chemical processes. State-of-the-art quantum chemical calculations relying on CCSD(T)-based composite schemes and hybrid CCSD(T)/DFT approaches are used to obtain an accurate prediction of the structural, rotational and vibrational spectroscopic properties. In addition, the equilibrium geometry is obtained by exploiting the semi-experimental method. The theoretical predictions are used to guide the analysis of the experimentally recorded gas-phase infrared spectrum, which is assigned in the 400–6500 cm−1 region. Furthermore, absorption cross sections are accurately determined over the same spectral range. Finally, by using the obtained spectroscopic data, a first estimate of the global warming potential of R1122 vibrational spectra is obtained.

Promoting effect of Ru-doped Mn/TiO2 catalysts for catalytic oxidation of chlorobenzene

Mn/TiO2 catalysts were synthesized using deposition-precipitation method. Ru-doped Mn/TiO2 catalysts were prepared by incipient-wetness impregnation method. To investigate the effect of Ru and Mn species, the catalytic performances of Mn/TiO2...

Liquid-phase catalytic hydrodechlorination of chlorinated organic compounds in a continuous flow micro-packed bed reactor over a Pd/AC catalyst

A continuous flow system based on a micro-packed bed reactor was developed for hydrodechlorination, and the hydrogenation of chlorobenzene was selected as the model reaction. With the optimal reaction conditions, a conversion and selectivity of 100% were obtained.

Fabrication of a multi-dimensional CoFeOx catalyst for the efficient catalytic oxidation elimination of o-dichlorobenzene

Multi-dimensional CoFeOx/CoOx with a 2D/1D structure exhibited outstanding catalytic activity and thermal stability in the catalytic elimination of o-DCB.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

3D dynamic model empowering the knowledge of the decontamination mechanisms and controlling the complex remediation strategy of a contaminated industrial site

Knowledge of the geology and hydrogeology of the polluted site emblematize a key requirement for environmental remediation, through assembling and synthesizing findings from various sources of physical evidence. In an increasingly virtual era, digital and geo-referenced metadata may serve as tools for collecting, merging, matching, and understanding multi-source information. The main goal of this paper is to emphasize the significance of a 3D hydrogeochemical model to the portrayal and the understanding of contamination dynamics and decontamination mechanisms at a highly contaminated industrial site. Some remediation measures are active on-site, due to the evidence-based presence of chlorinated solvents in groundwater. These are attributable to a slow-release source of pollutants in the saturated zone associated with very low permeability sediments. Therefore, in this research, a new technique for the remediation of secondary sources of dense non-aqueous phase liquid (DNAPL) contamination was investigated for the first time on a full-scale application. The combination of groundwater circulation wells (IEG-GCW®) and a continuous electron donor production device was set up to boost in situ bioremediation (ISB). A multi-phase approach was followed handling and releasing data during various remediation stages, from site characterization via pilot testing to full-scale remediation, thus allowing users to monitor, analyze, and manipulate information in 3D space-time. Multi-source and multi-temporal scenarios reveal the impact of ongoing hydraulic dynamics and depict the decontamination mechanisms in response to the interventions implemented over time, by quantifying the overall performance of the adopted strategies in terms of removal of secondary sources of pollution still active at the site.

Enhanced trichloroethylene biodegradation: Roles of biochar-microbial collaboration beyond adsorption

Trichloroethylene (TCE) is a pollutant widely found in groundwater, especially in the heavily contaminated industrial sites. Biological dechlorination method is environmentally friendly and low-cost. However, microorganisms grow slowly and their activity is susceptible to environmental fluctuations. This study used biochar as an additive to promote anaerobic biodegradation of TCE with mixed culture. Results showed that biochar with dose of 0.1-0.4% (w/v) brought a rapid initial decrease of TCE concentration by 39.4-88.8% in 24 h via adsorption mechanism. Biochar produced at 500 °C pyrolysis temperature (BC500) achieved the highest TCE adsorption in comparison to BC300 and BC700. Subsequently, a significantly shortened microbial stagnation phase (from 85 h to 37 h) was observed in the system with the presence of biochar. During the exponential growth phase, BC700 outperformed BC300 and BC500 in terms of TCE degradation efficiency. Electrochemical analysis demonstrated that BC700 possessed the greatest electron transfer capability. Finally, biochar shortened the time for achieving 100% removal of TCE by 54.5-69.7% (from approximate 330 h to 100-150 h). Even at high concentration of TCE (20-30 mg·L^-1) that could lead to serious microbial growth inhibition, the TCE degradation efficiency could be recovered in the presence of BC500. The high-throughput sequencing data revealed that biochar promoted the relative abundance of co-metabolizing dechlorinating microorganisms (Pseudomonas, Burkholderia) in the aqueous solution, and simultaneously led to the selective colonization of reductive dechlorinating microorganisms (Enterobacteriaceae, Clostridium) attached on biochar surface. On the other hand, biochar addition decreased the relative abundance of hydrogen-competing microorganisms, thereby forming an efficient co-metabolism-reductive dechlorination system. These findings allow a better understanding of the promotion mechanism of biochar for microbial dechlorination technology supporting the biochar-assisted bioremediation in practice.

Enhanced biodegradation of chlorobenzene via combined Fe3+ and Zn2+ based on rhamnolipid solubilisation

Biotrickling filters (BTFs) for hydrophobic chlorobenzene (CB) purification are limited by mass transfer and biodegradation. The CB mass transfer rate could be improved by 150 mg/L rhamnolipids. This study evaluated the combined use of Fe³⁺ and Zn²⁺ to enhance biodegradation in a BTF over 35 day. The effects of these trace elements were analysed under different inlet concentrations (250, 600, 900, and 1200 mg/L) and empty bed residence times (EBRTs; 60, 45, and 32 sec). Batch experiments showed that the promoting effects of Fe³⁺/Zn²⁺ on microbial growth and metabolism were highest for 3 mg/L Fe³⁺ and 2 mg/L Zn²⁺, followed by 2 mg/L Zn²⁺, and lowest at 3 mg/L Fe³⁺. Compared to BTF in the absence of Fe³⁺ and Zn²⁺, the average CB elimination capacity and removal efficiency in the presence of Fe³⁺ and Zn²⁺ increased from 61.54 to 65.79 g/(m³⋅hr) and from 80.93% to 89.37%, respectively, at an EBRT of 60 sec. The average removal efficiency at EBRTs of 60, 45, and 32 sec increased by 2.89%, 5.63%, and 11.61%, respectively. The chemical composition (proteins (PN), polysaccharides (PS)) and functional groups of the biofilm were analysed at 60, 81, and 95 day. Fe³⁺ and Zn²⁺ significantly enhanced PN and PS secretion, which may have promoted CB adsorption and biodegradation. High-throughput sequencing revealed the promoting effect of Fe³⁺ and Zn²⁺ on bacterial populations. The combination of Fe³⁺ and Zn²⁺ with rhamnolipids was an efficient method for improving CB biodegradation in BTFs.

Understanding Hydrodechlorination of Chloromethanes. Past and Future of the Technology

Chloromethanes are a group of volatile organic compounds that are harmful to the environment and human health. Abundant studies have verified that hydrodechlorination might be an effective treatment to remove these chlorinated pollutants. The most outstanding advantages of this technique are the moderate operating conditions used and the possibility of obtaining less hazardous valuable products. This review presents a global analysis of experimental and theoretical studies regarding the hydrodechlorination of chloromethanes. The catalysts used and their synthesis methods are summarized. Their physicochemical properties are analyzed in order to deeply understand their influence on the catalytic performance. Moreover, the main causes of the catalyst deactivation are explained, and prevention and regeneration methods are suggested. The reaction systems used and the effect of the operating conditions on the catalytic activity are also analyzed. Besides, the mechanisms and kinetics of the process at the atomic level are reviewed. Finally, a new perspective for the upgrading of chloromethanes, via hydrodechlorination, to valuable hydrocarbons for industry, such as light olefins, is discussed.

Non-Thermal Plasma-Modified Ru-Sn-Ti Catalyst for Chlorinated Volatile Organic Compound Degradation

Chlorinated volatile organic compounds (CVOCs) are vital environmental concerns due to their low biodegradability and long-term persistence. Catalytic combustion technology is one of the more commonly used technologies for the treatment of CVOCs. Catalysts with high low-temperature activity, superior selectivity of non-toxic products, and resistance to chlorine poisoning are desirable. Here we adopted a plasma treatment method to synthesize a tin-doped titania loaded with ruthenium dioxide (RuO2) catalyst, possessing enhanced activity (T90%, the temperature at which 90% of dichloromethane (DCM) is decomposed, is 262 °C) compared to the catalyst prepared by the conventional calcination method. As revealed by transmission electron microscopy, X-ray diffraction, N2 adsorption, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction, the high surface area of the tin-doped titania catalyst and the enhanced dispersion and surface oxidation of RuO2 induced by plasma treatment were found to be the main factors determining excellent catalytic activities.

Evaluation for the Removal Efficiency of VOCs and Heavy Metals by Zeolites-Based Materials in the Wastewater: A Case Study in the Tito Scalo Industrial Area

The current study was designed to demonstrate the efficiency of selected zeolites in the immobilization of heavy metals and volatile organic compounds from water in the industrial area of Tito Scalo (Basilicata Region in Southern Italy). The efficiency of zeolite materials has been evaluated by analyzing real water samples, by a multi-technique approach. Gas chromatography (GC) and inductively coupled plasma optical emission spectrometry (ICP-OES) were selected for the detection of volatile organic compounds (VOCs) and heavy metals respectively, and then by thermal analysis (TG, DTA) and X-ray powder diffraction (XRD) to verify the presence of contaminants in the structural channels of the adsorbents. ZSM-5 zeolite (MFI topology) was suitable for volatile organic compounds, showing removal efficiencies 87%. 13X (FAU topology) was more selective for in situ abatements of heavy metals, with efficiencies up to 100%. After VOCs and heavy metals removal, structure refinements of loaded zeolites highlighted variations of both lattice parameters and extraframework content confirming the pollutants immobilization in the framework microporosities. The occurrence of these species was also confirmed by DTA curves showing different phenomena explained on the basis of the nature and number of extraframework species hosted in the zeolite micropores.

A review and perspective of recent research in biological treatment applied in removal of chlorinated volatile organic compounds from waste air

Chlorinated volatile organic compounds (Cl-VOCs) waste air is a kind of typical recalcitrant organic compounds, which poses a great threat to the ecological environment and human health. At present, the biotechnology is considered as a potential strategy for the Cl-VOCs removal due to the advantages of low energy consumption and less possibility of secondary pollution. This work summarizes the recent researches on strains, bioreactors and technology integration. The dominant pure strains for biodegradation of Cl-VOCs are first outlined with a special focus on the co-metabolism of multi-components. It then summarizes two bioreactors (optimized airlift reactor (ALR) and two-phase partitioning bioreactor (TPPB)) and strategy (addition of surfactant) for improvement of biotrickling filter (BTF), which are benefit to achieve the mass transfer enhancement in the removal of hydrophobic Cl-VOCs from waste air. After that, the integration technologies, such as magnetic field (MF)-BTF, non-thermal plasma (NTP)/ultraviolet light (UV)-BTF, and microbial electrolytic cells (MEC), are elucidated, which provide opportunities for complete mineralization of Cl-VOCs in a more efficient, energy-saving and economical way. Finally, current challenges and a perspective of future research on biotechnology for Cl-VOCs removal are thoroughly discussed.

Utilization of metal-organic frameworks for the adsorptive removal of an aliphatic aldehyde mixture in the gas phase

Considerable efforts have been undertaken in the domain of air quality management for the removal of hazardous volatile organic compounds, particularly carbonyl compounds (CCs). In this study, the competitive sorptive removal of six CCs (namely, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) was assessed using selected metal-organic frameworks (MOFs: MOF-5, MOF-199, UiO-66, and UiO-66-NH₂) and inexpensive commercial activated carbon as a reference sorbent. The sorption experiments were conducted using a mixture of the six CCs (formaldehyde and acetaldehyde at ∼1 Pa and propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde at ∼0.2 Pa) together with 15 Pa water and 2.6 Pa methanol in 1 bar nitrogen. For all of the carbonyl compounds other than formaldehyde, MOF-199 showed the best 10% breakthrough performance ranging from 34 L g^-1 and 0.14 mol kg^-1 Pa^-1 for acetaldehyde to 1870 L g^-1 and 7.6 mol kg^-1 Pa^-1 for isovaleraldehyde. Among all the sorbents tested, UiO-66-NH₂ exhibited the best 10% breakthrough performance metrics towards the lightest formaldehyde which remains to be one of the most difficult targets for sorptive removal (breakthrough volume: 285 L g^-1 and partition coefficient: 1.1 mol kg^-1 Pa^-1). Theoretical density functional theory (DFT)-based computations were also conducted to provide better insights into the adsorbate-adsorbent interactions. Accordingly, the magnitude of adsorption energy increased with an increase in the CC molar mass due to an enhancement in the synergetic interaction between C[double bond, length as m-dash]O groups (in adsorbate molecules) and the MOF active centers (open metallic centers and/or NH₂ functionality) as the adsorbent. Such interactions were observed to result in strong distortion of MOF structures. In contrast, weak van der Waals attraction between the hydrocarbon "tail" of CC molecules and MOF linkers were seen to play a stabilizing role for the sorbent structure. The presence of the NH₂ group in the MOF structure was suspected to play a key role in capturing lighter CCs, while such an effect was less prominent for heavier CCs. Overall, the results of this study provided a basis for the establishment of an effective strategy to enhance the sorption capacity of MOFs against diverse carbonyl species.

Halocarbon Emissions from Hazardous Waste Landfills: Analysis of Sources and Risks

Landfills are sources of fugitive volatile organic carbon (VOC) emissions, including halocarbons. The objective of this study was to evaluate the contribution of halogenated VOCs to the health risks associated with the exposure of workers operating in landfills, gathering information on the role of endogenous/exogenous sources present in anthropized areas. A hazardous waste landfill located in Turin, Italy was used as a case study. Ambient concentrations of 10 pollutants (BTEX, styrene, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, 1,2-dichloroethane, and 1,2-dichloropropane), measured in 10 points of the landfill area, were considered and analyzed. The data had a monthly frequency and covered two years. A cumulative health risk analysis was conducted by applying a Monte-Carlo method. The results showed that the contribution of 1,2-dichloroethane and 1,2-dichloropropane was 17.9% and 19.4% for the total risk and hazard index respectively. Benzene and ethylbenzene gave the highest contribution to the total risk (56.8% and 24.8%, respectively). In the second phase of the study, waste typologies that are possibly responsible for halocarbon emissions were investigated. Halocarbon concentration trends and waste disposal records were compared. Although further investigation is needed, some waste typologies were not excluded to contribute to halocarbon emissions, in particular sludge coming from wastewater treatment plants.

Revealing adsorption and the photodegradation mechanism of gas phase o-xylene on carbon quantum dots modified TiO2 nanoparticles

Here, we report the photocatalytic oxidation (PCO) of o-xylene on carbon quantum dots (CQDs) modified TiO₂ nanoparticles. The results demonstrated that with 1 wt% CQDs loading, 87 % of o-xylene (50 ppm) can be photodegraded, which is 55.3 % higher than pure TiO₂ (56 %) under UV/visible light. This improved photocatalytic activity is associated with the important role of CQDs on TiO₂ surface, which increased the o-xylene adsorption and facilitated the photogenerated hole-electron separation process. Also, the 1 wt%CQDs/TiO₂ nanocomposite showed photocatalytic activity in the visible region (λ > 400 nm) compared to pure TiO₂ (inactive). The DFT study revealed that o-xylene strongly adsorb on TiO₂ (001) surface than (101) through π electrons of the aromatic ring. The in situ DRIFTS study showed that free OH groups on the photocatalyst surface could act as effective Lewis sides for the o-xylene adsorption. The interaction of π electrons of the aromatic ring and isolated OH groups was also observed. The FTIR peaks for CO₂ increased in the case of CQDs/TiO₂ nanocomposite contrasted to pure TiO₂, which suggested that the presence of CQDs improved the mineralization potency of TiO₂. These findings should affect the quest for a better photocatalyst to photodegrade VOCs.

Removal of Carbon Monoxide from Simulated Flue Gas Using Two New Fenton Systems: Mechanism and Kinetics

Two novel removal processes of carbon monoxide using two new Fenton systems (i.e., Cu²⁺/Fe²⁺ and Mn²⁺/Fe²⁺ coactivated H₂O₂ systems) were developed. The effect of several process parameters (concentrations of H₂O₂, Fe²⁺, Cu²⁺, and Mn²⁺, reagent pH value, solution temperature, and simulated flue gas components) on CO removal was studied in a bubbling reactor. The mechanism and kinetics of CO removal were also revealed. Results show that adding Cu²⁺ or Mn²⁺ obviously enhances the removal process of CO in new Fenton systems. The measured results of free radical yield demonstrate that the enhancing role is derived from producing more ·OH (they are produced due to the synergistic activation role of Cu²⁺/Fe²⁺ or Mn²⁺/Fe²⁺ in new Fenton systems. The removal efficiency of CO is raised by increasing concentrations of Fe²⁺, Cu²⁺, and Mn²⁺ and is reduced by raising concentrations of CO, NO, and SO₂. Increasing H₂O₂ concentration, reagent pH, and solution temperature demonstrates a dual impact on CO absorption. Three oxidation pathways are found to be responsible for CO removal in new Fenton systems. Results of mass-transfer reaction kinetics reveal that CO removal processes are located in a fast-speed reaction kinetics region (the CO removal process is controlled by the mass transfer rate).

Emulsified polycolloid substrate biobarrier for benzene and petroleum-hydrocarbon plume containment and migration control - A field-scale study

The objective of this field-scale study was to assess the effectiveness of applying an emulsified polycolloid substrate (EPS; containing cane molasses, soybean oil, and surfactants) biobarrier in the control and remediation of a petroleum-hydrocarbon plume in natural waters. An abandoned petrochemical manufacturing facility site was contaminated by benzene and other petroleum products due to a leakage from a storage tank. Because benzene is a petroleum hydrocarbon with a high migration ability, it was used as the target compound in the field-scale study. Batch partition and sorption experiment results indicated that the EPS to water partition coefficient for benzene was 232 mg/mg at 25 °C. This suggests that benzene had a higher sorption affinity to EPS, which decreased the benzene concentrations in groundwater. The EPS solution was pressure-injected into three remediation wells (RWs; 150 L EPS in 800 L groundwater). Groundwater samples were collected from an upgradient background well, two downgradient monitor wells (MWs), and the three RWs for analyses. EPS injection increased total organic carbon (TOC) concentrations (up to 786 mg/L) in groundwater, which also resulted in the formation of anaerobic conditions. An abrupt drop in benzene concentration (from 6.9 to below 0.04 mg/L) was observed after EPS supplementation in the RWs due to both sorption and biodegradation mechanisms. Results show that the EPS supplement increased total viable bacteria and enhanced bioremediation efficiency, which accounted for the observed decrease in benzene concentration. The first-order decay rate in RW1 increased from 0.003 to 0.023 d^-1 after EPS application. Injection of EPS resulted in significant growth of indigenous bacteria, and 23 petroleum-hydrocarbon-degrading bacterial species were detected, which enhanced the in situ benzene biodegradation efficiency. Results demonstrate that the EPS biobarrier can effectively contain a petroleum-hydrocarbon plume and prevent its migration to downgradient areas, which reduces the immediate risk presented to downgradient receptors.

Application of an emulsified polycolloid substrate biobarrier to remediate petroleum-hydrocarbon contaminated groundwater

Emulsified polycolloid substrate (EPS) was developed and applied in situ to form a biobarrier for the containment and enhanced bioremediation of a petroleum-hydrocarbon plume. EPS had a negative zeta potential (-35.7 mv), which promoted its even distribution after injection. Batch and column experiments were performed to evaluate the effectiveness of EPS on toluene containment and biodegradation. The EPS-to-water partition coefficient for toluene (target compound) was 943. Thus, toluene had a significant sorption affinity to EPS, which caused reduced toluene concentration in water phase in the EPS/water system. Groundwater containing toluene (18 mg/L) was pumped into the three-column system at a flow rate of 0.28 mL/min, while EPS was injected into the second column to form a biobarrier. A significant reduction of toluene concentration to 0.1 mg/L was observed immediately after EPS injection. This indicates that EPS could effectively contain toluene plume and prevent its further migration to farther downgradient zone. Approximately 99% of toluene was removed after 296 PVs of operation via sorption, natural attenuation, and EPS-enhanced biodegradation. Increase in total organic carbon and bacteria were also observed after EPS supplement. Supplement of EPS resulted in a growth of petroleum-hydrocarbon degrading bacteria, which enhanced the toluene biodegradation.