Removal of hexanal in cooking fume by combination of storage and plasma-catalytic oxidation on alkali-modified Co-Mn solid solution

Improving benzene catalytic oxidation on Ag/Co3O4 by regulating the chemical states of Co and Ag

Silver (9 wt.%) was loaded on Co3O4-nanofiber using reduction and impregnation methods, respectively. Due to the stronger electronegativity of silver, the ratios of surface Co3+/Co2+ on Ag/Co3O4 were higher than on Co3O4, which further led to more adsorbed oxygen species as a result of the charge compensation. Moreover, the introducing of silver also obviously improved the reducibility of Co3O4. Hence the Ag/Co3O4 showed better catalytic performance than Co3O4 in benzene oxidation. Compared with the Ag/Co3O4 synthesized via impregnation method, the one prepared using reduction method (named as AgCo-R) exhibited higher contents of surface Co3+ and adsorbed oxygen species, stronger reducibility, as well as more active surface lattice oxygen species. Consequently, AgCo-R showed lowest T90 value of 183°C, admirable catalytic stability, largest normalized reaction rate of 1.36 × 10-4 mol/(h·m2) (150°C), and lowest apparent activation energy (Ea) of 63.2 kJ/mol. The analyzing of in-situ DRIFTS indicated benzene molecules were successively oxidized to phenol, o-benzoquinone, small molecular intermediates, and finally to CO2 and water on the surface of AgCo-R. At last, potential reaction pathways including five detailed steps were proposed.

A comprehensive review and perspective research in technology integration for the treatment of gaseous volatile organic compounds

Volatile organic compounds (VOCs) are harmful pollutants emitted from industrial processes. They pose a risk to human health and ecosystems, even at low concentrations. Controlling VOCs is crucial for good air quality. This review aims to provide a comprehensive understanding of the various methods used for controlling VOC abatement. The advancement of mono-functional treatment techniques, including recovery such as absorption, adsorption, condensation, and membrane separation, and destruction-based methods such as natural degradation methods, advanced oxidation processes, and reduction methods were discussed. Among these methods, advanced oxidation processes are considered the most effective for removing toxic VOCs, despite some drawbacks such as costly chemicals, rigorous reaction conditions, and the formation of secondary chemicals. Standalone technologies are generally not sufficient and do not perform satisfactorily for the removal of hazardous air pollutants due to the generation of innocuous end products. However, every integration technique complements superiority and overcomes the challenges of standalone technologies. For instance, by using catalytic oxidation, catalytic ozonation, non-thermal plasma, and photocatalysis pretreatments, the amount of bioaerosols released from the bioreactor can be significantly reduced, leading to effective conversion rates for non-polar compounds, and opening new perspectives towards promising techniques with countless benefits. Interestingly, the three-stage processes have shown efficient decomposition performance for polar VOCs, excellent recoverability for nonpolar VOCs, and promising potential applications in atmospheric purification. Furthermore, the review also reports on the evolution of mathematical and artificial neural network modeling for VOC removal performance. The article critically analyzes the synergistic effects and advantages of integration. The authors hope that this article will be helpful in deciding on the appropriate strategy for controlling interested VOCs.

Superior performance of oxygen vacancy-enriched Cu-Co3O4/urushiol-rGO/peroxymonosulfate for hypophosphite and phosphite removal by enhancing singlet oxygen

The treatment of wastewater containing hypophosphite [P(I)] and phosphite [P(III)] is challenged by limitations of traditional Fenton oxidation such as low efficiency, secondary pollution and high costs. This study introduced a facile solvent-thermal method to synthesize Cu-Co3O4 nanoparticles uniformly loaded on graphene (Cu-Co3O4/U-rGO) through the reduction and coordination effects of urushiol (U). As prepared Cu-Co3O4/U-rGO exhibited excellent activity in activating peroxymonosulfate (PMS) for the oxidation of P(I)/P(III) to phosphate [P(V)] (0.229 min-1), along with high stability and reusability (91.5 % after 6 cycles), low metal leaching rate (Co: 0.2 mg/L, Cu: 0.05 mg/L), insensitivity to common anions in water and a wide pH range (3-11). The activation mechanism involved the synergistic effects from both urushiol and graphene, which promoted redox of Cu+/Cu2+ and Co2+/Co3+ and induced abundant oxygen vacancies for PMS activation to produce singlet oxygen. Furthermore, the Cu-Co3O4/U-rGO/PMS was also excellent in the oxidative removal of organic phosphorus. This study is expected to advance strategies for the treatment of P(I)/P(III)-rich wastewater and provide new insights for the development of low-cost, highly efficient heterogeneous catalysts with abundant oxygen vacancies.

Promoted catalytic performance of Ag-Mn bimetal catalysts synthesized through reduction route

VOCs can exert great harm to both human and environment, and catalytic oxidation is believed to be an effective technique to eliminate these pollutants. In this paper, Ag-Mn bimetal catalysts with 10 wt.% of silver were synthesized using doping, impregnation, and reduction methods respectively, and then they were applied to the catalytic oxidation of benzene. Through series of characterizations it showed that the loading of silver using reduction method significantly resulted in improved physico-chemical properties of manganese oxides, such as larger surface area and pore volume, higher proportion of surface Mn3+ and Mn4+, stronger reducibility and more active of surface oxygen species, which were all beneficial to its catalytic activity. As a result, the Ag-Mn catalysts synthesized by reduction method showed a lower T90 value (equals to the temperature at which 90% of initial benzene was removed) of 203°C. Besides, both the used and fresh Ag-Mn catalysts synthesized by reduction method showed preferable stability in this research.

Chemical characterization of volatile organic compounds (VOCs) emitted from multiple cooking cuisines and purification efficiency assessments

Cooking process can produce abundant volatile organic compounds (VOCs), which are harmful to environment and human health. Therefore, we conducted a comprehensive analysis in which VOCs emissions from multiple cuisines have been sampled based on the simulation and acquisition platform, involving concentration characteristics, ozone formation potential (OFP) and purification efficiency assessments. VOCs emissions varied from 1828.5 to 14,355.1 µg/m³, with the maximum and minimum values from Barbecue and Family cuisine, respectively. Alkanes and alcohol had higher contributions to VOCs from Sichuan and Hunan cuisine (64.1%), Family cuisine (66.3%), Shandong cuisine (69.1%) and Cantonese cuisine (69.8%), with the dominant VOCs species of ethanol, isobutane and n-butane. In comparison, alcohols (79.5%) were abundant for Huaiyang cuisine, while alkanes (19.7%), alkenes (35.9%) and haloalkanes (22.9%) accounted for higher proportions from Barbecue. Specially, carbon tetrachloride, n-hexylene and 1-butene were the most abundant VOCs species for Barbecue, ranging from 8.8% to 14.6%. The highest OFP occurred in Barbecue. The sensitive species of OFP for Huaiyang cuisine were alcohols, while other cuisines were alkenes. Purification efficiency assessments shed light on the removal differences of individual and synergistic control technologies. VOCs emissions exhibited a strong dependence on the photocatalytic oxidation, with the removal efficiencies of 29.0%-54.4%. However, the high voltage electrostatic, wet purification and mechanical separation techniques played a mediocre or even counterproductive role in the VOCs reduction, meanwhile collaborative control technologies could not significantly improve the removal efficiency. Our results identified more effective control technologies, which were conductive to alleviating air pollution from cooking emissions.

Green synthesis of a hydrophobic metal-organic gel for the capture of trace odorous hexanal from humid air

The development of superhydrophobic adsorbents for the capture of trace volatile organic compounds (VOCs) from humid indoor air is still a challenge. Herein, we reported the formation of a granular zinc-based metal-organic gel, i.e., ZIF-412(gel) by optimizing the synthesis conditions. The thermally stable xerogel exhibited high surface area (1008 m²/g), hydrophobicity, and viscosity for self-depositing on the substrate such as non-woven fibers. Dynamic adsorption experiments under various humidity conditions demonstrated as-synthesized ZIF-412(gel) owned excellent VOC (hexanal) adsorption performance with adsorption capacity higher than commercial activated carbon and some water-stable MOFs including ZIF-8, ZIF-67, MIL-101(Cr) and ZIF-414. ZIF-412(gel) could be regenerated at temperature as low as 358 K without obvious loss in adsorption capacity. The adsorption mechanism of hexanal over ZIF-412(gel) is also simulated by Grand canonical Monte Carlo (GCMC).

Tetracycline-Induced Release and Oxidation of As(III) Coupled with Concomitant Ferrihydrite Transformation

Cocontamination with tetracycline (TC) and arsenic (As) is very common in paddy fields. However, the process and underlying mechanism of arsenite (As(III)) transformation on iron mineral surfaces in the presence of antibiotic contaminants remain unclear. In this study, the release and oxidation of As(III) on ferrihydrite (Fh) surfaces and Fh transformation in the presence of TC under both aerobic and anaerobic conditions were investigated. Our results indicated that the TC-induced reductive dissolution of Fh (Fe(II) release) and TC competitive adsorption significantly promote the release of As, especially under anaerobic conditions. The release of As was increased with increasing TC concentration, whereas it decreased with increasing pH. Interestingly, under both aerobic and anaerobic conditions, the addition of TC enhanced the oxidation of As(III) by Fh and induced the partial transformation of Fh to lepidocrocite. Under aerobic conditions, the adsorbed Fe(II) activated the production of reactive oxygen species (·OH and ¹O₂) from dissolved O₂, with Fe(IV) being responsible for As(III) oxidation. Under anaerobic conditions, the abundant oxygen vacancies of Fh affected the oxidation of As(III) during Fh recrystallization. Thus, this study provided new insights into the role of TC on the migration and transformation of As coupled with Fe in soils.

A critical review on plasma-catalytic removal of VOCs: Catalyst development, process parameters and synergetic reaction mechanism

It is urgent to control the emission of volatile organic compounds (VOCs) due to their harmful effects on the environment and human health. A hybrid system integrating non-thermal-plasma and catalysis is regarded as one of the most promising technologies for VOCs removal due to their high VOCs removal efficiency, product selectivity and energy efficiency. This review systematically documents the main findings and improvements of VOCs removal using plasma-catalysis technology in recent 10 years. To better understand the fundamental relation between different aspects of this research field, this review mainly addresses the catalyst development, key influential factors, generation of by-products and reaction mechanism of VOCs decomposition in the plasma-catalysis process. Also, a comparison of the performance in various VOCs removal processes is provided. Particular emphasis is given to the importance of the selected catalyst and the synergy of plasma and catalyst in the VOCs removal in the hybrid system, which can be used as a reference point for future studies in this field.

Trends and Challenges Regarding the Source-Specific Health Risk of PM2.5-Bound Metals in a Chinese Megacity from 2014 to 2020

Identifying the health risk of PM_2.5 is essential for urban air pollution control. In 2013, China announced the ever-strict national Air Pollution Prevention and Control Action Plan, and its health benefit should be evaluated to provide reference for future policymaking. In this study, we conducted a seven-year (2014-2020) continuous observation of PM_2.5 in Shenzhen, the third largest city in China, which has relatively good air quality. The results showed that the annual mean PM_2.5 and total concentration of 21 associated metals dropped from 37.7 to 18.5 μg/m³ and from 2.4 to 1.1 μg/m³, respectively. Combining methods for source apportionment and health risk assessment, we found that the total carcinogenic risk (CR) of five hazardous metals (Cd, Cr, Ni, Co, and Pb) showed a clear decreasing trend. However, the total CR (1.8 × 10^-6) in 2020 still exceeded the widely acceptable risk level (i.e., 1 × 10^-6), with the primary contributor changing from industrial emissions (61%) to vehicle emissions (63%). Further analysis indicated that the CR of vehicles mainly came from Cr and Ni released by braking and tire wearing and has fluctuated in recent years, highlighting a great challenge of controlling nonexhaust emissions of vehicles (including electric cars) in the future.

2D confinement freestanding graphene oxide composite membranes with enriched oxygen vacancies for enhanced organic contaminants removal via peroxymonosulfate activation

Introducing membrane filtration into advanced oxidation processes to decrease energy and cost consumption has been considered as a promising direction in environmental remediation. In this work, we firstly developed a kind of novel lawn-like Fe₂O₃@Co_0.08Fe_1.92@nitrogen-doped reduced graphene oxide@carbon nanotube composites (FeCo@GCTs) through in-situ pyrolysis of self-assembly of Prussian blue analogues and GO, followed through a vacuum-assisted filtration strategy to fabricate 2D confinement freestanding GO composite membrane. Electrochemical analysis and H₂-TPR revealed the superiority of FeCo@GCTs as ideal electron acceptor, and this unique lawn-like structure concentrated active sites with a confined space and enriched oxygen vacancies that realized 98.5% (0.128 min^-1) sulfamethoxazole degradation via peroxymonosulfate activation, and accelerated the reduction of Cr(VI). Owing to the increasing interlayer spacing of GO nanosheets, the permeation flux of FeCo@GCTs/GO membrane has not only been attained to 487.3 L·m^-2·h^-1·bar^-1, which was more than 7.5-fold of GO membrane (64.6 L·m^-2·h^-1·bar^-1), but also achieved the synergistic membrane filtration and catalytic degradation of pollutants. Furthermore, scavenger experiments and EPR tests were conducted to confirm the active radicals, of which SO₄·^- and ¹O₂ were responsible for SMX degradation. Therefore, these features demonstrated great potential for the fabricated 2D confinement catalytic membrane with enriched oxygen vacancies in wastewater purification.

Collection and decomposition of oil mist via corona discharge and surface dielectric barrier discharge

Oil mist emitted during cooking is one of the major sources of atmospheric particulate matter in urban areas. A conventional electrostatic precipitator (ESP) is used in some large restaurants; it requires regular electrode cleaning to maintain particle collection performance. However, oil mist generated during cooking is viscous and difficult to clean with water. Herein, we introduce a methodology and a device for cleaning collected oil mist using surface dielectric barrier discharge (surface-DBD) plasma. Our device uses corona discharge for the collection of oil mist. Subsequently, the oil mist collected is decomposed to gas-phase species by surface-DBD plasma. A maximum collection efficiency of 93.25% (for 230 nm di-ethyl hexyl sebacate (DEHS) particle) is obtained at a flow velocity of 1.5 m/s. The maximum oil mist decomposition efficiency is 96.4%. More than 80% of the decomposed oil mist is converted to CO₂ and CO under all test conditions. Some of the byproducts other than CO and CO₂ are released as particles. Higher frequency results in higher oil mist decomposition efficiency, but also higher byproduct formation of particles. The mechanism of oil mist decomposition by surface-DBD plasma is discussed using optical emission spectroscopy data.

Enhanced Catalytic Oxidation of Toluene over Manganese Oxide Modified by Lanthanum with a Coral-Like Hierarchical Structure Nanosphere

Coral-like lanthanum manganese oxides (La_yMnO_x) with a hierarchical structure nanosphere were successfully prepared using a simple method, which presented a high-efficiency catalytic performance for toluene combustion. Among them, La_0.08MnO_x with the Mn₃O₄ phase exhibits superior catalytic activity, such as a lower T₉₅ value (218 °C), excellent H₂O resistance, and catalytic stability. The effects of La addition on the bulk and surface physicochemical properties of La_yMnO_x were investigated by sorts of characterization including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption-desorption, temperature-programmed reduction with H₂, temperature-programmed desorption of O₂, X-ray photoelectron spectroscopy, and so forth. The results demonstrate that the doping of La can induce the variation of physicochemical properties and the formation of more surface oxygen species and high valence state amorphous manganese oxides, improving low-temperature reducibility, which facilitates good catalytic activity for La_0.08MnO_x. A series of in situ diffuse reflectance infrared Fourier transform spectroscopy experiments for toluene adsorption were performed on the La_0.08MnO_x catalyst pretreated under different atmosphere conditions to investigate the role of oxygen species and the reaction processes. The results indicate that the abundant surface oxygen species over La_0.08MnO_x can make the rapid formation of benzoic acid species, further transfer into CO₂ and H₂O, which is considered as the key factor in the activation and oxidation of toluene.

Immobilized Co2+ and Cu2+ induced structural change of layered double hydroxide for efficient heterogeneous degradation of antibiotic

In this study, MgMn-layered double hydroxide (MgMnLDH) exhibited excellent remediation functionality for heavy metals-antibiotics combined pollution. On the one hand, Co²⁺ and Cu²⁺ was efficiently immobilized on MgMnLDH with maximum quantity of 4.30 and 10.65 mmol g^-1, respectively. A series of characterizations reflected the changes in structure and physicochemical properties of MgMnLDH after the immobilization. Density functional theory calculations (DFT) confirmed that the binding modes were lattice substitution for Co²⁺ and surface precipitation for Cu²⁺. On the other hand, the immobilized heavy metals enhanced the heterogeneous degradation for sulfamethoxazole (SMX) by peroxymonosulfate (PMS) activation. Complete degradation was achieved within 10 min in MgMnLDH-Co-4/PMS system and 60 min in MgMnLDH-Cu/PMS system, while only 20% in MgMnLDH/PMS system. The pH adaptability, reusability, stability and activation mechanism of two systems were systematically compared. The superior degradation performance of MgMnLDH-Co-4 benefited from the intense Co/Mn synergism and abundant oxygen vacancies, which could accelerate electron transfer during PMS activation process. The applicability of two catalysis system was confirmed in purifying other antibiotics and actual wastewater. The results highlight the importance of structural control in heterogeneous catalysis and provide new idea for environmental remediation.

Catalytic activity of porous manganese oxides for benzene oxidation improved via citric acid solution combustion synthesis

Various manganese oxides (MnO_x) prepared via citric acid solution combustion synthesis were applied for catalytic oxidation of benzene. The results showed the ratios of citric acid/manganese nitrate in synthesizing process positively affected the physicochemical properties of MnO_x, e.g., BET (Brunauer-Emmett-Teller) surface area, porous structure, reducibility and so on, which were in close relationship with their catalytic performance. Of all the catalysts, the sample prepared at a citric acid/manganese nitrate ratio of 2:1 (C2M1) displayed the best catalytic activity with T₉₀ (the temperature when 90% of benzene was catalytically oxidized) of 212℃. Further investigation showed that C2M1 was Mn₂O₃ with abundant nano-pores, the largest surface area and the proper ratio of surface Mn⁴⁺/Mn³⁺, resulting in preferable low-temperature reducibility and abundant surface active adsorbed oxygen species. The analysis results of the in-situ Fourier transform infrared spectroscopy (in-situ FTIR) revealed that the benzene was successively oxidized to phenolate, o-benzoquinone, small molecules (such as maleates, acetates, and vinyl), and finally transformed to CO₂ and H₂O.

Facile Synthesis of Hydrophobic Metal-Organic Gels for Volatile Organic Compound Capture

Exploring a synthesis method for preparing hydrophobic metal-organic gels (MOGs) is highly desirable for air purification. Here, we present a rapid heating-up synthetic route to hydrophobic MOG denoted CAU-3(gel) with hierarchical micro/mesoporosity. CAU-3(gel), which features water and thermal stability, high surface area, and hydrophobicity, exhibits excellent performance for the capture of three representative volatile organic compounds (hexanal, toluene, and p-xylene), higher than BPL activated carbon, zeolite 13X, and some representative metal-organic frameworks including ZIF-8, HKUST-1, MIL-101(Cr), and UiO-66 under wet conditions. Furthermore, CAU-3(gel) could be easily coated on a nonwoven fabric by a simple dip-coating method without using any binder, which exhibits outstanding hexanal removal performance and regenerability at low temperature. Grand canonical Monte Carlo simulations show that hexanal preferentially enters relatively small tetrahedral cages and occupies two adsorption sites at low pressure and then a new site appears in octahedral cages with increasing pressure.

“Storage-Discharge” Ethanol Cold Plasma for Synthesizing High Performance Pd/Al2O3 Catalysts

Atmospheric pressure cold plasma is an environmentally friendly and novel method to synthesize supported metal catalysts, which usually uses active hydrogen species to reduce metal ions. Ethanol is a hydrogen-rich renewable liquid hydrogen source, and it is more convenient to store and transport than H2. In this study, a “storage-discharge” ethanol cold plasma was used to prepare Pd/Al2O3-EP catalysts, and the obtained catalysts are used for CO oxidation. The complete oxidation of CO temperature (T100) over Pd/Al2O3-EP was 145 °C, which was comparable to the performance of Pd/Al2O3-HP that was synthesized by atmospheric pressure hydrogen cold plasma. Pd/Al2O3-EP-C obtained by calcining Pd/Al2O3-EP at 450 °C for 2 h in air atmosphere in order to remove residual carbon species showed much higher CO oxidation activity, and T100 was 130 °C. The Pd/Al2O3 catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron diffraction (XPS), Brunauer–Emmett–Teller (BET), and transmission electron microscopy (TEM), and the structure-performance relationship was analyzed. The results indicate that the “storage-discharge” ethanol cold plasma can reduce the Pd precursor ions into metallic Pd state, and the dissociation of ethanol forms lots of highly active chemisorbed oxygen species, which can enhance the performance of Pd/Al2O3-EP for CO oxidation. In contrast, Pd/Al2O3-EP-C shows much higher CO oxidation activity, which is mainly attributed to the removal of the residual carbon species, and the exposure of more Pd active sites and chemisorbed oxygen species. The “storage-discharge” ethanol cold plasma is a safe and efficient novel method for synthesizing supported Pd catalysts, and it has important potential for the preparation and application of supported metal catalysts.

The emission characteristic of VOCs and the toxicity of BTEX from different mosquito-repellent incenses

Three types of mosquito-repellent incenses including disc, electric liquid and electric mat, were selected to investigate the emission of volatile organic compounds in the respiratory zone during the burning. VOCs were analyzed by GC-MS. Results showed that the average concentration of TVOCs released by using the disc, electric liquid and electric mat mosquito repellent incense types were 7.760 ± 4.724, 3.122 ± 0.866 and 1.192 ± 0.062 mg/m³, respectively. The TVOCs pollution level produced during the burning of different mosquito-repellent incense types was ranked in the order: disc > electric liquid > electric mat. The concentration of TVOCs produced by Q-liquid, L-liquid, Q-disc and L-disc mosquito-repellent incense types could cause discomfort to the human body. Other types of mosquito-repellent incense induced multiple synergistic effects on human response. About 230 kinds of VOCs including 14 types of VOCs, were found in the smoke of mosquito-repellent incense. The number and content of alkanes was the highest, followed by aromatic hydrocarbons and esters. The level of non-carcinogenic health risk presented by exposure to BTEX (toluene, xylene and ethylbenzene) was acceptable. The chronic daily intake of VOCs decreased with age for both males and females, with male exposure being higher than that of the female, except for children.