Recent advances in the abatement of volatile organic compounds (VOCs) and chlorinated-VOCs by non-thermal plasma technology: A review

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.

Simultaneously enhancing toluene adsorption and regeneration process by hierarchical pore in activated coke: a combined experimental and adsorption kinetic modeling study

Activated coke is a type of commonly used adsorbent for benzene series VOCs such as toluene, but traditional microporous activated coke usually faces the challenge of poor regeneration performance. Herein, based on self-made activated cokes with typical pore configuration, we found that adsorption and regeneration of toluene can be simultaneously enhanced by constructing hierarchical pore in activated coke. Correlations of pore configuration with toluene adsorption capacity and regeneration efficiency reveal that micropore contributes for strong toluene adsorption; meso-macropore provides mass transfer channel for toluene desorption and regeneration process. Hierarchical porous activated coke prepared from Zhundong subbituminous coal not only achieves the highest toluene adsorption capacity of 340.92 mg·g-1, but also can retain more than 90% of initial adsorption capacity after five adsorption-regeneration cycles. By contrast, micropore-dominant activated cokes can only retain 70% of initial adsorption capacity. Adsorption kinetic modelling on adsorption breakthrough curves shows that hierarchical porous activated coke prepared from Zhundong subbituminous coal exhibits high adsorption and diffusion rate constants of 14.39 and 33.45 min-1, respectively, much higher than those of micropore-dominant activated cokes. Due to the accelerated surface adsorption and diffusion processes induced by meso-macropore, toluene adsorption and regeneration behavior can be simultaneously improved. Results from this work validated the role of pore hierarchy in toluene adsorption-regeneration process, providing guidance for designing high-performance activated coke with synergistically improved toluene adsorption capacity and regeneration performance.

Catalytic Oxidation of Benzene over Atomic Active Site AgNi/BCN Catalysts at Room Temperature

Benzene is the typical volatile organic compound (VOC) of indoor and outdoor air pollution, which harms human health and the environment. Due to the stability of their aromatic structure, the catalytic oxidation of benzene rings in an environment without an external energy input is difficult. In this study, the efficient degradation of benzene at room temperature was achieved by constructing Ag and Ni bimetallic active site catalysts (AgNi/BCN) supported on boron-carbon-nitrogen aerogel. The atomic-scale Ag and Ni are uniformly dispersed on the catalyst surface and form Ag/Ni-C/N bonds with C and N, which were conducive to the catalytic oxidation of benzene at room temperature. Further catalytic reaction mechanisms indicate that benzene reacted with ·OH to produce R·, which reacted with O2 to regenerate ·OH. Under the strong oxidation of ·OH, benzene was oxidized to form alcohols, carboxylic acids, and eventually CO2 and H2O. This study not only significantly reduces the energy consumption of VOC catalytic oxidation, but also improves the safety of VOC treatment, providing new ideas for the low energy consumption and green development of VOC treatment.

Current trends on dry photocatalytic oxidation technology for BTX removal: Viable light sources and highly efficient photocatalysts

One of the main gaseous pollutants released by chemical production industries are benzene, toluene and xylene (BTX). These dangerous gases require immediate technology to combat them, as they put the health of living organisms at risk. The development of heterogeneous photocatalytic oxidation technology offers several viewpoints, particularly in gaseous-phase decontamination without an additional supply of oxidants in air at atmospheric pressure. However, difficulties such as low quantum efficiency, ability to absorb visible light, affinity towards CO2 and H2O synthesis, and low stability continue to limit its practical use. This review presents recent advances in dry-phase heterogeneous photodegradation as an advanced technology for the practical removal of BTX molecules. This review also examines the impact of low-cost light sources, the roles of the active sites of photocatalysts, and the feasible concentration range of BTX molecules. Numerous studies have demonstrated a significant improvement in the efficiency of the photodegradation of volatile organic compounds by enhancing the photocatalytic reactor system and other factors, such as humidity, temperature, and flow rate. The mechanism for BTX photodegradation based on density functional theory (DFT), electron paramagnetic resonance (EPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations is also discussed. Finally, the present research complications and anticipated future developments in the field of heterogeneous photocatalytic oxidation technology are discussed.

Promotion of Cobalt Oxide Catalysts by Acid-Etching and Ruthenium Incorporation for Chlorinated VOC Oxidation

In this work, Ru-promoted cobalt oxide catalysts with a nanotube morphology were prepared by a synthesis route based on the Kirkendall effect followed by an acid treatment and subsequent optimized Ru impregnation. The resulting samples were thoroughly characterized by means of N2 physisorption, X-ray energy-dispersive spectroscopy, X-ray diffraction, scanning electron microscopy techniques, X-ray photoelectron spectroscopy, and temperature-programmed techniques (O2-temperature-programmed desorption, H2-temperature-programmed reduction, and temperature-programmed oxidation) and evaluated in the gas-phase oxidation of 1,2-dichloroethane. It has been demonstrated that Ru addition improves the oxygen mobility as well as the amount of Co2+ and Oads species at the surface by the formation of the Ru-O-Co bond, which in turn governs the performance of the catalysts in the oxidation reaction. Moreover, the acid-etching favors the dispersion of the Ru species on the surface of the catalysts and strengthens the interaction among the noble metal and the cobalt oxide, thereby improving the thermal stability of the Ru-promoted oxides. Thus, the resulting catalysts are not only active, as the chlorinated pollutant is efficiently converted into deep oxidation products at relatively low temperatures, but also quite stable when operating for 120 h.

Antibacterial and deodorizing effects of cold atmospheric plasma-applied electronic deodorant

Axillary odor is a malodor produced by bacterial metabolism near the apocrine glands, which often causes discomfort in an individual's daily life and social interactions. A deodorant is a personal care product designed to alleviate or mask body odor. Currently, most deodorants contain antimicrobial chemicals and fragrances for odor management; however, direct application to the underarm skin can result in irritation or sensitivity. Therefore, there is a growing interest in technologies that enable disinfection and odor control without the antiperspirants or perfumes. The cold atmospheric plasma temporally generates reactive radicals that can eliminate bacteria and surrounding odors. In this study, cultured Staphylococcus hominis and Corynebacterium xerosis, the causative bacteria of axillary bromhidrosis, were killed after 90% plasma exposure for 3 min. Moreover, the electronic nose system indicated a significant reduction of approximately 51% in 3-hydroxy-3-methylhexanoic acid and approximately 34% in 3-methyl-3-sulfanylhexan-1-ol, the primary components of axillary odor, following a 5-min plasma exposure. These results support the dual function of our deodorant in eliminating bacteria and axillary odors without the chemical agents. Therefore, cold atmospheric plasma-applied deodorant devices have great potential for the treatment and management of axillary odors as a non-contact approach without chemical use in daily life.

Differential Etching of Rays at Wood Surfaces Exposed to an Oxygen Glow Discharge Plasma

Basswood samples were exposed to oxygen glow-discharge plasmas for 30 min, and etching of radial and tangential longitudinal surfaces was measured. It was hypothesized that there would be a positive correlation between etching and plasma energy, and differential etching of wood surfaces because of variation in the microstructure and chemical composition of different woody tissues. Etching at the surface of basswood samples was examined using profilometry. Light and scanning electron microscopy were used to examine the microstructure of samples exposed to plasma. There was a large effect of plasma energy on etching of basswood surfaces, and radial surfaces were etched to a greater extent than tangential surfaces. However, rays at radial surfaces were more resistant to etching than fibers, resulting in greater variation in the etching of radial versus tangential surfaces. The same phenomenon occurred at radial surfaces of balsa wood, jelutong and New Zealand white pine subjected to plasma etching. The possible reasons for the greater resistance of rays to plasma etching are explored, and it is suggested that such differential etching of wood surfaces may impose a limitation on the use of plasma to precisely etch functional patterns at wood surfaces (raised pillars, grooves), as has been done with other materials.

Removal of toluene via non-thermal plasma generated by applying rare-earth tungsten electrode and nanosecond pulsed power supply

The objective of this investigation is to evaluate the characteristics associated with degradation of toluene through the utilization of non-thermal plasma (NTP) generated via application of a low-work-function electrode and nanosecond pulsed power supply. Initially, a comparative analysis is made between toluene removal efficiency utilizing the low-work-function electrode and that achieved with the conventional stainless-steel electrode. The outcomes demonstrate that NTP generated by the low-work-function electrode exhibits markedly superior removal efficiency for toluene in comparison to the stainless-steel electrode operating at the same voltage. Subsequently, the impacts of voltage, pulse frequency, and initial concentration of toluene on the removal efficiency and production of by-products are investigated. It is found that as the voltage and frequency increase, the removal efficiency also increases, and a maximum toluene removal efficiency of 87.2% is achieved at a voltage of 12,000 V and pulse frequency of 2000 Hz. The removal efficiency first increases and then decreases with increasing toluene initial concentration. The investigation also finds that energy yield is negatively correlated with voltage and pulse frequency and positively correlated with the initial concentration. Finally, the reaction products were subjected to quantitative analysis using GC-MS. Based on the analysis results, potential reaction pathways are inferred.

Neural network-based performance assessment of one- and two-liquid phase biotrickling filters for the removal of a waste-gas mixture containing methanol, α-pinene, and hydrogen sulfide

The performance of one- and two-liquid phase biotrickling filters (OLP/TLP-BTFs) treating a mixture of gas-phase methanol (M), α-pinene (P), and hydrogen sulfide (H) was assessed using artificial neural network (ANN) modeling. The best ANN models with the topologies 3-9-3 and 3-10-3 demonstrated an exceptional capacity for predicting the performance of O/TLP-BTFs, with R2 > 99%. The analysis of causal index (CI) values for the model of OLP-BTF revealed a negative impact of M on P removal (CI = -2.367), a positive influence of P and H on M removal (CI = +7.536 and CI = +3.931) and a negative effect of H on P removal (CI = -1.640). The addition of silicone oil in TLP-BTF reduced the negative impact of M and H on P degradation (CI = -1.261 and CI = -1.310, respectively) compared to the OLP-BTF. These findings suggested that silicone oil had the potential to improve P availability to the biofilm by increasing the concentration gradient of P between the air/gas and aqueous phases. Multi-objective particle swarm optimization (MOPSO) suggested an optimum operational condition, i.e. inlet M, P, and H concentrations of 1.0, 1.1, and 0.3 g m-3, respectively, with elimination capacities (ECs) of 172.1, 26.5, and 0.025 g m-3 h-1 for OLP-BTF. Likewise, one of the optimum operational conditions for TLP-BTF is achievable at inlet concentrations of 4.9, 1.7, and 0.8 g m-3, leading to the optimum ECs of 299.7, 52.9, and 0.072 g m-3 h-1 for M, P, and H, respectively. These results provide important insights into the treatment of complex waste gas mixtures, addressing the interactions between the pollutant removal characteristics in OLP/TLP-BTFs and providing novel approaches in the field of biological waste gas treatment.

Recent advances in the control of volatile organic compounds emissions from indoor wood-based panels: A comprehensive review

Wood-based panels provide efficient alternatives to materials such as plastics derived from traditional petroleum sources and thereby help to mitigate greenhouse gas emissions. Unfortunately, using indoor manufactured panel products also results in significant emissions of volatile organic compounds including olefins, aromatic and ester compounds, which negatively affect human health. This paper highlights recent developments and notable achievements in the field of indoor hazardous air treatment technologies to guide future research toward environmentally friendly and economically feasible directions that may have a significant impact on the improvement of human settlements. Summarizing and synthesizing the principles, advantages, and limitations of different technologies can assist policymakers and engineers in identifying the most appropriate technology for a particular air pollution control program based on criteria such as cost-effectiveness, efficiency, and environmental impact. In addition, insights into the development of indoor air pollution control technologies are provided and potential areas for innovation, improvement of existing technologies, and development of new technologies are identified. Finally, the authors also hope that this sub-paper will raise public awareness of indoor air pollution issues and promote a better understanding of the importance of indoor air pollution control technologies for public health, environmental protection, and sustainable development.

Nonthermal plasma coupled with liquid-phase UV/Fe-C for chlorobenzene removal

Catalyst poisoning problems limit the application of gas-solid non-thermal plasma (NTP) catalyzed decomposition of chlorinated volatile organic compounds (Cl-VOCs). To mitigate the catalyst deactivation, catalyst iron-loaded activated carbon (Fe-C) was added to the UV-activated liquid phase downstream of the NTP reactor (NTP + UV/Fe-C(L)) for the degradation of chlorobenzene (CB) in this study. The CB removal efficiency and mineralization efficiency (MR) of NTP + UV/Fe-C(L) were up to 94% and 68%, respectively, which were increased by 39% and 30% compared with the single NTP system. Compared with the conventional gas-solid NTP + UV/Fe-C(S) system, the stability of the NTP + UV/Fe-C(L) system was significantly improved due to the dissolved organic intermediates and low residuals on the catalyst surface. Reactive oxygen species ·OH and ·O₂^- dominated the decomposition of CB in the liquid phase, and with the help of UV, much more ·OH and ·O₂^- were produced by Fe-C catalytic O₃. In addition, Fe-C improved the removal of CB by increasing its absorption mass transfer coefficient from 0.0016 to 0.0157 s^-1. The degradation pathway of CB in the NTP + UV/Fe-C(L) system was proposed based on the detected organic intermediates. Overall, this study provides a new tactic to solve the catalyst poisoning problem in the NTP catalytic oxidation of Cl-VOCs.

Plasma-Catalytic CO2 Reforming of Toluene over Hydrotalcite-Derived NiFe/(Mg, Al)O x Catalysts

The removal of tar and CO₂ in syngas from biomass gasification is crucial for the upgrading and utilization of syngas. CO₂ reforming of tar (CRT) is a potential solution which simultaneously converts the undesirable tar and CO₂ to syngas. In this study, a hybrid dielectric barrier discharge (DBD) plasma-catalytic system was developed for the CO₂ reforming of toluene, a model tar compound, at a low temperature (∼200 °C) and ambient pressure. Periclase-phase (Mg, Al)O _x nanosheet-supported NiFe alloy catalysts with various Ni/Fe ratios were synthesized from ultrathin Ni-Fe-Mg-Al hydrotalcite precursors and employed in the plasma-catalytic CRT reaction. The result demonstrated that the plasma-catalytic system is promising in promoting the low-temperature CRT reaction by generating synergy between DBD plasma and the catalyst. Among the various catalysts, Ni4Fe1-R exhibited superior activity and stability because of its highest specific surface area, which not only provided sufficient active sites for the adsorption of reactants and intermediates but also enhanced the electric field in the plasma. Furthermore, the stronger lattice distortion of Ni4Fe1-R provided more isolated O^2- for CO₂ adsorption, and having the most intensive interaction between Ni and Fe in Ni4Fe1-R restrained the catalyst deactivation induced by the segregation of Fe from the alloy to form FeO _x . Finally, in situ Fourier transform infrared spectroscopy combined with comprehensive catalyst characterization was used to elucidate the reaction mechanism of the plasma-catalytic CRT reaction and gain new insights into the plasma-catalyst interfacial effect.