Absorption of toluene by vegetable oil–water emulsion in scrubbing tower: Experiments and modeling

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.

Cooperative and Bifunctional Adsorbent-Catalyst Materials for In-situ VOCs Capture-Conversion

Volatile organic compounds (VOCs) are gases that are emitted into the air from products or processes and are major components of air pollution that significantly deteriorate air quality and seriously affect human health. Different types of metals, metal oxides, mixed-metal oxides, polymers, activated carbons, zeolites, metal-organic frameworks (MOFs) and mixed-matrixed materials have been developed and used as adsorbent or catalyst for diversified VOCs detection, removal, and destruction. In this comprehensive review, we first discuss the general classification of VOCs removal materials and processes and outline the historical development of bifunctional and cooperative adsorbent-catalyst materials for the removal of VOCs from air. Subsequently, particular attention is devoted to design of strategies for cooperative adsorbent-catalyst materials, along with detailed discussions on the latest advances on these bifunctional materials, reaction mechanisms, long-term stability, and regeneration for VOCs removal processes. Finally, challenges and future opportunities for the environmental implementation of these bifunctional materials are identified and outlined with the intent of providing insightful guidance on the design and fabrication of more efficient materials and systems for VOCs removal in the future.

Enhancements on volatile organic compounds (VOCs) adsorption and desorption performance of ZSM-5 by fabricating hierarchical MCM-41

ZSM-5 zeolite has been considered a promising adsorbent for capturing VOCs. However, its hydrophilicity and narrow micropore structure limit their practical application especially under humid atmospheres. In this study, the pure silica mesoporous molecular sieve MCM-41 was assembled on ZSM-5 zeolite with different SiO2/Al2O3 ratios (SARs) via a surfactant-mediated recrystallization method. Then, its adsorption-desorption behaviors were investigated using n-hexane, toluene, and ethyl acetate as VOC model molecules. The results showed that the hydrophobicity of ZSM-5/MCM-41 composites and their VOC diffusion behaviors were significantly improved. Furthermore, the SARs of the ZSM-5 precursors had a remarkable influence on the adsorption performance of ZSM-5/MCM-41 composites. ZSM-5/MCM-41(130) was the optimum option, and its dynamic adsorption capacity for ethyl acetate (111.30 mg·g-1) was higher than that of the corresponding ZSM-5 zeolites even under statured humidity. Meanwhile, the ratios of dynamic adsorption capacities at humid and dry atmospheres (qs,wet/qs,dry) of ZSM-5/MCM-41(130) for n-hexane, toluene, and ethyl acetate were 84.89%, 61.46%, and 73.81% respectively. The results will provide guidelines for the preparation of hydrophobic adsorbents.

A review of whole-process control of industrial volatile organic compounds in China

Volatile organic compounds (VOCs) play an important role in the formation of ground-level ozone and secondary organic aerosol (SOA), and they have been key issues in current air pollution prevention and control in China. Considerable attention has been paid to industrial activities due to their large and relatively complex VOCs emissions. The present research aims to provide a comprehensive review on whole-process control of industrial VOCs, which mainly includes source reduction, collection enhancement and end-pipe treatments. Lower VOCs materials including water-borne ones are the keys to source substitution in industries related to coating and solvent usage, leak detection and repair (LDAR) should be regarded as an efficient means of source reduction in refining, petrochemical and other chemical industries. Several types of VOCs collection methods such as gas-collecting hoods, airtight partitions and others are discussed, and airtight collection at negative pressure yields the best collection efficiency. Current end-pipe treatments like UV oxidation, low-temperature plasma, activated carbon adsorption, combustion, biodegradation, and adsorption-combustion are discussed in detail. Finally, several recommendations are made for future advanced treatment and policy development in industrial VOCs emission control.

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.

Evaluation of the absorption performance of new compound absorbents for toluene under extremely high load

Absorption is an effective way to control volatile organic compound (VOC) industrial air pollution, and the key variable in this process is the selection of suitable liquid absorbents to absorb as many organic pollutants as possible. The objective of this study was to prepare a series of high-efficiency absorbents with different proportions of vegetal oil, mineral oil, and waste engine oil, which can be used for toluene absorption. The absorption efficiency (AE), saturated absorption (SA), and effective absorption time (EAT) of various absorbents were systematically analyzed. The results showed that when the inlet concentration of toluene was 8000 mg/m³ and the inlet flow was 1 L/min, the SA capability of vegetal oil, mineral oil, and waste engine oil was 7.15, 12.43, and 18.16 mg/g, respectively. With the 4000 mg/m³ inlet concentration, the SA of the absorber which was made in the ratio of 2:3:1 was increased to 50.93 mg/g. According to the thermodynamic equilibrium and absorption results, it is proved that the influence of the composition of the absorbent on absorption is greater than viscosity. It is also to be noted that the AE of the composite absorbent can still reach more than 80% after three times of heating and air purification.

Comprehensive evaluation of ionic liquid [Bmim][PF6] for absorbing toluene and acetone

Absorption is an eminent technology for volatile organic compounds (VOCs) elimination with the merits of high efficiency and low cost. Absorbent plays a critical role in the absorption process, and the thermal stability, saturation capacity, and regeneration performance should be concerned. As a kind of green and eco-friendly solvent, ionic liquid (IL) is expected to be a substitute for the conventional VOCs absorbent. In this study, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) is employed to absorb the modeling VOCs (toluene and acetone). Moreover, the used [Bmim][PF₆] is recovered by thermal distillation and the reusability is then conducted by consecutive batch experiments. Based on that, the thermal stability of [Bmim][PF₆] is comprehensively examined, in which the kinetic and thermodynamic parameters are also calculated. Results reveal that [Bmim][PF₆] owned promising toluene absorption performance with inlet concentration of 3000 mg/m³ and flow rate of 300 mL/min at 20 °C, it possesses the saturated adsorption capacity of 5.16 mg/g. [Bmim][PF₆] also shows satisfying thermal stability up to 610 K. In addition, thermal distillation is proved to be a reliable regeneration route on account of the recovered [Bmim][PF₆] remained satisfying capacity even after five cycles.

Hierarchical porous biochar from plant-based biomass through selectively removing lignin carbon from biochar for enhanced removal of toluene

This study proposed a simple and green air oxidation (AO) method to prepare hierarchical porous biochar by selectively removing lignin carbon from biochar after the pyrolysis of plant-based biomass, based on the fact that the thermal decomposition temperature in air between lignin carbon and cellulose/hemicellulose carbon was different. Three kinds of biomass with different lignocellulose contents were used, including walnut shell, cypress sawdust and rice straw. The results found that AO treatment could effectively improve the pore structure of the three biochar. The specific surface area of WCO-4, CCO-4 and RCO-4 was 555.0 m²/g, 418.7 m²/g and 291.9 m²/g, respectively, which was significantly higher than those of WC (319.5 m²/g), CC (381.7 m²/g) and RC (69.6 m²/g), respectively. Among these, walnut shell biochar with air oxidation (WCO) had higher surface area of 555.0 m²/g and mesopore volume of 0.116 cm³/g, this was related to its high content of lignin, which could facilitate the formation of mesopores by AO treatment with high selectivity. The toluene adsorption capacity of WCO reached 132.9 mg/g, which increased by 223.4% from that without AO treatment. The kinetics study indicated that the diffusion rates of toluene molecule were improved due to the increased mesopores volume of biochar and micropores also play an important role in the adsorption of toluene. The results demonstrate that AO treatment is a promising method to develop hierarchical porous structure for lignocellulose-rich plant-based biomass with low cost and environmental-friendly, which greatly enhanced the toluene adsorption capacity.

Efficient removal of volatile organic compound by ball-milled biochars from different preparing conditions

Adsorption is an important technology to deal with volatile organic compounds (VOCs), and biochar has attracted much attention as a new type of adsorbent for VOCs. In this study, rice husk, corn stover and pine wood sawdust biochars from different pyrolysis temperatures (300 °C, 500 °C and 700 °C) were synthesized and treated by ball milling. The pristine and ball-milled biochars were used as adsorbents for acetone and toluene removal. Results showed that wood biochar had higher adsorption capacity for VOCs. After ball milling, the BET specific surface area and the oxygen functional group content of biochars increased. With these changes, all the ball-milled biochars showed higher adsorption rate than the pristine biochars. The ball-milled biochars under pyrolysis temperature of 300 °C showed the best adsorption performance for acetone (304 mg g^-1), which was 1.7-fold greater than that of pristine biochar. Increasing the surface area by ball milling is conducive to the diffusion of hydrophobic VOCs molecules such as toluene to the adsorption sites in the biochar. However, for hydrophilic VOCs such as acetone, higher oxygen functional groups were the main reason for the enhanced adsorption by ball milling. Therefore, ball-milled biochar can be used as a potential adsorbent material in VOCs treatment.

Highly efficient capture of odorous sulfur-based VOCs by ionic liquids

This study proposes the capture of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) from waste gas using an ionic liquid (IL), namely, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf₂N]), and examines the process from a molecular level to the laboratory scale, which is then scaled up to the industrial level. The binding energy and weak interactions between DMS/DMDS and the anion/cation in [EMIM][Tf₂N] were investigated using quantum chemistry calculations to identify the capture mechanism at the molecular scale. A thermodynamic model (UNIFAC-Lei) was established by the vapor-liquid equilibrium data of the [EMIM][Tf₂N] + DMS/DMDS systems measured at the laboratory scale. The equilibrium and continuous absorption experiments were performed, and the results demonstrated that [EMIM][Tf₂N] exhibits a highly efficient capture performance at atmospheric conditions, particularly, absorption capacities (AC) for DMS and DMDS are 189.72 and 212.94 mg g^-1, respectively, and partial coefficients (PC) as more reasonable evaluation metrics for those are 0.509 × 10^-4 and 6.977 × 10^-4 mol kg^-1 Pa^-1, respectively, at the 100 % breakthrough. Finally, a mathematical model of the strict equilibrium stage was established for process simulations, and the absorption process was conceptually designed at the industrial scale, which could provide a decision-making basis for chemical engineers and designers.

Characteristics and mechanism of toluene removal by double dielectric barrier discharge combined with an Fe2O3/TiO2/γ-Al2O3 catalyst

Removal of volatile organic compounds (VOCs) by non-thermal plasma technology produced by dielectric barrier discharge has become a hotspot due to its rapid reaction at room temperature, convenience without preheating and high removal efficiency of VOCs. Ways to improve the removal efficiency of mixed VOCs and the control of by-products in the discharge process are urgently needed. In view of the above shortcomings, the research progress of catalysts to improve the removal efficiency of VOCs by dielectric barrier discharge in recent years was reviewed; in this research, we conducted experiments on the removal efficiency of toluene under high flow rate experimental conditions (input voltage 10-70 V, mixed gas flow rate 1 m³ h^-1, inlet concentration 1000 mg m^-3). The research used γ-Al₂O₃, TiO₂/γ-Al₂O₃ and Fe₂O₃/TiO₂/γ-Al₂O₃ as catalysts in a reactor with a discharge gap of 6 mm, and a 99.7% removal efficiency of toluene was achieved when the SIE was 183.4 J L^-1. The added catalyst significantly reduced the concentration of by-products. At the same time, experiments with multiple catalysts to improve the removal efficiency and the performance of the discharge effect under different humidity levels were analyzed. The removal efficiency of toluene was optimized and recyclable, the formation of by-products was effectively suppressed when the catalysts in the reactor were Fe₂O₃/TiO₂/γ-Al₂O₃, TiO₂/γ-Al₂O₃ and γ-Al₂O₃, and the discharge length was 1 : 2 : 2.

A facile synthesis of nitrogen-doped porous carbons from lignocellulose and protein wastes for VOCs sorption

This paper reported a sustainable method for preparation of N-doped porous carbon from co-hydrothermal carbonization (co-HTC) of lignocellulose (as a carbon skeleton) and sludge protein (as a nitrogen dopant). The HTC pretreatment could improve the yield of porous carbon. Although a low mass ratio (1:1) of biochar and KOH was employed, large specific surface areas of the porous carbons could be achieved. The porous carbon from rice husk (RH) by the HTC (260 °C) combined with the KOH activation (750 °C) had a largest BET surface area up to 1396.5 m²/g. Meanwhile, its adsorption capacity on toluene could reach 394.2 mg/g. It was attributed to its large specific surface area and hierachical porous structure with higher meso-porosity. The porous carbons with N-doping also improved the adsorption capacity. Furthermore, thermal desorption at around 300 °C had a high potential for regeneration of the saturated porous carbons.

Capturing of gaseous and particulate pollutants into liquid phase by a water/oil column using microbubbles

A new method was developed to remove gaseous and particulate pollutants by capturing them in water using microbubbles. The capture efficiency of gaseous toluene and ultrafine carbon particles, which are hydrophobic substances, was remarkably improved compared to water-only conditions by adding a small amount of oily substances (4% volume fraction of water) to the water surface. The physicochemical properties of four types of oily substances were investigated. Rapeseed and mineral oil exhibited good capture efficiency during a capture experiment of high-concentration gaseous toluene for 96 h. Additionally, a long-term continuous capture experiment for 24 days revealed that the capture mechanisms of rapeseed and mineral oil were different. The toluene concentration in rapeseed oil reached saturation in the middle of the experiment while the capture efficiency of mineral oil remained constant. It was also shown that the emulsion formation greatly affected the capture of rapeseed oil. Thus, it is expected that a new gaseous pollutant treatment technology that can capture and remove gaseous/particulate pollutants regardless of their hydrophilic/hydrophobic properties could be developed in the future.

Sorption of volatile organic compounds on non-activated biochar

This work dealt with the determination of the suitability of sorption of Volatile Organic Compounds (VOCs) on biochars prepared from neem, sugarcane and bamboo feedstocks. Six different VOCs namely benzene, toluene, methyl chloride, xylene, chloroform and carbon tetrachloride were used in a laboratory-scale set-up on non-activated biochars prepared via slow pyrolysis (350-550 °C). Although all the chars showed considerable sorption but amongst them N3 (neem-based biochar) showed the maximum removal efficiency (65.5 mg g^-1 for toluene). Variation in pyrolysis temperature and feedstock type showed significant change in the porosity and specific surface area of the biochar, which is favorable for VOC sorption efficiency. With higher surface area and contact time, the sorption capacity of char enhanced. However, the extent of sorption capacity of biochars differed with changing VOC type. Pseudo-Second-Order model fitted well with the results obtained from VOC sorption kinetics.

Catalytic removal of toluene over manganese oxide-based catalysts: a review

It is necessary to control the emissions of toluene, which is hazardous to both human health and the atmosphere environment and has been classified as a priority pollutant. Manganese oxide-based (Mn-based) catalysts have received increased attention due to their high catalytic performance, good physicochemical characteristic, availability in various crystal structures and morphologies, and being environmentally friendly and low cost. These catalysts can be classified into five categories, namely single manganese oxide, Mn-based composite oxides, Mn-based special oxides, supported Mn-based oxides, and Mn-based monoliths. This review focused on the recent progress on the five types of Mn-based catalysts for catalytic removal of toluene at low temperature and further systematically summarized the strategies improving catalysts, including improving synthetic methods, incorporating MnO_x with other metal oxides, depositing Mn-based oxides on proper supports, and tuning the supports. Moreover, the effect of coexisting components, the reaction kinetics, and the oxidation mechanisms toward the removal of toluene were also discussed. Finally, the future research direction of this field was presented.

Synthesis of ZSM-5/Siliceous Zeolite Composites for Improvement of Hydrophobic Adsorption of Volatile Organic Compounds

In this research, we investigated the hydrophobicity and dynamic adsorption-desorption behaviors of volatile organic compounds (VOCs) by applying different optimized coating dosage (25, 50, and 75%) on designed novel ZSM-5/MCM-41 and ZSM-5/Silicalite-1 hierarchical composites. The relatively large specific surface area and pore volume of adsorbents ZSM-5/MCM-41 and ZSM-5/Silicalite-1 composites with excellent stability were affirmed by ex-situ XRD, FTIR, BET, SEM, and water contact angle analyses. Regarding, toluene adsorption-desorption investigation, ZSM-5/MCM-41 composite lead a longer stable toluene breakthrough time no matter under dry or 50% humid conditions. However, under different loading dosage condition, the breakthrough time of 75% coating ratio was the longest, which was 1.6 times as long as that of pure ZSM-5 under wet adsorption. Meanwhile, the complete elimination of toluene for ZSM-5/MCM-41-75% was done by largest desorption peak area and the lowest desorption temperature of 101.9°C, while, the largest contact angle of ZSM-5/MCM-41-75% was 17.0° higher than pure ZSM-5 zeolite. Therefore, we believe that the present hydrophobic sorbent will provide new insight with great research potential for removing low concentration of VOCs at industrial scale.

Facile synthesis of porous carbons from silica-rich rice husk char for volatile organic compounds (VOCs) sorption

This work reported a facile synthesis of porous carbons from the silica-rich rice husk biochar via a ball-milling-assisted KOH activation for sorption of tar compounds and volatile organic compounds (VOCs) (i.e., toluene, phenol). The textural properties of activated biochars can be greatly influenced by the mass ratio of KOH and biochar. The high-performance biochar with a large specific surface area (S_BET: 1818 m²/g) was produced as the mass ratio was 3. This activated biochar exhibited a hierarchically meso-microporous structure, which benefited for the adsorption process. Particularly, it had long breakthrough time of 2784 min and high adsorption capacity of 264 mg/g for toluene, while it had short breakthrough time of 724 min and low adsorption capacity of 6.53 mg/g for phenol. Significantly, the mixed VOCs of toluene and phenol can be effectively adsorbed. Further, thermal desorption will be an alternative route for regeneration of waste activated biochar.

Mass Transfer in Multiphasic Gas/Liquid/Liquid Systems. KLa Determination Using the Effectiveness-Number of Transfer Unit Method

The Effectiveness-Number of Transfer Unit method (ε-NTU method) was applied to determine the overall mass transfer coefficient, KLa, of operating gas-liquid absorbers treating Volatile Organic Compounds (VOCs). This method requires the knowledge of the operating conditions (gas flow rate, QG; liquid flow rate, QL; scrubber volume V), the measurement of gaseous concentrations at the inlet, CGin, and at the outlet, CGout, of the contactor (in order to determine the effectiveness of the absorber ε) and the calculation of the Henry coefficient of the VOC between the gas and the liquid phases (HVOC). Coupled with the “equivalent absorption capacity” concept, the ε-NTU method was used to determine KLa of absorbers contacting a gas and a mixture of water and a Non Aqueous Phase, successfully. The method, validated from literature data for configurations countercurrent scrubbers and stirred tank reactors, could be used to simply determine the overall mass transfer coefficient of systems for which the standard KLa determination methods still remain non-reliable or inaccurate (viscous solvents, mixture of immiscible liquids, fermentation broths…).

Biochar for volatile organic compound (VOC) removal: Sorption performance and governing mechanisms

Sorption is one of the most efficient and low cost strategies for volatile organic compound (VOC) removal, but VOC sorption by biochar has seen limited research. In this work, gas phase sorption experiments were conducted to determine the sorption potential and mechanisms of VOCs onto biochar. A total of 15 biochars produced from 5 common feedstocks at 300, 450, and 600°C were evaluated as sorbents. Three common VOCs (acetone, cyclohexane, and toluene) were chosen as sorbates. The results showed that all the tested biochars had VOC sorption capacity in the range of 5.58-91.2mgg^-1. The sorption capacities were mainly influenced by both the surface area of biochar and its noncarbonized organic matter content. The vapor sorption process was exothermic, and the removal of VOCs by the biochars decreased with increasing feedstock temperature. Both the physical adsorption and partition mechanisms played important roles in controlling the VOC removal by the biochars. Biochar maintained its VOC removal ability after five consecutive sorption-desorption cycles, which indicated good reusability. Findings of this work suggest that biochar is a promising alternative sorbent for gaseous VOC removal.

Mathematical equations combined with the MHE-GC method to study desorption kinetics of contaminants from food-package paper to air

Online sample incubation techniques are convenient, sensitive and safe, additionally, mathematical equations are applicative for other similar contaminants in paper.