With the development of industrial production, the emission of volatile organic compounds and pollutant components continue to increase, and a variety of treatment processes have emerged for volatile organic compounds with complex treatment components and great harm to human body. The characteristics of typical volatile organic compounds were introduced, the three treatment processes of physics, chemistry and biology were compared, and the treatment technologies such as adsorption method, plasma method and biological trickle filtration method were reviewed. By analyzing the composition and treatment technology of current volatile organic compounds, the improvement direction and trend of future volatile organic compound treatment methods are proposed.
China's 2020 Volatile Organic Compounds Control Plan points out that in 2019, the concentration in 337 cities at the prefecture level and above in China was 148μg/, an increase of 21% compared with 2015; The number of days of excess for the primary pollutant accounted for 41.8% of the total number of days exceeded, second only to (45%). Volatile organic compounds (VOCs) are an important part of industrial exhaust gas, with high photochemical reactivity, and are easy to react with nitrogen oxides under light conditions. More than 300 VOCs have been identified, 33 of which are listed as priority pollutants by the U.S. Environmental Protection Agency (EPA). In addition, VOCs are diverse, mainly including alkanes, aromatics, olefins, halocarbons, esters, aldehydes, ketones and other organic compounds. At present, there are a variety of treatment methods for VOCs treatment, and how to improve the degradation efficiency of VOCs has become a research hotspot.
Characteristics of VOCs
VOCs have a complex composition
The common organic waste gas in the industry is usually a mixture of a variety of VOCs, and VOCs composed of different components have different physical and chemical properties, especially VOCs containing sulfur, chlorine and bromine may produce more harmful secondary pollutants in the atmosphere. Therefore, suitable industrial treatment methods should be selected for different types of VOCs. In addition, there are also harmful substances such as nitrogen oxides and sulfur oxide dust in the industrial exhaust gas, some of which are corrosive and easy to cause corrosion of pipelines. The use of suitable process technology can not only effectively remove VOCs, but also carry out preliminary purification of other harmful gases.
VOCs come from a wide range of sources and emit large emissions
In recent years, China's industry has developed rapidly, VOCs emissions have also shown an increasing trend year by year, the total amount of VOCs processed has continued to rise, and different types of VOCs come from different industrial industries and links. According to the statistics of China's ecological environment statistical annual report in 2020, the emission of industrial waste gas in China reached 17.8531 million tons in 2020. Among them, the use of VOCs-containing products accounts for 59% of the total industrial source emissions; Industrial coating, printing and packaging printing, manufacturing of basic chemical raw materials, gasoline storage and transportation, and petroleum refining are the top five sources of pollution, accounting for 54% of total industrial source emissions.
VOCs are volatile and harmful
VOCs are liquids or solids at room temperature and pressure, but their saturated vapor pressure is high and the boiling point is only 50~260 °C. This characteristic makes VOCs leakage and escape in many industrial production links and transportation links. Studies have found that VOCs in the air have a pungent odor, which can easily trigger allergic symptoms and, in severe cases, acute and chronic respiratory diseases. In addition to serious harm to the human body, VOCs have strong bioaccumulation potential and photochemical reaction, and are easy to react with nitrogen oxides, causing greenhouse effect and polluting the atmosphere.
Treatment technology for VOCs
Physical Law
The physical method is to use the corresponding appropriate technology to achieve gas separation and then removal according to the physical and chemical properties of VOCs, and use filtration, adsorption, membrane separation, absorption and other technologies to remove VOCs.
1. Liquid absorption method
The liquid absorption method is a technology that fully contacts the absorbed gas with the absorbing liquid, uses the interaction force between similar particles to dissolve VOCs gas molecules in the liquid phase, and then recovers or eliminates the gas molecules in the liquid phase. A key factor in improving treatment efficiency is finding the right absorbent that can dissolve large amounts of hydrophobic VOCs to maximize mass transfer from gas to liquid.
Ionic liquids are a new type of absorbent formed by connecting organic cations with a counterion with a melting point of 373K or below, which has shown many advantages in the treatment of hydrophobic VOCs. Alfredo et al. determined the partition coefficients of toluene and dichloromethane in 23 hydrophobic ionic liquids at 298K by static headspace method. The results showed that ionic liquids had a high affinity for hydrophobic VOCs, especially toluene. Wang et al. used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylsulfonyl) salt ionic liquid as an absorbent, achieving a toluene removal rate of 98.3%. Xu et al. used quantum chemical calculations to study the binding energy and weak interaction between anions/cations in dimethyl sulfide (DMS)/dimethyldisulfide (DMDS) and 1-ethyl-3-methylimidazole bis(EMIM][Tf2N]), so that the removal efficiency of DMS and DMDS of the new ionic liquid at 25°C and 60000Pa can reach 90.86% and 93.12%, respectively. However, the complex synthesis steps and high cost of ionic liquids limit their practical application. Deep eutectic solvents (DESs) are composed of suitable hydrogen bond acceptors (HBAs), which can form intermolecular hydrogen bonds with hydrogen bond donors (HBD), and have many common physicochemical properties with ILs (Ionic Liuide, ionic liquids), such as low volatility, wide liquid contact range, tunable structure and non-flammability. Compared with ILs, DESs have lower production costs, simple manufacturing procedures, and high biocompatibility and biodegradability. The study found that at 30°C and 8×mL of toluene (HBA:HBD molar ratio of 1:3), 1g DES could absorb 9.94mg of toluene.
It can be seen that ILs have limitations, and deep eutectic solvents are becoming new green solvents. However, at present, there are few studies on them by experts at home and abroad, and there are still some gaps that need to be filled: (1) some hydrophilic DESs may decompose before and after extraction, resulting in changes in their properties, which should be further studied; (2) The viscosity of DES is large, and it is usually necessary to extract by vortexing, ultrasonic and other methods, which makes the extraction efficiency low and the steps are complicated. Attempts can be made to combine DESs and magnetic nanotubes to simplify the procedure and improve extraction efficiency.
2. Adsorption method
The adsorption method is to separate the surface of substances from one phase to another through chemical adsorption or physical adsorption, and is often used to remove macromolecular organic pollutants. Physical adsorption refers to the transfer of gas molecules through the van der Waals force interaction between molecules when they are in contact with the solid adsorbent. Chemical adsorption means that the adsorbent breaks chemical bonds and reacts with polluting gases. The adsorption capacity of the adsorbent is the key to determine the treatment effect of the adsorption method, and the research overview of commonly used adsorbents is shown in Table 1. How to improve the adsorption capacity by changing the physical and chemical properties of adsorbed materials (i.e. surface area, pore volume, pore size and high ligand density) has become a hot spot in current research.
Table 1 Classification and characteristics of main adsorption materials
By introducing functional groups (carboxyls, carbonyls, hydroxyl, phenols, etc.) or active substances (metal oxides, organic compounds, etc.), the affinity between the surface of activated carbon and VOCs can be improved, and the adsorption capacity can be increased. Hou et al. modified coconut shell activated carbons (ACs) impregnated with the non-polar fatty amino acid glycine and studied the effects of the modification method on the physicochemical properties and adsorption performance of VOCs. The results showed that the penetrating adsorption capacity of 10% glycine-modified activated carbon p-toluene was 147% and 139% higher than that of original activated carbon and 5% ammonia-modified activated carbon, respectively, and the saturated adsorption capacity of glycine-modified ACs was less than that of 5% ammonia-modified ACs, and glycine could be deposited on the surface of ACs, forming polypeptide chains through self-condensation, which was conducive to increasing the nitrogenous groups of ACs and increasing the surface polarity of ACs. However, ammonia modification was more conducive to improving the specific surface area and pore volume, and the penetrating adsorption capacity and saturated adsorption capacity of modified activated carbon reached 75.02mg· and 451.48mg·, respectively. Liu et al. used mechanically and chemically coupled manganese dioxide to modify fly ash by ball milling to improve its oxidation activity against VOCs. Microscopic characterization showed that ball milling increased the specific surface area of fly ash, increased the dispersion of manganese dioxide in fly ash, and was conducive to the adsorption and oxidation of toluene, from 5% (untreated fly ash) to more than 90%, and the carbon balance efficiency exceeded 94%.
In summary, by introducing functional groups to change the surface chemistry of the adsorbent, the adsorbent material can have a large specific surface area and stability, thereby improving the adsorption capacity of the adsorbent. However, when modifying materials with chemical solutions, it may cause environmental pollution and increase costs.
3. Membrane separation method
The membrane separation method is a physical method of gas separation by using different components in the gas with different diffusion coefficients on the polymer material driven by pressure difference, concentration difference or potential difference, and the specific components covered by the raw material mixture can penetrate to the permeation side with the help of membrane pores, while other components remain on the raw material side, so as to realize the physical method of gas separation.
Ionic liquid membrane is a combination of membrane process and ionic liquid, which can promote the solvent performance of ionic liquids and improve the separation performance of membranes, which has received more and more attention in the removal of VOCs. Abraham et al. proposed an IL-modified multi-walled carbon nanotube (MWCNT)-based styrene-butadiene rubber (SBR) composite membrane, which has the highest separation coefficient of 128.4 when toluene selectively separates methanol/toluene mixture, which is 1.6 times that of SBR membrane. Studies have shown that the selectivity of the membrane depends mainly on the partition coefficient between the IL gel and the aqueous solution. Vopicka et al. studied a gel film consisting of 20% vinylidene fluoride-hexafluoropropene by mass and 1-ethyl-3-methylimidazole bis(C2mim][NTf2]) by mass 80% for the removal of ethanol from polluted air. In this system, when the ethanol vapor activity is the highest, the permeability of ethanol in the membrane reaches about 25×kPa. In addition, the adsorption of ethanol vapor in the membrane increases with increasing temperature. In addition to the partition coefficient, feed concentration, temperature, permeation flux and VOCs water separation factor are also important factors affecting membrane performance. Yi et al. used vinyltriethoxysilane (VTES) grafted silicon molecular sieve-1/PDMS mixed matrix membrane (MMM) to recover methanol by osmotic evaporation from methanol-containing binary , ternary and quaternary wastewater solutions. The results showed that with the increase of VOCs concentration in the feed, the total permeation flux increased, and the selectivity first changed slightly, and then decreased.
In summary, the membrane separation method overcomes the problem that other VOCs treatment methods can only be limited to specific VOCs, and can be applied to a variety of mixed VOCs. However, most of the membranes currently studied are not resistant to high temperatures, and some industrial waste gases (such as coal-fired power plants) are high-temperature exhaust gases, so how to apply the membrane separation method to high temperature and high pressure conditions is a problem to be solved in the future.
Chemical method
Chemical method is a method of efficient removal of VOCs by breaking the chemical bonds of pollutants by incineration, high-pressure discharge, etc. It mainly includes the following processing technologies.
1. Incineration method
Incineration method is a method of heating waste gas above the ignition point temperature, so that organic matter is incinerated to generate substances that are less harmful to the environment and achieve the purpose of purification. There are five main types of common incineration methods in the industrial field: direct incineration method, regenerative heat incineration method, porous medium incineration method, thermal incineration method and catalytic incineration method, etc., and the characteristics of the five incineration processes are shown in Table 2.
Table 2 Comparison of five incineration processes
The incineration method has high requirements for the reactor and usually requires a high reaction temperature, so it is necessary to investigate a suitable catalyst to catalyze the oxidation reaction and reduce the operating temperature of the direct incineration process. Liu et al. electrostatically adsorbed the ammonium ions in the quaternary ammonium surfactant between the ZSM-5 monolayer sheets, controlled embedding Pt nanoparticles, and then calcined to prepare a novel catalyst Pt@ZSM-5 nanosheets (Pt@PZN-2). The study found that the temperature of Pt@PZN-2 at the toluene conversion rate was only 176°C, which was 24°C and 34°C lower than Pt/ZN-2 and ZSM-5, respectively. Fang et al. successfully synthesized various three-dimensional (3D) cobalt-based metal-organic frameworks by modulating and reconstructing secondary structural units (SBUs) with cobalt ions. It was found that CeCoOx-MNS precursors formed by wet impregnation of Cerium ions by Co-MOFs-MNS precursors had more species and abundant surface weak acid sites, enabling 100% conversion of toluene at 249°C.
In summary, a suitable catalyst can reduce the reaction temperature of the incineration method and improve the degradation efficiency of VOCs, but the catalyst is easily inactivated, and the catalyst may react to produce more harmful toxic gases. Therefore, in practical applications, suitable catalysts need to be selected for different types of VOCs to avoid the formation of secondary pollution.
2. Low temperature plasma method
Low-temperature plasma is mainly through high-pressure discharge to produce high potential energy and high kinetic energy free electrons, and background gas molecules or atoms inelastic collision, so that gas molecules excitation, ionization and free radicalization, the production of O·, OH·, N· and, etc., these active groups and pollutant molecules undergo redox reaction, and degrade it into small molecule organic matter in a short time.
The technology of individual degradation of VOCs has technical shortcomings such as low degradation efficiency, high by-product generation rate, and low energy utilization. Zhang et al. prepared nickel oxide and pyrite (NiO/Pyr) composites by impregnation method, and applied them to dielectric barrier discharge (DBD) to catalyze the removal of gaseous styrene. The results showed that when the initial concentration of styrene was 240mg·, the degradation efficiency of the catalytic system was increased by 11.7% compared with that of the naked DBD system. Wu et al. prepared a series of perovskite catalysts with high specific surface area (68.6~85.6·) by simple Ce in-situ doping method, and the results showed that the decomposition efficiency of toluene can reach up to 100%, and the selectivity is 98.1%, which greatly reduces the formation of by-products. This is because the molecules are adsorbed and activated at the active site (-VO) through the electron transfer process, forming superoxide on the perovskite surface, enhancing the decomposition and intermediate oxidation capacity of toluene.
It can be seen that the introduction of catalyst can change the discharge state, stimulate new active radicals, and provide reaction sites, thereby improving degradation efficiency, reducing the selectivity of carbon oxides, and reducing the formation of by-products, so low-temperature plasma synergistic catalysts have increasingly become the focus of people's research.
3. Photocatalytic oxidation method
Ultraviolet light decomposition method is high-energy ultraviolet light radiates a large number of high-density light quanta, organic waste gas in the collision with light quanta, the bond energy less than the light quantum chemical bond will break and other reactions, the formation of low harm to the environment, or intermediates. The photocatalytic oxidation method adds a catalyst on the basis of photodecomposition, but the accumulation of residues on the surface of the catalyst will lead to a gradual decrease in the photocatalytic activity, thereby reducing the degradation efficiency of VOCs.
Zhou et al. constructed a quantum-level catalytic region on the surface of nanoparticles by loading quantum dots (QDs) in 2 steps. The results show that an upward band bending is formed from the particles to the quantum dots, which promotes the accumulation of holes on the side of the quantum dots (Figure 1 In addition, the quantum dots and the surrounding substrates form a quantum-scale catalytic region, which increases the reaction probability of electron-hole pairs and corresponding intermediates, and makes the mineralization efficiency of toluene in quantum dot loading reach 95.8%). To further improve degradation efficiency, Mahmood et al. developed and optimized carbon quantum dot-modified nanoparticles to investigate their potential use in photocatalytic oxidation of gas-phase mixed VOCs (such as benzene, toluene, and paraxylene) containing aromatic rings. The experiment found that when the carbon quantum dots with a mass percentage of 0.5% were modified on it, 64% of the mixed VOCs were photodegraded, while the pure only showed a photodegradation efficiency of 44%.
Fig. 1 Schematic description of band structure and reaction mechanism of titanium dioxide quantum dots
In summary, catalysts can improve the degradation efficiency of VOCs, but there is also the phenomenon of catalyst inactivation. In catalytic reactions, the inactivation of the catalyst may be affected by changes in catalyst performance, poisoning of intermediate or by-products. For example, during the catalytic degradation of benzene, coke is deposited on the surface of the catalyst, causing the catalyst to be inactivated. Catalyst inactivation results in a significant reduction in overall removal efficiency and the production of intermediate by-products. In addition, when catalysts are frequently regenerated or replaced, the operating costs of the system will increase significantly. Since regeneration is difficult and uneconomical, how to improve the performance of catalysts is the focus of future research.
Biological law
The biological method mainly uses microorganisms such as bacteria and fungi to oxidize and decompose the organic components in the exhaust gas into simple inorganic substances. In the biological system, microorganisms act as catalysts, using organic waste gas as nutrients for their own reproduction, producing carbon dioxide and water, mainly including biological filtration, biological washing method, biofilm method, etc.
1. Biological filtration method
The biological filtration method is to first pass the waste gas into the water to remove soluble waste gas and particulate matter, and then the heated and humidified waste gas is passed into the reactor body with microbial filler attached to it to be adsorbed and degraded. The biological filtration method has no spray device, and it is difficult to transfer mass by itself, and how to improve its mass transfer capacity has become the focus of research. Han et al. established a new two-phase biological filter (TLPB) using silicone oil and water as raw materials to treat gaseous dichloromethane (Figure 2: The experiment sprayed water and silicone oil at the top of the biological filter to form a silicone oil-water film between the gas phase and the microbial phase, reduce the mass transfer resistance of the poorly soluble waste gas, and enhance the removal performance of the two-liquid phase biological filter. Studies have shown that the average removal of TLPB is 85% during operation at 200 days, which is higher than that of single-phase biofilters (63%).
Figure 2 Schematic principle of two-liquid biological filter (TLPB).
2. Biofilm method
The biofilm method is to pass the waste gas into the membrane bioreactor, and the pollutants on the gas phase side pass through the membrane, diffuse to the liquid side, and then be degraded by microorganisms in the biofilm close to the membrane surface on the other side, and finally generate sum. Air membrane bioreactor is a new type of reactor, which has a large gas-liquid surface area, which can enhance the mass transfer capacity of pollutants and oxygen, so that it can operate at high loading rate under small reactor configuration, which solves the shortcomings of traditional bio-filled bed and other shortcomings.
Prikyai et al. evaluated the performance of an air-membrane bioreactor for treating gas-phase methanol under laboratory conditions. Under steady-state conditions, the lower the empty bed residence time, the lower the degradation efficiency of vapor phase methanol. When the inlet loading rate is increased, the gaseous methanol is dissolved into the water, and part of the gaseous methanol is stripped to the liquid phase, and the removal efficiency is also increased. When the inlet loading rate reaches 1916g·· (empty bed residence time 30s), the removal efficiency is the greatest (98%). The results of microbial structure analysis showed that the conversion of Candida TBRC217 strain to flavobacterium during methanol degradation helped to improve the removal rate of vapor-phase methanol.
3. Biological drip filtration method
The biological drip filtration method is based on the biological filtration method to add nutrient solution to the drip filtration system, which is used to regulate the pH value and humidity of the microbial phase and leach out difficult metabolites. Fillers mainly provide microorganisms with an adherent living environment and increase the gas residence time to fully absorb pollutants by biofilms, so the development of fillers with high mechanical strength and biological affinity is the key to improving degradation efficiency.
Dewidar et al. studied biodrip filters (BTFs) inoculated with mixed fungal aggregates to simultaneously remove 2-ethylhexanol and propylene glycol monomethyl ether (PGME). The results show that when the tensile inlet load ratio (LR) is 15.7 and 32g·, respectively. , the removal efficiency was 98.5% and 99%, respectively. Once the LR of 2-ethylhexanol increased to 48 g··, the substrate was inhibited while the removal efficiency suddenly decreased from 99.2% to 62.3%. However, by reinoculating 100 mg·surfactant, the removal efficiency of 2-ethylhexanol can be restored to 92.7%.
In summary, different VOCs can exhibit antagonistic, synergistic and neutral interactions. Mixed VOCs are more difficult to remove due to factors such as toxicity, competition between microorganisms, catabolic inhibition, hysteresis phase, and substrate competition for enzymes. Therefore, increasing the abundance of certain species, inducing effects, and/or co-metabolism through solubilization is the key to improving degradation efficiency.
Combined technology
Due to the complex composition of exhaust gas, the single treatment technology has certain limitations (Table 3 is to improve the purification efficiency and reuse rate of VOCs, and the combined process is often used in practical applications. For example, for high-concentration VOCs waste gas, recovery technologies such as absorption or condensation are usually used to recover the gas, and then low-concentration organic molecules are removed by destruction technologies such as adsorption or photocatalysis.
Table 3 Technical characteristics of various types
Wantz et al. discovered a non-aqueous liquid (NAPL) that can be absorbed by a separate gas-liquid contactor to remove VOCs from the gas stream before entering a two-phase distribution bioreactor (TPPB). NAPL target VOCs have a higher affinity than water and contain an aqueous phase of microorganisms and nutrients needed for their growth. Therefore, such reactors can be used to remove less soluble compounds in water and increase their biodegradation rate. Saoud et al. studied a commercial glass fiber structure (coated glass fiber structure) suitable for photocatalytic plasma processes to reduce butane-2,3-dione and heptane-2-one in the coupling of photocatalysis and dielectric barrier discharge (DBD), the principle is shown in Figure 3. It is found that the synergistic effect of these two oxidation technologies can improve the degradation efficiency (plasma degradation efficiency is 5.09%, photocatalytic degradation efficiency is 24.39%, binding degradation efficiency is 35.15%) and control the formation of oxidation by-products.
Fig. 3 Mechanism of removal of butane-2, 3-dione and heptane-2-one under DBD-plasma/photocatalytic system
conclusion
The composition of VOCs is complex, the harm to the human body and the environment is great, due to the wide variety of VOCs and different characteristics, corresponding treatment technologies should be adopted for different kinds of pollutants. However, various technologies have different degrees of limitations, in addition to the development of new materials and technologies to improve the degradation efficiency of a single technology, we should explore the adsorption method, plasma discharge technology, membrane modification technology and a variety of coexistence of joint technology, improve the carrier characteristics and give play to the synergistic effect of different technologies, so as to achieve better degradation effect and reduce secondary pollution. Although the combination technology has a better processing effect, its expensive cost and operating cost make it unable to be widely used in the industrial field, how to reduce costs, make the process operation tend to be simple, is the direction of focus in the future.
The authors of this article: Yu Miaofei, Mi Junfeng, Du Shengnan, Zhang Xuejia
About author:MIU Miaofei, School of Civil Engineering, Liaoning Shihua University, Master candidate, research direction is low-temperature plasma and gas pollution control engineering; Mi Junfeng (corresponding author), Associate Professor, School of Petroleum Engineering, Liaoning Shihua University, specializes in low-temperature plasma and wastewater control engineering.
The original article was published in the 11th issue of Science and Technology Review in 2023, welcome to subscribe to view.