VOC degradation by microwave-induced metal discharge and thermal destruction: a comparative study
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The effective treatment of volatile organic compounds (VOCs) is essential because of their direct effects on air pollution and human health. This paper introduces microwave-induced metal discharge as a highly effective and byproduct value-added approach to degrade high-concentration toluene as a model VOC. The effect of the factors that influence the discharge intensity, including the metal type (Fe, Cu, Ni, Zn) and amount, was investigated. Degradation efficiency of toluene can reach 79.76% under optimal discharge condition. In addition, the discharge method was compared with traditional thermal destruction at 700 °C, 900 °C and 1100 °C. The gaseous and liquid cracking products of toluene produced by the microwave-induced metal discharge method were almost similar to those obtained under thermal destruction at 900 °C; however, the solid-phase discharge products were nanoparticles that demonstrated good graphitization, while the thermal destruction products were amorphous microparticles. This work offers an effective and flexible way to degrade high-concentration VOCs and also provides an application reference for biomass tar cracking and removal of other organic pollutants.
KeywordsVolatile organic compounds Toluene Three-phase products Degradation
Volatile organic compounds (VOCs) are a type of pollutants with boiling points between 50 and 250 °C . There are more than 500 different VOCs present in the air , and they are released from a wide range of sources , including petrochemical industries and transport activities [4, 5]. In addition to generating harmful substances that pollute the environment, such as ozone [6, 7], particulate matter with a particle size less than 2.5 μm [8, 9], and photochemical smog , VOCs put human beings at risk of various diseases , even cancer [12, 13]. It is imperative to establish appropriate regulations and measures to control the emission of VOCs , and more researches need to be conducted to develop new methods for effectively removing VOCs from the atmosphere.
In recent years, microwave technology has received extensive attention due to its special mechanism in many fields, such as plasma processing [26, 27], functional material preparation [28, 29, 30], and pollutant treatment [31, 32, 33]. During discharge, a vast amount of energy can be concentrated if controlled well. Hussain found that a huge amount of heat can be produced in the interaction between microwaves and Fe, and the temperature can reach over 1100 °C . The temperature of discharge zone is extremely high and the heating effect is remarkable . Moreover, several studies have proven that microwave has plasma effects  and can affect the photocatalytic reaction . Both plasma and photocatalytic effects can take place during microwave-induced metal discharge.
Due to these special effects, a series of applications of microwave discharge are emerging, including enhanced destruction of tires , recycling of waste circuit boards , synthesis of organic magnesium compounds , and destruction of bituminous coal . The microwave-induced metal discharge phenomenon can also degrade gaseous organic compounds. Our group has conducted numerous studies in optimizing the efficiency of VOCs degradation by microwave-induced metal discharge [42, 43]. However, no study has been conducted to compare the microwave-induced metal discharge with other methods from product analysis aspects.
This work was carried out to get a comprehensive understanding on VOCs degradation by microwave-induced metal discharge. Firstly, the effects of different factors on the microwave-induced metal discharge were tested, and the optimal discharge conditions were determined. Secondly, the gas chromatograph–mass spectrometry was used to determine the toluene degradation efficiency and the type of liquid products. Thirdly, refinery gas analyzer was used to analyze the proportion of gaseous products. Lastly, Fourier-transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy were used to analyze the type and microscope morphology of solid products. This research gives a novel approach to control the industrial VOCs emission and presents its advantages in detail, which may provide a reference for the elimination of other pollutants as well.
Materials and methods
A high concentration of toluene vapor was obtained by bubbling method, and the vapor was then diluted with N2 from another channel to obtain toluene steam of an appropriate concentration in the cracking system. The experimental pipeline was heated (150 °C) and insulated to prevent toluene from condensing. We used both microwave-induced metal discharge for toluene cracking and a conventional thermal destruction method for comparison. The microwave power was set at 700 W. The cracking products were condensed in liquid collection bottles by circulating ice brine (− 10 °C), the uncondensed gas was collected in Tedlar sampling bags, and the solid products were collected from the quartz reactors (shown in Fig. 2) for further analysis. The H2 yield was used as an indicator to measure the degree of toluene degradation and to determine the best discharge condition. In general, higher degrees of macromolecular carbon cracking resulted in a higher percentage of H2 in the gaseous products.
Determination of toluene evaporation
To determine the reliability of the evaporation system, two temperatures, 60 °C and 70 °C, were tested for comparison, and the corresponding toluene evaporation rates were 0.96 g min−1 and 1.34 g min−1, respectively. Therefore, to obtain more products, 70 °C was chosen as the evaporation temperature for all subsequent experiments.
Characterization of gaseous products
The gaseous products of toluene cracking were analyzed using a refinery gas analyzer (RGA, Clarus 500). The specific parameters were as follows. N2 and H2 were used as the carrier gases; thermal conductivity detector (TCD) and hydrogen flame ionization detector (FID) were used as the detector, and the detector temperature was 200 °C and 250 °C.
Characterization of liquid products
The liquid products collected by condensing system were analyzed by gas chromatograph–mass spectrometry (GC–MS, ISQ™ 7000). The liquid products were diluted with ethanol at a v/v ratio of 8:1 and fully mixed prior to injection as the high viscosity of the liquid products would cause the detector to become contaminated. The specific parameters were as follows. An electron bombardment source (EI) was used as the ion source and the ion source temperature was 230 °C. The capillary column was 50 mm long and 0.25 mm in diameter, and non-polar HP-5 (5% biphenyl and 95% polysiloxane) was used as the stationary phase. The column heating rate was 10 °C min−1, and the column temperature rose from 60 to 230 °C and maintained for 10 min. Helium was used as the carrier gas at a flow rate of 2 mL min−1. GC–MS was performed in the full-scan mode.
Characterization of solid products
Firstly, Fourier-transform infrared spectroscopy (FTIR, Nicolet 6700) was used to obtain the infrared spectra of the solid products. The samples were ground to sizes of less than 2 μm prior to analysis, and the specific parameters were as follows. KBr and MCT/A were used as the beam splitter and detector, respectively. Scans were run 100 times, and the spectra had a resolution of 4 cm−1.
Secondly, scanning electron microscopy (SEM, SUPRA™ 55) was used to obtain the surface morphology of carbon on the quartz sand bed, which was one of the main solid products formed as a result of toluene degradation, and a representative area was selected for energy-dispersive X-ray spectroscopy (EDS) to acquire the elemental composition. The specific parameters were as follows. The imaging magnification was 25–650,000 times. The resolution was 15 kV/1.0 nm or 1 kV/2.2 nm. The sample stage moving range was 70 mm in the x direction, 50 mm in the y direction, 1.5–25 mm in the z direction, with R = 360° and T = − 5° to 60°.
Finally, transmission electron microscopy (TEM, JEM-2100) was used to obtain the surface morphology and microstructure of the carbon. To prepare the sample for TEM analysis, a small amount of the carbon deposit was first ground and then dispersed in absolute ethanol by ultrasonication. The dispersion was then dropped onto a microgrid, dried and placed in a high-resolution electron microscope. The TEM images were obtained using an accelerating voltage of 200 kV and a beam spot size of less than 0.5 nm. The electron microscopy point and line resolutions were 0.23 nm and 0.14 nm, respectively.
Results and discussion
Analysis of gaseous products
When microwave-induced metal discharge was used to degrade toluene, the cracking depth differed significantly between different metal types. Fe had the best cracking effect compared with other types of metals, and the proportion of H2 in total gaseous products was approximately 60% (v/v). However, when the selected metal was zinc (Zn), the proportion of H2 was only 22% (v/v). Consequently, five Fe strips were determined to be the optimal conditions for toluene cracking and used for subsequent toluene cracking experiments.
Under traditional destruction conditions, the volume fraction of H2 gradually increased as the destruction temperature increased. At 700 °C, the volume fraction of H2 was only 23.41% (v/v), but when the temperature rose to 1100 °C, the H2 volume fraction increased to 78.92% (v/v). Toluene destruction is an endothermic reaction, so the increase in temperature can promote the destruction reaction.
Overall, the degree of toluene cracking under discharge with five Fe strips was comparable to that of thermal destruction at 900 °C. This result indicates that the degradation by discharge in a simple quartz reactor can be parallel to destruction at high temperatures, which demonstrates the efficiency of microwave-induced metal discharge.
Analysis of liquid products
GC-MS results of toluene cracking condensate obtained under different conditions
Relative content (%)
Total product types
The toluene cracking efficiency was less than 40% when the thermal destruction temperature was 700 °C. Besides, there were only three isomers of dimethylbiphenyl appeared in the products, which was the result of polymerization after toluene cracking. When the temperature rose to 900 °C, the toluene cracking efficiency exceeded 90% with nine main liquid products produced. When microwave-induced metal discharge was used, the cracking efficiency was still high (79.76%), and nine types of products were detected.
Table 1 shows that these cracking products are mainly fused ring compounds of two benzene rings. In toluene cracking, chemical bonds were broken to generate small-molecule gases, and polycondensation reaction occurs simultaneously. Therefore, toluene cracking can also be considered as a process that increases the degree of aromatization. These macromolecular compounds require more energy to be cracked, which makes it difficult to completely degrade toluene into small molecular substances.
Analysis of solid products
In this section, a variety of analytical characterization methods were used to detect the solid products generated from toluene cracking under microwave-induced metal discharge and thermal destruction. It was difficult to collect the solid products directly because of the small amounts produced during the cracking reaction, and most of the solid products adhered to the surface of the Fe strips, the inner wall of the quartz reactor and the bed of quartz sand. Therefore, the quartz sand adhering to the carbon deposit was ground until a uniform, fine mixed powder was formed for further analysis.
FTIR results and analysis
Detection was also carried out with separate coke heated at 1100 °C on the quartz sand bed as a comparison. The intensity of each peak decreased, and the peak at 1350 cm−1 also shifted. However, the spectrum of 1100 °C is more in accordance with that of SiO2 and 700 °C than 900 °C. According to these results, toluene will be effectively degraded if the temperature exceeds 1100 °C, and less amount of solid products will be found on the bed surface.
The carbon produced by microwave-induced metal discharge was similar to that produced by thermal destruction at 900 °C. However, the peak at 1350 cm−1 was consistent with the peaks of coke at 1100 °C, which indicates that microwave-induced metal discharge was more effective in degrading toluene than thermal destruction at 900 °C.
SEM and EDX results and analysis
Figure 6f shows the surface structure of the solid products obtained by microwave-induced metal discharge. We found that microwave-induced metal discharge produced spherical solid particles on the surface of the quartz sand, but these solid particles were different from those obtained by pyrolyzing coke at 1100 °C. Most of the surface of the particles obtained by microwave-induced metal discharge were extremely matte and damaged. This indicates either that the bombardment of high-energy particles generated during the discharge process destroy some of the structures or that the spherical carbon was formed by the agglomeration of smaller particles. Further experiment such as EDX was necessary to determine the specific reason.
TEM results and analysis
This paper introduced a novel method for the degradation of high-concentration VOCs via microwave-induced metal discharge. In experiments, the degree of toluene degradation by microwave-induced metal discharge reached 79.76%, with only five Fe strips used to induce discharge at a microwave power of 900 W. The H2 yield (61%) of microwave-induced metal discharge was close to that of thermal destruction at 900 °C (58%). The liquid products of toluene degradation by microwave-induced metal discharge, mainly including dimethylbiphenyl and phenanthrene, were also similar to the products of thermal destruction at 900 °C. The solid products obtained by microwave-induced metal discharge were smaller but more prone to agglomeration than those obtained by thermal destruction. This study provides a convenient and effective approach for the degradation of VOCs and may have practical application potential in industries.
This work was generously supported by the National Key Research and Development Program of China (Grant No. 2018YFB0605200), Natural Science Foundation of China (Grant No. 51976110), Young Scholars Program of Shandong University (Grant No. 2018WLJH75), Fundamental Research Funds of Shandong University (Grant No. 2017GN009), and Natural Science Foundation of Shandong Province (Grant No. ZR2019MEE035).
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