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Waste Disposal & Sustainable Energy

, Volume 1, Issue 4, pp 261–270 | Cite as

VOC degradation by microwave-induced metal discharge and thermal destruction: a comparative study

  • Yuting Lv
  • Yuli Zhou
  • Wenlong WangEmail author
  • Jing SunEmail author
  • Zhanlong Song
  • Ke Wang
Article
  • 63 Downloads

Abstract

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.

Keywords

Volatile organic compounds Toluene Three-phase products Degradation 

Introduction

Volatile organic compounds (VOCs) are a type of pollutants with boiling points between 50 and 250 °C [1]. There are more than 500 different VOCs present in the air [2], and they are released from a wide range of sources [3], 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 [10], VOCs put human beings at risk of various diseases [11], even cancer [12, 13]. It is imperative to establish appropriate regulations and measures to control the emission of VOCs [14], and more researches need to be conducted to develop new methods for effectively removing VOCs from the atmosphere.

Currently, there are many different methods to remove VOCs, which can be classified into recovery or destruction types [15] as summarized in Fig. 1. Each method can achieve good degradation efficiency under suitable conditions and some of them have been used in practice [16, 17, 18, 19, 20]. However, few of the existing methods are widely applicable with further developments or optimizations [21, 22]. For example, high temperature and humidity may significantly reduce the adsorption efficiency [23], reactor overheating and irreversible catalyst damage are long-standing problems with the combustion method [24] and further studies are still needed to address the potential toxicity problems of biodegradation [25]. Therefore, a new approach to remove VOCs that does not suffer from the issues of the previous methods is needed.
Fig. 1

An overview of methods to remove VOCs [15]

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 [34]. The temperature of discharge zone is extremely high and the heating effect is remarkable [35]. Moreover, several studies have proven that microwave has plasma effects [36] and can affect the photocatalytic reaction [37]. 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 [38], recycling of waste circuit boards [39], synthesis of organic magnesium compounds [40], and destruction of bituminous coal [41]. 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

Experimental setup

A schematic of the experimental setup is presented in Fig. 2. Analytically pure toluene (Sigma-Aldrich) was used as the model VOC. The carrier gas was high purity nitrogen purchased from Deyang Special Gas Co., Ltd. (Jinan, China). The modified microwave oven was purchased from Midea (Model M3-L233C; 2.45 GHz) to conduct the microwave-induced metal discharge, and the tube furnace was purchased from Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd. (Shanghai, China) to conduct thermal destruction of toluene at 700 °C, 900 °C and 1100 °C. To induce the discharge phenomenon, 6-mm long strips of Fe, Ni, Cu, and Zn (1 mm in diameter) were used as the metal sources.
Fig. 2

Schematic diagram showing the analysis of the three-phase products of toluene degradation. 1—Mass flow controller; 2—gas-washing bottle; 3—thermostat water bath; 4—Tedlar sampling bags; 5—microwave oven; 6—quartz reactor; 7—furnace; 8—RGA; 9—SEM/TEM/FTIR; 10—GC–MS

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

The evaporation bottle with a certain amount of toluene was placed into a water bath and heated until the temperature reached 60 °C and 70 °C separately. At this time, the toluene was weighed and the mass was recorded as M1 (g). Toluene evaporation was continued at the same temperature and, after t minutes, the toluene was weighed again and the mass recorded as M2 (g). The rate of toluene evaporation per minute M (g min−1) is expressed by Eq. (1) below
$$M = \frac{{M_{1} - M_{2} }}{t},$$
(1)
where M was measured by averaging three experimental results. The results and relevant parameters are shown in Fig. 3.
Fig. 3

Toluene evaporation rates

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 methods

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

After the system stabilized for 40 min, uncondensed gaseous products were collected and analyzed using the RGA. The gaseous product was a mixture of H2 and CH4. The results are shown in Fig. 4.
Fig. 4

Degree of toluene cracking under different conditions

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

The condensed liquid products from both the microwave-induced metal discharge and the thermal destruction processes were collected and analyzed by GC–MS. As the temperature in the thermal destruction procedure increased, the amount of condensable liquid product components and the degree of aromatization also increased. At 700 °C, the degradation products consisted of mainly monocyclic organic compounds, while at 900 °C, the condensate consisted mainly of bicyclic organic compounds. Moreover, the degree of toluene cracking under microwave-induced metal discharge was far more thorough than destruction at 700 °C, and almost equivalent to destruction at 900 °C. Compounds that were detected in the GC–MS spectrum within 30 min and accounted for more than 2% (v/v) are presented in Table 1.
Table 1

GC-MS results of toluene cracking condensate obtained under different conditions

 

Time (s)

CAS No.

Relative content (%)

Formula

Substance

700 °C

900 °C

Discharge

1

3.03

108-88-3

61.93

7.01

20.24

Open image in new window

Toluene C7H8

2

3.08

100-41-4

2.73

Open image in new window

Ethylbenzene C8H10

3

5.78

100-42-5

3.14

2.65

Open image in new window

Styrene C8H8

4

16.98

92-52-4

3.48

Open image in new window

Diphenyl C12H10

5

17.02

85-01-8

3.09

Open image in new window

Phenanthrene C14H10

6

18.07

101-81-5

2.84

Open image in new window

Diphenylmethane C13H12

7

19.29

605-39-0

2.23

Open image in new window

2,2′-Dimethylbiphenyl C14H14

8

19.51

644-08-6

10.7

2.38

Open image in new window

4-Phenyltoluene C13H12

9

20.15

103-29-7

4.67

2.2

Open image in new window

1,2-Diphenylethane C14H14

10

20.31

611-61-0

2.51

2

Open image in new window

2,4-Dimethylbiphenyl C14H14

11

21.79

605-39-0

2.57

Open image in new window

2,2′-Dimethylbiphenyl C14H14

12

21.7

612-75-9

2.1

Open image in new window

3,3′-Dimethylbiphenyl C14H14

13

21.86

86-73-7

2.08

Open image in new window

Fluorene C13H10

14

22.04

605-39-0

2.29

Open image in new window

2,2′-Dimethylbiphenyl C14H14

15

22.23

611-43-8

3.09

Open image in new window

2,3-Dimethylbiphenyl C14H14

16

22.40

611-61-0

2.46

Open image in new window

2,4-Dimethylbiphenyl C14H14

17

26.41

85-01-8

8.86

5.09

Open image in new window

Phenanthrene C14H10

Total product types

  

3

9

9

  

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

Figure 5 shows that an obvious vibration peak (799 cm−1) corresponding to Si–O appeared in the SiO2 blank sample. The spectrum of thermal destruction at 700 °C is similar to the SiO2 sample because the temperature is not high enough to produce abundant solid products from toluene cracking. When the temperature rose to 900 °C, the peak intensity changed sharply, and significant vibrations, including C = O (2000–1900 cm−1), C = C (1700–1620 cm−1), and benzene skeleton vibration (1620–1450 cm−1), were detected. Furthermore, the main peak at 1350 cm−1 was shifted, indicating that some cracking products adhered to the surface of SiO2.
Fig. 5

FTIR patterns of different solid products

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 6 shows the microstructure of a quartz sand-carbon mixture obtained under different cracking conditions. Figure 6a suggests that the pure-quartz sand particles were sharp and angular with relatively flat surfaces. Figure 6b shows the microstructure of solid products obtained by thermal destruction at 700 °C and the surface of these products had a corrugated carbon layer. There was a tendency to form smaller solid particles when the destruction temperature rose to 900 °C (Fig. 6c) and 1100 °C (Fig. 6d). At the same time, the SEM image of coke obtained at 1100 °C thermal destruction (Fig. 6e) shows that the carbon particles were round with a relatively uniform size.
Fig. 6

SEM patterns of different solid products: a quartz sand, b products obtained by thermal destruction at 700 °C and c 900 °C, d 1100 °C, e coke obtained at 1100 °C, and f products obtained by microwave-induced metal discharge

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.

EDX analysis of the solid product samples was performed to determine their elemental composition, and the results are shown in Fig. 7. Obviously, the degree of toluene cracking under microwave-induced metal discharge (Fig. 7a) was not as deep as that of 1100 °C (Fig. 7b). The products generated by the microwave-induced metal discharge process contained a trace of Fe that was not present in the products obtained by thermal destruction.
Fig. 7

EDX patterns of solid products obtained by a toluene cracking under microwave-induced metal discharge and b thermal destruction of toluene at 1100 °C

TEM results and analysis

Figure 8 shows the TEM images of the solid products collected after toluene cracking under microwave-induced metal discharge (Fig. 8A/a) and thermal destruction at different temperatures (Fig. 8B/b–D/d). There were great differences between the microstructures of the solid products obtained by microwave-induced metal discharge and thermal destruction. Firstly, the solid products produced by microwave-induced metal discharge were blurred at lower magnification (Fig. 8a), while the carbon produced by thermal destruction at different temperatures was utterly clear. However, the sizes of the particles in Fig. 8A/a were much smaller than those in Fig. 8B/b–D/d when the TEM magnification increased. Further, as the temperature decreased, the number of particles in the field of view gradually decreased.
Fig. 8

TEM images of solid products produced under different conditions: A/a microwave-induced metal discharge, B/b thermal destruction at 1100 °C, C/c thermal destruction at 900 °C, and D/d thermal destruction at 700 °C

Figure 9 shows the sizes of the carbon particles obtained by the different methods. The destruction conditions clearly affected the size of the solid products, and the carbon particles produced by toluene cracking under microwave-induced metal discharge were significantly smaller than those produced by thermal destruction. It was concluded that the higher the temperature in the thermal destruction process, the deeper the degree of toluene cracking, and the smaller the size of the solid products obtained. However, none of the thermal destruction procedures produced smaller sized products than microwave-induced metal discharge did.
Fig. 9

Size distribution of the solid particles obtained by toluene cracking

Conclusion

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.

Notes

Funding

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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Copyright information

© Zhejiang University Press 2019

Authors and Affiliations

  1. 1.National Engineering Laboratory for Coal-fired Pollutants Emission ReductionShandong UniversityJinanChina
  2. 2.Department of Engineering PhysicsTsinghua UniversityBeijingChina

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