Volatile Organic Compound (VOC) Reduction from Face Mask Wastes via a Microwave Plasma Reactor

Since the outbreak of the COVID-19 pandemic, the driven of face masks as personal protective equipment has increased significantly. Thus, disposed face masks from users should be handled properly for preventing the contamination of medical waste and the potential spread of viruses to the environment. This study gives information for dealing with face masks and assessing the volatile organic compounds (VOCs) concentrations via an atmospheric-pressure microwave plasma reactor. Face mask samples were mixed with the flux agents, namely cullet/glass, Al 2 O 3 , SiO 2 and CaCO 3 . Samples were compared with control (no addition of flux agents) and the addition of only cullet. Moreover, microwave power, gas flow rate and pyrolysis duration were controlled at 1000 W, 9 standard liter per minute (SLM) and 5 min, respectively. The total concentration of VOC with the absence of flux agents was 448.04 ppm. Furthermore, the fuse of cullet and SiO 2 -Al 2 O 3 -cullet in the mask reduced the concentration of VOCs by 314.77 ppm and 54.7 ppm, respectively. Furthermore, the combination of CaCO 3 -SiO 2 -Al 2 O 3 -cullet creates the vitrification of material with the presence of crystalline structure, where the compositions of Ca and Si were 13.55% and 19.12%, respectively. Moreover, the final composition of carbon from the flux agents was 17.92 ± 10.08%. This study confirms that the fuse of CaCO 3 -SiO 2 -Al 2 O 3 -glass/cullet reduced the VOC via plasma technology, which is a promising method to be implemented in order to reduce the concentration of VOC from the face mask waste.


INTRODUCTION
Since the outbreak of COVID-19 pandemics, the driven of face mask as the personal protective equipment has increased significantly and should be concern as the medical waste. Also, face masks have been a concern as untreated medical waste since the COVID-19 spread, which has caused an effect on the environment (Zambrano-Monserrate et al., 2020;Ray et al., 2022). Akarsu et al. (2021) noted that face mask waste can be found around several locations, such as hospitals, bus stops and playgrounds, should be considered as the medical waste. Urban and Nakada (2021) reported that Brazil had generated face mask more than 35% of the medical waste. Torres and De-La-Tore (2021) confirmed that 74.9 tons day -1 (27,344.7 tons year -1 ) of face masks are generated in Peru. Nzediegwu and Chang (2020) noted that there are approximately 700 million face masks from a total of 15 countries in Africa. It is estimated that approximately 10 million masks waste are discarded in Italy from users every month (Singh et al., 2022). Zand and Heir (2020) confirmed that the highest amount of face mask waste was generated at a number of 10.78 million. In Taiwan, approximately 7 million mask wastes were generated since the outbreak of the COVID-19

Collection and Preparation of Samples
One box of medical face masks (Chiu Fu Yu Co., Ltd.) was purchased from the medical shop in Taiwan (Represent face mask wastes in this study). The size of medical mask is 17.5 cm × 9.5 cm. The mask was cut using a scissor to obtain the smaller size (approximately 4 cm × 4 cm). After that, the smaller fragments of the masks were crushed using a pulverizing machine (Rong Tsong 0-2B, Taiwan). Furthermore, samples, which were obtained with the smaller fragment size, were put in containers. Approximately 2 grams of fraction from the mask was measured using the analytical balance (Shimadzu AUY-220, Japan). Fig. 1(a) shows the preparation of the facemasks prior to treatment in an atmospheric-pressure microwave plasma reactor.
Meretrix meretrix shell samples were collected from the restaurant in Taiwan. The use of Meretrix meretrix as the source of CaCO3 (flux agents). The samples were washed with deionized a b Fig. 1. Preparation of face mask before treatment and atmospheric-pressure microwave plasma system. (a) Preparation of face mask wastes, plasma system and final residue, and (b) An atmospheric-pressure microwave plasma reactor system. water (DI Water) for removing all the dirty parts of shells. In addition, all the shells were washed with soap for cleaning the particles. Furthermore, samples were dried in the oven with the temperature 100°C for evaporating the water for around 1-2 days. Afterwards, samples were crushed using a pulverizing machine for obtaining the smallest size of samples. To obtain a 0.074 mm shell powder, a 200 mesh sieve was used for obtaining the samples, CaCO3 from shell powder, SiO2, Al2O3, and glass/cullet were used as the flux agents for building the crystalline network from the final residue.

VOC Assessment from the Samples
VOC emission from face mask pyrolysis was analyzed via a gas chromatography (GC; Agilent 6890 N-USA). The gas from the plasma pyrolysis was captured in the 1 L gas bag collector (Tedlar). The oven temperature of GC was set up at 32°C and increased to 200°C in 3 min. The column size of the GC was 60 mm × 0.25 mm × 1.00 µm. The number of R 2 was 0.99, which indicates a suitable linear gas standard.

Pyrolysis of Mask
Samples were put in the plasma crucible from metal materials. For creating the vitrification of materials, the flux agents, namely, CaCO3, SiO2, Al2O3 and glass/cullet, were added to the samples. Flux agents contained 3 g of glass/cullet, 2 g of Al2O3, 2 g of CaCO3, and 2 g SiO2. The pyrolysis was compared with only the addition of glass/cullet (3 g) and the absence of flux agents (control). Table 1 explains the design details information of the flux agents. Mixing samples were put inside the crucible. As a comparison, the treatment of face mask waste with the absence of flux agent was performed. Fig. 1(b) shows an atmospheric-pressure microwave plasma reactor system for this experiment.
To create the plasma jet, nitrogen gas was used as the carrier gas due to the high efficiency results (Sanito et al., 2022d). Nitrogen gas was purchased with from the company in Taiwan with the size of tube at 40 L. The flow rate was controlled at 9 L min -1 . Furthermore, the duration and microwave power were set up at 5 min and 1000 watt, respectively. The pressure of the gas was maintained at 30 psi. The position of the crucible was maintained at approximately at 4-5 cm from the plasma ignition (Sanito et al., 2020a). A plasma discharge was generated from a copper wire.

Analysis of Sample Characterization
A scanning electron microscope (SEM-EDX; Hitachi, S-4800, Japan) was used for identifying the elemental composition elements before and after treatment. The elemental analyzer (Vario el Cube, Germany) was used for analyzing the mass fraction of carbon, nitrogen, and hydrogen of the mask before and after treatment. The characterization of crystalline structure was analyzed via the X-ray diffraction (XRD; Bruker D8 Advance Eco, Germany) before and after treatment. The samples were scanned at the X-rays for the 5 minutes, where the 2θ scanning ranging from 10°-80°. XRD's voltage and electric current were set up at 40 kV and 25 mA, respectively. Then, the crystalline structures appear at 2θ.
The functional group in the sample was determined by Fourier transform infrared spectroscopy (FTIR: IRSpirit, Shimadzu, Japan). The ranges of wavenumber were 400-4000 cm −1 (radiation of infrared). Samples are pelleted with the kBr. The sample was pressed with a manual hand press until it reached a thickness of 1 mm. A manual hand press was used to press the sample until a 1 mm thickness can be obtained. Samples were scanned in 2 min, via the infrared rays, generated by the FTIR system. Table 1. Experiment using flux agents combine with face mask wastes via an atmospheric-pressure microwave plasma reactor.

Experiments
Flux Agents Weight of Samples (g) 1 No flux agents (Control) 2 2 Face mask waste with glass/cullet 5 3 Face mask waste with combination of CaCO3, Al2O3, SiO2 and glass/cullet

Analysis of Samples
Some elements from the mask fraction were analyzed via the ICP-AES (Shimadzu ICPE 9820, Japan). The flow rate of the argon gas was controlled at 10 L min -1 . The controls of auxiliary gas and carrier gas were set at 0.60 L min -1 and 0.31 L min -1 , respectively. ICP AES power was controlled at 1.20 kW. Furthermore, the exposure time was controlled at 30 s. Samples were tested two times. The result of an ICP-AES gives the concentration from all the elements from solid phase to liquid phase (Sanito et al., 2020a).

Statistical Analysis Using ANOVA
To analyze the different effects of the flux agents on the VOC degradation, analysis of Variance (ANOVA) was performed. Minitab software version 16 was used to analyze the data obtained from the experimental data (Sanito et al., 2022b). The number of p-values represents the results the effect of parameters. Comparisons of pairwise of each parameter were analyzed via the Tukey, Fisher and Hsu multiple comparison with the best (MCB) (Khan, 2013). Table 2 shows the comparison results of VOC from face masks waste after treatment via an atmospheric-pressure microwave plasma reactor. From the study, the total VOC concentration with the absence of flux agents after treatment of face masks via an atmospheric-pressure microwave plasma reactor was 448.04 ppm. Moreover, the total concentration of VOC was 133.27 ppm with only the addition of cullet. The best results on the elimination of total VOC were obtained at the value of 54.7 ppm, where face mask wastes were combined with Al2O3, CaCO3, SiO2 and cullet, confirming lower than control and face masks with cullet at the values of 314.77 ppm and 393.34 ppm, respectively. Fig. 2 shows the comparison results for the treatment of face masks waste with the absence and addition of flux agents. The highest concentration of pollutants in the pyrolysis of face masks was benzene. The concentration of benzene was obtained at a value of 237.73 ppm (53.05%) in control. Moreover, it was followed by toluene with a concentration 75.27 ppm (16.80%). Propylene can be found at the concentration of 56.63 ppm (12.64%) from the study. Lastly, other pollutants Table 2. Comparison results of VOC concentration from face masks from the post-treatment with different composition of flux agents.

Volatile Organic Compounds (VOCs)
Control (   were found with a concentration of less than 36 ppm (less than 8%). Kerkeling et al. (2021) confirmed that the mask samples consist of xylenes, terpenes, aldehydes and siloxanes. Compared to the control, the addition of cullet and flux agents has better results. Final concentrations of benzene from face masks with cullet and face mask with flux agents (combination of CaCO3-Al2O3-SiO2-glass/cullet) were 58.48 ppm and 43.28 ppm, respectively, which are, respectively, 179.25 ppm (53.05%) and 194.45 ppm (43.88%) less than result of control (face mask only). Furthermore, the high percentage composition of benzene at 79.12% (43.28 ppm) in Fig. 2(c) dominates the composition of VOC due to the successful reduction of tetrahydrofuran, chlorobenzene, m-xylene, o-xylene, p-xylene, styrene, 4-ethyltoluene, benzene and toluene. Moreover, cullet and flux agents also showed a good result on degrading toluene with the value of 17.56 ppm (13.18%) and 5.54 ppm (10.13%), respectively, which are, respectively, lower than 57.71 ppm and 69.73 ppm, compared to control with a concentration of 75.27 ppm. The statistical model confirms that (F(4.34) = 0.04, p > 0.05), confirming the addition of flux agents has a significant impact on the reduction of the total VOC concentration (Table 3). Table 4 gives the results of the Tukey and Fisher analyses. The total concentrations of VOC and flux agent type levels significantly overlap based on the statistics. The grouping results of Tukey and Fisher confirm that the addition of flux agents had a significant effect to reduce the concentration of VOC, where the mean can be obtained at the value of 13.25 (Khan, 2013). The Hsu MCB confirms the best group from the analysis where it refers to the best result on the degradation of VOC with the addition of flux agents (CaCO3, Al2O3, SiO2 and glass/cullet). Also, it confirms that the high degradation of VOC with the addition of flux agent has the significant impact on degradation of VOCs from the face mask waste compared to control and glass (the best results). From this study, the lowest is the best because the decrease of total VOC concentration confirms the best result of plasma pyrolysis. The values of VOC in the gas emission and flux agents from the statistical evaluations are 0.00 and -20.22, respectively (Table 5). It shows that final concentration on the reduction of VOC with the addition of flux agents was better than in other groups, where the mixing of different flux agents (CaCO3, Al2O3, SiO2 and glass/cullet) plays a major role in degrading the total VOC concentration from the pyrolysis and has a significant impact during the degradation process.
Pollutants, namely, chloromethane, 1.3 butadiene, tetrahydrofuran, chlorobenzene, and 4ethyltoluene were degraded perfectly at the value of 100%, with the addition of cullet and flux agents (CaCO3, Al2O3, SiO2, and glass/cullet) (Table 2). Furthermore, the addition of flux agents showed a significant difference compared to the absence of flux agents (p-value < α, where 0.04 < 0.05) based on the statistical analysis. It indicates that the addition of flux agents (Al2O3, CaCO3, SiO2 and cullet) has a significant impact on the destruction of VOC from the pyrolysis via an atmospheric-pressure microwave plasma reactor. The degradation patterns of pollutants of VOC  Fig. 3(a) and Fig. 3(b). Overall, the results showed that the addition of flux agents indicates a better result compared to the control on the degradation of VOC. Pollutants, such as chloromethane, benzene, toluene, and 1,3-butadine, have concentrations of less than 100 ppm after the pyrolysis treatment with the addition of flux agents. Some pollutants indicate a trend lower than 4 ppm, such as chloromethane, tetrahydrofuran, chlorobenzene, ethylbenzene, m-xylene, o-xylene, p-xylene, styrene, and 4-ethyl toluene. Therefore, the degradation of pollutants can be obtained via the addition of flux agents during plasma pyrolysis. Sanito et al. (2020a) confirmed that the addition of flux agents indicates an excellent result on degrading benzene and propylene from the treatment of resin via an atmospheric-pressure microwave plasma reactor. Sanito et al. (2022b) also stated that the flux agent addition from shell powder, especially from Babylonia formosae indicates a promising result on decreasing of the volatile organic compounds (VOC) from the plasma pyrolysis. In this study, the fuse of diverse flux agent gives a good result on degrading VOC from the face mask wastes, comparing with cullet and control because of the decrease of VOC. Thus, the addition of flux agents can be considered  on the reduction of total VOC concentration. Degradation of VOC may be due to the presence of OH radicals (Wessenbeeck, 2016) in the flux agent, reacting with electrons from plasma jets and colliding with molecules of VOCs (Sanito et al., 2020a(Sanito et al., , 2022b. It is supported by the statements of some authors (Saunders et al., 2003;Bloss et al., 2005;Waring and Wells, 2015) that VOCs can be degraded owing to the presence of OH radicals. Degradation of VOC is therefore having a better result with the presence of flux agents compared with the absence of flux agents. The detail mechanism of VOC degradation is showed in Fig. 4. Fig. 5 shows the result of vitrification after the treatment of face mask wastes via an atmosphericpressure microwave plasma reactor. From this study, it can be seen in Fig. 5(a) that the pyrolysis of face mask waste has no vitrification results, comparing with the addition of flux agents with combinations, such as SiO2, CaCO3, Al2O3 and cullet. This is because of flux agent melted with materials, creating encapsulation and establish the silicate network. Based on the vitrification results, formation of crystalline structure can be obtained as follow: Silicon (PDF Number 1-791), Quartz (PDF number 1-649), calcium aluminium oxide/Ca(AlO2)2 (PDF number 1-888), Rankinite/ 3CaOSiO2 (PDF number 2-323) and wollastonite/CaSiO3 (PDF number 1-720) (Fig. 5(b)). It can be seen from Fig. 5 that there are many formations of crystalline structures can be established from the addition of flux agents compared to control, where some peaks appear from the analysis with the fuse of flux agents. ICP tests confirmed that the concentrations of Ca were more than the detection limit (Table 6). Also, the harmful elements are not detected from the ICP test, where the concentrations are 0 mg L -1 with the fuse of flux agents. Cubas et al. (2015) confirmed that the addition of SiO2 creates the silicate network from the face mask wastes. Du et al. (2018) and Sanito et al. (2020a) also confirmed that the fuse of flux agent improves the vitrification of materials from the hazardous waste. Interestingly, the presence CaSiO3 because the using of Meretrix meretrix shell powder as the flux agents. Sanito et al. (2020a) found that the addition of shell powder (Babylonia formosae) creates the CaSiO3 formation in the material from the plasma pyrolysis. Shell powder, therefore, has contributed to the vitrification of face mask wastes during the pyrolysis. The addition of CaCO3, Al2O3, SiO2 and cullet gives variation of material structures from the face masks.   (Fig. 6(a)). In addition, the analysis of FTIR from the samples with addition of glass confirms the presence of SiO2. Akarsu et al. (2021) stated that polypropylene had a percentage of up to 27% and polyethylene with the value of 51% from the masks. Thus, main compositions are polypropylene and polyethylene in the face mask wastes. In this study, the degradation of carbon affects the different composition or the functional groups in the face masks due to the transformation of materials during plasma pyrolysis. Szefer et al. (2021) indicated that polyethylene fiber is the main composition of the face mask wastes. Moreover, it consists of the microfibers (Saliu et al., 2021). Thus, the polypropylene is the main composition of face mask waste. Wavenumbers at value of 469 cm −1 and 995.7 cm −1 represent Si-O-Si functional   (Fig. 6(c)). Reig et al. (2002) confirmed that the presence of Si can be obtained at the value of 709 cm −1 -831 cm −1 . Naayi et al. (2018) also confirmed that the presence of OH can be obtained at a value of 3550 cm −1 . In this aspect, vitrification occurs because of the addition of flux agents, which transforms the functional groups of face masks (control), such as carbon to the vitrify materials. Thus, vitrification can be obtained from this study with the presence of Al=O, Si-O-Si, and Si-O functional groups.

Organic Compounds of Face Mask Wastes
In this study, the percentages of carbon from control, addition of glass/cullet and addition of flux agents were obtained at values of 91.86 ± 2.24%, 8.48 ± 2.08% and 17.92 ± 10.08%, respectively. Table 7 shows the elemental analysis of the face mask wastes. A total percentage of hydrogen was obtained at the value of 0.56 ± 0.056% (control). The percentage hydrogen from face mask wastes combined with cullet and face masks with CaCO3-Al2O3-SiO2-cullet/glass were obtained at the value of 0.85 ± 0.42% and 0.13 ± 0.01%, respectively. Moreover, the compositions of nitrogen from control, face mask wastes combined with cullet and face mask wastes combine with flux agents were 0.590 ± 0.056%, 0.03 ± 0.001%, and 0.13 ± 0.008%, respectively. The presences of C, H. and O from the face mask are associated with the polypropylene with the combining structure of C, H, and O. Also, the presence of polypropylene with a high density related to the inner and outer layers is always related to the presence of plastics (Fadare and Okoffo, 2020).
Information about SEM-EDX is outlined in Fig. 7. From this study, it can be seen that the surface formation of the mask after treatment via the plasma pyrolysis is more irregular and not spherical ( Fig. 7(a)). The material from the plasma post-treatment with the addition of cullet indicates the conversion of materials with the spherical formation ( Fig. 7(b)). Moreover, the compact structures can be obtained with the addition of flux agents with the combination of SiO2, Al2O3, CaCO3 and cullet, which successfully convert the face mask waste to different material formations, confirming the proper vitrification result of material from the plasma post-treatment, and it is linked to each other (Fig. 7(c)). From the plasma post-treatment, the Ca formation increases from 1.76% to 13.55% because of the addition of shell powder in the flux agent, where the percentage of C was 25.21%-47.16% (Table 8). In this case, the addition of more flux agents confirmed the high vitrification material of the residue. Sanito et al. (2020aSanito et al. ( , 2022b confirmed that the addition of shell powder plays an integral role in the vitrification of hazardous waste. In this study, the addition of shell powder as the source of CaCO3 from the Meretrix metertix shells combined with SiO2, Al2O3, and cullet/glass may be suggested for the suitable vitrification of face mask wastes.

Future Perspectives of the Face Mask Wastes Treatment
Near the future, the prevention of the pandemics will become a huge issue because related to the high usage of face masks during the pandemic era. In this aspect, the use of face masks will increase because for preventing the outbreak the virus to the environment, where will become a problem to the capacities of the systems for treating the face mask wastes . Technologies, such as incinerator (Kerkeling et al., 2021) and plasma (Gomez et al., 2009;Cai and Du, 2021;Zaluska et al., 2022) will be used to treat the medical waste. The proper type of flux agents should be suggested for dealing with the medical waste, considering the cost issues and