Application of Graphene Oxide Aerogel to the Adsorption of Polycyclic Aromatic Hydrocarbons Emitted from the Diesel Vehicular Exhaust

Journal Pre-proof Application of Graphene Oxide Aerogel to the Adsorption of Polycyclic Aromatic Hydrocarbons Emitted from the Diesel Vehicular Exhaust Haolin Hsu, Chungyih Kuo, Jihmirn Jehng, Chientai Wei, Chingfeng Wen, Jeanhong Chen, Lungchuan Chen

PII:

S2213-3437(19)30537-8

DOI:

https://doi.org/10.1016/j.jece.2019.103414

Reference:

JECE 103414

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

1 August 2019

Revised Date:

1 September 2019

Accepted Date:

13 September 2019

Please cite this article as: Hsu H, Kuo C, Jehng J, Wei C, Wen C, Chen J, Chen L, Application of Graphene Oxide Aerogel to the Adsorption of Polycyclic Aromatic Hydrocarbons Emitted from the Diesel Vehicular Exhaust, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103414

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Application of Graphene Oxide Aerogel to the Adsorption of Polycyclic Aromatic Hydrocarbons Emitted from the Diesel Vehicular Exhaust

Haolin Hsua, Chungyih Kuob,1,*, Jihmirn Jehngc, Chientai Weib, Chingfeng

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Wenb, Jeanhong Chena, **, Lungchuan Chena

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a Green Energy Technology Research Center, Kun Shan University, Tainan, Taiwan,

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71070

b Department of Public Health and Health Technology Center, Chung Shan Medical

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University, Taichung, Taiwan, 40201

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Taiwan, 40201

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c Department of Chemical Engineering, National Chung Hsing University, Taichung,

** Co-Corresponding author: Prof. Jeanhong Chen

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E-mail address: [email protected] Tel.: +886 920 431177 * Corresponding author: Prof. Chungyih Kuo E-mail address: [email protected] Tel: +886 4 24730022 ext. 11304 1

Fax: +886 4 22854734

Corresponding authors contributed equally to this work.

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Graphical Abstract

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Highlights

1. The adsorption and sieving efficiency of GOA applied to the PAHs emitted from the

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vehicular exhaust by the dynamometer was first investigated. 2. More oxidation time enhanced the crystalline structure between the GOA layered sheets.

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3. The carcinogenic BAP adsorption of the GOA was much better than that of XAD resin. 4. The GOA had better adsorption efficiencies of total PAHs in the engine idling condition. 5. The GOA adsorbent can be reused after three runs of the Soxhlet extraction and drying.

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Abstract For reducing the carcinogenic BaP and polycyclic aromatic hydrocarbons (PAHs) from the moving and stationary pollution source in the atmosphere, this study demonstrated useful adsorbent of graphene oxide aerogel (GOA). The application of GOA to the

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adsorption of polycyclic aromatic hydrocarbons (PAHs) emitted from diesel vehicular exhaust via the dynamometer was investigated. The surface morphology and microstructure

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of GOA were characterized by SEM, TEM, elemental analysis, Raman, and TGA.

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According to the result of XRD, the grain size of GOA after 5 hours of oxidation time increased to 28.0 nm with 38 layers estimated. It demonstrated that more oxidation time

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greatly enhanced the crystalline structure and reformation between the GOA sheets. The

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GOA sheets possessed the molecular sieving property for the filtration and adsorption of carcinogenic PAH molecules due to the strong hydrogen bonding interaction along the

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edges and the Van der Waal's force interactions. Naphthalene, acenaphthene, benzo[a]pyrene (BaP), and dibenz[a,h]anthracene, were chosen to carry out the pre-test

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adsorption experiments. Especially, the BaP compound was known to be carcinogens and mutagens for humans because of their carcinogenicity and genotoxicity. From the result of pre-test adsorption of PAHs, the BAP adsorption capacity of the GOA adsorbent (310 ng BaP / 1g GOA) in 1 hour was much better than that of XAD16 resin (0.060 ng / 1g

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XAD16), which is the standard adsorbents of PAHs announced by the Environmental Protection Administration of Taiwan. Furthermore, the sample collection of PAHs emitted from the diesel exhaust was performed by using the vehicle dynamometer. At the idling speed, the best adsorption capacity of total PAHs per gram GOA was 12.1 μg g−1, and

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better than that of XAD16 (1.03 μg g−1). Moreover, the adsorption sleeve and adsorbent of GOA material could be recycled and reused repeatedly. The structure of the GOA

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adsorbent has maintained after three runs of the Soxhlet extraction and drying. Nevertheless,

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the structure of XAD16 resin is decomposed obviously and decreased after the Soxhlet extraction. Owing to the superior adsorption capacity of PAHs and low synthesis cost, the

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adsorbent material in the future.

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GOA material is possible to be one of the excellent environmental and air pollution

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Keyword: Graphene; Graphene oxide aerogel; Polycyclic aromatic hydrocarbons;

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Benzo[a]pyrene; Vehicle dynamometer

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1. Introduction Graphene derivative materials, such as graphene oxide (GO) [1], reduced graphene oxide [2], carbon aerogel [3] and graphene oxide aerogel (GOA) [4], have attracted worldwide attention in recent years because of their outstanding properties. Graphene exhibits

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excellent physical and chemical properties and can be easily modified [5]. Recent researches have been reported that graphene-related materials present outstanding

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performance in many potential applications, including biosensors [6-8], supercapacitors

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[9-11], contamination removal, and extraction [12-15]. Graphene and GO are not similar to the Perovskite adsorbent [16], and retain the layer structure. Graphene demonstrates high

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affinity to the polycyclic aromatic hydrocarbons (PAHs), and moreover, the hydrophobic

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properties and molecular sizes of the PAHs affect the adsorption of graphene and GO [15]. It is suggested that the high affinities of the PAHs to graphene and GO are achieved mainly

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by π–π electrostatic interactions and the sieving properties of the broad groove regions formed by wrinkles on surfaces [17, 18]. Therefore, graphene and graphite oxide materials

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exhibit excellent performance in adsorption with PAHs and their derivatives [19]. PAHs are distinctive classes of steady organic compounds and one of the most hazardous

pollutants with two or more fused benzene rings in linear or cluster arrangements from natural as well as anthropogenic sources. They can pass easily through human cell

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membranes and into cells [20-21]. They are known to be carcinogens and mutagens for humans because of their carcinogenicity and genotoxicity. In Taiwan, the number of car and motor vehicles has been progressively increasing. Moreover, the exhaust gases emitted by vehicles often contains lots of pollutants, including metal elements, suspended particular

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matter, carbon monoxide [22], volatile organic pollutants [23], and PAHs, etc [24]. PAHs can exist in the gas phase ubiquitously once emitted into the atmosphere. Besides, they can

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also be adsorbed onto particle surfaces, such as soot and particulate matters. Therefore, they

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spread between gas and particle phases widely [25]. Vehicular emissions of particulate and gas-phase PAHs are of particular interest and importance because of their potentially toxic

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and probable human carcinogenic compounds [26]. Furthermore, the rapid vehicular

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growth rate has increased the vehicular emissions, which have become one of the principal anthropogenic sources of PAHs in Taiwan. Nowadays, the removal of PAHs in the

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environment atmosphere is a significant worldwide matter. In this study, the main purpose was to examine the adsorption and sieving efficiency of

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PAHs, which were emitted from vehicular exhaust via dynamometer. The reusing ability of GOA adsorbent for the PAHs was also performed. The GO was prepared according to the modified Hummer’s method [1, 4] as the raw material in the study. Different oxidation time (1h, 2h, 3h, 4h, and 5h) was particularly considered as an important controlling factor for

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the crystalline of GOA nanosheets. Naphthalene (NAP), acenaphthene (ACP), benzo[a]pyrene (BaP), and dibenz[a,h]anthracene (DBA) were chosen to carry out the pre-test adsorption experiments. Sample collection of PAHs for exhaust gas was performed by using the vehicle dynamometer experiments. The polyurethane foam and commercial

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XAD polymer resin are the standard adsorbents of PAHs announced by the Environmental Protection Administration of Taiwan. Owing to the superior adsorption capacity of PAHs

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and low synthesis cost, however, the GOA material is possible to be one of the excellent

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environmental and air pollution adsorbent material in the future.

2. Experimental

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2.1. Materials and preparations of GOA The powdered flake graphite was purchased from Lancaster Synthesis. ACS reagent

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grade concentrated nitric acid, sulfuric acid and hydrochloride acid (Fisher chemical) were

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used in this work. 35% hydrogen peroxide solution was obtained from Tokyo Chemical Industry Co., Ltd. Amberlite® XAD16 was purchased from Sigma-Aldrich Products. From the product information sheet, the surface area is up to 800 m2 g−1. All other chemicals were purchased in analytical grade purity and used without further purification. Milli-Q water was used in all experiments. 6

Basically, the GO was prepared according to the modified Hummer’s method [1, 4] by using powdered graphite as the raw material in our laboratory. It is an oxidation process that can be used to produce graphite oxide and commonly used as a method of generating quantities of graphene oxide via various modifications. Scheme 1 shows the manufacturing

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process of GOA. In particular, various acidification and oxidation time (1h, 2h, 3h, 4h, and 5h) was considered as an important parameter in this process. Then the raw GO gel is

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mixed with ethylene glycol and heated at 120 °C for a few hours to prepare a hydrogel.

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Then the resulting GO hydrogel was washed several times with a large amount of deionized water to neutral. Afterward, the GO solution was frozen in an air-conditioned environment

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for 24 h and became a freezing hydrogel. After that, the freezing hydrogel was transferred

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to the freeze-drying machine set (FD3-12P-80, KINGMECH, Taiwan) at −85 ℃ for 72 h, and the GOA was prepared. With different oxidation time, the black GOA and light brown

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one were obtained in this research.

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Position of Scheme 1

2.2. Sample collection of PAHs from the pre-test and vehicle dynamometer experiments Four PAH compounds, NAP, ACP, BaP, and DBA are chosen to carry out the pre-test

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experiments. These PAHs were placed in the middle of the horizontal tube furnace and then heated to evaporate. The PAH concentration introduced to a detection column filled with a GOA or a XAD16 resin under room temperature at a low volumetric flow rate for 5, 10, 15, 30, and 60 minutes. Finally, samples of the PAH concentrations for the various

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gas-introducing duration were collected and analyzed. GO is evenly distributed in

deionized water, and then the solution is lyophilized after being filled in a glass

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sleeve. In such a manner, the carbon aerogel can overcome the assembly difficulties

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to the sleeve with a small diameter. It shows that the carbon aerogel has excellent filling properties to containers having various shapes or various sizes.

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Sample collection of PAHs for exhaust gas was carried out by using the vehicle dynamometer experiments as shown in Fig. 1. The test vehicle was positioned on the

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chassis dynamometer, and then its engine idled and warmed up for constant duration

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by a stand-by mode. After which, an exhaust vent of the vehicle was directly connected to a quartz tube. In order to simulate the real driving emission of the

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vehicle, a dilution chamber and a condensation machine were not considered, and an adsorbing sleeve was connected to the quartz tube. A low-volume air sampler pump was used to ensure the adsorbing sleeve fully collecting a constant volume of the exhaust gas for various driving rates and various driving durations. The total exhaust

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gas volume was 75, 225 and 375L, respectively. The collection was performed at idle speed, a low speed, or a high speed to estimate the adsorption efficiency, the recycling efficiency, and the reuse efficiency of the adsorbent materials.

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Position of Fig. 1

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2.3. Analysis of PAHs from pre-experiment and vehicle dynamometer experiments

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Total PAHs in the samples of the pre-test and vehicle dynamometer experiments were Soxhlet-extracted using a 300 and 600 mL high-purity mixture of organic solvents

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(n-hexane and dichloromethane in a 1:1 volumetric ratio) for 24 h, respectively. The

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extracted PAHs were dried using a rotating evaporator and then purified using a silica gel cleanup set. The samples that remained in the silica gel columns were diluted with 60 ml of

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n-hexane. The runoff solution was filtered through a 0.45 μm PTFE membrane and dried using a rotating evaporator. 30 PAHs were quantified by gas chromatography/mass

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spectrometry (Varian CP 3900/Saturn 2100 T) with a capillary column (30 m × 0.25 mm × 0.10 μm). Compounds monitored in this work included naphthalene (NAP), acenaphthylene (ACPy), acenaphthene (ACP), fluorene (FLU), phenanthrene (PHE), anthracene (ANTHR), fluoranthene

(FLT),

pyrene

(PYR),

benzo(c)phenanthrene

(BcPH),

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benzo(b)napth(2,1-d)thiophene (BNT), cyclopenta(cd)pyrene (CPP), benz(a)anthracene (BaA), chrysene (CHR), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(e)pyrene (BeP), Benzo[a]pyrene (BaP), pyridine (PYL), Indeno[1,2,3-cd]pyrene (IND), dibenz(ah)anthrancene (DBA), BghiP, anthanthrene (ANTHA), Dibenzo[a,l]pyrene

(2Me-NAP),

1-methylfluorene

(1Me-FLU),

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(DBalP), Coronene (COR), 1-methylnaphthalene (1Me-NAP), 2-methylnaphthalene 2-methylphenanthrene

(2Me-PHE),

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3-methylphenanthrene (3Me-PHE), and 3,6-dimethylphenanthrene (3,6Me-PHE). A

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standard solution consisting of known amounts of thirty PAHs dissolved in dichloromethane was prepared to spike with a known volume of naphthalene-d8,

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phenanthrene-d10, and perylene-d12 as the internal standard. An internal standard was also

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added to the collected samples of each analytical batch. Known volumes of PAHs standard were spiked into GOAs to determine the efficiency of the extraction and analysis. The

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extraction efficiency yielded agreements between 75 ‒ 125 %, except for the lower molecular weight species of NAP, ACP, ACPy, and Me-NAP. The extraction method was

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assessed for linearity, recovery, reproducibility, the limit of detection (LOD) and limit of quantification (LOQ) before sample analysis. LOD value was estimated from sequential injections of diluted standard solutions by using a signal-to-noise ratio of 3. The measured concentrations from most samples were higher than the limit of LOQ.

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2.4. Characterization Surface morphology and structure of GOA were investigated with a field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL) equipped with an energy-dispersive X-ray spectroscopy (EDX) detector (Oxford Instruments). Besides,

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high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010, operated at 200 kV) was also employed to monitor the morphology and layers of the GOA. Elemental

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analysis (EA) results were obtained with a Heraeus CHN-O-S-Rapid Analyzer F002 to

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determine the contents of carbon, hydrogen, and oxygen in the GOA. Raman spectra were recorded using a Tokyo Instruments spectrometer with He–Ne 632.8 nm laser source. The

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thermal stability of GOA was performed by using a thermogravimetric analysis (TGA) system (PerkinElmer Pyris 1 series). Typically, 10 mg of sample was heated from 100 to

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800 ℃ at a heating rate of 10 ℃ min−1 in N2 atmosphere. The Brunauer-Emmett-Teller

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(BET) specific surface area of the GOA before and after combustion was determined by N2 adsorption/desorption at −196 °C using a Micromeritics ASAP 2020, America. The X-ray

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diffraction (XRD) data was investigated by using the instrument (MAC Science, MO3XHF) with a CuKα radiation source (λ= 1.54056 Å), and recorded from 2º to 80º at a scanning rate of 2º min−1.

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3. Results and discussion The typical FESEM and HRTEM images of GOA are presented in Fig. 2a-b and c-d, respectively. The surface morphology of GOA can be confirmed by the broad wrinkled sheet without clear crack and gap structure, and obtained at low (Fig. 2a and Fig. 2c) and

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high (Fig. 2b and Fig. 2d) magnification images. The TEM images revealed that GOA film was more smooth and transparent with fewer wrinkles (red arrows) as shown in Fig. 2c and

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Fig. 2d. Besides, it appears that the GOA sheet consists of about 8-10 graphene layers from

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the edges of GOA sheets [16, 27]. Because of the broad wrinkled sheet without clear crack and gap structure, and the hydrogen bond between GOA layers, the measured surface area

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of GOA via Hummer's method is usually small [28]. As shown in Fig. S5a, N2

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adsorption-desorption isotherms of the GOA before and after short ambient combustion were measured to evaluate the specific surface area. The BET specific surface area of raw

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GOA was 5 m2 g‒ 1 and with H4 loop with the narrow slit-like pore [29]. Fig. S5b shows the pore-size (diameter) distributions with a major pore-size ~ 1nm. However, the specific

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surface area of GOA after combustion raised to 200 m2 g‒ 1 and obtained a type IV isotherm with H3 and H2 hysteresis loops, indicating mesoporous pore mainly. It demonstrates that the gaseous nitrogen molecules adsorbed and desorbed easily because strong affinity and weak hydrogen bonding interaction between graphene layers. Furthermore, we suggest that

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GOA sheets possess the molecular sieving property for applying to the filtration and adsorption of carcinogenic PAH molecules due to the low specific surface area. The surface morphology of the announced commercial XAD16 resin adsorbent with a high surface area about 800 m2 g‒ 1 is presented in Fig. 2e-f. From EDX results, the XAD16 were mainly

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composed of carbon and oxygen elements.

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Position of Fig. 2

Elemental analysis results of the graphite powder, GOA and XAD16 are listed in Table 1

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to indicate the contents before and after modified Hummer's method. The relative atomic

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molar ratios of C/O were approximately 168.2：1, 7.7：1 and 9.0：1 for the raw graphite powder, GOA, and GOA after ambient combustion, respectively. The C/O ratio of graphite

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powder after oxidation was decreased to 7.7：1. Moreover, the C/O ratio of GOA after combustion was further increased from 7.7：1 to 9.0：1, owing to the removal of surface

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functional groups (e.g., carboxylic groups, epoxy groups, hydroxyl groups, and carbonyl groups). Therefore, these results indicate that the content of oxygen atoms after the oxidation process increased. The relative C/O ratio of polymeric resin XAD16 is 17.8：1. It is significantly different compared to GOA.

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Position of Table 1

Raman spectra of the raw graphite powder, GOA, and XAD16 with a laser wavelength at

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632.8 nm are shown in Fig. 3(a). The characteristic disorder and graphitic peaks of graphite and GOA were D band (1350 cm− 1) and G band (1580 cm− 1), respectively. In addition, the

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peaks named G' band which is the overtone of the D band was also observed at 2700 cm− 1

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[30]. According to the observation of the Raman shifts of 1000, 1604, 2904, and 3054 cm−1, the composition of XAD16 resin were determined mainly polystyrene [31]. The relative

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intensity ratio of G band to D band, IG/ID, demonstrates the quality of graphitization or

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defective disorders on carbon materials. In this study, the IG/ID ratio of the raw GOA (0.881) is lower than that of graphite powder (3.379), indicating that the functional groups formed

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onto the surface and edge of graphene sheets via the modified Hummer's method. Fig. 3b shows the TGA weight loss curves of the graphite powder and GOA. From the TGA result,

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the graphite powder is stable with higher temperature treatments from 100 to 800 °C, and about 5.8% weight loss. The GOA begins to lose weight from 100 to 300 °C because of the decomposition of functional groups and hydrogen bound between GOA layers. The GOA material is lower thermal stable than graphite powder, and its estimated weight loss is ~

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49.2% at 800 °C. It reveals that the GOA is more thermal stable than other carbon material we synthesized before [32], showing the better crystalline structure in GOA layers.

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Position of Fig. 3

Fig. 4a shows the powder XRD patterns of the graphite powder and GOA. For the

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graphite powder, the basal spacing (d002) reflection obtained at a 2θ value of 26.5º

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corresponding to an interlayer distance of 3.40 Å. The basal spacing (d004) reflection also obtained at a 2θ value of 54.6º corresponding to an interlayer distance of 1.68 Å. After the

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oxidation via the Hummer’s method, a larger basal spacing (d001) reflection of GOA

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observed at 2θ = 9.8º corresponding to an interlayer distance of 9.03 Å. The characteristic basal spacing reflections of d002 and d004 almost not observed. The Scherrer equation is a

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widely used tool to determine the nano-scale crystallite size of polycrystalline samples [33]. The crystallite size (D) of the GOA was estimated by the Scherrer equation:

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D = 0.91 × λ / ( B × cosθ ),

where 0.91 is the shape factor constant; λ is the wavelength of the Cu K radiation source (0.15418 nm); B is the half-width of the strongest diffraction peak, and θ is its diffraction angle (in radian unit) of the strongest diffraction peak. Furthermore, the full width at half

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maximum (FWHM) of XRD profiles is used to characterize different material properties and surface integrity features. Fig. 4b-c shows the effect of oxidation time on the diffraction angle and the basal d spacing. It shows a steady increase in diffraction angle and a decrease in basal d spacing

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with increasing oxidation time. Peak width due to crystallite size varies inversely with crystallite size. By calculation with the XRD result, FWHM and the Scherrer equation, the

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grain sizes of graphite powder and GOA were estimated in 39.4 nm and 14.0 nm,

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respectively. In addition, it shows that the crystalline graphite powder and GOA possessed approximate 116 and 20 layers, respectively. Moreover, the grain size of GOA after 5 hours

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of oxidation time increased to 28.0 nm with 38 layers estimated. We suggest that more

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GOA layers.

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oxidation time extremely enhanced the crystalline structure and reformation between the

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Position of Fig. 4

Four PAH compounds (NAP, ACP, BaP, and DBA) with high toxic equivalency

factors (TEFs) were chosen to carry out the pre-test experiments [34]. The possible adsorption mechanism and diagram is drawn as scheme 2. Due to the hydrogen bonding

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interaction along the edge, the surface morphology of GOA became the broad wrinkled sheets. In Scheme 2, large molecular size of the PAHs filtered on the surface of GOA, and besides, the PAHs adsorbed onto the surface of GOA by the Van der Waal's force interaction. Molecular sieving effect and the Van der Waal's force interaction were mainly

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physical adsorption mechanism between PAHs and GOA. Fig. 5 shows the PAH concentrations of NAP, ACP, BaP, and DBA adsorbed by XAD16

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and GOA material adsorbents in 5, 10, 15, 30, and 60 minutes. Additionally, these four

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PAHs concentrations analyzed in detail are shown in Table 2. For these PAH compounds, the GOA material adsorbent had higher adsorption concentrations than the XAD resin one.

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Especially, the XAD resin adsorbent has poor adsorption affinity for the linear five fused

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benzene rings compound (DBA). The NAP compound was a low molecule weight PAH and easier to vaporize than the DBA compound, which was a high molecule weight PAH of

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five fused benzene rings. In addition, the DBA concentrations were not detected for any time by using the XAD resin adsorbent. It reveals that the XAD resin material does not

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have the ability to adsorp the high molecular weight PAH compound. Furthermore, BaP is considered to be highly carcinogenic to humans and adsorbed by the XAD resin announced by the Environmental Protection Administration of Taiwan. However, the BAP adsorption capacity in 1 hour of the GOA adsorbent (310ng BaP / 1g GOA) was a thousand times

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better than that of XAD resin one (0.060 ng BaP / 1g XAD16). It demonstrates that the GOA adsorbent material has excellent adsorption efficiency for BaP than the XAD resin according to the pre-test result. From the result of FESEM and pre-test, GOA layered sheets possess the molecular sieve property to filtrate and adsorb PAHs compounds efficiently.

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Furthermore, we suggested that the GOA adsorbent material seems to be an excellent material to use as the PAHs adsorbent for the following vehicle dynamometer

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Position of Fig. 5, scheme 2, and Table 2

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dynamometer simulation experiments.

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experiments. The further work of adsorption efficiency was carried out by real vehicle

Fig. 6 shows the total PAHs concentrations adsorbed by per gram XAD16 (Fig. 6a) and

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GOA (Fig. 6b) adsorbent at 0 (idling speed), 30 and 60 km h− 1 speed. The adsorption quantity of total PAHs by the adsorbents of XAD16 and GOA all increased with the engine

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running time. Compared to three sampling speeds, the highest adsorption quantity of total PAHs is obtained at idling speed. It demonstrates that the higher engine temperature makes the combustion reaction of the diesel oil more complete, and generate more carbon dioxide and water. Besides, the PAHs compound emissions will be significantly improved and

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reduced due to the higher speed engine temperature. At the idling speed, the best adsorption efficiencies of total PAHs per gram GOA is 12.1 μg g− 1, and better than that of XAD16 (1.03 μg g− 1). In comparison with previous literature, the adsorption of PAH compound of fluoranthene was investigated and possessed adsorption capacity of 2.212 mmol g− 1 ( ~

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1.09 μg g− 1) [18]. Nevertheless, the sum of total PAHs by using the solid phase extraction [35-36] was lower than the result of vehicle dynamometer simulation experiments. It shows

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that the GOA material adsorbent has excellent adsorption capacity. At 60 km h− 1 speed, the

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total PAHs weight adsorbed by the XAD16 and GOA material is shown in Fig. 6c. Because the amount of XAD16 used as the sampling adsorbent is more than that of GOA, the total

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PAHs concentration adsorbed by XAD16 is higher than GOA one. The total PAHs

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adsorbed efficiency based on per gram adsorbent is further shown in Fig. 6d. After evaluation and calculation, the adsorbed total PAHs concentration based on per gram GOA

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is much higher than that of XAD16. The GOA material has great potential to substitute the expensive commercial XAD resin adsorbent to lower the manufacturing cost in the future.

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As is known to all, the adsorbent's reusability is decisive and crucial to the practical

application, due to the manufacturing expense and mechanical property. The tests of the reusability of XAD16 and GOA were conducted repeatedly at the same condition, and the adsorption plug tubes were also recycled and reused again. Because of the limitation of

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frequency and time for operating vehicle dynamometer, the efficiency of the reusability at 60 km h− 1 speed was mainly achieved. Fig. 6e shows the reusability of XAD16 and GOA adsorbent material. First of all, the manufacturing cost of the GOA adsorbent via this process is much lower than the XAD16 resin. The real vehicle dynamometer temperature of

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test adsorbent is lower than 150℃. According to the TGA result at 150℃ in Fig. 3, the structure of GOA material decomposed about 3 weight percent. The structure of GOA

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adsorbent has maintained after three runs of the Soxhlet extraction and drying. Nevertheless,

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the structure of XAD16 resin is decomposed apparently and decreased after the Soxhlet extraction. The total PAHs concentration analyzed in the first run was the lowest during

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these three reused runs because there were a regular vehicle inspection and mechanical

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adjustment of the exhaust black smoke detection. However, it obviously demonstrates that the adsorption sleeve and adsorbent of GOA material can be recycled and reused repeatedly.

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There is not much difference in the concentration of total PAHs adsorbed by the second and third adsorption runs. Nevertheless, the adsorption efficiency of XAD16 resin material in

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recycling reuse significantly reduces.

Position of Fig. 6

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From the result of analysis FESEM, XRD, TGA and vehicle dynamometer experiment data, it appears that the GOA sheets possess the molecular sieve property to the filtration and adsorption of carcinogenic PAH molecules. In addition, it shows that the crystalline graphite powder and GOA possessed approximate 116 and 20 layers, respectively.

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Moreover, the grain size of GOA after 5 hours of oxidation time increased to 28.0 nm with 38 layers estimated. We suggested that more oxidation time extremely enhanced the

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crystalline structure and reformation between the GOA layers. From the result of pre-test

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adsorption of PAHs, the BAP adsorption capacity of the GOA adsorbent in 1 hour was a thousand times better than that of XAD resin. The highest adsorption quantity of total

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PAHs is obtained at idling speed via the vehicle dynamometer experiments. At the idling

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speed, the best adsorption efficiencies of total PAHs per gram GOA is 12.1 μg g − 1, and better than that of XAD16. The structure of GOA adsorbent has maintained after three runs

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of the Soxhlet extraction and drying. Nevertheless, the structure of XAD16 resin is decomposed obviously and decreased after the Soxhlet extraction. In apparent, the GOA

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material has great potential and opportunity to substitute the expensive commercial XAD resin adsorbent to lower the manufacturing cost in the future.

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4. Conclusions In this study, the GOA layers particularly become more crystalline structure enhanced by more oxidation time. We demonstrate that the GOA sheets possess the molecular sieve property to the filtration and adsorption of carcinogenic PAH molecules. Especially, the

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XAD resin adsorbent has poor adsorption affinity for the linear five fused benzene rings compound (DBA). Besides, that GOA material has outstanding adsorption efficiency for

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human carcinogenic BaP. Additionally, GOA presents excellent adsorption quantity of total

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PAHs than that of XAD one. Furthermore, the structure of GOA material decomposed about 3 weight percent at 150℃. The structure of GOA adsorbent still maintains after three

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runs of the Soxhlet extraction and drying. Nevertheless, the structure of XAD16 resin is

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decomposed apparently and decreased after the Soxhlet extraction. The adsorption sleeve and adsorbent of GOA material can be recycled and reused repeatedly. Because of the

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superior adsorption capacity of PAHs and low synthesis cost, the GOA material is possible

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to be the perfect useful environmental and air pollution adsorbent material in the future.

Acknowledgments Financial support of the National Science Council of Taiwan (104-2622-E-040-001-CC2 and 105-2622-E-040-001-CC2) and the Featured Areas Research Center Program within

22

the Higher Education Sprout Project by the Ministry of Education of Taiwan (107-N-270-EDU-T-142) is gratefully acknowledged. The support of vehicle dynamometer experiments from the Taichung City Government Diesel Vehicle Smoke Exhaust

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Inspection Station is also acknowledged and appreciated.

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29

Captions: Fig. 1. Schematic illustration for vehicle dynamometer sampling tests.

Fig. 2. (a-b) FESEM and (c-d) TEM images of GOA; and (e-f) FESEM images of XAD16

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resin.

e-

weight loss curves of the graphite powder and GOA.

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Fig.3. (a) Raman spectra of the graphite powder, GOA, and XAD16 resin; and (b) TGA

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Fig. 4. (a) Powder XRD patterns of the graphite powder and GOA; (b) XRD diffraction

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patterns of GOAs after the oxidation time of 1h, 2h, 3h, 4h and 5h; and (c) the plot of the

ur

effect of oxidation time on the diffraction angle and basal d spacing.

Fig. 5. PAH concentrations of NAP, ACP, BaP and DBA adsorbed by (a) XAD16 and (b)

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GOA adsorbent in 5, 10, 15, 30, and 60 minutes.

Fig. 6. Total PAHs concentration adsorbed by (a) XAD16 and (b) GOA adsorbent at 0 (idling state), 30 and 60 km h− 1 speed; (c) Total PAHs weight amount adsorbed by XAD16

30

and GOA at 60 km h− 1 speed and (d) after calculation based on per gram XAD16 and GOA; and (e) The reusability of XAD16 and GOA adsorbent material.

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Table 1 EA results of raw graphite powder, GOAs, and XAD16

Table 2 The PAH concentrations of NAP, ACP, BaP, and DBA adsorbed by XAD16 and

e-

pr

GOA adsorbent in 5, 10, 15, 30, and 60 minutes

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Scheme 1 The manufacturing process of GOA

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ur

na l

Scheme 2 Physical adsorption mechanisms between PAHs and GOA

31

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ur

na l

Pr

e-

pr

Scheme 1.

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Scheme 2.

32

na l

ur

Jo

oo f

pr

e-

Pr

Fig. 1.

33

na l

ur

Jo Fig. 2.

34

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pr

e-

Pr

(a) D

G'

GOA G

IG / ID = 3.379

G'

D

pr

Graphite Powder

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IG / ID = 0.881

XAD16

1000

1500

2000

2500

3000

Pr

500

e-

Relative Intensity (a.u.)

G

Jo

ur

na l

Raman Shift (cm-1)

35

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Fig. 3.

(a)

na l ur

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Relative Intensity (a.u.)

Graphite (002) 2 theta = 26.5o d = 0.340 nm

Graphite (004) 2 theta = 54.6o

Graphite Powder

10

2 theta = 9.8o, d = 0.903 nm 20

30

GOA 40

50

60

Diffraction Angel (2 theta) 36

2 theta = 9.8o, d = 0.903 nm

1 hour

2 theta = 10.2o, d = 0.867 nm

2 hours

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Relative Intensity (a.u.)

(b)

2 theta = 10.3o, d = 0.860 nm

10

20

4 hours

5 hours

e-

2 theta = 10.7o, d = 0.827 nm

pr

2 theta = 10.5o, d = 0.843 nm

3 hours

30

40

Pr

Diffraction Angel (2 theta)

Jo

ur

na l

Fig. 4.

37

(c) 12 Diffraction Angel d spacing 11

10

10

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d spacing (10

-10

m)

11

9

9

8

pr

8

7

0

1

2

3

e-

Diffraction Angel (2 theta/deg.)

12

4

5

7 6

Jo

ur

na l

Fig. 4. (continue)

Pr

Oxidation Time (hour)

38

(a) 700

NAP

600

AcP BaP

ppm

500 400

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300 200

0 0

20

40

pr

100

60

80

60

80

(b) NAP

600

AcP

ppm

500 400

BaP

DBA

ur

300

na l

700

Pr

e-

Time (min)

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200 100 0

0

20

40

Time (min) Fig. 5.

39

(a) total PAHs ( μg / g )

15

60 km / h 30 km / h

12

0 km / h

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9 6

0 0

10

pr

3

20

30

Pr

e-

Time (min)

(b)

na l

60 km / h 30 km / h

12

0 km / h

9

Jo

ur

total PAHs ( μg / g )

15

6 3 0

0

10

20

30

Time (min) Fig. 6.

40

(c) 70

GOA

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50 40 30 20

pr

total PAHs ( μg )

60

XAD16

e-

10 0 5

15

25

Jo

ur

na l

Pr

Samplng time (min)

41

(d) 12

GOA XAD16

10 8 6 4

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total PAHs ( μg / g )

14

2 0 15

25

pr

5

Samplng time (min)

Jo

ur

na l

Pr

e-

Fig. 6. (continue)

42

(e) 15

XAD16

12 9 6

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total PAHs (μg / g)

GOA

3

1

2

3

pr

0

Reused number

Jo

ur

na l

Pr

e-

Fig. 6. (continue)

43