Introduction of amino and rGO into PP nonwoven surface for removal of gaseous aromatic pollutants and particulate matter from air

Applied Surface Science 511 (2020) 145631

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Full Length Article

Introduction of amino and rGO into PP nonwoven surface for removal of gaseous aromatic pollutants and particulate matter from air

T

⁎

Xiaolong Tanga,b, Junfu Weia,b,d,e, , Zhiyun Konga,c,d, Huan Zhanga,c,d, Jian Tiana,e a State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, Tiangong University, Tianjin 300387, China b School of Materials Science and Engineering, Tiangong University, Tianjin 300387, China c Tianjin Engineering Center for Safety Evaluation of Water & Safeguards Technology, Tianjin 300387, China d School of Environmental Science and Engineering, Tiangong University, Tianjin 300387, China e School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China

A R T I C LE I N FO

A B S T R A C T

Keywords: rGO Polypropylene nonwoven Gaseous aromatic pollutants Adsorption PM filtration

Gaseous aromatic pollutants such as benzene, toluene or xylene and particulate matter (PM) from an individual’s living environment potentially cause serious and life-threatening health problems. A new bifunctional individual protection material was prepared by grafting polymerization and electrostatic self-assembly method. The individual protection material has a dual function layer consisting of an amino-functionalized nonwoven grafted with dimethylaminoethyl methacrylate (DMAEMA) and a gaseous aromatic pollutant adsorption layer made up of reduced graphene oxide (rGO). The surface composition and structure of materials were obtained by a series of characterization methods. Adsorption and filtration performance for each function layer and double function layer Polypropylene (PP) nonwoven were intensively studied. As indicated by the experiments, the adsorption efficiency of PP-g-DMAEMA/rGO increased with the decreasing oxygen content of GO and increasing the initial concentration of toluene, filter performance and aperture parameters data showed that the surface Zeta potential and pore size had great influence on filtration efficiency of the filter material, the resulting PP nonwoven exhibited prominent multifunctional performance, including a higher adsorption amount (39.1 mg/g) and filter efficiency (72.2%). Consequently, it is expected that the bifunctional PP nonwoven will have potential applications in air purification for the removal of gaseous aromatic pollutants and PM.

1. Introduction In recent years, the quality and security of air receive increasing global attention, air pollution has become one of the most serious threats that human face, and thus it is important to carry out individual protection effectively in a polluted environment [1]. Actually, air pollutants are highly complicated and can be divided into particulates, liquid droplets, gases, or mixtures of the above [2]. Among the diverse technologies that exist for removing toluene and capturing PM, adsorption filtration technology holds considerable application prospect due to its advantages of simplicity, economy and high efficiency [3,4]. At present, various materials have been developed and researched for their adsorption and filtration performance, however, most purification materials (absorbent and filtration material), such as activated carbons [5,6], porous microspheres [7–9], and fiber [10–12], show single removal performance. On the one hand, PM, with complex components (organic matter, nitrate, sulfate, ammonium, chloride, elemental ⁎

carbon, etc., including diverse ions and metal elements) and potential perniciousness to the human body, has a strong polarity [1,13]. In addition, the surface of PM can be easily polarized by positive charges and thus shows negatively charged. Meanwhile, PM can be captured by these positive charges via the electrostatic interactions. On the other hand, gaseous pollutants include volatile organic compounds (VOCs) and other hazardous gas. Gaseous aromatic pollutants have been proven to be one of the gaseous pollutants causing various problems of human health and environment [14], aromatic pollutants are mostly adsorbed by π-π interaction [15]. In fact, air pollutants originating from human activities such as vehicle emission, burning fuels, and pollutions from some industrial sectors, are always multifarious and complex [16]. At present, existing purification materials still face challenges in an environment filled with complex pollutants because of VOC adsorption and PM capture are two completely different removal mechanisms. As a result, development of multi-functionalized material with well filtration efficiency and high-density adsorption sites as well as a strong binding

Corresponding author at: School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China. E-mail address: [email protected] (J. Wei).

https://doi.org/10.1016/j.apsusc.2020.145631 Received 30 September 2019; Received in revised form 5 January 2020; Accepted 2 February 2020 Available online 04 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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mixture solution. 1.5‰ mass percent of sodium diethyldithiocarbamate was added so as to decrease the homopolymerization of DMAEMA. The solutions were bubbled by purity nitrogen for 20 min to remove oxygen and immediately subjected to ultraviolet ray irradiated immediately at room temperature. The ultraviolet ray irradiation grafted polymerization reaction occurred on the surface of PP nonwoven and the reaction time was 15 min. Following this, the grafted PP nonwoven was taken out and put it into a beaker containing ethanol by ultrasonic cleaning for 1 h to remove the unreacted monomers and homopolymers. Ultimately, the PP-g-DMAEMA nonwoven was obtained after drying at 323 K for 12 h. The degree of grafting was obtained from the following Eq. (1):

affinity toward gaseous aromatic pollutants is extremely needed. In order to meet this demand, we propose the creation of a double function layer nonwoven possessing the following features: abundant adsorption sites achieve high affinity for gaseous aromatic pollutants, high -load positively charged functional groups provide electrostatic adsorption sites, proper pore size distribution ensures satisfactory air permeability, excellent stability guarantees multiple uses, and efficient adsorption and filtration performance offers potential possibilities for practical applications. At the same time, the obtained double function layer nonwoven might serve as a mask filter material for protecting VOC and PM simultaneously. To reach that goal, a double layer detachable base material is needed, for which polypropylene nonwoven fabric is an excellent choice due to its porous structure and interception function [17]. The abundant penetrating channels and micron-level polypropylene fibers mean that PP nonwoven can be designed as a PM captured substrate [18]. However, the functional group on the PP nonwoven surface is extraordinarily poor, limiting its application as a double function layer nonwoven. Grafting DMAEMA onto the surface of PP nonwoven fabric can make the surface of the fibers charged positively and improve the efficiency of capturing PM [19]. Furthermore, rGO with aromatic ring structure is a widely used adsorbent for various pollutants. Moreover, GO with abundant oxygen functional group presenting negatively charged, can be assembled into PP-g-DMAEMA by electrostatic selfassembly and further translated into rGO after chemical reduction [20]. A step-by-step modification method is considered to prepare double function layer nonwoven materials. Herein, a rGO and amino-functionalized based double function layer PP nonwoven (DFLPP) was prepared through a step-by-step modification method including grafting polymerization, self-assembly, and chemical reduction process, respectively. The favorable change of charged characteristics because of DMAEMA molecule on fiber surface could improve the filtration performance of the substrate with little change of filtration resistance. By loading these GO sheets into PP fiber and via reduction process by ascorbic acid, a rGO and amino-functionalized based double function layer PP nonwoven was attained. We constructed double function layer PP nonwoven based on the following trait: (i) penetrating channels structure thereby assuring rapid gas transfer without extremely high gas resistance, (ii) the rGO could offer abundant adsorption sites for gaseous aromatic pollutants, and (iii) the DMAEMA molecule could capture PM better.

G=

Wg − W0 W0

× 100%

(1)

where Wo and Wg were the mass of the original and grafted nonwoven, respectively. The obtained PP-g-DMAEMA nonwoven was split into two parts, one part was used as PM filtration layer. 2.2.2. Preparation of rGO-functionalized adsorption layer The other part was used as a carrier for loading GO, the rGO-functionalized adsorption layer(PP-g-DMAEMA/rGO) was prepared by the electrostatic self-assembly and chemical reduction method [19]. Firstly, 10 mg of graphene oxide was dispersed in deionized water and sonicated for 3 h. Typically, the concentration of GO was 10 mg/L. Then, PP-g-DMAEMA nonwoven was immersed in homogeneous GO aqueous solution and shaken at 50 rpm for 3 h to allow GO self-assemble into PP-g-DMAEMA nonwoven. The resulting product was washed several times with deionized water and immersed in solutions of ascorbic acid at 70 ℃. Afterward, the reduction products were washed several times with deionized water to remove excess reduction agent. Ultimately, the PP-g-DMAEMA/rGO nonwoven was obtained after drying at 323 K for 12 h. Generally, relative weight gain (RWG) of PP-g-DMAEMA was quantitatively ascertained mass fraction of rGO and calculated according to the following Eq. (2):

RWG =

Wg −W1 W1

× 100%

(2)

where Wg and W1 were the mass of the grafted PP after and before electrostatic interaction, respectively. 2.2.3. Preparation of DFLPP nonwoven In the above introduction, two functional nonwoven fabrics with filtration properties and gaseous aromatic pollutants adsorption properties were successfully prepared by introducing functional molecules and rGO into a two-layer detachable PP nonwoven fabric, respectively. To get DFLPP nonwoven, two types of modified nonwoven were further consolidated using thermocompressor (R3202, Wuhan) at 80 ℃ and 0.3 MPa. In addition, the double layer PP-g-DMAEMA and PP-DMAEMA/rGO nonwoven were prepared at the same condition, which was used as control samples.

2. Materials and methods 2.1. Materials and reagents PP nonwoven fabric was obtained from the Haidexin Chemical Factory, Jiangsu, China. The nonwoven was washed by ethanol and distilled water and dried at 323 K before using. 2-(Dimethylamino) ethyl methacrylate (DMAEMA, analytical grade), benzophenone (analytical grade), sodium diethyldithiocarbamate (analytical grade) and ascorbic acid (analytical grade) were purchased from Aldrich. Toluene (analytical grade), benzene(analytical grade), xylene(analytical grade) and ethanol (analytical grade) were purchased from Tianjin Guangfu Fine Chemical Research Institute. Graphene oxide was purchased from Tianjin Kermel Chemical Regent Ltd, China.

2.3. Characterization To analyze the functional groups on the surface of modified PP nonwoven fabrics, the FTIR spectra was recorded on Fourier transform infrared spectrometer (Necolet 6700, USA) in the wave number range of 600–4000 cm−1 under ambient condition. The surface chemical composition was characterized by X-ray photoelectron spectroscopy (XPS), and analysis was carried out on an AEM PHI PHI5300X spectrometer with an Al K X-ray source (1486.71 eV of photons) to determine the C, N and O. The surface morphologies of the modified PP nonwoven fabrics were observed using the scanning electron microscopy (SEM) by a S-2500C microscope (Hitachi, Japan). μm-sized pore

2.2. Preparation of double function layer PP nonwoven 2.2.1. Preparation of amino-functionalized PM filtration layer A sample of amino-functionalized PP (PP-g-DMAEMA) nonwoven was prepared according to our previous work [21]. A double layer detachable PP nonwoven with the basis weight of 25 g/m2 was adopted, the PP nonwoven fabric was weighed and put it into the polyethylene bags. Next, the nonwoven was immersed in solutions of DMAEMA with mass concentrations of the DMAEMA/water (1/4, m/m) 2

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Fig. 1. Experimental apparatus for column adsorption of gaseous aromatic pollutants.

2.5. PM filtration performance of the modified PP nonwovens

size and its distribution of original PP and PP-g-DMAEMA were analyzed with a capillary flow porometer (POROLUX 1000, Germany). The N2 adsorption-desorption isotherms of original PP and modified PP were obtained on a surface area and porosity analyzer (ASAP 2020 PLUS HD88, America). Universally, surface charge (zeta potential ζ) of nonwoven was evaluated by measuring the streaming potential on the basis of the applied pressure, the SurPASS instrument (Anton Paar GmbH, Austria) was used to investigate the ζ-potential of PP-gDMAEMA and PP-g-DMAEMA/rGO based on the streaming potential and streaming current method within a PH range of 6.5–8.5.

The PM filtration performance of original PP, PP-g-DMAEMA nonwoven, and double function layer PP nonwoven were detected by Automatic Filter Detector (TSI, TSI 8130, America). In this experiment, the dry air velocity was 32 L/min and the indoor relative humidity during the experiences was 42 ± 2%. The PM was simulated by sodium chloride aerosol produced by instruments. Different modified PP nonwovens were measured at setting conditions. 2.6. Safety assessment of DFLPP nonwoven

2.4. Adsorption experiments

It is necessary to consider the factor of safety when applying the polypropylene material with double function layer to air purification. That is to say, we need to explore whether rGO can be loaded firmly on the fiber. If the graphene falls off and enters the body with breathing, it will be harmful to the body. Therefore, as shown in Fig. 2, a simple method was designed. First, purging the double-functional layer polypropylene material with nitrogen continuously at a high flow rate, which was 2 L/min. And then putting the gas into the aqueous solution (V = 50 ml). Finally, whether rGO could be detected in the solution or not was taken as the basis of safety.

Gaseous aromatic pollutants, one typical indoor air pollutant, were adopted to investigate PP-g-DMAEMA/rGO nonwoven and double function layer PP nonwoven adsorption performance. Column adsorption experiments were performed using methods published previously [22], the experimental apparatus had been shown in Fig. 1. The filter with a certain amount of adsorbent was connected to a gas chromatograph (GC, 3420A, Beifen, China) with a flame ionization detector (FID), the VOC concentration at the upstream was controlled by the temperature of water bath, the target compound continuously passed the filter at room temperature with various flow rates controlled by means of a mass flow controller. The inlet and outlet concentration of target compound was detected by GC. Toluene, benzene, and xylene were selected as target VOCs in this study, these compounds could be commonly found in the indoor environment. The properties of these compounds were summarized in Table1. Experiments were performed to develop breakthrough curves for toluene on three nonwovens (PP, PP-g-DMAEMA/rGO and DFLPP) at three different levels of concentration from 40 mg/m3 to 200 mg/m3 and three different types of flows rate from 100 ml/min to 400 ml/min. Similar experiments were also conducted at 200 mg/m3 concentration of benzene and xylene. The adsorption equilibrium and breakthrough curves were obtained when the outlet concentration was similar to the inlet concentration and could be held steady for a while. Each experiment was repeated 3 times and final values were the mean.

3. Results and discussion 3.1. Preparation of double function layer PP nonwoven Polypropylene (PP) non-woven fabric with technical and economical merits had been produced for a wide application of adsorption and filtration. The nonwoven fabric has a unique heterogeneous macroporous structure because of its random overlapping fibers, and has the advantages of low gas resistance, rapid mass-transfer, low cost, and preeminent mechanical strength. Although the polymer displays excellent properties, the absence of adsorption sites and the strong interaction functional groups limit its application. In Fig. 3, the synthesis of PP-g-DMAEMA, PP-g-DMAEMA/rGO and double functional layer PP nonwoven fabric were summarized. A double layer of detachable amination-functional PP nonwoven was obtained after DMAEMA micromolecule was successfully introduced onto the surface of double-layer detachable PP nonwoven via a UV radiation process, and one of them was used as PM filtration layer. Indeed, it was surely beneficial to load GO and capture PM when the potential of PP nonwoven changed from −86.05 mV to 36.81 mV. The dispersed GO in water was negatively charged due to some ionization of carboxyl and hydroxyl functional groups on the GO surface, and the TEM image of Supp. Fig. 1(A) showed the few-layer sheet shape structure of GO. Choosing different graft rate of PP-g-DMAEMA nonwoven could make various kinds of PP-g-DMAEMA/GO with unique

Table 1 Some properties of the selected VOCs. Compound

Category

Formula

Molar mass (g/mol)

Toxicity

Toluene Benzene Xylene

Aromatic Aromatic Aromatic

C7H8 C6H6 C8H10

92.17 78.11 106.17

Low toxicity Carcinogenic Toxic

3

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Fig. 2. Experimental apparatus for safety assessment of DFLPP nonwoven.

performance. Ascorbic acid, a common essential nutrient with exhibiting reducibility properties, was adopted in the chemical reduction process. By controlling the reaction time, PP-g-DMAEMA/rGO with varying degrees of a reduction described by the oxygen content was obtained, as shown in Table 2. The TEM image revealed a wrinkled conformation of rGO platelets was shown in Supp. Fig. 1(B). A simple hot-pressing process was used to fabricate double function layer PP nonwoven and control samples.

Table 2 Relationship between reaction time and oxygen content (RWG% =6.3%). Reaction time (h)

Oxygen content (%)

0 0.5 1 1.5

31 26.29 17.26 13.22

oxide on the surface of PP nonwoven was reduced. To further prove that PP-g-DMAEMA/rGO was obtained successfully, the chemical compositions of original PP nonwoven and functional PP nonwoven were characterized by using XPS (Fig. 5). Compared with original PP nonwoven, two prominent peaks in the C1s Core-level spectra of PP-g-DMAEMA could be observed at 285.8 eV and 288.4 eV, corresponding to CeO and C]O, respectively. Utilizing the peak separation method, the fractions of chemical bonds (CeC, CeO, and C]O) were calculated and shown in Table 3. The fraction of CeC bond, CeO, and C]O for PP-g-DMAEMA/GO nonwoven were 63.30%, 32.01%, and 5.69%, respectively. After the reduction reaction, the fraction of CeO decreased to 10.96%. slight decrease of C]O bond fraction to 2.49%. However, the fraction of C-O and C]O were 21.12% and 4.37% for PP-g-DMAEMA nonwoven, indicating that most of

3.2. Characterization of functionalized PP nonwovens Fig. 4 showed the FITR spectra of the original PP, PP-g-DMAEMA, PP-g-DMAEMA/GO, and PP-g-DMAEMA/rGO. Compared with original PP nonwoven (Fig. 4a), the spectra of DMAEMA grafted PP nonwoven (Fig. 4b) showed the existence of some functional groups, 1727 cm−1 was for C]O vibration peak, 1147 cm−1 was for CeN vibration absorption peak that indicated DMAEMA monomer was grafted into PP nonwoven successfully. After the self-assembly process (Fig. 4c), the broad peak at 3023–3642 cm−1 was corresponding to eCOOH and eOH stretching vibration and 1633–1568 cm−1 assigned to stretch vibration of aromatic C]C. These peaks indicated that the graphene oxide was introduced into the surface of PP nonwoven. The variation of a broad peak at 3023–3642 cm−1 in Fig. 4d indicated that the graphene

Fig. 3. Synthetic route to prepare the DFLPP nonwoven. 4

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Table 3 Fractions of chemical bonds in 3 type of materials. Materials

PP-g-DMAEMA PP-g-DMAEMA/GO PP-g-DMAEMA/rGO

Fractions of chemical bonds (%) C-C

C-O

C=O

74.51 63.30 86.55

21.12 32.01 10.96

4.37 5.69 2.49

nonwovens were prepared by a chemical reduction after PP nonwoven were covered with GO layers via electrostatic interaction. From images in Fig. 6(C, D), the skeletons of PP nonwoven covering with GO and rGO could be observed, respectively. N2 adsorption-desorption isotherms at 77 K and BET surface area of original PP, PP-g-DMAEMA and PP-g-DMAEMA/rGO with various RWG were compared in Fig. 7 and Table4. BET surface area of all nonwovens showed the following order: original PP < PP-g-DMAEMA < 1.9% PP-g-DMAEMA/rGO < 4.0% PP-g-DMAEMA/rGO < 6.3% PP-gDMAEMA/rGO. The BET surface area of the original PP and PP-gDMAEMA was very low and mainly contributed by the surface of each solid fiber. After loading rGO into PP nonwoven, the BET surface area increased sharply from 1.03 m2/g of original PP to 45.36 m2/g of PP-gDMAEMA/rGO, the incremental BET surface area was obviously contributed by the loading rGO on the modified PP nonwoven. The isotherms of PP-g-DMAEMA/rGO show typical type Ⅰ nitrogen adsorptiondesorption isotherms with a steep increase of adsorbed amount at a low relative pressure (P/P0 < 0.1), corresponding to the microporous structure in the PP-g-DMAEMA/rGO. Besides, the hysteresis loop was H4 type, indicating the mesoporous structure existed in the sample. Moreover, the observed hysteresis extended to P/P0 ≈1, which indicated the presence of macropores. Currently, we found that zeta potential might play a crucial role in

Fig. 4. FT-IR spectra of (a) original PP nonwoven, (b) PP-g-DMAEMA, (c) PP-gDMAEMA/GO, (d) PP-g-DMAEMA/rGO.

graphene oxide on the surface of PP nonwoven were reduced. The SEM images of original PP nonwoven and modified PP nonwoven morphology were shown in Fig. 6. The original PP nonwoven (Fig. 6A1 and A2) exhibited a comparatively smooth surface and multiply connected pores, these multiple connected pores could strengthen gas permeability. Compared with original PP nonwoven, the surface of PP-g-DMAEMA nonwoven covered by a layer of polymer (Fig. 6B1 and B2) became coarser and maintained network structures of original PP nonwoven after grafting with DMAEMA. PP-g-DMAEMA/rGO

Fig. 5. (a) C1s spectra of original PP nonwoven, (b) C1s spectra of PP-g-DMAEMA, (c) C1s spectra of PP-g-DMAEMA/GO, (d) C1s spectra of PP-g-DMAEMA/rGO. 5

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Fig. 6. SEM images of (A) original PP nonwoven, (B) PP-g-DMAEMA, (C) PP-g-DMAEMA/GO, (D) PP-g-DMAEMA/rGO.

Fig. 8. Zeta potential of PP, PP-g-DMAEMA and PP-g-DMAEMA/rGO.

Fig. 7. Nitrogen adsorption–desorption isotherms of PP nonwoven, PP-gDMAEMA and PP-g-DMAEMA/rGO.

3.3. Adsorption properties of the PP-g-DMAEMA/rGO nonwovens Table 4 Structural characteristics and properties. Material

Grafting degree (%)

rGO RWG (%)

Surface area (m g−1)

PP PP-g-DMAEMA 1.9% PP-g-DMAEMA/rGO 4.0% PP-g-DMAEMA/rGO 6.3% PP-g-DMAEMA/rGO

– 42.9 43.4 43.6 44.6

– – 1.9% 4.0% 6.3%

1.03 3.13 19.3 39.99 45.36

In our previous work, a type of porous microsphere self-assembled nonwoven fabric (PM/PP) was prepared, which owned excellent adsorption capacity for styrene because of π-π interactions and porous structure [22]. We have successfully prepared PP-g-DMAEMA/rGO adsorption materials, due to rGO on the surface of PP nonwoven, it was expected to be potential adsorbents for gaseous aromatic pollutants. To investigate the adsorption behavior of gaseous aromatic pollutants, toluene was used as a typical example of gaseous aromatic pollutants to determine the capture performance of PP-g-DMAEMA/rGO nonwoven. The equilibrium adsorption capacity was obtained by calculating the area above the breakthrough curve. The effect of the reduce time of PP-g-DMAEMA/GO on toluene sorption (Fig. 9) was studied while maintaining the flow rate constant at 300 ml/min and inlet concentration containing 200 mg/m3 during the operation. The completely reduced PP-g-DMAEMA/GO exhibited preferable adsorption capacity than the other two types of PP-gDMAEMA/rGO, which indicated the morphology of aromatic ring in graphene affected the toluene adsorption capacity. With the increasing of reduce time, oxygen functional groups were more likely to be replaced, there were more adsorption sites occurring on the surface of PP nonwoven fabrics. The effect of rGO content of PP nonwoven surface of toluene sorption (Fig. 10) was examined, it was found that the breakthrough time of original PP and PP-g-DMAEMA nonwovens happened just 1.5 and 3.9 min, respectively, after start of toluene exposure, indicating that the original PP and PP-g-DMAEMA nonwoven had not enough effective

2

filtering PM through experiments. The Zeta potential was tested in solutions at weakly acidic and weakly alkaline to analyze the surface charge. It could be expected that the grafting DMAEMA micromolecule and coating rGO would change the surface charge of the PP nonwoven (Fig. 8). After grafting polymerization, the surface charge became more positive, approximately −90 mV (at PH-7) to 38 mV (PH-7). This phenomenon of positive charge could be ascribed to the ionization of tertiary amine groups in response to pH changes. Hence, the self-assembly could be implemented because the surface of grafted nonwoven fabric was charged positively and graphene oxide presented electronegativity. Furthermore, PP nonwovens with the positive charged surface could effectively capture the PM with negative charged and molecules. Because of the presence of rGO, the zeta potential value of PP nonwoven decreased to 10 mV (PH-7), approximately.

6

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Fig. 9. Breakthrough curves of gaseous toluene on PP-g-DMAEMA/GO adsorbent at different reduction degree of GO (T = 298 K, Flow rate = 300 ml/ min, C0 = 200 mg/m3, RWG = 6.3%).

Fig. 11. Breakthrough curves of gaseous toluene on PP-g-DMAEMA/rGO adsorbent at different initial concentrations (T = 298 K, Flow rate = 300 ml/min, RWG = 6.3%).

Fig. 10. Breakthrough curves of gaseous toluene on PP-g-DMAEMA/rGO adsorbent with different RWG of rGO (T = 298 K, Flow rate = 300 ml/min, C0 = 200 mg/m3).

Fig. 12. Breakthrough curves of gaseous toluene on PP-g-DMAEMA/rGO adsorbent at different flow rates (T = 298 K, C0 = 200 mg/m3, RWG = 6.3%).

with the inlet concentration containing 200 mg/m3. Fig. 10 showed the breakthrough curve, a faster break time was obtained with a higher flow rate. The larger influent volume of gaseous toluene passing through the filter per minute resulted in the shorter contact time of the organic molecules with the PP-g-DMAEMA/rGO nonwoven and increased adsorption capacity. The higher adsorption capacity maybe explained by an increase in driving force to the mass transfer within the multiple interconnect porously, as well as to the interaction between the toluene and the PP-g-DMAEMA/rGO nonwoven. We explored the adsorption performance of PP-g-DMAEMA/rGO nonwoven for other aromatic gas pollutants with toxicity and carcinogenicity. Xylene was widely used in plastic processing and paint application which might influence the respiratory tract and central nervous system [23]. Benzene, a known human carcinogen generated from paint, and petrochemical industries, was harmful to human health [24]. The removal of these aromatic gas pollutants by PP-g-DMAEMA/rGO nonwoven was also compared with original PP and PP-g-DMAEMA, Adsorption ability of original PP and modified PP nonwoven to 3 types of aromatic gas pollutants was shown in Fig. 13. PP-g-DMAEMA/rGO nonwoven showed excellent adsorption performance of xylene and

adsorption sites to capture toluene molecule. For the PP-g-DMAEMA/ rGO nonwovens, the introduction of rGO significantly increased toluene adsorption, it could be seen that 6.3% PP-g-DMAEMA/rGO nonwoven obtained the largest adsorption amount (approximately 51.2 mg/g), which was 76 times greater than that of original PP. In addition, the 1.9% and 4.0% PP-g-DMAEMA/rGO nonwoven obtained adsorption amounts lower than 6.3% PP-g-DMAEMA/rGO nonwoven because of fewer adsorption site and smaller surface area of the former. The effect of inlet concentration of toluene sorption of PP-gDMAEMA/rGO nonwoven (Fig. 11) was researched while maintaining the flow rate constant at 300 ml/min during the operation. The effect of inlet concentration of toluene in the adsorption performance of PP-gDMAEMA/rGO nonwoven was shown in Fig. 9, It was obvious that with the increasing of inlet concentrations, the breakthrough time decreased, the breakthrough curves became steep. As the inlet concentration increasing, the number of adsorption sites were increasing in per unit time, the breakthrough time decreased, and the adsorption capacity increased. The effect of flow rate ranged from 100 to 400 ml/min on toluene sorption (Fig. 12) was examined using PP-g-DMAEMA/rGO nonwoven 7

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Fig. 13. Adsorbing capacity of benzene, toluene and xylene on PP, PP-gDMAEMA and 6.3%PP-g-DMAEMA/rGO (T = 298 K, C0 = 200 mg/m3, flow rate = 300 ml/min). Fig. 15. filtration performance and adsorption capacity of original PP, PP-gDMAEMA, PP-g-DMAEMA/rGO and DFLPP nonwoven (RWG = 6.3%).

Fig. 14. (a) Pore diameter distribution of original PP and modified PP nonwovens; (b) Changes of main pore size and thickness of PP-g-DMAEMA nonwoven at various grafting ratios; (c) Pressure drop versus face velocity of the original PP and modified PP nonwoven; (d) PM filtration performance of PP-g-DMAEMA with different grafting ratio. 8

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Table 5 Comparison of gaseous aromatic pollutants adsorption performance and PM filtration efficiency of various material from the literature. Materials

Filtration efficiency (%)

CS/PVA membrane βCD/PVA membranes PEI–SiO2 fibrous membrane HNTs@CM250 F-Z15 PVP/NaY zeolite composite fibers Cellulose foam HKUST-1 28.2% PM/PP nonwoven DFLPP nonwoven

95.59 99.00 99.99 96.77 93.71 ± 1.03

Not mentioned 72.20

Pressure drop (Pa)

Face velocity

633.5 45.0 61.0 143.9 109.2 ± 2.4 Not mentioned Not mentioned 63.4 58.0

5.3 cm s 0.3 L/min 32 L/min 5.3 cm s−1 4.0 cm s−1

32.0 L/min 32.0 L/min

Zeta potential (mV, PH = 7)

Filtration efficiency (%)

PP PP-g-DMAEMA/rGO PP-g-DMAEMA

−90 mV 10 mV 38 mV

59.8 65 76.4

Not Not Not Not Not

mentioned mentioned mentioned mentioned mentioned 667.0 416.47 52.8 39.1

Ref. [25] [26] [27] [28] [13] [29] [30] [22] This work

diameter simultaneously. In consequence of the difference of diameter sizes and random distribution of fibers, the mean pore size with various DMAEMA content also exhibited significant differences. To ensure the veracity of the experimental results, all experiments were repeated 3 times and averaged. As shown in Fig. 14a and b, the mean pore size with an increase in DMAEMA content decreased steadily. This was because that the action of chemical reagents and heat caused the fibers to move and the pore size to become smaller during irradiation, furthermore, the thickening of fibers was another factor leading to the decrease of pore size. After grafting polymerization, the mean pore size becomes smaller, approximately 50.8 μm (original PP) to 20.8 μm (DMAEMA content 58.8%), additionally, the minor pore size decreased and even disappeared gradually when the DMAEMA content was up to 58.8 percent. Moreover, the PP-g-DMAEMA nonwoven grew ever thicker with an increase in DMAEMA content. As can be seen in Fig. 14c, the slope of the PP-g-DMAEMA nonwoven was 15.5524, larger than that of original PP nonwoven (11.1784), which indicated that the pressure drop of modified PP nonwoven was moderately higher than that of original PP nonwoven. This can be attributed to the reduction of mean pore size after grafting polymerization. It was well accepted that the grafting ratio of PP nonwoven had a significant effect on the filtration efficiency and pressure drop. Thus, the influence of the grafting ratio of PP nonwoven was further explored. As shown in Fig. 14d, the filtration efficiency with various DMAEMA content firstly increased with the adding of DMAEMA content, at the same time, there was no obvious change in pressure drop, which was

Table 6 Relationship between Zeta potential and filtration efficiency. Material

Gaseous aromatic pollutants adsorbing capacity (mg/g)

−1

benzene, in stark contrast to the original PP and PP-g-DMAEMA nonwoven. There were few adsorption sites on the original PP and PP-gDMAEMA nonwoven surface limited the adsorption of xylene and benzene.

3.4. PM filtration performance of PP-g-DMAEMA nonwoven. Pore diameter distributions were a crucial parameter for the filtration performance of PP nonwoven fabric. Nonwoven fabric applications in filtration commonly required appropriate air permeability property. For instance, original PP filter material must be introduced functional groups with strong electrical properties to preferably capture PM, meanwhile, the modified PP nonwovens should be supposed to maintain suitable pore size distribution to present low gas resistance. Chemical methods could be applied in order to enhance the filtration efficiency of materials, for example, one useful chemical approach was ultraviolet radiation. The DMAEMA grafted layers on PP fiber usually provided a functional group with a positive charge and enhanced fiber

Fig. 16. Schematic illustration air purification process. 9

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due to that the change of electrical properties on the fiber surface was easier to capture PM with little variation in pore size distribution. However, with DMAEMA content increasing continuously, the main pore size decreased obviously, which increased the gas resistance of the filter material. What is more, filtration efficiency began to decline when DMAEMA content was greater than 23.3 percent. This result might be explained that the airflow passed through the surface of the filter material at the same surface velocity, the local velocity would increase when passing through the channel, which caused the slightly decreased filtration efficiency since it lowered the chance for particles to collide with PP-g-DMAEMA nonwoven.

was not falling off the nonwoven surface. We also analyzed the changes in mass of the DFLPP nonwoven before and after testing shown in Table S1, minimum graphene had fallen off the nonwovens during the stability tests, in other words, the risk for the user from falling graphene in the course of actual use of DFLPP nonwoven was small. 4. Conclusion In summary, the dual-functionalized layer PP nonwoven containing positively charged functional groups and abundant of aromatic organic molecular adsorption sites were synthesized successfully by following steps: Firstly, PP-g-DMAEMA and PP-g-DMAEMA/rGO nonwoven were prepared through a step-by-step modification method including grafting polymerization, self-assembly, and chemical reduction process. Then, two types of functionalized nonwoven were consolidated using a hot press technology. The PP nonwoven showed higher filtration efficiency after grafting polymerization process, results from Zeta potential and filtration efficiency revealed that the filtration efficiency was related to the strength of Zeta potential. Moreover, in the adsorption experiment, the reduction degree of GO had a great influence on the adsorption capacity of gaseous aromatic pollutants. Compared with single functionalized PP nonwoven, the dual-functionalized layer PP nonwoven presented superior adsorption, filtration, and safety performance, and had great potential in an application for air purification and individual protection.

3.5. Evaluation of filtration and adsorption performance of DFLPP Obviously, the filtration properties of PP-g-DMAEMA and PP-gDMAEMA/rGO nonwovens were distinguished from the original PP. It was clearly seen that both the filtration efficiency and pressure drop of the functionalized PP nonwovens were higher than that of original PP (Fig. 15). Particularly, the PP-g-DMAEMA nonwoven presented the best filtration performance compared with the original PP and PP-gDMAEMA/rGO nonwoven with the same basis weight of 25 g/m2, revealing that DMAEMA micromolecule on the nonwoven fabric surface presented positive electricity. Similarly, the PM filtration performance of dual-functionalized layer PP nonwoven was distinctly superior to original PP and PP-g-DMAEMA/rGO nonwovens, this furtherly implied that DMAEMA played an important role in capturing PM. It could be seen in Supp. Fig. 2 that after filtering PM, the fiber surface changed significantly. After filtering for 40 min, various components (solid particles, harmful gases) in the flue gas accumulated on the fiber surface. The DFLPP nonwoven possessed PM filtration and gaseous aromatic pollutants adsorption performance compared with original PP and PPg-DMAEMA nonwoven, furthermore, the DFLPP nonwoven showed a little bit higher filtration efficiency than PP-g-DMAEMA/rGO nonwoven, this could be interpreted as the amino-functionalized layer replaced one part of PP-g-DMAEMA/rGO nonwoven to improve the filtration performance and simultaneously weaken adsorption performance. Table 5 presented a comparison between the DFLPP nonwoven and other materials in the literature, including membranes [25–28], hierarchical composite material [13], composite fibers [29], cellulose [30] and nonwoven [22]. It could be observed that the DFLPP nonwoven possessed bifunctional individual protection performance. To comprehend the role of surface potential played in influencing the filtration properties of PP nonwoven, the Zeta potential of functionalized PP and original PP nonwovens were studied (Fig. 8). The result of filtration efficiency and the Zeta potential value for three types of filter materials were listed in Table 6. As the Zeta potential was increasing, the filtration efficiency became higher. The air purification process was shown in Fig. 16. DMAEMA molecules on the PP nonwoven changed the charge characteristics of the fibers. Graphene, an allotrope of carbon consisting of a monolayer of sp2-hybridized conjugated carbon atoms with its intrinsic merits outstanding adsorptive property [31]. The purification process could be divided into two steps, In the filtration step, PM was polarized by a positively charged functional group on the amino-functional layer, followed by toluene adsorption and PM filtration step including toluene molecules and PM, which were captured via π-π interaction and electrostatic interaction, respectively.

CRediT authorship contribution statement Xiaolong Tang: Investigation, Methodology, Writing-original draft, Formal analysis, Data curation. Junfu Wei: Supervision, Resources, Project administration. Zhiyun Kong: Conceptualization, Funding acquisition. Huan Zhang: Resources, Project administration. Jian Tian: Methodology. Acknowledgements This research was supported by the National Natural Science Foundation of China (51678409 and 51708406), the Program for Innovative Research Team in University of Tianjin (No. TD13-5042). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145631. References [1] Y. Chen, S. Zhang, S. Cao, S. Li, F. Chen, S. Yuan, C. Xu, J. Zhou, X. Feng, X. Ma, B. Wang, Roll-to-roll production of metal-organic framework coatings for particulate matter removal, Adv. Mater. 29 (2017). [2] H. Tian, X. Fu, M. Zheng, Y. Wang, Y. Li, A. Xiang, W.-H. Zhong, Natural polypeptides treat pollution complex: moisture-resistant multi-functional protein nanofabrics for sustainable air filtration, Nano Res. 11 (2018) 4265–4277. [3] X. Wang, C. Ma, J. Xiao, Q. Xia, J. Wu, Z. Li, Benzene/toluene/water vapor adsorption and selectivity of novel C-PDA adsorbents with high uptakes of benzene and toluene, Chem. Eng. J. 335 (2018) 970–978. [4] X. Zhang, Y. Yang, L. Song, J. Chen, Y. Yang, Y. Wang, Enhanced adsorption performance of gaseous toluene on defective UiO-66 metal organic framework: equilibrium and kinetic studies, J. Hazard. Mater. 365 (2019) 597–605. [5] A.T. Poortinga, C.J.M. van Rijn, Gas-shell-encapsulation of activated carbon to reduce fouling and increase the efficacy of volatile organic compound removal, Colloid Interface Sci. Commun. 18 (2017) 1–4. [6] J. Zhu, P. Zhang, Y. Wang, K. Wen, X. Su, R. Zhu, H. He, Y. Xi, Effect of acid activation of palygorskite on their toluene adsorption behaviors, Appl. Clay Sci. 159 (2018) 60–67. [7] S.W.L. Ng, G. Yilmaz, W.L. Ong, G.W. Ho, One-step activation towards spontaneous etching of hollow and hierarchical porous carbon nanospheres for enhanced pollutant adsorption and energy storage, Appl. Catal. B 220 (2018) 533–541. [8] J. Yang, W. Xu, C. He, Y. Huang, Z. Zhang, Y. Wang, L. Hu, D. Xia, D. Shu, One-step synthesis of silicon carbide foams supported hierarchical porous sludge-derived activated carbon as efficient odor gas adsorbent, J. Hazard. Mater. 344 (2018)

3.6. Stability measurements of rGO on DFLPP nonwoven surface The potential stability of DFLPP for filtration/adsorption process is also crucial for practical application in individual protection material. As shown in Supp. Fig. 4, the comparability of UV–visible spectra of the samples treated with N2 at various time, the absorbance had no significant changes with an increase in the purge time, suggesting that rGO 10

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