Environ Eng Res > Volume 24(4); 2019 > Article
Patil, Jeong, Lim, Byun, and Han: Removal of volatile organic compounds from air using activated carbon impregnated cellulose acetate electrospun mats

### Abstract

Volatile organic compounds (VOCs) are released from various sources and are unsafe for human health. Porous materials are promising candidates for the adsorption of VOCs owing to their increased ratio of surface area to volume. In this study, activated carbon (AC) impregnated cellulose acetate (CA) electrospun mats were synthesized using electrospinning for the removal of VOCs from the air mixture of ACs, and CA solution was electrospun at different proportions (5%, 10%, and 15%) in a single nozzle system. The different AC amounts in the electrospun mats were distributed within the AC fibers. The adsorption capacities were measured for acetone, benzene, and dichloromethane, using quartz crystal microbalance. The results elicited an increasing adsorption capacity trend as a function of the impregnation of ACs in the electrospun mats, while their capacities increased as a function of the AC concentration. Dichloromethane resulted in a faster adsorption process than acetone and benzene owing to its smaller molecular size. VOCs were desorbed with the N2 gas purging, while VOCs were adsorbed at higher temperatures owing to the increased vapor pressures. The adsorption analysis using Dubinin-Astakhov equation showed that dichloromethane is more strongly adsorbed on mats.

### 1. Introduction

Volatile organic compounds (VOCs) are gaseous compounds associated with high vapor pressures at room temperature. Chemical processing industries involved with the manufacturing, handling, and the distribution of paints, lubricants, and liquid fuels, constitute the primary sources of VOCs [1]. These entities often leak from pipes, heat exchange systems, and processing vessel vents, and cause loading losses in storage tanks. VOCs are not only present in air but they are also the product of volatilization of building materials, detergents, pesticides, and cosmetics [2], which consist of liquid fuels, cleaning agents, lubricants, solvent thinners, and degreasers. Prolonged exposure to VOCs can potentially affect human health based on their effects on the central nervous system and liver, and can potentially lead to harmful and irritating effects on the eyes and throat [3].
The porous materials with increased internal surface areas up to 7,000 m2/g and with very small pores [4] are potential candidates for sensing and gas storage applications, separation, and catalysis [5]. Porous materials, such as zeolite, activated carbon (AC), and metal-organic framework (MOF) [4], have been used for the removal of VOCs in membrane processes, adsorption, and chromatographic separation. Das et al. [5] concluded that AC fibers are potential adsorbents for the removal of VOCs from the air, Chiang et al. [6] studied the effects of pore structure and temperature on the adsorption of VOCs on AC. Khan and Ghoshal [3] studied the VOC removal by zeolites from polluted air. In turn, Kim and Ahn [7] studied the adsorption properties of zeolites, like benzene, toluene, xylene, and methanol, using microwave heating (for desorption). Additionally, Yang et al. [8] reported the effects of the molecular size and the shape on the adsorption of gaseous VOCs by MOF MIL-101.
Electrospinning is a type of process that produces nanofibers using electrostatic force. Nanofibers produced by electrospinning possess a high-surface area owing to their interconnected structures. Polymer solution charged with a high-voltage power is ejected from the tip of the needle to a collector at a controlled flow rate when a repulsive electric force overcomes the surface tension. The solution jet is converted to a solid fiber while travelling from the needle to the collector [9, 10]. Accordingly, fiber morphology can be influenced by many parameters relevant to solution properties (viscosity, elasticity, conductivity, and surface tension) [11], process parameters (electric potential, flow rate, concentration, and the distance between the capillary and collector), and ambient parameters (temperature, humidity, and air velocity). The electrospinning techniques have been applied to produce adsorbents for VOC removal. Shim et al. [12] found that electrospun polyacrylonitrile (PAN) mats were effective for the removal of VOCs owing to their shallow pore structures [13]. However, Scholten et al. [14] demonstrated that polyurethane solutions can be electrospun to produce fiber mats to adsorb acetone and toluene depending on the building blocks used in fiber synthesis with high-adsorption capacities. Feng et al. [11] studied chloroform removal by membrane gas stripping using electrospun polyvinylidene fluoride (PVDF) nanofiber membranes. Additionally, electrospun mats were used for other separation processes, such as thermoregulated gas transport [15], particulate matter filtration, and seawater desalination [16], which mainly depends on the large surface area of the electrospun mats. In general, as the specific area increased, the separation properties improved.
Condensation, thermal oxidation, catalytic oxidation, absorption, adsorption, and many other techniques are used to control the concentration of VOCs in the air [5, 17]. Among these, adsorption has been used during the past several years in bulk separation or purification processes, and was found to be effective at low-concentration levels (in ppm) [18]. Zhou et al. enhanced the VOC adsorption capacity using porous composites. They prepared ZIF-8 and graphene oxide composites to study the adsorption of VOCs. They showed an enhancement in the adsorption capacity following increases of graphene oxide content [19]. In other studies, Liu et al. investigated the adsorption of trichloroethylene and benzene vapors onto hyper crosslinked polymeric resins [20]. Goss and Eisenreich studied the adsorption of VOCs from the gas phase onto various minerals and mineral mixtures [21].
In the present work, we used AC powders with a specific surface area up to 2,000 m2/g to increase the adsorption capacities of electrospun fiber mats. A solution mixture of cellulose acetate (CA) and AC powder was electrospun to produce CA fiber mats impregnated with AC particles. Given that quartz crystal microbalance (QCM) has elicited significant improvements for online and in situ detection of VOCs in comparison to conventional detection techniques, including gas chromatography, mass spectrometry, and Fourier transform infrared spectroscopy [22], the adsorption performance of the mats was evaluated by the QCM system which can measure the real time data of the adsorption and desorption of VOCs. In this study, acetone, benzene, and dichloromethane, were used as model VOCs. These compounds are extensively used in manufacturing at varying temperatures and concentrations, coating, painting, and electronic industries [23], to evaluate the applicability of the CA electrospun mats with AC for the removal of VOCs. Most previous research studies using electrospun mats with AC have focused on energy applications, such as supercapacitors, solar cell, batteries, etc. [2426]. These techniques require intensive energy for carbonization in electrospun polymer fibers to produce carbon nanofibers, while the fabrication technique proposed in this study can be simply applied to produce the AC-impregnated mats without carbonization. To our knowledge, this is the first demonstration of AC-powdered, impregnated, electrospun mats, using a mixture solution of AC powder and CA that also enhances their applicability in human mask and filter development.

### 2.1. Materials

CA (Mn = 30,000 g/mol), AC (100 mesh particle sizes), and N, N-dimethyl acetamide, were purchased from Sigma Aldrich (USA). Acetone and N, N-dimethyl acetamide (DMAC) were purchased from Samchun Chemicals (Republic of Korea). Additionally, AC and CA were used in polymer solutions at different proportions of AC. CA and AC were dissolved in a solvent of acetone (67%) and dimethyl acetamide (DMAC) (33%).

### 2.2. Preparation of Polymer Solution and Synthesis of Fiber Mats Using Electrospinning

AC was prepared in compositions of 5%, 10%, and 15% with respect to the proportion of CA. Specifically, AC (0.1 g, 0.2 g, 0.3 g) and CA (1.9 g, 1.8 g, 1.7 g) were respectively dissolved in a combined solution of acetone (7.6 g) and DMAC (3.8 g). The prepared solution was added in a syringe to draw fibers. The fibers were collected on an aluminium foil collector which was placed at a distance of 210 mm near the needle tip. The applied voltage was 24.1 kV and the flow rate was controlled at 0.02 mL/min with the use of a syringe pump (Pump 11 Elite, Harvard Apparatus) up to 6 h. At the end of electrospinning, the electrospun mat was kept in a vacuum oven at 140°C for 3 h for removing the solvents and moisture. Fig. S1 shows the color changes after the impregnation of AC. Polystyrene (PS, 1 g) was dissolved to produce 20 wt% of dimethylformamide (DMF) solution during stirring at 50°C for 2 h. Similarly, a PS electrospun mat was produced by applying the voltage of 30 kV with a flow rate of 0.02 mL/min.

### 2.3. Measurement of VOCs Adsorption Capacity Using QCM

QCM was used to detect micro changes in the physical properties of the thin layers deposited on the crystal surface. A thin quartz crystal plate with metal electrodes deposited on the plate was the active element of the QCM. The voltage applied between the electrodes resulted in shear deformation within the quartz crystal owing to the piezoelectric properties and crystalline orientation of the quartz. QCM is useful in measuring the gas solubility in polymers and has a reversible nature in real time monitoring [27]. Furthermore, the reusability of the piezoelectric crystal is also one of the most important benefits of QCM. Sauerbery’s equation provides a linear relationship between the resonant frequency shift of QCM and the mass attached on the electrode surface [28].

### 2.4. Measurement of Surface Area Using BET

Surface structure characteristics of the electrospun mats with ACs were determined by the Brunauer-Emmett-Teller (BET) method using the BELSORP-Mini-II BET equipment (M/s Microtrac BEL Co., Japan). Samples were pretreated at 140°C for 2 h in vacuum to remove moisture and impurities. The specific surface area of BET was measured using BET plots using liquid nitrogen. Nitrogen gas was injected into a sample and a reference chamber to calculate the relative pressures during the monolayer deposition on the samples.

### 2.5. Measurement of the AC Amount Using TGA

Thermogravimetric analysis was carried out by a TGA instrument (N-1000 Scinco Co., Ltd.) to identify the composition of AC in the CA fiber membranes after the electrospinning process. The prepared mats were cut into very tiny pieces to fit into the measuring pan, and equal samples were measured for each proportion to ensure the accuracy of the results. Temperature gravimetric analyses of the samples were conducted in the presence of nitrogen at the flow rate of 50 cm3 min−1. The temperature was set at 600°C with the heating rate of 20°C min−1 and the sample was held at 600°C for about an hour.

### 3. Results and Discussion

To justify the AC impregnation in the electrospun mats, the surface areas were measured using the BET method where the adsorbed N2 monolayer was accounted to estimate the surface area and the pore volume [29]. Table 1 shows the characteristic analysis of the electrospun mats at different AC percentages. As the table results show, the specific surface area increases as the AC percentages increases in the CA electrospun mat. The mats with AC with a composition of 15% yielded the highest surface area, total pore volume, and the specifically adsorbed amount owing to the highest AC percentage, thereby resulting in the maximum adsorption capacities in the AC 15% mats. Scanning electron microscopy (SEM) images were acquired to verify whether the ACs were dispersed within the fibers or whether they were combined inside the fiber materials. As shown in Fig. 4, the ACs were formed into droplet-like shape structures and were dispersed within the fiber structures, thereby leading to enhanced adsorption properties owing to the increased interaction between VOCs and ACs compared to the case where ACs were impregnated in the fiber.
It is very important to predict the interaction between the solid adsorbent and the gaseous adsorbate for the analysis and design of the adsorption and separation in real life applications. Hence, it is vital to examine the experimental data with the appropriate equation. The adsorption data can be linearized by the Dubinin-Astakhov (DA) equation to overcome the problem faced in the characterization of the material. The DA equation includes additional parameters, which can linearize the adsorption data for a broad range of materials [30]. To explain the physical adsorption of gases on microporous materials, the DA equation was extensively used as follows
##### (1)
$a=ao exp [-(A/E)n]$
where a is the adsorption capacity, ao is the limiting adsorption capacity, and A is the Polanyi adsorption potential, which is described by the following equation
##### (2)
$A=RTln (Po/P)$
where R is the universal gas constant, T is the equilibrium temperature, E is the characteristic energy of the adsorbent-adsorbate system, Po and P are the saturation vapor and equilibrium pressures, respectively, and n is the Astakhov’s exponent [4]. By plotting ln(a) vs. ln(Po/P)n we can obtain the results listed in the Table 2.

### Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (#NRF-2017R1C1B1002851) and a cooperative R&D between Industry, Academy, and the Research Institute, funded by the Korean Ministry of SMEs and Startups in 2018 (Grant No. S2606715).

#### Nomenclature

a

a

A

E

n

Astakhov’s exponent

P

Equilibrium pressure

Po

Saturation vapor pressure

R

Universal gas constant

T

Equilibrium temperature

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##### Fig. 1
VOCs measurement system using QCM.
##### Fig. 2
Adsorption of acetone, benzene, and dichloromethane on AC with a composition of 15%.
##### Fig. 3
Adsorption capacities on electrospun mats as a function of the AC composition for (a) acetone, (b) benzene, and (c) dichloromethane. Error bars correspond to the minimum and maximum values (n = 3).
##### Fig. 4
SEM images for electrospun mats for an AC composition of 10%. (a) CA electrospun mat at 100× magnification, and (b) CA electrospun mat at a 1,000× magnification.
##### Fig. 5
Desorption behaviors of acetone, benzene, and dichloromethane by N2 gas purging (a) following a temperature increase of 40°C, (b) for a mat with an AC composition of 5%.
##### Table 1
Surface Area, Pore Volume, and Adsorbed Amount of Liquid N2 for Electrospun Mats at Different AC Compositions
% of AC Specific surface area, m2/g Specific amount adsorbed, cm3/g
0 14.23 12.765
5 18.73 15.434
10 19.62 16.231
15 21.57 17.945
##### Table 2
Characteristic Energies and Limiting Adsorption Capacities Obtained by Fitting Based on the DA Equation
Acetone 0 2.88 75 68 5.14
5 2.88 283 290 5.72
10 2.88 403 406 6.09
15 2.88 406 408 7.39

Benzene 0 2.88 165 165 4.25
5 2.88 255 255 4.35
10 2.88 350 335 4.37
15 2.88 352 350 4.41

Dichloromethane 0 2.88 245 258 7.47
5 2.88 391 405 8.63
10 2.88 431 435 8.28
15 2.88 534 541 11.28
##### Table 3
TGA Analysis for AC/CA Samples (5, 10, and 15%) Samples
AC/CA samples Sample weight with impurities (mg) Dry sample weight without impurities (mg) AC weight after CA degradation (mg) % of the AC in electrospun mat
5% 17.04 15.75 0.77 4.60
10% 17.18 16.31 2.05 11.15
15% 17.49 16.96 3.09 15.40
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