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Original article
13 (
1
); 1741-1751
doi:
10.1016/j.arabjc.2018.01.011

Air pollution control: The evaluation of TerphApm@MWCNTs as a novel heterogeneous sorbent for benzene removal from air by solid phase gas extraction

, ,
a School of Public Health, Shahroud University of Medical Sciences, Shahroud, Iran
b Research Institute of Petroleum Industry, West Entrance Blvd., Olympic Village, P.O. Box: 14857-33111, Tehran, Iran
c Department of Inorganic Chemistry, Faculty of Chemistry, Tabriz University, Tabriz, Iran

⁎Corresponding author at: Department of Chemistry, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. hamidshirkhanloo@yahoo.ca (Hamid Shirkhanloo), hamidshirkhanloo@gmail.com (Hamid Shirkhanloo),

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Abstract

  • Solid phase gas extraction method (SPGE) used for benzene removal from air.

  • The TerphApm@MWCNTs were synthesized as a novel heterogeneous sorbent.

  • Benzene volatile was absorbed on the TerphApm@MWCNTs based on SPGE method.

Abstract

Benzene is one of the most harmful VOCs pollutions which its control emission is a vital purpose in industries and environment. A new and efficient method based on solid phase gas extraction (SPGE) with Schiff base immobilized on MWCNTs (TerphApm@MWCNTs) as a novel sorbent was used for benzene removal from artificial air after loaded in Robson quartz tubes (RGT). The TerphApm@MWCNTs were synthesized and purified via heat treatment at 60 °C as a new heterogeneous sorbent. In bench scale set up, a system was designed and developed for standard gas generation of benzene in air with difference concentrations, and then was passed through RGT by SKC pump at optimized flow rate. Benzene volatile was absorbed on the TerphApm@MWCNTs and desorbed from it at 80 °C before determined by GC/FID. The important factors such as, temperature, humidity, benzene concentration, amount of adsorbent and flow rate were optimized. The recovery and capacity of benzene capture with 20 mg of TerphApm@MWCNTs were obtained 98% and 492 mg g−1 (25 °C). The flow rate and temperature had highly impact on the removal efficiency and adsorption capacity. According to results, the unique characterization of TerphApm@MWCNTs can be considered as a best adsorbent for benzene removal from air as compare to MWCNTs, SWCNTs, and NAC.

Keywords

Benzene
Adsorption
Air pollutant
Schiff base immobilized on MWCNTs
Solid phase gas extraction
1

1 Introduction

Volatile organic compounds (VOCs) are one of the most important environmental pollutants that cause adverse effects on human health and environment, even at very low concentrations (Uria-Tellaetxe et al., 2016). Recently, a large amounts of the volatile organic compounds from process industries such as oil refining, paint, pharmaceuticals, solvent in manufacturing, transportation and exhaust vehicles can be released into the atmosphere (Mohammed et al., 2015). It has been well documented that many of these compounds, such as benzene, are well-known to cause leukemia, carcinogenic and mutagenic in human body (Chen et al., 2015). Benzene is the most important risk factor in workplace air and caused the acute myeloid leukemia in humans (Carlos-Wallace et al., 2016). The international agency for research on cancer (IARC, USA) has classified benzene as A1 group (carcinogenic effect) which can cause many diseases in human such as; lymphoma, aplastic anemia and bone marrow damage (Pearce et al., 2015; Stenehjem et al., 2015). Benzene has been widely used in many petrochemical industries as a solvent in raw product and by-product (Mohammed et al., 2015). The human exposure to benzene caused to consider a toxic substance which was strictly monitored around the world (Zhao et al., 2015). As a consequence, many government regulations have been established to reduce occupational and environmental exposure to benzene and other VOCs (Barbieri et al., 2008). In conjunction with, the implementation of high-performance methods to removal and control such emissions are inevitable in industries and environment (Romero-Anaya et al., 2010; Shih and Li, 2008). The different methods, including adsorption (physical and chemical), condensation (low temperature and dew point), thermal oxidation, catalytic oxidation, photo catalytic oxidation and bio filtration are used in reduction of VOCs like benzene emissions in air (Karimnezhad et al., 2014; Bouazza et al., 2008; Hort et al., 2009; Shahidi et al., 2015) which are able to concurrently compete in a market characterized by low cost, efficiency, and incremental innovation due to operating condition by efficient methods (Bouazza et al., 2008; Hort et al., 2009). Hence, the applicable methods can be used to absorb gases and vapors with low concentration, low operating costs, environmentally friendly and appropriate cost effectiveness (Ruan et al., 2014). The solid adsorbents such as zeolites, silica gel and activated carbons (ACs) which trapped adsorbate materials (BTEX) on the surface are widely used (Hu et al., 2009). Among the different adsorbents, carbon adsorbents such as; graphene oxide, activated carbons, carbon nanotubes and porous carbon have demonstrated more advantage due to their low density, chemical stability, variety of structural forms, high capacity, desire chemical structure, and texture properties (Silvestre-Albero et al., 2010; Seredych and Bandosz, 2010; Dhaouadi et al., 2010; Wang et al., 2010). The adsorption process using activated carbon to remove organic contaminants in the gas phase has been widely studies (Zhang et al., 2011) and more information about chemical and physical propertied of activated carbon and carbon nanotubes was introduced and compared in electronic supplementary material (ESM, Text S1) (Ahmaruzzaman, 2008, Xiang et al., 2010; Sone et al., 2008; Ren et al., 2011; Pan and Xing, 2008; Agnihotri et al., 2005; Hussain et al., 2008; Chen et al., 2009a; Lin et al., 2009; Gaur and Shim, 2008; Fu and Wang, 2011; Chen et al., 2008; Chen et al., 2009b; Goering et al., 2008; Hyung and Kim, 2008). In gas phase, a few studies showed that the carbon nanotubes as a favorite sorbent absorbed benzene vapor from environmental and workplace air as a pollution removal technique. In addition, the adsorption of BTXs (benzene, toluene and xylenes) on carbon structures including SWCNT, MWCNT (2 types), nanoporous carbon, carbon nanofibers, double-walled carbon nanotubes and activated carbon were studied (Jahangiri et al., 2011). In this study, the structure and surface properties of the carbons were examined, however, the important adsorption parameters such as flow rate, initial concentration, temperature and humidity were assumed constant (Jahangiri et al., 2011). Over the last two decades, the multi-wall carbon nanotubes (MWCNTs) have been extensively studied in different applications of nanotechnology due to their extraordinary properties and wide variety of applications such as mechanical, electrical, optical and sensoring (Popov, 2004). The surface modification of MWCNTs with different compounds is the major interest for enhancing sensitivity and ameliorating selectivity to different toxic vapors or gases (Luo et al., 2011; Fan et al., 2011; Lu et al., 2006; Leghrib et al., 2011). The sidewall functionalization of MWCNTs not only allows better interactions with gas species, but also leads to improve their surface reactivity and dispersion. In general, it is possible to modify them by non-covalent (Chen et al., 2001; Manoso et al., 2013; Zhang et al., 2015) or covalent functionalization (Niu et al., 2007; Mu et al., 2014). The sorption of volatile and semi-volatile compounds (benzene, phenol and naphthalene) based on C60 and C60-OH was determined with headspace analysis which was described by Hüffer et al. (2014). They have previously investigated the effect of UV-induced surface oxidation on the sorption behavior of C60 for removal of volatile compounds from gas/air (Hüffer et al., 2013). If the aforementioned theory of enhanced sorption being due to π–π interactions was also relevant for sorbents with functionalized surfaces, such surface oxidation would be expected to result in an increase in sorption. However, the oxidation of C60 surfaces actually led to a significant reduction in both the maximum sorption capacity and the sorption affinity, much likely due to the decrease in the surface hydrophobicity (Wang et al., 2014; Hüffer et al., 2013).

In this study, the gas sensing properties of Schiff bases immobilized on MWCNTs (TerphApm@MWCNTs) as a novel sorbent for benzene removal from air was studied. Based on proposed method, benzene vapor was removed from air with TerphApm@MWCNTs by SPGE method. Experimental parameters affecting on the benzene removal from air such as, temperature, flow rate and relationship between temperature/humidity/benzene concentrations were studied and optimized. The performance of the proposed method in bench scale was evaluated by GC-FID and GC–MS.

2

2 Experimental

2.1

2.1 Reagents and instruments

For analysis of benzene, gas chromatography equipped with flame ionization detector (GC-FID) and air sample loop injection was used (Agilent GC, 7890A, FID, Netherland). More information about GC-FID, pilot system as a dynamic system and generation of benzene vapor in air was described in ESM (Text S2 and Fig. S1). TGS 2180 (Apollo Electronic Co., Ltd. China) and Dräger Pac 3500 (Lübeck, Germany) detectors were used for continuous measurement of H2O vapor and O2 concentrations in gas fluid, respectively. The TGS 2180 has relatively high sensitivity to water-vapor and its conductivity depends on absolute humidity (0.7–150 g m−3). The temperature increased in preheat chamber and water droplets was captured. The benzene was evaporated and mixed with purified air, then, the mixture introduced to sampling chamber which was warm with heater accessory at 69 °C. For validation, the benzene concentration in sampling chamber and polyethylene bag was determined by GC–MS before passed through connection tube to TerphApm@MWCNTs and other sorbents. All chemicals were purchased from Merck, Fluka, and Sigma Aldrich chemical companies (Germany). Benzene was diluted by filtered air and carried to the standard chamber. Uniformity of air velocity in the sampling chamber was evaluated by measuring the velocity of air flow inside the standard chamber by Hotwire Thermo-anemometer VT110 (KIMO, France), in three heights of in the holes. The accuracy and precision of the system was investigated by injecting a known concentration of benzene in the vessel and measure it in the holes of sampling chamber by direct reading equipment (Phocheck Tiger, England). Benzene (99.5% purity) was obtained from Fluka. Five calibration solutions of benzene were prepared and the approximate concentrations of benzene were 0.1, 0.5, 1.0, 1.5, and 2.0% (v/v). The other chemicals with high purity (99%) and GC grade were purchased from Merck (Germany). The TerphApm@MWCNTs as novel sorbent was synthesized by Shahroud University. The multi walled carbon nanotubes (≈50 nm), SWCNTs (10–30 nm), nanoactivated carbon (NAC, ≈30 nm) with 80% purity were synthesized. The identified compounds were analyzed by comparison of their physical properties and spectral data with reliable samples in the literature.

2.2

2.2 Synthesis of MWCNT and TerphApm@MWCNTs

Synthesis of MWCNT was mainly explained in ESM (Text S3) (Sayes et al., 2006; Rashidi et al., 2007). After Synthesis of MWCNT, Carboxylate of MWCNTs were obtained based on the procedures described by Lee et al. (2008b) with a slight modification. The raw MWCNTs were refluxed in the mixture of H2SO4 and HNO3 (ratio 3:1, 98% and 70%, respectively) for 4 h at 60 °C. The solid phase was separated by filtration technique, washed with deionized water and dried in vacuum oven. The resulting materials were immersed in a sodium borohydride/methanol solution, the suspension of solid phase was stirred at 60 °C for 2 h and then, the carboxyl groups (CH2-COOH) were reduced to hydroxyl groups (CH2-OH). The MWCNTs@OH was added to a solution of xylene (100 mL), 2 mL of 3-aminoropropyltrimethoxysilane (APTMS) and then, the suspension was refluxed for 4 h under dry nitrogen atmosphere. Finally, an ethanolic solution of terephthalaldehyde and Amp@MWCNTs was refluxed for 2 h to give TerphApm@MWCNTs nanomaterial. The solid was then separated by filtration and exhaustively washed with ethanol and dried in a vacuum. The meshed/purified MWCNT, SWCNT, NAC and TerphApm@MWCNTs were packed into a quartz capillary tube (11.5-cm length, 4-mm ID and 6-mm OD). Each tube was loaded with 20 mg of the sorbent. In order to create a uniform surface sorbent, mechanical shaker was applied. Both ends of the tube were closed by treated glass wool. The sorbent tubes were stabilized according to procedure which was reported by Shih and Li (2008).

2.3

2.3 Characteristics

After heat-treatment of synthesis nano materials, the SEM and TEM images of the SWCNTs, TerphApm@MWCNTs/MWCNTs and NAC were shown in Fig. 1a and b, respectively. The structure of carbon nanotubes including length, diameter and surface area were provided in ESM (Table S1). The surface area and heat treatment of nanostructure of CNTs was described in text S4, ESM (Gangupomu et al., 2016; Lv et al., 2011; Chin et al., 2010; Yang and Xing, 2010; Zhang et al., 2008). In this study, the surface area of MWCNT, was found 382 m2 g−1 which was similar to previous literature for MWCNT (156–471 m2 g−1) (Lin et al., 2009; Ncibi and Sillanpää, 2015; Yu et al., 2012). The low specific surface area in CNTs depended on the large diameters and many walls. The surface area of TerphApm@MWCNTs (351 m2 g−1) is little lower than simple MWCNT (382 m2 g−1) because of CNTs functionalized. The surface area of NAC (624 m2 g−1) and SWCNTs (433 m2 g−1) was higher than MWCNT/TerphApm@MWCNTs. The X-ray diffraction (XRD) patterns of parent MWCNTs/TerphApm@MWCNTs was explained in ESM (Fig. S2). The X-ray diffraction pattern of TerphApm@MWCNTs is similar to MWCNTs and the crystallinity/morphology of MWCNTs were preserved during grafting method. Fig. S2 shows that the XRD pattern of MWCNTs is similar with that of highly oriented pyrolytic graphite (HOPG). The MWCNTs showed typical peak of (0 0 2), (1 1 0), and (4 0 0) at 2θ = 26, 43, and 53°, respectively. The surface modification of MWCNTs was confirmed by FTIR a spectrum which was shown in Fig. 2. The FT-IR spectrum of MWCNTs-COOH shows a strong band at 1708 cm−1 and a weaker band at 1229 cm−1 attributed to asymmetric stretching of carboxylic and hydroxyl groups in oxidized MWCNTs, respectively. The FTIR spectrum of ApmMWCNTs had a broad band at 1000–1100 cm−1 assigned to Si–O stretching vibrations which were not present in pristine MWCNTs and/or MWCNTs-COOH. The presence of anchored propyl chain was confirmed by CH2 stretching vibrations appearing at 2922 cm−1 (asymmetric CH2 stretch) and 2855 cm−1 (symmetric stretch). The presence of these bands suggested that during the anchoring of APTMS, a condensation occurred between the OH surface groups of the oxidized MWCNTs and methoxy groups of APTMS to form the stable covalent Si—O—C linkage, leading to attachment of aminopropyl groups on the surface of MWCNTs. Thus, this treatment leads to disappearance of the bands at 1708 and 1229 cm−1 and provides further evidence for the desired reaction. In the FT-IR spectrum of TerphApm@MWCNTs, the bands observed at 1630 and 1705 cm−1 were assigned to C⚌N and C⚌O vibrations due to the anchoring of the terephthaldehyde on the modified MWCNTs with APTMS. More characterization such as; Raman spectroscopy, the pore characteristics and nitrogen physisorption isotherms was explained in ESM (Text S5, Table S2, Figs. S3-S7).

(a) SEM images of the SWCNTs (I), TerphApm@MWCNTs/MWCNTs (II) and NAC (III), (b) TEM images of the SWCNTs (I), TerphApm@MWCNTs/MWCNTs (II) and NAC (III).
Fig. 1 (a) SEM images of the SWCNTs (I), TerphApm@MWCNTs/MWCNTs (II) and NAC (III), (b) TEM images of the SWCNTs (I), TerphApm@MWCNTs/MWCNTs (II) and NAC (III).
FT-IR spectrum of Schiff base immobilized on MWCNTs; (a) HOOC-MWCNTs, (b) Apm@MWCNTs and (c) TerphApm@MWCNTs.
Fig. 2 FT-IR spectrum of Schiff base immobilized on MWCNTs; (a) HOOC-MWCNTs, (b) Apm@MWCNTs and (c) TerphApm@MWCNTs.

2.4

2.4 General procedure

The 20 mg of different sorbent tubes such as, TerphApm@MWCNTs, MWCNTs, SWCNTs and NAC was connected to a universal sampling pump (SKC, UK) and calibrated using the calibrator defender 530 (Mesa Labs, USA). The flow rate was adjusted to 50, 100, 150, 200 and 300 mL min−1. Then, the sorbent tubes were placed in the hole of DSB which was the best hole of the uniformity of velocity. The volume of benzene with different concentration in air was passed through the TerphApm@MWCNT/MWCNT/SWCNT/NAC sorbents and absorbed on its. Then, the sorbents were heated by thermal accessory at 80 °C and the benzene was online desorbed from sorbents and flowed to polyethylene bag with Argon gas for storage and sampling. Finally, the 1 mL of air of polyethylene bag was injected to injector of GC-FID by Hamilton syringes and the concentration of benzene was determined by GC-FID (Agilent 7890A, USA). In addition, for validation, carbon disulfide instead thermal accessory was used for back extraction benzene from sorbent and remained solution was injected by GC syringe to the sampling valves that introduce into the carrier gas stream for determining benzene by GC-FID. The both of method for back extraction of benzene from sorbents were compared in different sorbents from 20 to 50 °C. Based on purposed procedure, the result showed; TerphApm@MWCNTs (chemically and physically) had efficient extraction, capacity and recovery in proposed temperature than other sorbents (physically) for removal of benzene from air. The concentration of benzene according to the calibration curve was calculated and evaluated. The conditions of SPGE method for benzene removal from air was shown in ESM (Table S3).

3

3 Results and discussion

The recoveries of proposed method were measured with the ratio of adsorption/desorption of signal peak area by injecting the standard concentration of benzene in each of sorbents before determined with GC-FID which was shown in Eq. (A1) (Li et al., 2004). In addition, adsorption capacity (AC) and removal efficiency (RE) was calculated by Eqs. (A2) and (A3) (Ahmad and Haseeb, 2017). More information has introduced in text S6 (ESM)

(A1)
Recovery = X Y × 100
(A2)
AC = ( C i - C f ) × v W s
(A3)
RE ( % ) = C i - C f C i × 100

As Eq. (A1), X is the initial concentration of benzene in sorbents and Y is the concentration of benzene which determinate by GC-FID. Where AC (mg g−1) is the adsorption capacity of benzene, RE (%) is the removal efficiency of benzene, C i (mg L−1) and C f (mg L−1) are the concentration of benzene before and after adsorption experiments, respectively, V (L) is the air volume which passed through the sorbent tube, and W s (g) is the amount of sorbent.

3.1

3.1 Proposed mechanism

Engineered carbon-based nanomaterials (CNMs), including carbon nanotubes (CNTs), have been attracting increasing attention due to their unique physicochemical properties. Strong interactions between CNTs and organic compounds, such as polycyclic aromatic hydrocarbons and benzene derivatives have led to the recognition of CNTs as promising sorbent materials for removal of organic contaminants from air. The most frequently discussed interactions sorption by CNTs are hydrophobic, π–π electron donor–acceptor (EDA), and hydrogen-bond interactions. The new sorbent based on TerphApm@MWCNTs had chemical adsorption between TerphApm and MWCNTs with molecular of benzene has been ascribed to π–π and n–π EDA interaction at room temperature which was more than physical adsorption. In addition, the interaction TerphApm-benzene (interaction 2) was stronger than MWCNTs-benzene (interaction 1). The novelty of TerphApm@MWCNTs referred to functionalization in modified MWCNTs may be a key tool to improve the compatibility between benzene vapor in air and desired nanomaterial with highly aromatic nature to provide the nanotube based sensors with a high sensitivity for gas revealing or fast and simple benzene removal from air with high efficiency in optimized conditions as compared to other sorbents. We proposed that chemically adsorption mechanism of benzene via TerphApm@MWCNTs is mainly due to π-π interaction between benzene and the surface terephthalaldehyde moieties of the TerphApm@MWCNTs as shown in Fig. 3(interaction 2 and 1). For benzene removal, the sorption of hydroxyl-CNTS, amino-CNTs and TerphApm@MWCNTs was stronger than the sorption of non-functionalized CNTs. The results showed us, benzene vapor was efficiently removed from air by TerphApm @MWCNTs as compared to MWCNTs and others function groups of MWCNTs (MWCNTs < OH–MWCNTs < NH2─MWCNTs ≪ TerphApm─MWCNTs). In addition, the interaction between organic chemicals such as benzene and CNTs/TerphApm─CNTs may act by simultaneously different mechanisms which can note such as hydrogen bands, interaction of electrostatic, bands, π-π and hydrophobic interaction (Hamadanian et al., 2016).

Adsorption mechanism of benzene based on TerphApm@MWCNTs by SPGE method.
Fig. 3 Adsorption mechanism of benzene based on TerphApm@MWCNTs by SPGE method.

3.2

3.2 Breakthrough volume

The breakthrough volume of TerphApm@MWCNTs is one of important feature in sorbents which must be evaluated. The breakthrough volume depends on some absorbent features such as type of adsorbent, the adsorption capacity, and the amount of adsorbent, temperature and air flow rate which had examined and optimized. The breakthrough point was set to be C/C0 = 0.1. The effect of temperature and flow rate on breakthrough volume was completely explained in text S7, Table S4 of ESM (Saridara et al., 2010).

3.3

3.3 Thermal gravimetric analysis

For determining of benzene concentration which was absorbed on sorbents, the temperature of thermal accessory was optimized. The results showed us, the optimized temperature was achieved at 80 °C for back removal of benzene from sorbents. The effect of increasing temperature on structure of NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs was investigated. The sorbents such as; NAC, SWCNTs and MWCNTs had any problem with increasing temperature at 480 °C and effect of temperature on TerphApm@MWCNTs was studied. The thermal gravimetric analysis (TGA) was used for thermal behavior of the TerphApm@MWCNTs. The TGA curve of TerphApm@MWCNTs, had several weight losses up to 500 °C which was shown in ESM (Fig. S8). The small mass loss below 250 °C probably reflects loss of humidity. The main weight loss at 250–350 °C was assigned to the combustion of organic compounds and removing from MWCNTs system which is in agreement with endothermic DTA peak. The DTA curve demonstrates the stepwise loss in weight, where one distinct weight loss has been observed at 300 °C. The temperature for back extraction of benzene from TerphApm@MWCNTs was obtained in 80 °C which was lower than 250 °C. So, TerphApm@MWCNTs as a novel sorbent can be used in many times for benzene removal from air without destroyed TerphApm on MWCNTs at optimized temperature.

3.4

3.4 Removal efficiency and adsorption capacity

Under equilibrium conditions, adsorption capacity of sorbent in a gas phase is the amount of adsorbate molecule (mg) on sorbent per unit mass (g). The removal efficiency of sorbent is the ratio of removed molecules to initial molecules. The removal efficiency and adsorption capacity are depended on the important parameters such as; temperature, flow rate, initial benzene concentration and relative humidity which were optimized. In this study, the effects of parameters on removal efficiency and adsorption capacity were evaluated in different temperatures (20–80 °C), relative humidity (20%, 50% and 80%), flow rates (50–300 mL min−1) and initial concentrations from 1 to 10 mg L−1(ppm). Based on previous study, the efficiency and capacity adsorption of sorbents such as TerphApm@MWCNTs, MWCNTs, SWCNTs, and NAC for benzene can be increased at the low temperature like 25 °C to 0 °C or dew point by condensing benzene on sorbents. In special condition at low temperature (0°C), the benzene vapor in gas/air condensed to liquid form (solid-liquid phase) and many interactions such as; π–π/n–π EDA, physical adsorption, hydrogen bands, electrostatic, hydrophobic interaction increased as compare to gas phase (solid-gas phase) for benzene.

3.4.1

3.4.1 Effects of temperature

The temperature has a critical role in adsorption capacity/efficiency of sorbents for benzene removal from air. In this study, the heating accessory was used in sampling chamber for controlling the temperature up to 70 °C, for prevent of condensing benzene vapor. As evaluation of sorbents efficiency, the effect of temperature was studied and optimized between 20 and 60 °C. The results showed us, the absorption efficiency of benzene by NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs was depended to temperature. Desorption of benzene from sorbents was occurred at 80 °C. In optimized temperature, the average of removal efficiency of benzene with TerphApm@MWCNTs was more than other sorbents and adsorption capacity had steady state up to 50 °C. Sone et al. (2008) found that the adsorption capacity of VOCs include benzene, decreases by increase temperature (Sone et al., 2008). They reported that the surface area and absorption capacity of SWCNT and MWCNT were obtained (254 m2 g−1, 90  μ g  g−1) and (91 m2 g−1, 505  μ g  g−1), respectively. By proposed method in optimized condition, the high temperature had negative effects on benzene removal efficiency and adsorption capacity of sorbents and temperature had more effect than humidity (Fig. 4a and b). In previous study, the adsorption capacity of NAC, SWCNTs and MWCNTs was lower than presented method (319 mg g−1, 308 mg g−1, 278 mg g−1) and TerphApm@MWCNTs (495 mg g−1) which was calculated at 20 °C. Also, the surface area of NAC, SWCNTs, MWCNTs (624 m2 g−1, 433 m2 g−1, 382 m2 g−1) and TerphApm@MWCNTs (351 m2 g−1) was higher than sorbents which was reported by Sone et al. at 20 °C. In addition, the surface area alone cannot explain the difference between absorption capacity and optimized conditions (e.g. temperature, humidity, flow rate and etc.). Other parameters such as; kind, porosity, source, size of adsorbates and chemical and physical bonding may be affected on their discrepancy of absorption capacity which must be studied and optimized. So, different of sorbents with specified characterizations, porosity, source and size was used. Previous research couldn’t show the role of accurate effect of flow rate and temperature on adsorption process by nano particles. They were prepared the SWCNT and MWCNT by chemical vapor deposition based on large amounts of amorphous carbon which was decreased absorption capacity even at low temperature for benzene removal from air (Sone et al., 2008). The poor adsorption capacity of SWCNT and MWCNT related to greater amounts of amorphous carbon with low surface area or increasing of temperature. According to Hussain et al. (2008), removal of carbonaceous impurities without significant loss of nanotubes and non-tubular carbonaceous impurities was significantly increased of adsorption capacity for benzene removal from air at room temperature (25 °C) (Hussain et al., 2008). Shih and Li (2008) reported that the adsorption capacity was considerably decreased by increasing temperature up to 90 °C (Shih and Li, 2008).

The relation between temperature with (a) Removal efficiency (b) Adsorption capacity for benzene removal from air by on TerphApm@MWCNTs.
Fig. 4 The relation between temperature with (a) Removal efficiency (b) Adsorption capacity for benzene removal from air by on TerphApm@MWCNTs.

3.4.2

3.4.2 Effects of relative humidity

In this study, the effect of relative humidity (RH) for benzene removal from air was discussed (Heidari et al., 2013). The effects of RH on removal efficiency and adsorption capacity of sorbents were completely explained in ESM (Text S8; Fig. S9).

3.4.3

3.4.3 Effect of flow rate

Optimization of the flow rate conditions were performed in order to obtain the maximum recovery with the maximal flow rate by proposed method. So, the effect of different flow rates between 100 and 500 mL min−1 was examined by TerphApm@MWCNTs and other sorbents. The flow rate was measured by a digital rotameter in input and output of RGT in different sorbents. The results showed us, the removal efficiency and adsorption capacity of TerphApm@MWCNTs was decreased in more than 200 mL min−1 of flow rate. So, 200 mL min−1 of flow rate was selected as optimum flow rate for efficient removal of benzene from air. Higher flow rate was significantly decreased the adsorption efficiency of sorbents. In proposed condition, the maximum adsorption of benzene by exterior and interior sites of nanoparticles adsorbents was constant up to flow rate of 200 mL min−1. Fig. 5(a) and (b) shows the effects of difference flow rate on the removal efficiency and adsorption capacity in optimized conditions. The trend analysis shows which the removal efficiency and adsorption capacity decrease by increasing the flow rate in optimized temperature and initial concentration. The slope of flow rate showed us, the flow rate from 200 to 300 mL min−1 is smoother than from 50 to 200 mL min−1. The same pattern observed in NAC, SWCNTs and MWCNTs with low efficiency for flow rate as compared to TerphApm@MWCNTs. The removal efficiency and adsorption capacity had been significantly improved for TerphApm@MWCNTs compared to other sorbents. According to the results, in optimum flow rate, the removal efficiency for NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs sorbents was 78%, 68%, 56% and 99%. When, the mass of adsorbents was increased the flow rate and removal efficiency of benzene was increased, but in optimized condition, TerphApm@MWCNTs had an efficient recovery with lower mass and length. Yao et al. (2009) reported that removal efficiencies for VOCs (Limonene, Toluene and Methyl ethyl ketone) adsorption at the three flow rates of 45, 92, and 184 L min−1 were 79.2, 38.5 and 21% respectively. They indicated by increasing the flow rate, the outlet concentration increased (Yao et al., 2009). Ghorai and Pant (2005) showed that the removal efficiency of fluoride was decreased by increasing of flow rate from 20 to 30 mL min−1 in activated alumina (Ghorai and Pant, 2005). Mohan et al. (2009) showed the breakthrough of toluene vapors and adsorption capacity of granular activated carbon (GAC) was decreased when the flow rate increased in optimized conditions. Therefore, benzene emission un-adsorbed in higher flow rate by sorbents but when the flow rate was decreased, the reaction time increased (Mohan et al., 2009).

Effects of flow rate on (a) removal efficiency and (b) adsorption capacity for benzene removal from air by on TerphApm@MWCNTs.
Fig. 5 Effects of flow rate on (a) removal efficiency and (b) adsorption capacity for benzene removal from air by on TerphApm@MWCNTs.

3.4.4

3.4.4 Effect of initial concentration

At optimized conditions, the air benzene removal based on TerphApm@MWCNTs was investigated in different benzene concentration from 0.5 to 10.0 ppm. The adsorption capacity depended on available adsorption sites in optimized concentration. In high benzene concentration, the active sites of sorbent were loaded and reduced the un-saturated sites. So, the maximum capacity was occurred in 9.84 mg L−1 (ppm) of benzene concentration for 20 mg of TerphApm@MWCNTs (RE = 99%). So, 10.0 ppm was selected as optimized concentration of benzene by proposed procedure (RE = 99.16). More information was explained in text S9 and Fig. S10 (ESM) (Mohan et al., 2009; Naghizadeh et al., 2013).

3.5

3.5 Effect of column

The inside of Robson quartz tubes (RGT) was filled with NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs nanoparticle. By proposed process, we used TerphApm@MWCNTs nanoparticle for removal and capture of benzene vapor from air. Electric heater power supply accessory and pneumatic valves with argon flow rate desorbed and moved the benzene from sorbent to bag and then determined by GC-FID. Diameter and length of column is important factor for adsorption efficiency of sorbents and depended to physical and chemical properties of sorbents which must be optimized. In this study, various diameter and length of RGT column (D: 0.2–0.5 cm and L: 1–4 cm) with different sorbents were studied and optimized. The maximum of adsorption efficiency was obtained with 0.2 cm in diameter and 3 cm in length of RGT column.

3.6

3.6 Method validation

Due to high adsorption capacity, the TerphApm@MWCNTs was selected as a novel sorbent for removal of benzene vapor (C6H6) from air. By proposed method, a mixture of 0.5–10 ppm of benzene vapor in artificial air with argon gas as a carrier gas passed through TerphApm@MWCNTs by SKC pump and absorbed on it. All standard of benzene in air was validated by high sensitive and accurate GC–MS instrument in different concentration before using by proposed method. Since no standard reference material (SRM) for air benzene are currently available, the spiked of validated benzene concentration in a bag (GC–MS, 10 ppm, bag 5Li, 0.2 L min−1) were prepared to demonstrate the reliability of the method by TerphApm@MWCNTs as a sorbent. At optimized conditions in 1.25 and 5.0 min, 0.5 ppm and 2.0 ppm of benzene vapor in air was almost removed by TerphApm@MWCNTs, respectively. The efficient recovery of spiked samples is satisfactorily reasonable and was confirmed using addition method, which indicates the capability of proposed method for removal of benzene from air. After thermal desorption of sorbent tube, on-line benzene concentration was determined by GC-FID. The validation of methodology was confirmed using ultra-trace benzene analyzer (GC–MS) (Tables 1, 2).

Table 1 Method validation based on TerphApm@MWCNTs/SGPE for benzene removal from air by spike of benzene concentration.
Samplec Time initial
(min)		 Benzene
(ppm)a 		Added
(Conc./time)b 		Found
(ppm)a 		Recovery (%)
Sample A 5.0 1. 93 ± 0.11 0.5/1.25 2.41 ± 0.12 96.0
Sample B 7.5 2.79 ± 0.14 1.0/2.5 3.81 ± 0.18 102.0
Sample C 12.5 4.82 ± 0.22 2.0/5.0 6.76 ± 0.32 97.0
Sample D 7.5 3.08 ± 0.13 4.0/10.0 6.98 ± 0.35 97.5
Sample E 2.5 0.94 ± 0.05 1.0/2.5 1.92 ± 1.28 98.0
Mean 7 2.71 ± 0.14 1.7/4.25 4.37 ± 0.61 98.1
a Mean of three determinations ± confidence interval (P = 0.95, n = 5).
b Conc./time = ppm/min−1).
c (Air bag; 10 ppm in 5Li bag, 0.2 mL min−1 air flow rate, Peak Area, 20 mg, T = 25 °C).
Table 2 Validation of methodology with GC-FID/SGPE for benzene vapor removal from air by GC–MS.
Sample* Time
(min)b 		Proposed method GC-FID
(ppm)a 		Proposed method GC–MS
(ppm)a 		CRM of air bag GC–MS
(ppm)a 		Recovery GC-FID
(%)		 Recovery GC–MS
(%)
1 5.0 1.86 ± 0.12 1.89 ± 0.11 1.91 ± 0.09 97.4 98.9
2 10.0 3.66 ± 0.25 3.81 ± 0.22 3.86 ± 0.18 94.8 98.7
3 15.0 5.54 ± 0.28 5.63 ± 0.25 5.79 ± 0.24 95.7 97.2
4 20.0 7.58 ± 0.37 7.68 ± 0.32 7.81 ± 0.28 97.1 98.3
5 25.0 9.67 ± 0.48 9.72 ± 0.43 9.84 ± 0.41 98.2 98.8
Mean 15 5.66 ± 0.32 5.74 ± 0.28 5.84 ± 0.26 96.64 98.38
* (Air bag; 10 ppm in 5Li bag, 0.2 mL min−1 air flow rate, Peak Area, 20 mg, T = 25 °C).
a Mean of three determinations ± confidence interval (P = 0.95, n = 5).
b Conc./time = ppm/min−1).

3.7

3.7 Comparing the developed method with others

Table 3 showed, the characterization of TerphApm@MWCNTs based on SPGE method as compared to the alternative methods for extraction/removal of benzene from air. To the best of our knowledge the solid phase gas extraction based on TerphApm@MWCNTs as a novel sorbent in air has not been explored so far, and no work has been documented in air pollution. However, some other techniques (the mote information provided in text S10, ESM) such as solid phase extraction (SPE) based on MWCNTs, SWCNTs and activated carbo and filtration have been already evaluated for this purpose (Yang and Xing, 2010; Gangupomu et al., 2016; Jahangiri et al., 2011). In this study the same behavior was evaluated for sorbents such as; NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs for benzene removal from air. The removal efficiency and adsorption capacity of NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs was calculated and compared to other sorbents. The performance of procedure based on SPGE for benzene removal from air was satisfactory by TerphApm@MWCNTs. The developed method based on TerphApm@MWCNTs with high surface area and capacity had a good efficiency (98%) for benzene removal from air as compare to reported methods (Table 3). According to results, the presented method with small amount of TerphApm@MWCNTs has high capacity, high efficient extraction, simple, fast, repeatability, and low cost for benzene removal from air than other methods.

Table 3 Comparison of proposed method based on TerphApm@MWCNTs with other reported methods for benzene removal from air.
Source Sorbent Matrix Surface m 2 / gr Recovery (%) Capacities (mg g−1)
Sone et al. (2008) Crystalline MWCNT Air 25 92.6 0.17
Li et al. (2004) Purified MWCNT Air/water 98 – –
Carbopack B – –
Shih and Li (2008) MWCNT 1 Air 108 – –
MWCNT 2 114 – –
Jahangiri et al. (2011) SWCNT Air 303 – 37.3
MWCNT 1 133 – 32.2
MWCNT 2 128 – 31.6
MWCNT-COOH 110.23 – 26.5
Pourfayaz et al. (2014) MWCNT-Iranian Air 195 91.7 12.5
MWCNT-Chinese 125 – 8.1
Liu et al. (2008) Tenax TA/MWCNT/Carboxen 564 Air 111.3 95.2 21.6
Lee et al. (2008a) Activated Carbon Air 1241 – 200
Long et al. (2010) Hypercrosslinked polystyrene Air 1020 – 284
Ramirez et al. (2005) ACFC Air 1604 – 622
CDAC 965 – 358
Britt et al. (2008) MOF-199 Air 1264 – 176
*This work TerphApm@MWCNTs Air 351 99.8 492
* Shirkhanloo et al., Benzne removal based on TerphApm@MWCNTs by Solid phase gas extraction.

4

4 Conclusion

In this study, many sorbents such as; NAC, SWCNTs, MWCNTs and TerphApm@MWCNTs was used for benzene removal from air by SPGE method. According to results, the simple, fast, reliable, sensitive, accurate, precise and inexpensive method based on SPGE/TerphApm@MWCNTs was demonstrated. In different conditions, benzene concentration, amount of sorbents, temperature, relative humidity and flow rate were optimized. The capacity adsorption, breakthrough volume, removal efficiency of sorbents was investigated. The results showed, the flow rate and temperature can consider as a desirable factor in the breakthrough volume and capacity adsorption. Also, the air humidity can be affected on removal efficiency and adsorption capacity of sorbents but the effects of air humidity on removal efficiency had lower than flow rate and temperature. So, the air temperature/flow rate and kind/amount of sorbent were more important factor than humidity for benzene removal from air which was optimized in SPGE method. In addition, the flow rate had significant effect on adsorption capacity and removal efficiency of sorbent and depended to temperature, column and sorbents. However, in optimized conditions, the breakthrough volume, the removal efficiency and adsorption capacity of TerphApm@MWCNTs were more than NAC, SWCNTs and, MWCNTs. Also, the wide range of temperature in chemical adsorption of benzene by TerphApm@MWCNTs caused to increase the adsorption capacity in different temperature (20─50 °C) as compared to physical adsorption by NAC, SWCNTs and MWCNTs at 20 °C. As a novelty of TerphApm@MWCNTs, based on special characteristics of sorbents, high adsorption capacity with chemically adsorption due to π-π interaction between benzene and the surface terephthalaldehyde moieties was lead to a more efficient removal of benzene vapor from air than other sorbents with physically adsorption.

Conflict of interest

The authors declare no conflict of interest

Acknowledgments

We are thankful to Shahroud University of Medical Sciences (SUMS), Research Institute of Petroleum Industry (RIPI) and Iranian Petroleum Industry Health Research Institute (IPIHRI).

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Appendix A

Supplementary material

The supplementary information contains supplementary text captions (STC) from text S1 to text S10; supplementary figures captions (SFC) from Figs. S1 to S10 and supplementary table captions (STC) from Tables S1 to S3. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.01.011.

Appendix A

Supplementary material

Supplementary data 1

Supplementary data 1


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