Anal. Methods Environ. Chem. J. 4 (4) (2021) 36-48
Research Article, Issue 4
Analytical Methods in Environmental Chemistry Journal
Journal home page: www.amecj.com/ir
AMECJ
Removal of benzene vapor from the air based on novel
tantalum metal-organic framework (Ta-MOF) adsorbent
by gas ow solid-phase interaction before determination
by gas chromatography
Mohammad Bagher Aghebat Bekheira, Mohammad Reza Rezaei Kahkhab, Nasser Hasheminejada
and Ali Faghihi-Zarandia,*
a Department of Occupational Health and Safety at Work, Kerman University of Medical Sciences, Kerman, Iran
b Department of Environmental Health Engineering, Faculty of Health, Zabol University of Medical Sciences, Zabol, Iran
ABSTRACT
Benzene has a carcinogenic effect on the human body and adsorption
from the air is the best way to control it. By this research, benzene
vapor was removed from the air based on a tantalum metal-organic
framework (Ta-MOF) by gas ow solid-phase interaction (GF-SPI).
Benzene adsorption with Ta-MOF was studied in the static and
dynamic systems at room temperature. The benzene concentration
was analyzed by gas chromatography equipped with an FID detector
(GC-FID). The factors affecting benzene removal efciency like
initial concentration of benzene, amount of adsorbent, exposure time,
ow rate, and temperature were studied and optimized. The results
showed us, the adsorption capacities range of Ta-MOF for benzene
in the static and dynamic system were obtained between 90-160 mg
g-1 and 65-135 mg g-1, respectively. Also, the high removal efciency
was achieved by more than 95% at 45°C, 67.5 mg L-1 benzene
concentration, 0.5 g of Ta-MOF, and the ow rate of 250 mL min-1
for a dynamic system. By dynamic system, the benzene is generated
in the chamber, stored in a bag, and then moved on the surface of
Ta-MOF. The GF-SPI method was validated by GC-MS and spiking
real samples.
Keywords:
Adsorption,
Gas ow solid-phase interaction,
Tantalum metal-organic framework,
Gas chromatography
ARTICLE INFO:
Received 12 Aug 2021
Revised form 15 Oct 2021
Accepted 17 Nov 2021
Available online 29 Dec 2021
*Corresponding Author: Ali Faghihi-Zarandi
Email: Alifaghihi60@yahoo.com
https://doi.org/10.24200/amecj.v4.i04.155
------------------------
1. Introduction
Volatile organic compounds (VOCs) contain a
carbon structure with a high vapur pressure at
room temperatures [1]. These compounds are
known air pollutants released through industrial
activities such as liquid fuels and cleaning supplies
[2]. Benzene is a hazardous pollutant in the air that
workers in various industries (oil and chemicals)
exposure to it [3]. Benzene has been classied as a
denitive carcinogen in humans since 1979 based
on sufcient evidence of leukemia. Benzene can
replace by toluene or derivatives with less toxicity
[4,5]. Benzene exists in fuels such as gasoline and
is used in the production of styrene, dyes, inks and
polymer products [6, 7]. Benzene enters into the air
through different ways such as gasoline leakage,
pipelines and petrochemical efuents[8]. Exposure
to benzene can affect human health and cause
cancer, CNS impairment, and kidney diseases [7,
9, 10]. The benzene is volatilized and distributed
in air, soil, water and foods [11,12]. Exposure to
BTEX products causes many problems in the
37
Removal of benzene from the air by Ta-MOF Mohammad Bagher Aghebat Bekheir et al
human body [13-15]. By increasing stringent
environmental standards, the actual control
for the removal of VOCs is the principal aim
[16]. Methods developed to control and remove
VOCs emission on industrial plants including,
condensation, adsorption, catalytic oxidation, and
biodegradation methods. Whereas adsorption onto
sorbents such as activated carbon, zeolites, and
cotton bres has been the reliable option so far
[17,18]. Adsorption is one of the most efcient
ways to control the emission of VOCs [19]. Some
common adsorbents for the adsorption process are
carbon quantum dots(CQDs), graphene(NG), resin,
zeolite, and carbon nanotubes [20]. Metal-organic
frameworks (MOFs) are relatively new compounds
made of porous crystal materials and are being
studied as adsorbents since 1998 [21]. These
materials are regular polymers that are formed
from different metals and bonds between metals
created by organic compounds [22]. Organic-
metal framework compounds use in clean energy,
as the most important storage devices for gases
such as hydrogen and methane, as well as high-
capacity adsorbents to meet various separation
needs23] ]. The previous studies have shown that
these adsorbents have excellent performance in
absorbing benzene vapours. For instance, in 2008,
Bright and colleagues studied benzene adsorption
efciency on various adsorbents made from MOFs.
The adsorption occurs at the same conditions at
25°C and 440 ppm benzene concentration. Also,
the adsorption capacities of the adsorbents varied
from 2 to 176 mg g-1. The MOF-199 sorbent had
the highest absorption rate [24]. In 2019, Vikrant
et al. Examined conventional adsorbents such as
activated carbon for the adsorption of gaseous
benzene compared to the new adsorbent of the
metal-organic framework. The results showed the
highest efciency of MOF-199 was 94.8 mg g-1,
which was slightly higher than the other adsorbent,
activated charcoal (93.5 mg g-1). Also, the adsorption
efciency of the adsorbent MOF(UiO-66) is 27.1
mg g-1, which is lower than the activated charcoal
adsorption efciency [25]. In 2011, Young and
colleagues studied the adsorption of VOCs by
the metal-organic framework, MIL-101(Cr),
inuenced by the shape and size of the molecule
and concluded that benzene adsorption capacity in
MIL-101 was 1291±77 mg g-1[21]. Ahmaruzzaman
and Xiang et al showed that activated carbon has
desirable physical and chemical properties, which
made it useful as an adsorbent and was used in the
industry for decades. Sone et al were reported the
CNTs can be used for BTEX removal from the air.
They showed the carbon nanotubes are promising
better absorbed than other carbon materials due
to their unique properties. The CNT has been
dened as cylindrical porous with walls made of
crystalline graphite layers. Many technologies,
such as bio-lter system [26], surface interaction
[27], separation [28], adsorption [29, 30], and
nanocatalyst [31], were used for VOCs removal. In
addition, the various adsorbents such as activated
carbon based on cellulose acetate [32], carbon
nanotube (MWCNTs) [33], Zeolite [34], the
graphene-modied by IL [35] were reported for
removal benzene and BTX from the air.
In this study, benzene vapour was removed
from the air based on a tantalum metal-organic
framework (Ta-MOF) by the gas ow solid-phase
interaction (GF-SPI). Benzene adsorption based on
Ta-MOF was evaluated in the static and dynamic
systems. The benzene concentration was analyzed
by the GC-FID. The main parameters such as
temperature, ow rate, the adsorbent mass and
benzene concentration were optimized.
2. Material and Methods
2.1. Instrumental and reagents
All of the chemical compounds such as
benzene anhydrous (99.8%, CAS N.: 71-43-2),
tantalum chloride (CAS N: 7721-01-9 TaCl5),
benzene tetracarboxylic acid (CAS N.: 89-
05-4), the cetyltrimethylammonium-Bromide
(CAS N.: 57-09-0), and propane had high purity
and were purchased from the Merck/Sigma
company(Germany). Five calibration solutions
of benzene were prepared. The approximate
concentrations of benzene were prepared from 0.1,
0.5, 1.0, 1.5, and 2.0% (v/v). The other chemicals
38 Anal. Methods Environ. Chem. J. 4 (4) (2021) 36-48
based on GC grade and 99% purity were purchased
from Merck (Germany). The characterization of the
synthesized MOF was carried out via SEM-Philips
XL 30, UK, and the X-ray diffraction equipment
(XRD) Rigaku XDS 2000. The Agilent 7890A
GC based on three detectors was used for benzene
determination. The FID detector chosen was
selected for benzene analysis in air. For injecting,
slide the plunger carrier down and tighten the
plunger thumb screw. Due to sampling valves, we
introduce a sample of xed size into the carrier gas
stream. Valves are most frequently used to sample
air or liquids. Gas sampling bags with valve and
septum port (Tedlar) and air sampling apparatus
(Bucket brigade) were prepared. The split/splitless
injector, FID, and a column coated (50 m × 0.2 mm
id.) was used by GC. The injector temperature was
adjusted to 200-210°C and the detector temperature
at 240-250°C. The GC oven temperature was tuned
at 220°C. Hydrogen as the carrier gas was used at a
ow rate of 1.0 mL min
–1
with a split ratio of 1:100.
Vials with PTFE air-tight cap (parker) were used as
a batch/static system.
2.2. Preparation of the tantalum metal-organic
framework
In this study, the MOF adsorbent was prepared by
Rezaie and colleagues’ research [36]. A solution
of tantalum chloride (0.027 g) and benzene
tetracarboxylic acid (0.011 g) was prepared in 17
mL double distilled water (DDW). The solution
prepared from the reaction above and then added
to the mixture of 0.77 g cetyltrimethylammonium-
Bromide and 8 mL propane in a 50 mL Pyrex tube.
This new compound is then put in a microwave
bath of 45°C and microwave bath with a power
of 220 watts for 30 minutes. After 45 minutes of
centrifuging, the MOF white crystals were formed,
and then the product was left to dry in an argon
atmosphere (Fig.1).
2.3. Benzene preparation in a dynamic system
First, the air was puried with an electro air cleaner
(EA-HEPA600M) based on HEPA and activated
carbon which was removed particles 200-300 nm
(99.97%) and VOCs from the air, respectively.
Then, the pure air passed through the chamber and
entered to PVC bag (5 Li) by an SKC pump. The
amount of H2O (vapor) was controlled by adjusting
the amount of water injection. All of the gas lines
and bags were covered with heating jackets at 50-
60 °C to prevent H2O (vapor) and benzene from
condensing (Fig.2).
2.4. Static and dynamic adsorption procedure
For the static experiment, 10 mL of different
concentrations of benzene were drawn with a syringe
and injected into the vials that were air-tightened with
a PTFE lid and contained the Ta-MOF adsorbent.
After a specic time, 100-500 microliter air from
the vial was extracted and injected into the GC-FID
analyzer for determining benzene. The effect of
four exposures, times (5, 10, 15, and 20 minutes),
the amount of Ta-MOF adsorbent in four amounts
(0.5, 1, 1.5, and 2 mg), four different concentration
levels (30, 50, 70, and 100 mg L-1 of benzene) and,
two temperatures of 25 and 45°C based on Ta-MOF
adsorbent were studied (Fig.3).
Fig.1. Preparation steps for synthesis of the tantalum metal-organic framework
39
Fig.2. The generation of benzene vapor in pure air by chamber
Fig.3. Static procedure for benzene removal from air by the Ta-MOF adsorbent
Removal of benzene from the air by Ta-MOF Mohammad Bagher Aghebat Bekheir et al
40
The adsorption efciency for static adsorption was
obtained from the following equation:
In the equation, Q is adsorption efciency in (mg
g-1), C0 is benzene’s primary concentration in (mg
L-1), Ce is benzene’s equilibrium concentration in
the vial in (mg L-1), m is the amount of adsorbent in
(g), and V is the vial’s volume in (L).
To draw the calibration curve from different
standard concentrations of benzene in the ranges
between 10 - 1000 mg L-1 were prepared in Tedlar
sampling bags. Then, 100-500 microliters of the
air vial containing benzene were prepared by a
micro GC syringe. At last, the calibration curve
was plotted by considering standard solutions and
adsorption peak areas by GC-FID (Fig.4).
By dynamic procedure, 20 mg of Ta-MOF sorbent
tubes connected to a sampling pump (SKC, UK).
The ow rate was adjusted to 50-400 mL min-1.
The benzene vapour was mixed with pure air in
the chamber, the different value of benzene in the
air was passed through the Ta-MOF sorbent. After
adsorption of benzene on Ta-MOF, the adsorbent
was heated by thermal accessory at 80-100 OC in
the presence of Ar gas and then, the benzene was
desorbed from Ta-MOF and owed/stored in a
polyethylene bag. Finally, the 100 -500 microliters
of air were aspired with Hamilton syringes and
injected into an injector of GC-FID. Based on
the dynamic procedure, Ta-MOF had efcient
extraction and recovery for the removal of benzene
from the air. The results showed that the absorption
capacity of the static system was higher than the
dynamic system at a ow rate of 250 ml min-1.
3. Results and Discussion
3.1. SEM, TEM and XRD analysis
The morphology and size distribution of Ta-MOF
nanoparticles were studied by a scanning electron
microscope (SEM) and transmission electron
microscopy (TEM). In this study, characterization
of tantalum MOF adsorbent was carried out by
SEM, TEM and XRD devices. SEM’s microscopic
images of Ta-MOF showed that the average size
of particles in tantalum MOF adsorbent is 48
nanometers (Fig.5a). TEM of Ta-MOF showed
a nanometric size of about 30 nm. The Ta-MOF
sample has homogenous morphology with similar
particle size (Fig.5b). Also, XRD images showed that
tantalum MOF adsorbent has a cubic crystal structure
(Fig.6). the X-ray diffraction pattern of porous Ta-
MOF that prepared by the ultrasonic method. A
comparison of Ta-MOF diffraction with the other
MOF revealed a triclinic crystalline structure of Ta-
Fig.4. The calibration curve of benzene based on Ta-MOF adsorbent by GC-FID
Anal. Methods Environ. Chem. J. 4 (4) (2021) 36-48
41
MOF. As TEM images any agglomeration wasn’t
seen in the structure of Ta-MOF.
3.2. The effect of benzene concentration on
adsorption efciency
The effect of benzene’s primary concentration
varying from 10 to 100 mg L-1 on tantalum MOF
adsorbent was studied. The results showed tantalum
adsorbent became saturated at a concentration of
70 mg L-1 (Fig. 7).
3.3. The Effect of exposure time on adsorbent
efciency
The effect of different times from 5 to 20 minutes
for benzene removal based on tantalum MOF in
different concentrations of benzene between 10-100
mg L-1 was studied. In this study, it was observed that
increasing the exposure time has a positive effect on
the recovery and adsorption capacity of Ta-MOF
adsorbent. The results showed, the adsorption rate
increases up to 10 minutes and then remains almost
constant (Fig.8).
3.4. The effect of the amount of Ta-MOF on
adsorption efciency
The effect of the amount of the adsorbent (Ta-MOF)
in the range of 0.1 to 2 mg for benzene removal
include the benzene concentration (10 - 100 mg L-1)
Fig. 5a. SEM images of tantalum Ta-MOF Fig. 5b. TEM images of tantalum Ta-MOF
Fig. 6. XRD images of the tantalum MOF adsorbent
Removal of benzene from the air by Ta-MOF Mohammad Bagher Aghebat Bekheir et al
42
was studied in optimized conditions. The results
showed the maximum adsorption efciency for
benzene was achieved for 0.5 mg of Ta-MOF by
70 mg L-1. Due to Figure 9, for an extra amount
of adsorbent, the adsorption capacity increased and
then constant (Fig. 9).
3.5. The effect of temperature on adsorption
efciency of Ta-MOF
For efcient removal of benzene from the air, the
effect of temperature on adsorption/desorption of
Ta-MOF must be optimized. The results showed
us, the optimized temperature for adsorption and
desorption of benzene from sorbent was achieved
at 45 OC and 90 OC, respectively. The effect of
temperature on benzene removal was studied in
optimized conditions. For this purpose, the different
concentrations of benzene in the range of 10 to
100 mg L-1 of benzene at a temperature between
25 - 60°C were evaluated. The results showed us,
the maximum absorption capacity and efciency
were obtained at the temperature of 45 °C. For
evaluating, 100-500 microliters of air containing
benzene in static and dynamic procedure injected
into GC. As shown in Figure 10, by increasing
temperature, the absorption capacity and efciency
decreased (Fig.10).
Fig.7. The effect of benzene concentration on Ta-MOF adsorption efciency
Anal. Methods Environ. Chem. J. 4 (4) (2021) 36-48
Fig. 8. The Effect of exposure time on adsorbent efciency
30 50 70 100
43
Removal of benzene from the air by Ta-MOF Mohammad Bagher Aghebat Bekheir et al
Fig. 9. The effect of the amount of Ta-MOF on adsorption capacity
30 50 70 100
Fig.10. The effect of temperature on adsorption efciency of Ta-MOF
20 30 45 60 80 90 100 120
44
3.6. Effect of ow rate
By procedure, the effect of different ow rates
between 50 -500 mL min-1 was studied by Ta-
MOF adsorbent. The ow rate was measured by
a digital rotameter. The removal efciency and
adsorption capacity of Ta-MOF were reduced
in more than 300 mL min-1 of ow rate. So, 250
mL min-1 of ow rate was used as optimum ow
rate for removal of benzene from the air. A higher
ow rate was signicantly reduced the adsorption
efciency of benzene. Figure 11 shows the
removal efciency and adsorption capacity
decreased by increasing the ow rate in optimized
temperature (blue, 45 °C). Therefore, benzene
cannot absorb at a higher ow rate by the sorbent.
The results showed us the removal efciency and
the adsorption capacity was decreased at 60 °C
(grey).
Table 1. Validation of GF-SPI method in a dynamic system based on Ta-MOF adsorbent
by spiking benzene (ppm)
Air Samples Added(ppm)
GF-SPI, GC-FID
ppm Recovery (%)
A ---- 11.9 ± 0.4 ----
10 21.7± 1.1 98.1
B ---- 18.5 ± 0.8 ----
20 38.2 ± 1.7 98.5
C---- 31.7 ± 1.4 ----
30 61.9 ± 2.9 100.6
a Mean of three determinations ± condence interval (P = 0.95, n = 5)
Anal. Methods Environ. Chem. J. 4 (4) (2021) 36-48
Fig.11. The effect of ow rate on adsorption efciency of Ta-MOF (blue, 45 °C; grey, 60 °C)
50 100 150 200 250 300 400 500
45
Table2. Comparing of adsorption capacities of Ta-MOF with other adsorbents
Refs.TQa
Asur(m2 g-1)AdsorbentsAdsorbate
[24]25° c2 mg g-1
2205(Zn4O(CO2)6) MOF-5
Benzene
[24]25° c56 mg g-1
1568(C8H5NO4-2)IRMO-3
[24] 25 ° c96 mg g-1
632(Zn2O2(CO2)2)MOF-74
[25]25 ° c1 mg g-1
3875(Zn4O(btb)2)MOF-177
[24,25]25 ° c176 mg g-1
1264(Cu2(CO2)4)MOF-199
[24]25 ° c109 mg g-1
1814IRMO-62
[17]25 ° c1291±77 mg g-1
3980MIL-101 (Cr)
This study45 ° c350 mg g-1
1200Ta-MOF
3.7. Validation of methodology
The Ta-MOF was used for the removal of benzene
vapour from the air. By the proposed method, a
mixture of 10-100 ppm of benzene vapour in
pure air (Ar gas) was passed through Ta-MOF
sorbent by an SKC pump. Before adsorption,
the standard of benzene in the air was validated
by GC-MS in different concentrations. For
validation, the spiked benzene concentration was
done to demonstrate the reliability of the method
by Ta-MOF (Table 1). At optimized conditions,
at different times based on 250 mLmin-1, the
different concentrations of benzene in the air
were generated and used for validation by spiking
samples. The efcient recovery of spiked samples
conrmed the capability of ow solid-phase
interaction (GF-SPI) for the removal of benzene
from the air.
3.8. Discussion
MOF adsorbent is one of the strongest adsorbents
regarding the removal of volatile compounds
from the air. In this study, the effect of tantalum
organic metal framework adsorbent (Ta-MOF) for
the removal of benzene vapour was investigated.
The efciency of different MOF adsorbents for
benzene adsorption is shown in Table 2. Due to
results, the tantalum MOF adsorbent has a higher
adsorption efciency as compared to the other
adsorbents. In 2010, Lu and colleagues conducted
a study regarding the removal of benzene, toluene,
ethylbenzene, and para-xylene with carbon
nanotubes oxidized by sodium hypochlorite
and concluded that at rst, with the increase of
exposure time, benzene adsorption efciency also
increased and then decreased [37]. As compared
to the Ta-MOF, by increasing exposure time,
the adsorption efciency increased. Moreover,
the maximum amount of adsorption occurred at
10 minutes and then the amount of adsorption
remained constant. Also, in 2018, Hua Xie and
colleagues carried out a study and concluded that
the increase in temperature when using the MOF
adsorbent, BUT-66 (Zr), lowered adsorption
efciency. In this study, it was found that the
adsorbent at 25 and 80 ° C had 2.54 mmol g-1 and
1.65 mmol g-1 of benzene adsorption, respectively
[5], while in the present study at 25 and 45 °
C, by increasing temperature, the amount of
adsorption has decreased slightly. Also, the effect
of temperature in the different adsorbents such as
nano activated carbons and activated carbon has
also been observed, which is conrmed by the
study of Golbabai et al. to remove xylene vapor
by these adsorbents. The results from Bright and
colleagues research in 2008 regarding benzene
adsorption efciency using the adsorbents such as
(MOF-5) (Zn4O(CO2)6), (MOF-3)(C8H5NO4-2),
(MOF-177) (Zn4O(btb)2) and (MOF-199)
(Cu2(CO2)4), at benzene concentration levels
of 440 ppm, showed that benzene adsorption
efciency for each 1gr adsorbent, was 2, 56, 1,
and 176 milligrams of benzene, respectively [38].
However, in the present study, a trace amount of
Ta-MOF can be absorbed by a high concentration
of benzene (70 ppm) with a signicant adsorption
rate of 155.3 mg g-1 which indicated the high
adsorption capacity of the Ta-MOF adsorbent.
Removal of benzene from the air by Ta-MOF Mohammad Bagher Aghebat Bekheir et al
46
4. Conclusion
In the present study, a new Ta-MOF adsorbent was
used for the removal of benzene from air and the
absorption efciency studied. For benzene analysis,
100-500 microliter air from the vial (static) or
polyethylene bag (dynamic) was extracted and
injected into the GC-FID analyzer. To evaluate the
adsorbent capacity, the effect of four independent
factors such as benzene concentration, the adsorbent
amount, the exposure time, and temperature on the
amount of benzene adsorption were investigated. The
absorption capacities of static and dynamic procedures
based on Ta-MOF were obtained 155.3 mg g-1 and
124.6 mg g-1, respectively based on 0.5 g of Ta-MOF
and the ow rate of 250 mL min-1. The results showed
the Ta-MOF adsorbent had more adsorption capacity
than other conventional adsorbents.
5. Acknowledgements
The authors wish to thank the Department
of Occupational Health and Safety at Work,
Kerman University of Medical Sciences, Kerman,
Iran for supporting this work.
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