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Functionalized graphene-trimetho=
xyphenyl
silane for toluene removal from workplace air by
sorbent gas extraction method
Majid Bagheri Hosseinabadi1, Sha=
hnaz
Timoori2 and Ali Faghihi Zarandi3,*
1- School of
Public Health, Shahroud University of Medical
Sciences, Shahroud, Iran.
2- Department =
of
Environment and Natural Resources, Islamic Azad University, Science and
Research Branch, Tehran, Iran.
3- Occupational
Health Engineering Department, School of Public Health, Kerman University of
Medical Sciences, Kerman, Iran.
Abstract
Inhalation
exposure to toluene in the environment and workplace causes that some concerns
about its adverse health effects on public and workers raise. So Key words: Toluene, Chemical adsorption, Graphene, N-Phenyl-3-aminopropyl trimethoxy silane, Sorbent
gas extraction procedure, 1.<=
span
style=3D'font:7.0pt "Times New Roman"'> =
Introduction Toluene is one=
of
the well-known air contaminants and widely used as an organic solvent in
various industries such as petroleum refining, shipping, rubber manufacturi=
ng,
automobile repairing, and shoe manufacturing (1). It is estima=
ted
that toluene is consumed about 2.3 million tons in worldwide annually(2). Moreover, public and occupational exposure to toluene can occur throu=
gh
the inhalation of toluene fumes from cigarette smoking and vehicle emission=
(3). In addition, it is likely to release from indoor sources such as pain=
ts,
paint thinners, and adhesives (4).The studies have been shown that chronic exposure to toluene can cause=
a wide
variety of health problems including neurotoxic effects (from headache and
fatigue to narcosis with increasing exposure level) and mucosal irritation =
(5, 6). Because of t=
he
harmful effects of toluene, the implementation of high-performance methods =
to
control toluene emission in industrial and environment settings is inevitab=
le (7). There are some different methods such as including adsorption,
condensation, thermal oxidation, catalytic oxidation, photo catalytic oxida=
tion
and bio filtration that can be applied to reduce the concentration of tolue=
ne in
air (8-11). It seems that
adsorption as a low operating cost and effective method can be used to remo=
ve
toluene from air at low concentration(12, 13). Adsorption is a process in which the molecules of pollutants are trap=
ped at
the surface of porous materials such as zeolites, silica gel and activated
carbons by physical adsorption (14-16). Among the
different adsorbents, carbon adsorbing materials such as graphene oxide,
activated carbons, carbon nanotubes, and porous carbon have demonstrated mo=
re
advantage due to their low density, chemical stability and variety of
structural forms (17-20). The founding=
of new carbon
materials is always one o=
f the
favorite subjects in the process of adsorption and sequestration (21)=
span>. Graphene, new two-dimensional carb=
on
nanomaterial, as a new member of the carbon family with desirable properties
such as low weight, small size, high surface area, and superior electrical,
thermal and mechanical properties has attracted much attention (22-24). The more
advantage of graphene is the adsorption ability of chemicals with benzene r=
ings
like toluene through strong π-π interaction =
span>(25, 26). In addition, some On the other h=
and,
graphene has high theoretic surface area up to 2620 m2
g-1 that it become an ideal adsorption (28, 29). Furthermore, graphene can be easily obtained from the che=
ap
natural graphite in large scale (29, 30). In this regard, the high adsorption
capacity of graphene and its derivatives was confirmed =
for
dyes (31), pharmaceutical antibiotics (32), heavy metals, and VOCs in water (33, 34). However, the studies on toluene adsorption characteristics and behavi=
ors
onto graphene from air are limited (35). In this study,=
toluene
removed from air based on G-PhAPTMS by SGEP. Th=
e different
experimental conditions such as mass, flow rate, and temperature were
investigated and optimized. By proposed procedure, the chemical=
ly
adsorption mechanism of toluene was achieved based on π-π interac=
tion
between toluene and the surface of the G-PhAPTMS. 2. Experimenta=
l procedure 2.1. Material =
and
Instruments Toluene was purchased from Fluka (Germany) with purity above 99.5%. In addition,
standard gas was obtained by injecting a certain amount of toluene into a 1=
0 ml
glass vial with PTFE cap to determine absorption capacity. In this study,
helium and pure air were used as the dilution gas. Standard gas was prepared
with a relative humidity of 32±5% for simulating the humidity in the workpl=
ace
air. Thus, before filling the vial, the dilution gas was passed through
deionized water. The concentrations of toluene in the standard gas were pre=
pared
from 9 to 75 mg L-1 (in batch system, high concentration up to 4=
00
mg L-1). In addition, a
dynamic standard gas generation was designed to measure removal efficiency.
This system consisted of impinger<=
/span>, adsorption tube, micro compressor, and sampling Tedlar bag (SKC Inc., USA). A certain amount of tolue=
ne was
injected to the impinger, and then the dilution=
gas
with a certain flow was passed through the adsorption tube. At the end of t=
he
system, unabsorbed toluene was collected into the Tedl=
ar
bag. The concentration of toluene =
in the
Tedlar bag was measured by gas chromatography
equipped with flame ionization detector (GC-FID) and air sample loop inject=
ion
(Agilent GC, 7890A, FID, Netherland). Also, GC-MS was used for validation
toluene concentration in air. The crystal structure studies of the
solids were carried out by X-ray diffractions (PW 1840, Phillips X-ray diffractmeter, Netherland) with Cu-Kα
radiation source. Raman spectroscopy was performed using an Almega
Thermo Nicolet and 532 nm =
Ar-ion
laser excitation source. The Fourier transform infrared (FT-IR) spectrum was
recorded on Bruker IFS 88 spectrometer (Bruker Optik=
span>
GmbH, Ettlingen, Germany) with KBr
pelleting method in the 4000–400 cm−1. The morphology of t=
he
sorbent was examined using scanning electron microscopy (SEM, Phillips, PW3=
710,
Netherland) and transmission electron microscopy=
(TEM,
CM30, Philips, Netherland). 2.2. Synthesis=
of functionalized
graphene with N-Phenyl-3-aminopropyl trimethoxy=
silane Graphene (G) was prepared by our special CVD (chemical vapor
deposition) method by an electrical furnace consisting of a quartz tube. The
furnace heating tuned up to 1000 ºC for 25 min. The reaction between methane
and hydrogen (4:1) was obtained. In order to pure grapheme without any meta=
ls,
the product was stirred in HCl solution (ultra-=
trace)
for about 20 h. The sample was then washed repeatedly with ultra-pure water
until the solution became neutral. The treated product was finally dried in
oven at 120 ºC. For carboxylation process, 1 g of the as-prepared G was fir=
st
mixed with a 200 mL mixture of concentrated H2SO4 and=
HNO3
(3:1 v/v) and stirred for 20 min at room temperature followed by sonication=
at 55
ºC for 4 hours in an ultrasonic bath (50 kHz and 100 W). After cooling to r=
oom
temperature, the reaction mixture was diluted with 500 ml of deionized
water and then vacuum-filtered through a filter paper (0.2 μm).
Finally, G-COOH was dried in oven at 70 °C. In order to the formation of th=
e free
hydroxyl group, 0.5 g of filtered G-COOH was added to a methanolic solu=
tion
of sodium borohydride. Then, the G bearing –OH group were allowed =
to
disperse in xylene followed by addition of N-Phenyl-3-aminopropyl trimethoxy
silane (PhAPTMS=
) to synthesize
functionalized G in refluxing. After washing with xylene to remove the
unreacted excess PhAPTMS, the product was dried for 10 h at 90 °C under reduced
pressure. Morphology of the G-PhAPTMS was studied usi=
ng
scanning electron microscopy (SEM) and transmission electron microscopy (TE=
M). Graphite
powder (5 g) and NaNO3 (2.5 g) were mixed with 120 mL of
concentrated H2SO4 and stirred for 30 min in an ice b=
ath
(0-5 °C). KMnO4 (15 g) was gradually added to the vigorously
stirred suspension and the temperature of the mixture was kept to be below =
15 °C.
The ice bath was then removed, and the mixture was stirred at 35 °C until it
gradually became brownish slurry, and then it was diluted slowly with 250 m=
L of
water. The reaction temperature was rapidly increased to 98 °C with
effervescence, and the color changed to brown <=
span
class=3DSpellE>color. Later, H2O2 solution (30=
%) was
added to stop the oxidation process, and the color
General proced=
ure
Because the removal efficiency is highly affected by the amount of sorb=
ent,
the effects of amount of sorbents were studied in 1, 2, 4, 5, 10, 15, 20, 2=
5,
and 30 mg of different sorbents including G-PhAPTMS, G, GO and AC. =
The
flow rate of the dilution gas was set from 50 to 500 ml min-1. T=
he
effects of temperature on the absorption of toluene were investigated in
different temperature from 10 to 90 °C. The same concentration of standard =
gas
of toluene was passed through the adsorption tube containing different amou=
nt
of each sorbents in various thermal and flow rate conditions. The concentra=
tion
of unabsorbed toluene collected in the Tedlar b=
ag was
analyzed by gas chromatography equipped with flame ionization detector. Rem=
oval
efficiency was calculated as seen in Equation 1. The removal of toluene was evaluated in present of pure air in differ=
ent
flow rate by G-PhAPTMS, G, GO and AC as nanosorbents. In optimized conditions, the static and
dynamic system was used by SGEP. The air was purified with electro air cleaner (EAC) and
mixed with toluene in pilot. By proposed pilot, the pure air with and witho=
ut
toluene gas based on SGEP method was measured (5 times) by GC-FID and valid=
ated
by GC-MS. The novel G-PhAPTMS
sorbent
had chemical and physical adsorption between G
and N-Phenyl with to=
luene,
respectively. The mechanism of che=
mical
adsorption was occurred based on π–π interaction of N-Phenyl with
toluene. Moreover, the G-PhAPTMS was more interaction than =
G for
toluene removal from air. B=
y new
synthesis, G was functioned by PhAPTMS and impr=
oved
the compatibility between toluene and surface of sorbent in optimized
conditions as compared to G, Go, and AC.
Removal
Efficiency =3D Cin-Cout/<=
span
class=3DSpellE>Cin × 100 Eq 1
Cin (mg L-1) and Cout (mg L-1) were the concentr=
ation
of toluene before and after adsorption respectively.
Results and
discussion
Morphology of =
the G-PhAPTMS has been studied by SEM and TEM (Fig. 1 and 2=
). After
functionalizing of graphene with N-Phenyl-3-aminopropyl trimethoxy
silane, it seems that the graphene has much more
active surface to absorb toluene.
=
Figure 1: SEM images of the G-PhAPTMS.
=
=
Figure 2: TEM image of the G-PhAPTMS.
Effect of
Temperature:
The removal
efficiency of G-PhAPTMS, G, GO and AC in temper=
ature
from 10 to 90 °C are shown in Fig. 3. Functionalizing graphene could improve
the properties of graphene in absorption of toluene from the air by increas=
ing
temperature. In addition, activated carbon had more removal efficiency rath=
er
than graphene oxide in the same conditions. The removal efficiency of G-
Figure 3: the effects of temperature on rem=
oval
efficiency of G-PhAPTMS, AC, and GO.
Effect of flow=
rate
The effect of =
flow
rate on removal efficiency in 25 °C was investigated to optimize the flow r=
ate
conditions. The flow rate was set on 50 to 500 ml min-1 for all
sorbents. The results are presented in Fig. 4.
Figure 4: the effects of flow rate on removal
efficiency of G-PhAPTMS, AC, and GO.
The removal
efficiency of G-PhAPTMS, G, GO, and AC decrease=
d by
increasing the flow rate, like temperature. The results showed, the more
removal efficiency of G-PhAPTMS as compared to =
other
sorbents, and the removal efficiency of G-PhAPTMS was
decreased up to 62% in 500 ml min-1. However, the removal effici=
ency
of AC and GO in this flow rate was 32% and 5% respectively. Increasing the =
flow
rate affected on the removal efficiency of all sorbents, but this effect was
lower for G-PhAPTMS. Similarly, the impact of f=
low
rate (45, 92 and 184 ml min-1) on the removal efficiencies of to=
luene,
limonene and methyl ethyl ketone was investigate=
d by
Yao et al in 2009. Their results showed that removal efficiency was decreas=
ed
from 79% to 21% by increasing the flow rate [39]. Moreover, declining removal efficiency of fluoride by increasing the =
flow
rate from 20 to 30 ml min-1 in activated alumina was reported by=
Ghorai and Pant [40]. The influence of flow rate on removal efficiency may be explained by
reaction time. When the flow is high, the molecules of toluene have a little
time to interact with the surface of absorbent, so they begin to escape from
the end of the sorption tube [41].
Effect of mass
sorbent
The amount of
sorbent is another important parameter which was investigated in this study=
. The
absorption tubes were loaded by different amounts of each absorbent from 1 =
to
30 mg by proposed method. The temperature and flow rate were set on 25 °C a=
nd 200
ml min-1. The results are shown in Fig. 5.
Figure 5: the effects of amount of sorbent on
removal efficiency of G-PhAPTMS, AC, and GO.
The removal
efficiency of toluene with G-PhAPTMS increased =
by increasing
the amount of sorbent and reached the highest pint in 5 mg of sorbent. In
addition, the removal efficiencies of AC and GO followed the same pattern as
the G-PhAPTMS, while reached the highest pint i=
n 15
and 30 mg of sorbents respectively. The GO had the lowest removal efficiency
rather than the G-PhAPTMS, G, and AC for all am=
ount
of sorbent. The positive impact of amount of sorbent on removal efficiency =
was
reported by Samarghandi et al (2017) in the rem=
oval
of reactive red from distillated water. They found that the removal efficie=
ncy
increased when the amount of active carbon and graphene increase from 0.2 t=
o 4
g L-1 and 0.02 to 0.4 g L-1 respectively [42]. Higher amount of sorbent provides much more chance for the molecule of
toluene to react with the available surface of sorbent. So, the removal
efficiencies of sorbents increase effectively. Based on the results, the mo=
re
active absorption sites of g L-1 allow the molecules of toluene =
to
absorb rapidly and functionalizing graphene creates this opportunity that in
the lower amount of sorbent, So we have maximum =
removal
efficiency by G-PhAPTMS as compared with other
sorbents. Repeatability of the sorbents was investigated in 25 °C, 200 ml m=
in-1
and from 2 to 30 mg of each sorbent (Figure 6). Each type of sorbent was us=
ed
for twenty times and the mean of removal efficiencies were calculated.
Figure 6: The removal efficiency of different
sorbents after using repeatedly.
The G-PhAPTMS had the highest removal efficiency for all in=
vestigated
amount of sorbent compared with G, GO and AC sorbents after twenty times ab=
sorption
and desorption. The mean of removal efficiency of AC experienced a downward
pattern from 5 to 30 mg. Decreasing the removal
efficiency for the GO happened from 20 mg to 30 mg where the removal effici=
ency
fell from 64 to 42%. According to the results, functionalizing graphene wit=
h N-Phenyl-3-aminopropyl
trimethoxy silane (=
G-PhAPTMS) can improve the absorption characteristics of
graphene to absorb toluene from air.
Validation
The G-PhAPTMS was used as a novel and low-cost sorbent for
removal of toluene vapor from air. By proposed method, a mixture of 5-100 p=
pm
of toluene vapor in artificial air with argon gas as a carrier gas passed
through sorbent (G-PhAPTMS) by a pump. The
concentration of toluene in standard gas was validated by highly sensitive =
and
accurate GC-MS instrument in different concentration before using by propos=
ed
method. Since standard reference material for toluene in air is not current=
ly
available, the spiked of validated toluene concentration in a bag (GC-MS, 50
ppm, bag 5 Li, 0.2 L min-1) were prepared to demonstrate the
reliability of the method by G-PhAPTMS. At opti=
mum
condition in one minute, 2.0 ppm of toluene vapor in air was almost removed=
by G-PhAPTMS. The efficient recovery of spiked samples was
reasonable and was confirmed using addition method, which it indicates the
capability of proposed method for removal of toluene from air (Table 1).
Table 1: <=
/b>The recovery of <=
/span>G-PhAPTMS for removing toluene from air.
Samples |
Toluene in air
(ppm)* |
Toluene in sorb=
ent (ppm)** |
Sorbent Recovery
(%) |
Sample A |
2.63 ± 0.07 |
2.57 ± 0.11 |
97.7 |
Sample B |
6.89 ± 0.14 |
6.48 ± 0.21 |
94 |
Sample C |
10.72 ± 0.19 |
10.51 ± 0.32 |
98 |
Sample D |
14.08 ± 0.34 |
13.52 ± 0.84 |
96 |
Sample E |
18.73 ± 0.17 |
17.35 ± 0.28 |
92.6 |
*
Initial concentration in standard gas
**
Measured concentration in sorbent
Conclusions
In this study,=
different
absorbents such as G-PhAPTMS, G, GO, and AC wer=
e used
to remove toluene from air. The results showed that functionalizing graphene
with N-Phenyl-3-aminopropyl trimethoxy silane (G-PhAPTMS) promot=
es the
removal efficiency of graphene by chemical bonding as π–π electron donor–accept=
or. Furthermore, increasing temperature and flow rate have negative effec=
ts
of the removal efficiency of all sorbents, but the results of G-PhAPTMS showed high removal efficiency of toluene com=
pared
to the AC, G, and GO. The amount of sorbent was another parameter which was=
investigated.
In optimized conditions, the amount of sorbent for the G-PhAPTMS,
AC, and the GO were obtained 5, 5, and 30 mg respectively. This result shows
that the G-PhAPTMS has many more available acti=
ve
sites for absorption of toluene. By lower amount of G-=
PhAPTMS,
high removal efficiency was achieved more than 95% by interaction between
toluene and N-Ph. The results of repeatability of sorbents also indicate th=
at functionalizing
graphene with N-Phenyl-3-aminopropyl trimethoxy=
silane allows graphene to be used repeatedly, and it =
can be
introduced as an environmentally friendly sorbent.
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