J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S 43 (2 0 1 6 ) 1 28–1 3 5
Available online at www.sciencedirect.com
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Adsorption and desorption of SO2, NO and chlorobenzene on
activated carbon
Yuran Li, Yangyang Guo, Tingyu Zhu⁎, Song Ding
Beijing Engineering Research Center of Process Pollution Control, National Engineering Laboratory for Hydrometallurgical Cleaner Production
Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected]
AR TIC LE I N FO
ABS TR ACT
Article history:
Activated carbon (AC) is very effective for multi-pollutant removal; however, the
Received 16 June 2015
complicated components in flue gas can influence each other's adsorption. A series of
Revised 23 August 2015
adsorption experiments for multicomponents, including SO2, NO, chlorobenzene and H2O,
Accepted 26 August 2015
on AC were performed in a fixed-bed reactor. For single-component adsorption, the
Available online 11 January 2016
adsorption amount for chlorobenzene was larger than for SO2 and NO on the AC. In the
multi-component atmosphere, the adsorption amount decreased by 27.6% for chloroben-
Keywords:
zene and decreased by 95.6% for NO, whereas it increased by a factor of two for SO2,
Activated carbon
demonstrating that a complex atmosphere is unfavorable for chlorobenzene adsorption
Multi-components
and inhibits NO adsorption. In contrast, it is very beneficial for SO2 adsorption. The
Functional groups
temperature-programmed desorption (TPD) results indicated that the binding strength
Binding force
between the gas adsorbates and the AC follows the order of SO2 > chlorobenzene > NO. The
Flue gas
adsorption amount is independent of the binding strength. The presence of H2O enhanced
the component effects, while it weakened the binding force between the gas adsorbates and
the AC. AC oxygen functional groups were analyzed using TPD and X-ray photoelectron
spectroscopy (XPS) measurements. The results reveal the reason why the chlorobenzene
adsorption is less affected by the presence of other components. Lactone groups partly
transform into carbonyl and quinone groups after chlorobenzene desorption. The
chlorobenzene adsorption increases the number of C = O groups, which explains the
positive effect of chlorobenzene on SO2 adsorption and the strong NO adsorption.
© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
Published by Elsevier B.V.
Introduction
The constant levels of huge energy consumption in China
have produced serious levels of air pollution. There is an
extremely urgent need for multi-pollutant control technologies with much stricter air pollutant emission standards for
application to industrial kilns and furnaces. For the existing
iron and steel industry, the effluent concentration of used
sintering flue gas is restricted to 200 mg/m3 for SO2, 300 mg/
m3 for NO, and 0.5 ng-TEQ/m3 for dioxin in GB 28662–2012.
Compared with traditional limestone-gypsum wet flue gas
desulfurization (WFGD) and selective catalytic reduction (SCR)
denitration with ammonia technologies, the activated carbon
(AC) adsorption method of simultaneously capturing SO2,
NOx, dioxin, mercury and other substances is more economical, consuming much less water and energy (Guo et al., 2013;
Li et al., 2014; Liu and Liu, 2013; Shahkarami et al., 2015).
The components of flue gas are very complicated, including large amounts of SO2 and NOx, with H2O, volatile organic
compounds (VOCs) and other substances as well. Thus, one
⁎ Corresponding author. E-mail: [email protected] (Tingyu Zhu).
http://dx.doi.org/10.1016/j.jes.2015.08.022
1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
J O U RN A L OF E N V I RO N M EN TA L S CI EN CE S 43 (2 0 1 6 ) 1 2 8–1 3 5
component may affect the adsorption of another component
over AC. Previous studies have revealed that SO2 and NOx
affect each other's adsorption. The presence of SO2 inhibits
NO adsorption due to its higher permanent dipole moment
and polarizability (Yi et al., 2012), whereas NO promotes SO2
adsorption through the formation of intermediate species
[(NO2)(SO3)]⁎ (Tang et al., 2005). The co-adsorption of butane
(C4H10) and NO2 or SO2 onto AC reduces the adsorption
capacity for oxide components, but the butane adsorption is
not influenced by SO2 or NO2 (Ahnert and Heschel, 2002). SO2
and NO removal on AC is a complicated process that involves
adsorption and catalysis with various adsorption sites occupied, whereas the adsorption of polychlorinated biphenyl over
AC is mainly physical adsorption (Kawashima et al., 2011).
Thus, co-adsorption can limit the adsorption amount for
certain components, and the adsorption behavior differs
because of the gas properties.
Water vapor is frequently present at high concentrations
in flue gases and may influence the AC adsorption behaviors.
Water adsorption isotherms in carbons are type V in the
IUPAC (International Union of Pure and Applied Chemistry)
classification, and the sharp rise in the water isotherm has
been ascribed to the coalescence of clusters of hydrogenbonded water molecules (Striolo et al., 2005). SO2 adsorption
on AC is marginally increased at above 30 vol.% of moisture
in the atmosphere, as the oxidation of SO2 to SO3 is followed
by hydration to H2SO4, and the presence of moisture can
promote the release of vacant sites (Gaur et al., 2006). Water
vapor has a partial inhibition effect on NO adsorption (Klose
and Rincón, 2007). The adsorption capacity for trichloroethylene is almost equal under dry and moisture atmospheric
conditions, but water vapor has a negative influence on
n-butane adsorption (Marbán and Fuertes, 2004; Lee et al.,
2005). The effects of moisture on single components have
been widely discussed. Moreover, the influence of water on
multi–components in situations involving their simultaneous
presence is worthy of investigation.
In this work, a series of adsorption experiments with SO2,
NO, H2O and chlorobenzene, as a volatile organic compound,
on AC under various gas components were performed in a
fixed-bed reactor to investigate the influence of the complicated flue gas components on each component's adsorption.
Temperature-programmed desorption coupled with mass
spectrometry (TPD-MS) measurements were adopted to analyze the adsorption sites of each component and the carbon
surface functional groups, and then to reveal the interaction
mechanisms between pairs of components.
1. Experimental
1.1. Activated carbon sample
The commercial coconut-shell AC used in the experiment was
from the Gongyi activated carbon plant in Henan province.
The AC with particle sizes of 38–62 μm was dried at 393 K for
10 hr before the experiment. The specific surface area was
980.7 m2/g, as calculated from the N2 adsorption isotherms
using the BET (Brunauer–Emmett–Teller) equation. N2 adsorption was performed at 77 K in an automatic surface area and
129
porosity analyzer (AutosorbiQ, Quantachrome, USA). The
total pore volume was 0.480 mL/g, derived from the amount
of N2 adsorbed at p/p0 = 0.95. The micropore volume was
0.385 mL/g, and the average micropore width was 1.449 nm as
determined by the DR (Dubinin–Radushkevich) equation.
1.2. Gas adsorption tests
The single component adsorption isotherms were obtained
by thermo–gravimetric (TG) analysis (Versa Therm HM,
Thermal, Germany). A 10 mg AC sample was placed in a
quartz crucible. The adsorption temperature was kept at
393 K, and the total gas flow was 100 mL/min. The mixed
gas included 5 mol% O2, a balance of Ar, and a single
adsorption gas (SO2, NO or chlorobenzene) with a concentration ranging from 0.01 to 0.7 mol%. The adsorption time
ranged from 1 to 8 hr as the concentration decreased.
The co-adsorption of SO2, NO and chlorobenzene on the AC
was investigated in a fixed-bed reactor. The quartz tube
reactor was 8 mm in diameter and 500 mm in height with a
sieve plate in the middle. A 300 mg AC sample was loaded on
the plate for each experiment. The reaction temperature was
393 K, and the total adsorption time was 180 min. The total
gas flow was 300 mL/min, and the gaseous hourly space
velocity (GHSV) was approximately 24,000 hr−1. The simulated
flue gas consisted of 0.1 mol% SO2, 0.05 mol% NO, 0.025 mol%
chlorobenzene, 5 mol% O2, water vapor and a balance of Ar with
the flow controlled by mass flow controllers. The chlorobenzene
vapor was generated by bubbling Ar through a container in
a 313 ± 0.1 K water bath. The chlorobenzene concentration
was calibrated using gas chromatography (7890 A, Agilent, USA)
at 0.025 ± 0.001 mol%. The water vapor was generated using
the Ar carrier by evaporating deionized water in a U-tube in a
323 ± 0.1 K bath. The relative humidity (RH) was 40%, controlled
by the Ar flow based on the water vapor Antoine equation. After
approximately 60 min, the water vapor concentration began to
stabilize with fluctuation less than 2%. After being mixed in the
mixing vessel, the gas was fed into the reactor with the effluent
gas continuously detected by a quadrupole mass spectrometer
(GAM200, IPI, Germany) and identified using the major mass ions
of 64 for SO2, 30 for NO, and 112 for chlorobenzene.
The gas adsorption amount was calculated based on the
integrated area between the blank line and the break-through
curve. The desorption area integral also could be calculated,
and the calculation results revealed that the desorption
amount was almost equal to the adsorption amount. Taking
chlorobenzene at a given situation as an example, the
chlorobenzene adsorption and desorption area integrals
were 3.71 × 10− 11 and 3.67 × 10−11 A·min, respectively, with a
deviation of 1%, whereas the error of the calculation method
was approximately 3%. Thus, the chlorobenzene could be
recognized as totally desorbed and detected.
1.3. Adsorption product tests
A Fourier transform infrared (FTIR) spectroscope (Nicolet 6700,
Thermal, USA) was used to determine the surface functional
groups of the adsorption products. The carbon–KBr mixtures
in a ratio of 1:1000 were ground in an agate mortar. The
spectra were collected on a Perkin-Elmer Spectrum One
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spectrometer operating in the infrared region of 500–4000 cm−1
with a resolution of 4 cm−1. The background spectrum was
recorded using the atmospheric vapor correction to eliminate
CO2 and H2O vapor during the experiment. Three scans were
collected for each sample.
TPD-MS was applied to investigate the adsorption products. The AC samples were placed in the quartz crucible of the
TG with Ar as the carrier gas with a flow rate of 300 mL/min.
Desorption was measured in the temperature range of 303 to
1273 K with a heating rate of 10 K/min. The outlet gases of
SO2, NO, chlorobenzene, CO2, CO and H2O were continuously
recorded by the mass spectrometer, with the major mass ions
of 44 for CO2, 28 for CO and 18 for H2O.
The surface functional groups were quantitatively measured using X-ray photoelectron spectroscopy (XPS) (ESCALab
250, Thermo Electron, USA) with an Al Kα X-ray source
(1486.6 eV) at a constant recording ratio of 40. The X-ray
source was operated at a reduced power of 150 W. The
pressure in the analysis chamber was maintained lower
than 1.33 × 10−6 Pa during each measurement. All of the
binding energies were referenced to the C1s hydrocarbon
peak at 284.6 eV to compensate for surface charge effects.
2. Results and discussion
2.1. Single component adsorption
Fig. 1 shows the SO2, NO and chlorobenzene adsorption
isotherms over the AC. The SO2 and NO adsorption amounts
increase as the adsorbate concentration increases, with the
line still climbing even as the concentrations increase over
0.5 mol%. The amount of chlorobenzene adsorption rapidly
increases at chlorobenzene concentrations below 0.1 mol%,
whereas it slows over 0.1 mol%. The adsorption amount at the
highest concentration is 178.3 mg/g for chlorobenzene,
75.8 mg/g for SO2, and 38.1 mg/g for NO. The molar adsorption
amount is calculated as 1.59 mmol/g for chlorobenzene,
1.18 mmol/g for SO2, and 1.27 mmol/g for NO. Higher boiling
point gases are more easily adsorbed. The boiling point is
404 K for chlorobenzene, 263 K for SO2, and 121 K for NO.
200
Adsorption amount (mg/g)
160
CB
SO2
NO
Langmuir fitting line
Freundlich fitting line
120
80
40
0
0
2000
4000
6000
Concentration (10-4 mol%)
8000
Fig. 1 – SO2, NO and chlorobenzene (CB) adsorption isotherms. Adsorption conditions: 5 mol% O2, a balance of Ar,
gas flow rate 100 mL/min, adsorption temperature 393 K.
Therefore, the adsorption amount for chlorobenzene is larger
than for SO2 and NO on the same amount of carbon.
The Freundlich and Langmuir equations, expressed as Eqs.
(1) and (2), were adopted to fit the experimental values in Fig.
1. The Langmuir equation considers a homogeneous surface
with equivalent adsorption sites and only one molecule
adsorbed per adsorption site. The Freundlich equation considers a heterogeneous surface with different adsorption
sites.
q ¼ K F C1=n
q ¼ qs KL C
1 þ KL C
ð1Þ
ð2Þ
where, q (mg/g) is the adsorption amount of the adsorbent; KF
is the adsorption or distribution coefficient for the Freundlich
isotherm (mg/(g·ppm1/n)). C (ppm, parts per million) is the
adsorbate concentration, and n is the Freundlich constant.
The magnitude of the exponent n gives an indication of the
favorability of adsorption; values n > 1 represent favorable
adsorption conditions. qs (mg/g) is the Langmuir constant
related to adsorption capacity, and KL is the Langmuir
constant related to rate of adsorption (ppm−1). The fitting
results are listed in Table 1.
The chlorobenzene adsorption isotherm on the AC is
obviously type I in the IUPAC classification; therefore, the
Langmuir isotherm fits well for this adsorption. It is also
possible that chlorobenzene adsorption belongs to micropore
adsorption, which can also be described by the Langmuir
isotherm. The average micropore width of the AC is 1.449 nm,
larger than the diameter of the chlorobenzene molecule,
0.77 nm. Larger pores provide access for the adsorbed gas to
enter the micropores to allow multilayer adsorption. The
Freundlich isotherm fits well for SO2 and NO adsorption. The
KF value for SO2 is 4.264, almost twice that of 1.998 for NO,
which is in accordance with the observed adsorption
amounts. The n value for SO2 is 3.040, which is slightly larger
than 2.945 for NO, indicating that SO2 is more favorably
adsorbed by AC than NO. In other studies, SO2 adsorption
equilibrium on cork-powder AC was successfully fitted to
Langmuir and Freundlich isotherms (Atanes et al., 2012),
whereas SO2 and NO adsorption isotherms on AC fiber were
described by the Freundlich equation (Zhou et al., 2012).
Different equations adopted for SO2 and NO adsorption were
strongly influenced by the chemical properties of the adsorbent. In this work, SO2, NO and chlorobenzene adsorption
on the AC belong to different types due to their different
adsorption isotherms.
The surface functional groups of the single-component
adsorption products were determined by FTIR, with the
results shown in Fig. 2. In all the recorded spectra, in the
3600–3200 cm− 1 range, a band of O-H stretching vibrations
was observed due to the surface hydroxylic groups and
chemisorbed water. The band at 1560 cm−1 can be mainly
attributed to the stretching vibrations of C = O moieties in
carboxylic, lactone and quinone groups. Another band at
1434 cm−1 can be ascribed to the symmetrical stretching
vibration of -COO− moieties in carboxylic and lactone groups.
The band at 1045 cm−1 can be assigned to the stretching
vibrations of C-O moieties in lactone (Ghimbeu et al., 2011).
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Table 1 – Parameters of the Freundlich and Langmuir fitting results.
Adsorbate
Freundlich model
Langmuir model
1/n
KL C
q ¼ qs 1þK
(mg/g)
LC
q = KFC
(mg/g)
KF (mg/(g·ppm1/n))
n
R2
qs (mg/g)
KL (ppm−1)
R2
23.98
4.264
1.998
4.153
3.040
2.945
0.912
0.983
0.978
187.2
77.98
39.98
27.88 × 10−4
9.764 × 10−4
9.132 × 10−4
0.997
0.946
0.969
Chlorobenzene
SO2
NO
q: the adsorption amount of the adsorbent; KF: adsorption or distribution coefficient for the Freundlich isotherm; C (ppm): the adsorbate
concentration; n: the Freundlich constant; qs: the Langmuir constant related to adsorption capacity; KL: the Langmuir constant related to rate of
adsorption amount.
CB+SO2+NO
CB+SO2+NO+H2O
4000
3500
3000
2500
2000
1500
Wavenumber (cm)
0
1000
500
Fig. 2 – FTIR (Fourier transform infrared spectroscopy spectra)
of the adsorbent products for single components. Adsorption
conditions: 0.1 mol% SO2 or 0.05 mol% NO or 0.025 mol%
chlorobenzene (CB), 5 mol% O2, and a balance of Ar in all
atmospheres, gas flow rate 100 mL/min, adsorption time
180 min, adsorption temperature 393 K.
Chlorobenzene
NO
Adsorption atmospheres
SO2+CB+H2O+NO
SO2+CB+H2O
SO2+CB+NO
95.6%
Blank
SO2+CB
NO+CB+SO2+H2O
20
SO2
CB
40
NO+CB+SO2
SO2
27.6%
NO+CB+H2O
Intensity (%)
NO
60
NO+CB
1434
1560
202%
NO
1045
3438
CB
Adsorption amount (mg/g)
80
CB+NO+H2O
100
CB+SO2+H2O
The adsorption amount for each component in a complex
atmosphere was compared with that under the singlecomponent atmosphere conditions, with the results shown
in Fig. 3. In the single-component atmosphere, the adsorption amount was 65.2 mg/g for chlorobenzene, 11.3 mg/g for
NO, and 30.9 mg/g for SO2. When all of the components were
present, the chlorobenzene adsorption amount decreased
27.6%, and the NO adsorption amount sharply decreased by
95.6%, whereas the SO2 adsorption amount increased by
202%, up to 93.3 mg/g. A complex atmosphere is unfavorable
for chlorobenzene adsorption and almost completely inhibits NO adsorption, whereas it is very beneficial to SO2
adsorption.
CB+SO2
2.2. Multi-component adsorption under various atmospheres
SO2 has a negative effect on chlorobenzene adsorption
with an adsorption amount decrease of 4.3%, whereas NO has
a positive effect on chlorobenzene adsorption with an
adsorption amount increase of 6.2%. Chlorobenzene decreases the NO adsorption amount by 14.3%, but increases
the SO2 adsorption amount by 7.4%. Chlorobenzene adsorption is slightly affected by the other components; meanwhile,
Chlorobenzene has less influence on the adsorption of other
components. When SO2, NO and chlorobenzene coexist, the
SO2 adsorption amount increases 132.2%, NO decreases 89.1%,
and chlorobenzene decreases 15.9% due to the negligible
positive effect of NO and the large negative effect (more than a
factor of two) of SO2. H2O can enhance these effects, both
positively and negatively. At room temperature, a large H2O
concentration has been verified to be favorable in achieving
the effective continuous removal of SO2 in the form of H2SO4
(Gaur et al., 2006). The activation of NO is strongly inhibited by
SO2 and H2O; however, it has been verified that inhibition by
H2O is not marked, as the NO adsorption is much more
restricted by other components (Mochida et al., 2000). NO
should be removed in an atmosphere with low SO2 and H2O
CB+NO
After chlorobenzene adsorption, the band peak declined at
1045 cm−1, indicating that the amount of lactone decreased.
After NO adsorption, the band peak declined at 1434 and
1560 cm−1, showing that the carboxylic, lactone or quinone
groups decreased, but which functional group decreased
cannot be determined. After SO2 adsorption, no obvious
band peak changes occurred.
SO2
Fig. 3 – Chlorobenzene (CB), SO2 and NO adsorption amounts
under various adsorption atmospheres. Adsorption conditions: 0.1 mol% SO2 (if present), 0.05 mol% NO (if present),
0.025 mol% chlorobenzene (if present), 40% relative humidity
(RH) (if present), 5 mol% O2 and a balance of Ar in the
atmosphere; gas hourly space velocity (GHSV) 24,000 hr−1,
adsorption temperature 393 K, adsorption time 180 min.
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concentrations. Chlorobenzene removal also should occur in
cases of low H2O concentration. The AC should be injected
downstream of the flue gas desulfurization to effectively
capture chlorobenzene and reduce the AC consumption.
2.3. TPD-MS profiles of the adsorption products
2.3.1. Desorption product analysis
The adsorption products were measured by TPD-MS to
analyze the binding strength between gas molecules and AC
according to the desorption temperature. The results are
shown in Fig. 4. The desorption peak temperature is related to
the adsorption bond strength. A stronger bond gives rise to a
3.0
540 K
550 K
CB ion current (×10-12 A)
2.5
2.0
1.5
1.0
0.5
0.0
300
404 K
450
600
T (K)
1.4
750
900
SO2
SO2+CB
SO2+CB+H2O
SO2+CB+NO
SO2+CB+NO+H2O
580 K
SO2 ion current (×10-12 A)
CB
CB+NO
CB+SO2
CB+NO+H2O
CB+SO2+H2O
CB+SO2+NO
CB+SO2+NO+H2O
1.2
1.0
590 K
8.0
6.0
0.4
615 K
0.2
300
450
600
T (K)
900
NO
440 K
4.0
NO ion current (×10-12 A)
750
NO+CB
450 K
NO+CB+H2O
NO+CB+SO2
3.0
NO+CB+SO2+H2O
2.0
580 K
1.0
300
higher TPD peak temperature (Yang, 2010). The desorption
peak temperatures for SO2, chlorobenzene and NO are 580–
615, 540–550, and 440–450 K, respectively, indicating that the
binding strength between gas molecules and AC follows
the order of SO2 > chlorobenzene > NO. The chlorobenzene,
SO2 and NO desorption peak areas are consistent with the
adsorption amounts in Fig. 3. It can be concluded that the
adsorption amount is independent of the binding strength.
Chlorobenzene begins to desorb at approximately 404 K.
The desorption peak temperature is 550 K, significantly
higher than its boiling point, which indicates that the binding
strength between the chlorobenzene and AC is larger than the
chlorobenzene intermolecular force. The desorption peak
temperature decreases from 550 to 540 K in the presence of
H2O, indicating that H2O weakens the binding strength
between chlorobenzene and the AC. The desorption peaks
have the same shape under various atmospheres, indicating
that the chlorobenzene molecule maintains its structure after
desorption.
SO2 begins to desorb above 480 K and almost finishes
desorption below 750 K, with the peak temperatures from 580
to 615 K. Desorption temperatures lower than 453 K are
related to weakly adsorbed or physically adsorbed SO2, and
desorption temperatures between 473 and 773 K are related
with strongly adsorbed SO2 or oxidized SO3 (López et al., 2007;
Raymundo-Piñero et al., 2001). In this work, the adsorption
species of SO2 can be categorized as strongly adsorbed SO2 or
oxidized SO3. The desorption peak temperature remains 615 K
in the presence of the chlorobenzene and H2O, indicating that
the SO2 adsorption state does not change. The desorption
peak temperature decreased 25 K with NO, indicating that NO
increases the amount of strongly adsorbed SO2 (Guo et al.,
2015). Both the strongly adsorbed SO2 and oxidized SO3
increase in the presence of all of the components.
The NO desorption displays two peak temperatures, 440
and 580 K, corresponding to the weakly adsorbed NO and
strongly adsorbed NO, mainly in the form of (NO)2 dimer
(Klose and Rincón, 2007). The desorption peak at 580 K
increases in the presence of only chlorobenzene, indicating
that chlorobenzene is beneficial for the strongly adsorbed NO.
The desorption peaks disappear with SO2 due to the much
lower amount of NO adsorption.
In regards to chlorobenzene, SO2 and NO desorption, H2O
results in a desorption peak shift to lower temperature,
indicating that the binding force between the adsorbate
and the AC is weakened, and the product tends to be easily
desorbed. Different from the case at 393 K, the adsorption
force is much stronger in moist than in dry conditions for SO2
adsorption at ambient temperature (Arcibar-Orozco et al.,
2013).
2.3.2. AC surface functional group changes
450
600
T (K)
750
900
Fig. 4 – TPD-MS profiles of the adsorption products for
chlorobenzene (CB), SO2 and NO in Ar. TPD: temperature
programmed desorption; MS: mass spectrometry; desorption
conditions: gas flow rate 300 mL/min, heating rate 10 K/min;
the adsorption products are the same as in Fig. 3.
To investigate the interactions of the adsorbates with surface
oxygen-containing groups, the TPD-MS profiles for H2O, CO
and CO2 from the adsorption products are shown in Fig. 5. The
H2O desorbed at approximately 380 K is from the free water in
the AC, and that at approximately 570 K is mainly from the
desorption of H2SO3 and H2SO4, which come from strongly
adsorbed SO2 and oxidized SO3. The desorption of strongly
adsorbed SO2 is accompanied by CO2 release at around
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--- With water
Without water
380 K 570 K
SO2+NO+CB
SO2+NO+CB
SO2+NO+CB
1060 K
NO+CB
NO+CB
460 K
720 K
SO2+CB
CO2 ion current (A)
CO ion current (A)
H2O ion current (A)
NO
CB
740 K
NO+CB
460 K
SO2+CB 580 K
SO2+CB
CB
580 K
1080 K
560 K
NO 450 K
600 K
CB
700 K
1030 K
NO
450 K
SO2
550 K
Blank
SO2
SO2
Blank
Blank
1000 K
300
600
900
1200
300
600
T (K)
900
T (K)
1200
300
1000 K
900
1200
600
T (K)
Fig. 5 – Profiles for H2O, CO and CO2 released from the surface functional groups on adsorption products in Ar. CB:
chlorobenzene; desorption conditions: gas flow rate 300 mL/min, heating rate 10 K/min; the adsorption products are the same
as in Fig. 3.
500–650 K, and then the release of CO above 800 K
(Raymundo-Piñero et al., 2001). The SO3 (H2SO4) desorption
produces carbon gasification and generates CO2 at approximately 673 K and CO at approximately 1073 K (Mochida et al.,
2000). In López's investigation, CO and CO2 are evolved over a
very wide temperature range from about 533 to more than
1173 K as SO2 is desorbed at temperatures between 533 and
873 K (López et al., 2007). The release temperature and
desorption amount of the CO and CO2 are related to the
adsorbed SO2 species. In this work, slight CO2 release at
580–600 K and release of CO and CO2 at 1000–1080 K were
observed. In the presence of H2O, the CO and CO2 desorption
peak temperature shifts a small amount, and the peak height
changes slightly. Water vapor does not change the adsorption
products. The CO and CO2 desorbed at 450–460 K are
consistent with the NO desorbed at 440–450 K. In the complex
atmosphere conditions examined, little NO is desorbed, so
only a small amount of CO and CO2 are desorbed. These
results indicate that NO desorption is accompanied by CO and
CO2 desorption. The release of CO detected at 560–580 K and
CO2 desorption observed at 700–740 K correspond to chlorobenzene desorption at 400–720 K.
In the presence of chlorobenzene, the CO desorption
peak temperature greatly increases from 1000 to 1060–
1080 K, and the CO2 desorption peak temperature shifts from
1000 to 1030 K, compared with the absence of chlorobenzene.
The release of CO and CO2 in various temperature ranges
corresponds to the decomposition of the various types of
oxygen functional groups. Carboxyl groups decompose into
CO2 at 370–730 K, and anhydride groups simultaneously
decompose into CO and CO2 at 650–900 K. In addition, phenol
groups decompose into CO at 870–970 K. Lactone groups
decompose into CO2 at 860–1080 K, and carbonyl and quinone
groups decompose into CO at 970–1250 K (Figueiredo and
Pereira, 2010; Shafeeyan et al., 2010). Therefore, the functional
groups from the adsorption products mainly include carboxyl,
lactone, carbonyl and quinone groups. The increase of the CO
and CO2 desorption temperature indicates that the lactone
group, with a lower decomposition temperature, decreases,
and carbonyl and quinone groups with higher decomposition
temperatures increase. It can be deduced that the lactone
group is partly transformed into carbonyl and quinone groups
after chlorobenzene desorption. The lactone decrease agrees
well with the results from the FTIR measurement.
According to the electron donor – acceptor mechanism, the
carbon atoms in the benzene ring of chlorobenzene tend to
combine with C = O moieties in lactone, quinone and carbonyl groups, to form weakly adsorbed chlorobenzene. When the
lactone groups combine with chlorobenzene, the adsorption
potential energy of organic gas is larger than that for inorganic
gas (Biniak et al., 1997); therefore, chlorobenzene adsorption is
less affected by the other components. SO2 and NO occupy the
weakly acidic functional groups such as the quinone groups
(Mochida et al., 2000). The greater interaction between SO2
and quinone groups decreased NO and chlorobenzene adsorption. After chlorobenzene adsorption, the increase of
quinone groups can promote only the strongly adsorbed NO,
based on the NO desorption curves in Fig. 4c. Previous studies
have verified that the electrostatic interaction of SO2 molecules with carbonyl groups having dipole moments can
increase the SO2 adsorption (Furmaniak et al., 2013). The
carbonyl group increase after chlorobenzene adsorption
agrees well with the positive effect of chlorobenzene on SO2
adsorption.
To quantitatively determine the changes of the oxygen
functional groups measured by TPD-MS, XPS was also used to
examine the C1s binding energies. The results are shown in
Fig. 6. Only the surface elements on the AC can be measured,
134
J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S 43 (2 0 1 6 ) 1 28–1 3 5
Cl
a
Intensity (a.u.)
O
Phenol
OH
C-C
Carboxyl Lactone
OH
O
O
O
NO
SO2
C-O
COOH
295
290
285
Binding energy (eV)
C-O
Carbonyl
O
SO2
NO
C=O
∗
Lactone
O
C=O
COOH
20
280
∗
Fig. 7 – Oxygen surface functional groups on AC (activated
carbon) and the combine mechanism between SO2/NO/CB
with carbon.
b
Percentage (%)
15
10
5
0
Blank
SO2
NO
CB
SO2+CB NO+CB SO2+NO+CB
Adsorbate
Fig. 6 – (a) C1s spectrum of the oxygen groups on one sample
and (b) area percentage of the functional groups on the
original activated carbon (blank) and adsorption products
under various gas components. CB: chlorobenzene; blank.
so XPS leads to an overestimation due to the microporous
nature of the materials (Shafeeyan et al., 2010). The primary
C1 peak was divided into five peaks based on the binding
energies of 284.6, 286.0, 287.3, 288.6 and 290.4 eV corresponding
to the C-C, C-O, C = O, COOH and π → π* groups, respectively
(Liu et al., 2014). The peak area percentages reflect the relative
content of each group, with the results shown in Fig. 6b.
Compared with the original AC, the C-O groups of the
products significantly increase in the presence of SO2 or SO2,
NO and chlorobenzene, whereas the C = O groups markedly
increase in the presence of NO or chlorobenzene or both. NO
adsorption remarkably increases C = O groups released at
450–460 K as NO is desorbed, which are beneficial for SO2
adsorption and oxidation (Guo et al., 2015; Lee et al., 2003). The
increase in C = O groups after chlorobenzene adsorption is
also verified by the CO desorption increase at 1060–1080 K, as
shown in Fig. 5. Therefore, NO and chlorobenzene have a
promotion effect on SO2 adsorption. The change of the COOH
group is not obvious. In the presence of a large number of
oxygen, containing acidic groups on the carbon surface,
π → π* groups show no marked change due to their contributions to the carbon basicity.
The binding site between SO2 or NO and AC is mainly C = O
and C-O like carbonyl, quinone and phenol groups, while the
chlorobenzene is mainly attached to the lactone groups, with
the schematic diagram shown in Fig. 7. Based on these
findings, NO is greatly affected by SO2 due to occupying the
same adsorption sites. The chlorobenzene adsorption is
strongly affected by the carbon pore structure, but is little
affected by the chemical properties (Guo et al., 2013);
therefore, chlorobenzene adsorption is slightly affected by
SO2 and NO. However, a water atmosphere inhibits chlorobenzene adsorption due to the water cluster formed on the AC
surface, which blocks the narrow pores for chlorobenzene
adsorption. The adsorption sites for chlorobenzene are not
homogeneous due to its high affinity for lactone; therefore,
chlorobenzene adsorption on activated carbon belongs to
micropore adsorption.
3. Conclusions
The SO2 and NO adsorption can be fit with Freundlich
isotherms, indicating that the adsorption sites are heterogeneous. Chlorobenzene adsorption, described by a Langmuir
isotherm, belongs to micropore adsorption. In a singlecomponent atmosphere, the adsorption amount is 65.2 mg/g
for chlorobenzene, 30.9 mg/g for SO2, and 11.3 mg/g for NO.
The binding strength between the gas molecules and AC
follows the order of SO2 > chlorobenzene > NO. The adsorption amount is independent of the binding strength. In
the complex atmosphere conditions tested, the adsorption
amount decreased 27.6% for chlorobenzene and 95.6% for NO,
whereas it increased 202% for SO2 compared with that under a
single-component atmosphere. The complex atmosphere is
quite beneficial for SO2 removal, whereas NO is adsorbed
much more in an atmosphere that has a lower concentration
of SO2.
Chlorobenzene adsorption is less affected by other components due to its high affinity for lactone groups as well as
micropore adsorption mode. After chlorobenzene adsorption,
lactone groups decrease and the increase of C = O groups
promotes SO2 adsorption and strong NO adsorption. NO
adsorption increases the number of C = O groups, which has
a promotion effect on SO2 adsorption. NO adsorption is
depressed by SO2 due to their competition for the same
adsorption sites. Water vapor can enhance the positive or
negative effects of a complex atmosphere on the adsorption of
each component, but it weakens the binding force between
the adsorbates and AC.
J O U RN A L OF E N V I RO N M EN TA L S CI EN CE S 43 (2 0 1 6 ) 1 2 8–1 3 5
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Nos. 21177129, 21207132) and the
Strategic Priority Research Program of the Chinese Academy
of Sciences (No. XDB05050502).
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