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Removal of o-xylene from off-gas by a combination of bioreactor and adsorption.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
Published online 29 July 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.172
Research Article
Removal of o-xylene from off-gas by a combination
of bioreactor and adsorption
L. Li,1 * S. B. Wang,2 Q. C. Feng1 and J. X. Liu1
1
2
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Haidian District, Beijing 100085, P. R. China
Department of Chemical Engineering, Curtin University of Technology, Perth, WA, Australia
Received 5 September 2007; Accepted 17 January 2008
ABSTRACT: Biofiltration is an effective technology for the treatment of gaseous waste. However, a biofiltration system
needs a bioreactor of large volume and it is slow to adapt to fluctuating concentrations in waste gas. To overcome these
disadvantages, a bench-scale system integrating a biofilter and an adsorption unit for the treatment of gases containing
o-xylene was investigated in this study. The adsorption unit was packed with granule active carbon (GAC). The results
showed that 90% of o-xylene could be removed in the biofilter at an inlet concentration below 900 mg/m3 . The
maximum elimination capacity was 80 g/m3 h when the o-xylene loading rate was less than 100 g/m3 h. High o-xylene
concentration in inlet gas resulted in an overload of the biofilter. Using the adsorption unit, the outlet concentration
of o-xylene could be reduced significantly. The surface properties of GAC and the factors affecting the adsorption
performance were investigated, and GAC regeneration method was also evaluated. The combination of adsorption and
microbial processes not only led to a high and stable efficiency of o-xylene removal, but also improved the capacity
of resisting the shock loads.  2008 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: integrated reactor system; biofiltration; adsorption; off-gas treatment; o-xylene; granule active carbon
INTRODUCTION
Volatile organic compounds (VOCs) are generally toxic
gases emitted from wastewater treatment plants and
many industries, such as printing and coating facilities, chemical industries, electronics, and paint manufacturing. Legislation has already been introduced to
reduce their emissions due to their potential threat
to environment and human health. Biofiltration, an
effective and economical method, has gained much
attention in eliminating VOCs. Compared with conventionally physicochemical technologies, biological
treatment technologies present an advantage of complete degradation of the contaminants into innocuous or less-contaminating products.[1] Various VOCs,
e.g. styrene,[2 – 4] xylene,[5] benzene, toluene, ethylbenzene, and xylenes (BTEX),[6 – 8] pentane and toluene,[9]
benzene,[10] toluene and styrene,[11,12] α-pinene,[13]
chlorinated compounds, and p-xylene,[14] have been
reported to be degraded effectively by biological treatment technologies. However, biological processes rely
solely on the capability of specific microbial species to
oxidize the targeted organic pollutants. Biodegradation
*Correspondence to: L. Li, Research Center for Eco-Environmental
Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Haidian District, Beijing 100085, P. R. China. E-mail: leel@rcees.ac.cn
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
of organic contaminants to mineral products occurs
in steps, producing intermediate compounds. An overloaded biofilter may result in high effluent concentration
of untreated gases.[15] For these reasons, bioreactors are
sensitive to surges in VOC loadings, and therefore biological methods are not suitable for treating waste gases
containing relatively high concentrations of VOCs.[16]
Conventional control technologies for VOCs, including incineration, condensation, adsorption, absorption,
ozonation, and membrane separation have been commonly utilized for the elimination of VOCs from waste
gases.[17] Activated carbon has undoubtedly been the
most popular and widely used adsorbent in exhaust
gas treatment throughout the world because of its high
surface area and pore volume.[4] The adsorption of various hydrocarbons, e.g. xylene with three isomers,[18]
toluene and benzene,[19,20] 1,1-dichloroethane and
chloroform,[21] benzene, toluene, and xylenes (BTX),[22]
and other organic materials[23 – 28] had already been
investigated. Activated carbon can also be used to
remove compounds in a once-through process with offsite regeneration. However, regeneration of carbon for
reuse will require further treatment before disposal or a
concentrated vapor stream, which is costly and energy
intensive.
The selection of treatment methods depends on
the nature of the compounds to be treated, their
490
L. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
concentration, flow rate, and mode of emission of the
gaseous waste stream. Despite the difference in emission standards of different countries, the general trend
leans toward a more strict regulation of air emission.
Attention has thus been focused on the combination of
various techniques to meet these strict standards.
In the present study, an integrated reactor system,
consisting of a biofiltration unit and an adsorption unit,
was used to treat gaseous waste containing o-xylene.
The objectives were to further improve the efficiency
in the removal of VOCs by integrating biological and
adsorption processes, and to make the biofiltration unit
strong enough to resist shocking inlet loads.
MATERIALS AND METHODS
Integrated system of biological adsorption
The experiments were conducted repeatedly using a
bench-scale integrated reactor system (Fig. 1). This
system includes a biofiltration unit (a column packed
with porous media) and an adsorption unit. As gases
flow through the biofilter, biodegradable compounds
are absorbed and subsequently biodegraded by the
growth of microorganisms on packing media; then the
gases with residual contaminants enter the adsorption
unit packed with granule active carbon (GAC). These
compounds are then adsorbed and separated from the
gas stream.
o-Xylene biodegradation test was carried out in the
biofiltration unit which is a single-stage plexiglass
column of 10 cm diameter and 100 cm height. The
working volume of the biofiltration unit is 6.28l. It
was packed with porous polyurethane foam cubes for
the growth of microorganisms. Each polyurethane foam
cube is 1 cm3 . Microorganisms were inoculated from
the packing media of the xylene-treating bioreactor
in our laboratory. The sampling ports are located
at both inlet and outlet of the biofiltration unit in
Nutrients
tank
Gas
analysis
Cleaned air
correspondence to compound concentrations prior to
and after the treatment.
A glass tube reactor of 6 cm diameter with a working
volume of 0.70l was used as adsorption unit. The
GAC was used as adsorbent. Sampling ports were
at the bottom and top of the adsorption unit for the
determination of compound concentrations in untreated
and treated gases.
Materials and chemicals
The adsorption on activated carbon is useful for the
recovery of VOCs with intermediate molecular weights
(typically about 45–130), while smaller compounds
cannot be adsorbed well in carbon. o-Xylene (MW =
106.16), one of the main toxic pollutants quoted by the
US Environmental Protection Agency (EPA), was chosen as a testing compound in this study. A commercially
available GAC was obtained from Sinopharm Chemical
Reagent Co. Ltd in China.
Experimental conditions
The synthetic gaseous waste stream was generated as
follows. A small stream of air was bubbled through
a vessel containing pure o-xylene solvent and then
mixed with a large gas stream, resulting in an inlet gas
with a concentration of o-xylene between 670.1 and
2394 mg/m3 . The desired concentration of o-xylene in
the influent air stream was maintained by adjusting the
rates of the two airflows in an up-flow mode.
The experiment was carried out in a laboratory, with
seasonal temperature changing from 23 to 30 ◦ C. The
pH value in the biofiltration unit was measured regularly
to maintain the pH in the range of 5–6. The growth of
microorganisms in the biofiltration unit was maintained
by the nutrients available in the packing material. The
packing material was first soaked in the nutrient solution
(pH 5.5–6.0) before packing, and then packed. Further
nutrient solution (K2 HPO4 : 0.5 g/l, KH2 PO4 : 9.0 g/l,
MgSO4 · 7H2 O: 0.1 g/l, NH4 Cl: 2.0 g/l) was added to
the biofiltration unit once every 2 weeks during the
running period. Redundant nutrient solution was drained
from the bottom of the unit.
The adsorption of o-xylene on the GAC was carried
out at a flow rate of 0.35 m3 /h, corresponding to the
empty bed residence time (EBRT) of 7.2 s.
Air
Biofiltration
unit
Adsorption
unit
o-Xylene Mix
chamber
Figure 1.
system.
Schematic diagram of the integrated reactor
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Analytical methods
Analysis of o-xylene, pH, and relative humidity
of the integrated bioreactor
The performance of the integrated system was monitored by measuring the changes of o-xylene concentration in gases at the inlet and the outlet, the pH, relative
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
REMOVAL OF O-XYLENE FROM OFF-GAS
humidity (RH), and temperature. A pH meter (PH-3C,
Shanghai, China) was used to measure the pH of the
biofiltration unit periodically. The Dewpoint Thermohygrometer (WD-35 612, OAKTON, Germany) was used
to measure RH and temperature. o-Xylene in gas phase
was analyzed by gas chromatography (Agilent 6890N,
USA) with a flame ionization detector (FID). An HP-5
column of Ø 0.32 mm × 30 m was used at 180 ◦ C with
nitrogen carrier gas at a pressure of 25 psi. Serial standard o-xylene gases (National Institute of Metrology,
P. R. China) were used to obtain the calibration curves.
Gas samples (1000 µl) were collected by a gas-tight
syringe (Agilent, 5182–9604) and then were directly
injected into the GC. Data acquisition and processing
were based on an HP Chemstation Data System.
impact at a lower stage. Air that impacts the last Petri
dish goes out.[29,30] Microorganisms in air stream of
six-dimension orders were impacted on nutrient agar
(for bacteria) or the rose Bengal medium (for fungi)
surface of individual Petri dish. All inside surfaces were
maintained in a sterile condition until sampling. After
sampling for 120 s, the plates were removed from the
sampler, covered, inverted, incubated at a temperature
of 30 ◦ C, and enumerated after 7 days. The counts of
all stages were summed and from this value the number
of colony forming units (CFUs/m3 ) was calculated.
Characterization of granule activated carbon
Electron microscopy was used to observe particle morphology of the GAC. Samples were dried with a critical
point dryer (CPD030 Critical Point Dryer) and gold
coated. Observation of samples was conducted with a
scanning electron microscope (SEM) (FEI QUANTA
200, Japan).
The surface area, total pore volume, and pore size
of GAC were determined by N2 adsorption using an
accelerated surface area and porosimetry (ASAP 2000,
Micromeritics, USA). Adsorbents were degassed at
350 ◦ C for 4 h, prior to the adsorption experiments.
The Brunauer, Emmett and Teller (BET) surface area
was determined by applying the BET equation to the
adsorption data. The pore size was obtained using the
Barrett-Joiner-Halenda (BJH) method.
Characterization of GAC
Analysis of microorganisms released from the
integrated bioreactor
Microorganisms in air stream were captured using a
six-stage impacting airborne microorganisms sampler
(Kangjie FA-1, Liaoyang, China). There are six Petri
dishes containing suitable growth medium, kept under
sieves of different pore sizes. Each has 400 pores.
The circular orifice at the top sucks air at a flow rate
of 28.3 l/min, passing through the sieve arranged in
decreasing order of pore size. Large particles (>7 µm
aerodynamic diameter) impact on the first stage and
smaller particles follow until accelerated sufficiently to
(a)
(b)
RESULTS AND DISCUSSIONS
The morphology of GAC was examined using SEM and
is presented in Fig. 2. GAC exhibited a rough surface.
Various sizes of pores could be observed on the surface
or inside the particle. The GAC could give surface area,
pore volume, and pore size of 340.2 m2 /g, 0.289 cm3 /g
and 16.68 Å, respectively. The porous structure would
favor o-xylene adsorption.
o-Xylene adsorption on GAC
A glass tube of 0.8 cm diameter and 10 cm length was
packed with GAC. A measure of 2.0 g GAC was used as
adsorbent in the adsorption tests. A stream with varying
concentrations of o-xylene was passed through the tube
continuously.
Dynamic adsorption at different contact time
Adsorption of o-xylene on GAC was tested to evaluate the adsorption capacity of the GAC. The effect of
contact time on the amount of o-xylene adsorption on
GAC is shown in Fig. 3. As shown, the uptake of oxylene increased rapidly in the beginning, leveled off
gradually, and approached equilibrium. The adsorption
of o-xylene on GAC was 206.6 mg/g and would reach
(c)
Figure 2. SEM of GAC (a) ×50, (b) ×500, (c) ×4000.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
DOI: 10.1002/apj
491
L. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
necessary to identify the types of adsorption mechanism
in a given system. The adsorption isotherm of o-xylene
on GAC at 25 ◦ C is shown in Fig. 5.
Two adsorption isotherms, the Langmuir and Freundlich models, had been used for the fitting of the
experimental data. For solid–gas system, these two
models were listed in following equations[22] :
240
Amount Absorbed (mg/g)
210
180
150
120
90
Qc = α Qe Cin /(1 + αCin )
Langmuir:
60
Freundlich: Qc = kf (Cin )n
30
0
0
20
40
60
80
100
120
140
160
Time (min)
Figure 3. Dynamic adsorption of o-xylene on GAC.
equilibrium at around 120 min with an inlet concentration of 1367.5 mg/m3 and a flow rate of 30 ml/min.
Effect of flow rate
The effects of flow rate on adsorption performance
were investigated at room temperature. The o-xylene
concentration in inlet gases was 2285 mg/m3 . The
results obtained at three different flow rates were
shown in Fig. 4. The GAC exhibited varying adsorption
behavior at different flow rates. The adsorption on
GAC reached equilibrium very quickly at approximately
65 min when the flow rate was 45 ml/min. When the
flow rate was 30 ml/min, it took 90 min to reach
equilibrium while it took a longer time of about 200 min
to reach equilibrium at the flow rate of 15 ml/min. As
the flow rate was increased, mass transfer zone was
shortened. Thus, a short time was required to reach the
bed saturation.
Adsorption isotherms
Although equilibrium sorption studies were important
in determining the effectiveness of adsorption, it was
(1)
(2)
Where, Cin is the inlet concentration of o-xylene
(mol/l); Qe is the amount of gas sorbed (g/g) at
equilibrium; and Qc is the amount of gas sorbed (g/g) at
any inlet concentration Cin ; α is adsorption coefficient
(l/mol); kf and n are Freundlich adsorption isotherms
model constant and exponent, respectively.
Equations (1) and (2) could be transformed to the
following formula:
Langmuir:
Cin /Qc = Cin /Qe + 1/(αQe )
(3)
Freundlich:
ln(Qc ) = n × ln(Cin ) + ln kf
(4)
In Langmuir, the adsorbed amount of monolayer
(Qe ) can be obtained from the slope of the plot of
Cin /Qc with respect to Cin . The Langmuir constant α,
the degree of surface adsorption, showed a positive
relationship with the extent of adsorbate; the adsorbed
amount increases as constant α increases. According to
Freundlich isotherm, the constant kf showed a positive
relationship with the adsorption capacity.[22]
To determine the values of these respective parameters, the relationships Cin /Qc vs Cin , and ln(Qc ) vs
ln (Cin ) were plotted in Figs 6 and 7.
Figures 6 and 7 show that the Langmuir isotherm
expressed relatively better adsorption of o-xylene on
250
3000
2500
200
2000
Qe (mg/g)
o-xylene concentration (mg/m3)
492
1500
1000
500
Q=45ml/min
Q=30ml/min
Q=15ml/min
0
0
50
100
150
200
Time (min)
250
300
350
Figure 4. Effect of flow rate on the o-xylene adsorption.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
150
100
50
0
0
200
400
600
800
1000 1200 1400 1600
Cin (mg/m3)
Figure 5. Adsorption isotherm of o-xylene on GAC at 25 ◦ C.
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
REMOVAL OF O-XYLENE FROM OFF-GAS
1.09 × 106 l/mol and the Gibbs energy of adsorption G ◦ = −34.11 kJ/mol (25 ◦ C), indicating that
o-xylene had a strong adsorption on GAC.
80
70
Cin/Qc (10-6)
60
50
o-Xylene removal by the integrated reactor
system
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
Cin (10-6)
Figure 6. Cin vs Cin /Qc .
-1.5
-1.6
-1.7
ln(Qe)
-1.8
-1.9
-2.0
-2.1
-2.2
-2.3
4.0
4.5
5.0
5.5
6.0
ln(Cin)
6.5
7.0
7.5
Figure 7. ln(Cin ) vs ln(Qc ).
GAC than the Freundlich model in data fitting.
Langmuir isotherm:
Cin /Qc = 1.2493Cin + 1.1465
Qe = 0.80 g/g
α = 1.09 × 106 l/mol (5)
The free energy G ◦ of o-xylene adsorption onto
GAC is calculated based on the adsorption isotherm by
Andersen and Das.[31,32]
α = exp (−G ◦ /RT )
(6)
Equation (6) could be transformed to Eqn (7)
G ◦ = −RT ln α
(7)
The adsorption of o-xylene on GAC obeys the
Langmuir isotherm with an adsorption coefficient α =
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
The capacity of the integrated reactor system for
o-xylene elimination was tested for nearly 1 month. The
o-xylene concentration in inlet gases was in the range of
670.1–2394 mg/m3 . The gas flow rate was 0.35 m3 /h,
corresponding to EBRT of 1.08 min for biofiltration unit
and 7.2 s for the adsorption unit.
The biodegradation of o-xylene in the biofiltration
unit was monitored continually and the adsorption unit
was operated only when the biofiltration unit met with
fluctuating loads. Figure 8(a) demonstrates the changes
in o-xylene concentrations of inlet gas (Cin ), effluent
gas of the biofiltration unit (Cout(B) ), and adsorption
unit (Cout(A) ) when the biofiltration met with fluctuating
loads. The removal efficiencies of the biofiltration unit
(RB ) and the total removal efficiencies of the integrated
reactor system (RT ) are also shown in Fig. 8(b).
The o-xylene concentration in inlet gases was
between 670.1 and 900 mg/m3 in the first week. The
results indicated that o-xylene effluent concentration
from the biofiltration unit could be maintained at
less than 90 mg/m3 , and removal efficiency reached
over 90%. The maximum elimination capacity was
80 g/m3 h when the o-xylene loading rate was less
than 100 g/m3 h. A lot of investigations have shown
that the removal efficiency of pollutants from exhaust
was concentration-dependent for a biofilter.[1,12] The
removal efficiency of a biofiltration unit remained high
at lower inlet concentration and had a decreasing trend
with increase in inlet concentration. In the present
study, the removal of o-xylene was over 91% with
inlet concentrations varying from 670 to 900 mg/m3
and decreased from 87 to 77.5% as the inlet concentrations were increased from 900 to 1914 mg/m3 . Further
increase in the inlet concentrations slightly reduced the
removal efficiency but maintained at 77% (Fig. 9).
As the o-xylene inlet concentration was suddenly
increased over 1000 mg/m3 on the 9th day, the concentration of o-xylene out of the biofiltration unit
was changed from 181 to 375 mg/m3 and the average removal efficiency of biofiltration unit dropped to
81.2%. The overloading occurred in the biofiltration unit
due to its deficient elimination capacity. In order to
ensure elimination efficiency, the adsorption unit was
used. The results shown in Fig. 8 revealed that the
average concentration of o-xylene coming out of the
adsorption unit was still kept at 24.6 mg/m3 , and 98.6%
of the total o-xylene removal efficiency was obtained
in the integrated system. By integrating the adsorption
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
DOI: 10.1002/apj
493
L. LI ET AL.
(a)
Asia-Pacific Journal of Chemical Engineering
confirmed the potential of GAC as a promising adsorbent for the integrated system in controlling o-xylene
emissions with high inlet concentrations.
o-xylene concentration (mg/m3)
3000
Cin
Cout(B)
Cout(A)
2500
2000
Effect of humidity and microorganisms
in stream
1500
1000
500
0
0
3
6
9
12
15
18
Time (days)
21
24
27
21
24
27
100
(b)
Removal (%)
80
60
40
R(T)
R(B)
20
0
0
3
6
9
12
15
18
Time (days)
Figure 8. The concentration (a) and removal (b) of o-xylene
in integrated system.
Biological technologies are based on using microorganisms to biodegrade gaseous contaminants and produce
innocuous end products. This biological degradation
takes place in a biofilter packed with porous solid particles on which pollutant-degrading cultures are immobilized. Packing media in the biofilter often contain
60–80% water to maintain the growth of microorganisms. The RH in gas coming out of the biofilter is
usually over 80%. Moisture could be adsorbed by the
GAC packed in the adsorption tube and leads to a reduction in o-xylene adsorption.
On the other hand, microorganisms growing on the
packing media may be released from the biofilter in
the gas stream. The emissions of microorganisms from
the biofiltration unit and adsorption unit are shown in
Fig. 10. The results revealed that more than half of
CFUs released from the biofilter were captured by the
GAC and retained in the adsorption unit. As the adsorption unit outlet was also the system outlet, it could
prevent the emission of the microorganisms from the
system when the stream passed through the adsorption unit. However, the accumulation of microorganisms
might result in clogging of the adsorption tube.
GAC regeneration
100
Adsorbents should be regenerated after saturated
adsorption. Two regeneration methods were compared
during this work. A stream gas with 23 067 mg/m3 of
90
80
Removal (%)
70
60
1200
50
40
30
0
500
1000
1500
2000
2500
3000
Inlet concentration (mg/m3)
Figure 9. Effect of inlet concentration on o-xylene removal
efficiency.
Microorganisms (CFU/m3)
494
1000
Outlet
Out of biofilter
800
600
400
200
0
unit with the biofilter, residual o-xylene could be effectively adsorbed and prevented from discharging, when
the biofiltration unit was in shocking loads. The findings
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
bacteria
fungi
Total
Figure 10.
CFUs from adsorption unit outlet and
biofiltration unit outlet.
Asia-Pac. J. Chem. Eng. 2008; 3: 489–496
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
REMOVAL OF O-XYLENE FROM OFF-GAS
Table 1. Results of o-xylene adsorption and adsorbent
regeneration.
o-Xylene
(mg/m3 )
23 067
GAC (g)
Sadsorbed
(mg/g)
Sair
(mg/g)
Sheat
(mg/g)
270
183.13
31.95
0.00
o-xylene was passed through the adsorption tube continuously at the ambient temperature and at a flow rate
of 0.35 m3 /h. It reached the saturated adsorption when
the o-xylene concentration in the outlet gas was equal
to that in the inlet gas. The adsorbed amount (Sadsorbed )
of o-xylene was analyzed by taking adsorbent samples
from the adsorption tube.
The residue adsorbents were divided equally into
two sections for different regeneration processes. One
portion of the adsorbent was regenerated using air
stripping. The other was treated at 120 ◦ C for 2 h.
In order to accelerate the process of regeneration, the
flow rate of air was 1.05 m3 /h, 3 times as high as
that in adsorption. It was found that the o-xylene
concentration in air stream decreased dramatically at the
very beginning. Then it dropped slowly and approached
gradually to equilibrium. The regeneration was stopped
when 20 mg/m3 of o-xylene could be detected from the
outlet stream.
The adsorbed and residue o-xylene concentration in
GAC are shown in Table 1. Sair and Sheat correspond to
the residue o-xylene of air stripping regeneration and
heating regeneration, respectively. From Table 1, it can
be seen that 31.95 mg/g of o-xylene remained in Sair
while no o-xylene was found in Sheat . As a large amount
of o-xylene was present, it could not be removed
completely during air stripping regeneration, and hightemperature regeneration had to be employed. It is
suggested that two adsorption tubes be arranged in the
adsorption unit, which could run for either adsorption
or desorption to ensure continuous purification of waste
gases.
The results from the adsorption tests after regeneration showed that the residue o-xylene, adsorbed moisture, and accumulated microorganisms will reduce the
adsorption ability of adsorbents. Thus, GAC could be
reused after heating regeneration, as o-xylene and moisture adsorbed on GAC could be removed and accumulated microorganisms could be degraded under 120 ◦ C
for 2 h while much more moisture, microorganisms, and
some o-xylene still remained in GAC after air stripping
regeneration.
CONCLUSIONS
The integrated reactor system with a biofilter and an
adsorption unit can effectively remove o-xylene with a
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
high removal efficiency. The total removal efficiency
was 98.6%. With the adsorption unit, the removal
efficiency of o-xylene increased, especially when the
biofiltration unit was overloaded.
GAC had a porous structure and a high adsorption
capacity for o-xylene at 206 mg/g. Langmuir model
described best the adsorption isotherm and the Gibbs
energy of adsorption G ◦ was −34.11 kJ/mol (25 ◦ C,
STP). Two regeneration methods for GAC were tested
and it was found that heat treatment under 120 ◦ C
would be better than air stripping and could be used
to effectively regenerate GAC.
Acknowledgement
This work was financially supported by the National
Natural Science Foundation of China (No. 50678171).
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