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DBDregeneration of GAC loaded with acid orange 7.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 649–653
Published online 14 May 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.310
Special Theme Research Article
DBD regeneration of GAC loaded with acid orange 7
Guang-Zhou Qu,1 and Jie Li,1,2 * Guo-Feng Li,1,2 Yan Wu,1,2 Na Lu1,2
1
2
Institute of Electrostatics and Special Power, Dalian University of Technology, 116024 Dalian, China
Key Laboratory of Industrial Ecology and Environmental Engineering, MOE, 116024 Dalian, China
Received 1 September 2008; Revised 23 February 2009; Accepted 25 February 2009
ABSTRACT: Activated carbon (AC) has been widely used as adsorbent in various industrial applications, such as
purification of water in sewage facilities and filtration of air in toxicity-treating factories. However, after exhaustion,
AC should be regenerated and reused because of the limited resources for AC production and additional secondary
pollution of spent-carbon dumped into water or soil. In this study, a process for regenerating AC based on high active
species (O3 , ·OH, HO2 , O2 , ·RO, etc.) generated by dielectric barrier discharge (DBD) oxidation was proposed. The
regeneration of granular-activated carbon (GAC) exhausted with azo dye acid orange 7 was investigated to assay this
method. The influences of the parameters, such as treatment time, electric field and gas kind, on the readsorption rate
were studied systematically. The results of structural properties of GAC analyses showed that the surface area, the
micropore area, external surface area, micropore volume and total volume of GAC after three cycles DBD treatment
decreased to different extent. The adsorption isotherms indicated that the regeneration efficiency was about 81% after
three times DBD plasma regeneration cycles, which confirmed the reuse feasibility of the regenerated GAC.  2009
Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: activated carbon; dielectric barrier discharge plasma; regeneration; acid orange 7
INTRODUCTION
Activated carbon (AC) adsorption has been extensively
and successfully applied to many fields because of
their high adsorption capacity, fast adsorption kinetics
and ease of regeneration. Especially in environmental field, it exhibits a considerable adsorption capacity
toward water and gas pollutions caused by both organic
and inorganic compounds in liquid and gas phases.[1]
However, when the AC reaches its adsorption capacity and no longer is able to adsorb contaminates, the
exhausted AC would simply be taken to the landfill
and discarded or be regenerated for reuse. From the
point of view of resources and environmental protection as well as cost reduction, developing an efficient
approach to regenerate AC is urgently required because
the resources of AC production are limited and the
cost of single use of AC may be so high that its use
is unjustified. Moreover, the discarded spent-carbons
dumping into water or soil can cause additional pollution, which would be considered highly hazardous in
certain cases.[2,3]
*Correspondence to: Jie Li, Institute of Electrostatics and Special
Power, Dalian University of Technology, 116024 Dalian, China.
E-mail: lijie@dlut.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
AC regeneration by dielectric barrier discharges
(DBD) might be a promising method since the mircodischarges between two electrodes are able to generate
ultraviolet light, O3 and plasma, which include highenergy electron and various higher oxidation potential
free radicals such as ·OH, ·HO2 , ·O2 , ·RO, etc. These
active species can oxidize the pollutants loaded on
AC to nontoxic small molecular substances. Kodama
et al .[4] applied DBD plasma to improve adsorbability of granular-activated carbon (GAC) and found that
DBD plasma could enhance the adsorption amount of
GAC to metal ions. Lee et al .[5] also reported that
adsorbability of the DBD plasma-treated AC for Fe2+
was about 3.8 times higher than that of untreated sample. But they only modified the virgin carbon with DBD
plasma. Moreover, their research objects all were metal
ions. So far as we know, there is very little literature
about the regeneration of GAC loaded on pollutions by
DBD plasma.
The purpose of the present study was to investigate
the regeneration of GAC exhausted with azo dye acid
orange 7 (AO7) by DBD plasma, and optimize the
regeneration condition by studying the effects of treatment time, electric field and gas kind on readsorption
rate. The adsorption capacities and structural properties
of GAC samples were also studied. The results obtained
with this procedure are expected to provide some useful
650
G.-Z. QU ET AL.
Asia-Pacific Journal of Chemical Engineering
information for regenerating azo dyes-exhausted GAC
with DBD plasma.
the dielectric barrier and the counter electrode (lowvoltage electrode) was varied from 1 to 20 mm. In
this experiment, the gap is maintained for 15 mm.
Therefore, the gap space between the dielectric barrier and low-voltage electrode had discharge area with
200 mm × 200 mm with 15-mm gap spacing resulting
in a cross-section area for a reaction volume of 600 cm3 .
At terminal of the reactor, an EG-2001 ozone tester was
used to evaluate the ozone concentration from the reactor and a bottle containing 50 ml of 10% KI solution
was used to collect the exhaust gas.
EXPERIMENTAL
Material
A commercial columned coal-based GAC (4-mm diameter, 6–8 mm length, supplied by Gongyi Zhulin Filtrate Material Factory, China) was used as the adsorbent. AO7 with purity greater than 98% (Jierda Dye
Chemicals, Jinzhou, Hebei, China), a widely used
anionic monoazo-dye, was chosen as target compound
in this investigation since it is a pollutant of many
textile industry wastewater and resistance to biological
degradation for adsorption and AC regeneration studies. The distilled water was used to prepare the aqueous
solutions.
The regeneration of GAC
As commented earlier, the regeneration of spent-AC
was carried out in the DBD reactor. Before regeneration,
a known mass of prepared exhausted GAC, the moisture
content was controlled at about 16%, was introduced
into the reactor and was evenly spread over the lowvoltage electrode’s surface of the reactor. In each
regeneration experiment, the thickness of the GAC
beds was kept at 4 mm. Unless special conditions
were required, all experiments were conducted at room
temperature and atmospheric pressure.
The DBD regeneration reaction system
The schematic diagram of the experimental setup was
given in Fig. 1, which mainly consisted of alternating
current high-voltage power supply and DBD reactor.
The frequency of alternating current high-voltage power
supply of this experiment was 50 Hz and the peak voltage was adjustable within 0–50 kV. The DBD experimental reactor was a parallelepiped plexiglas chamber (450 mm × 400 mm × 120 mm), containing two
rectangular parallel-plate electrodes made of stainless
steel, where the upper electrode (200 mm × 200 mm ×
2 mm) was high-voltage electrode, was covered by
a 2-mm-thick dielectric barrier (300 mm × 300 mm
quartz glass plate, the dielectric constant, ε = 7.0), the
nether grounded electrode was the low-voltage electrode
(200 mm × 200 mm × 2 mm). The gap space between
Analysis method
The kinetics adsorption of AO7 on virgin and regenerated carbon was carried out in a rapid small-scale
continuous cycle flow adsorption column (6-cm diameter and 20-cm length), as shown in Fig. 2, where
GAC sample was placed, 200 ml AO7 solution (initial concentration 50 mg/l) was circulated through the
adsorption column by a peristaltic pump at a flow rate
of 160 ml/min. The residual concentration was continuously monitored by a UV–vis spectrophotometer
oscilloscope
HV probe
quartz barrier
discharge
electrode
ozone tester
flow meter
AC
power supply
gas pump
ground
electrode
current
probe
10% KI
solution
Figure 1. The schematic diagram of experimental setup.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 649–653
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
DBD PLASMA REGENERATION OF GAC LOADED WITH AO7
increased slowly with the treatment time increasing
from 2 to 4 h. The possible reason was that pore
structure of GAC suffered from weak destroying though
the GAC was activated by DBD.
Effect of electric field
peristaltic pump
AC
adsorption
column
AO7 storage tank
Figure 2. The schematic diagram of AO7
circulating adsorption experiment.
The electric field significantly influences the production
of active species (such as O3 , OH, etc.), which may
in turn affect the AO7 oxidation. Therefore, it would
also affect the adsorption rate of regenerated carbon.
Fig. 4 showed the adsorption rate of regenerated GAC
under the flow rate of 6.7 l/min oxygen after treatment
for 2 h. It could be found that the adsorption rate
increased quickly with respect to the electric field,
which could be explained from the concentrations of
generated O3 by DBD because O3 was main active
species. Figure 5 showed the relationship between the
produced O3 concentration and the electric field under
60
55
Concentration (mg/l)
[UV-2102C, Unico (Shanghai, China) Instrument Co.,
Ltd.] at the characteristic wavelength of 485 nm.
The adsorption equilibrium isotherms of AO7 on
the virgin carbon and GAC after three different DBD
cycles were measured in accordance with the method
provided by Walker and Weatherley[6] for evaluation of
adsorption capacities and regeneration efficiency.
In order to investigate the structural changes of the
GAC during the DBD plasma regeneration process,
the determination of the physical properties of virgin
and DBD-treated GAC samples was performed with
an automated gas sorption system, using N2 as the
adsorbate at 77 K.
50
saturated carbon
regeneration 1h
regeneration 2h
regeneration 3h
regeneration 4h
45
40
35
30
25
20
15
10
0
20
40
60
80
100
120
Time (min)
Figure 3.
Effect of regeneration time on
adsorption rate of GAC. This figure is available
in colour online at www.apjChemEng.com.
RESULTS AND DISCUSSIONS
55
Effect of treatment time
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Concentration (mg/l)
Since treatment time decided the reaction time of active
species and pollution, it is very necessary to investigate
the influence of treatment time on the readsorption rate
of GAC. Fig. 3 showed the adsorption rate of AO7
on GAC after different treatment time when electric
field was 21.6 kV/cm and oxygen flow rate was 6.7
l/min. As shown in Fig. 3, since the adsorption reached
equilibrium, the readsorption rate for saturated carbon
was almost zero. After DBD plasma treatment, the
adsorption rate increased with regeneration time. It may
be the reason that increasing the treatment time can
prolong the reaction time of active species and AO7,
which is favorable to the degradation of AO7 on GAC.
In addition, it was found that the readsorption rate
19.3kV/cm
20.7kV/cm
21.6kV/cm
22.1kV/cm
30.4kV/cm
50
45
40
35
30
25
20
15
10
0
15
30
45
60
75
90
105 120
Time (min)
Figure 4. Effect of electric field on adsorption rate
of GAC. This figure is available in colour online at
www.apjChemEng.com.
Asia-Pac. J. Chem. Eng. 2009; 4: 649–653
DOI: 10.1002/apj
651
G.-Z. QU ET AL.
Asia-Pacific Journal of Chemical Engineering
6
5
Concentration (mg/l)
Concentration of O3 (g/m3)
4
3
2
1
0
18
20
22
24
26
28
30
55
50
45
40
35
30
25
20
15
10
5
32
6.7 l/min oxygen gas
6.7 l/min air
6.7 l/min nitrogengas
0
15
Electric field (kV/cm)
Figure 5. The relationship between the produced
O3 concentration and the electric field.
the condition of 6.7 l/min oxygen flow rate. It was
obvious that there was direct relationship between the
electric field and the concentration of O3 . As the electric
field increases, the concentration of generated O3 also
enhanced. The fact that it was comparatively easy to
decompose AO7 loaded GAC by large numbers of
O3 molecule. On the other hand, the ultraviolet light
generated by DBD was also an important aspect to the
recovery of adsorption rate of GAC.
45 60 75
Time (min)
90
105 120
of GAC. This figure is available in colour online at
www.apjChemEng.com.
12
10
8
6
virgin carbon
DBD3
4
Effect of gas kind
30
Figure 6. Effect of gas kind on adsorption rate
qe (mg/g)
652
2
0
50
100 150 200 250 300 350 400
Ce (mg/L)
As already known, different oxidation potential active
species would be produced in DBD process when different gas was impregnated into the reactor. Therefore,
gas kind was also another key parameter affecting the
regeneration efficiency and adsorption rate of GAC.
When the O2 , air and N2 of the same flow rate were
injected into the regeneration reactor, it was evident that
the adsorption rate of regenerated carbon had a remarkable enhancement when impregnating O2 (Fig. 6). Since
a little active species was generated by DBD in N2
atmosphere, the adsorption rate of regenerated GAC
was slower in N2 atmosphere than that in O2 and air
atmosphere. The result also proved that active species
oxidation generated by DBD plasma was the principal
reason for the regeneration of exhausted GAC.
Effect of DBD on adsorption capacity of GAC
A simple comparison of adsorption isotherms between
virgin and regenerated GAC after three cycles DBD
treatment allows us to get average information about
the effect of DBD on adsorption capacity, as the
results shown in Fig. 7. It could be found, from the
Fig. 7, that the adsorption isotherm of the regenerated
carbon after three DBD-treated cycles was clearly
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Adsorption isotherms of AO7 on
virgin carbon and three times regeneration cycle
GAC. This figure is available in colour online at
www.apjChemEng.com.
Figure 7.
located under that of virgin carbon. After three DBD
regeneration cycles, the adsorption capacity had an
obvious decrease. It was believed that the adsorption
capacity of GAC to adsorbate depended strongly on
the physical structure of the GAC.[7] It is indispensable
to investigate the effect of DBD on the structural
properties of GAC. After 2-h DBD treatment under the
conditions of 21.6 kV/cm electric field and 6.7 l/min
oxygen flow rate, some relevant structural properties of
virgin and DBD-treated GAC samples were summarized
in Table 1. In order to further investigate the change of
micropore regions (micropores were defined as pores of
width less than 2 nm) in detail, the pore size distribution
curves in micropore regions of GAC samples were also
presented in Fig. 8. The results from Table 1 showed
that the surface area of GAC after three cycles of
DBD treatment decreased from 594.9 to 518.9 m3 /g,
the micropore area, external surface area, micropore
volume and total volume also decreased to different
extent. The same as the change of surface area, the
Asia-Pac. J. Chem. Eng. 2009; 4: 649–653
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
DBD PLASMA REGENERATION OF GAC LOADED WITH AO7
Table 1. Structural properties of the virgin carbon and
three times DBD-treated GAC samples.
GAC
samples
Virgin
carbon
∗
DBD3
∗
SLangmuir SMicropore SExternal VMicropore VTotal pore
(m2 /g) (m2 /g) (m2 /g) (m3 /g)
(m3 /g)
594.9
346.9
66.9
0.1756
0.2899
518.9
290.4
57.1
0.1468
0.2510
GAC sample after three cycles of DBD treatment.
GAC after three cycles of DBD treatment had less
pores than that of virgin carbon in the micropore range.
(Fig. 8), which was consistent with the conclusions
of related literature.[5] But these changes were little,
which was not insufficient to arouse the tremendous
change of adsorption capacity of GAC. Therefore, it
could be predicated that the surface chemistry of GAC
may also play an important role in the adsorption
capacity of GAC aspect, but the causal relation and
exact mechanism between them still need further study.
The regeneration yield was also calculated using the
method provided by Narbaitz and Cen.[8] After three
times regeneration cycles, regeneration efficiency of this
0.035
0.030
technique was about 81%, which confirmed the reuse
feasibility of the regenerated GAC.
CONCLUSIONS
The DBD plasma regeneration of GAC loaded with
AO7 was studied. Longer regeneration time and higher
electric field could enhance the adsorption rate of the
exhausted GAC. Injection of O2 was favorable for
the regeneration of GAC with DBD plasma. After
three cycles of DBD regeneration, the surface area
and micropores volume of GAC decrease more or
less, and compared with virgin carbon, three times
regenerated GAC had less pores in the micropore range.
The results of adsorption isotherms indicated that the
regeneration efficiency was about 81% after three times
DBD plasma regeneration cycles, which confirmed the
reuse feasibility of the regenerated GAC.
Acknowledgements
Financial supports provided by Ministry of Science and
Technology, P.R. China (Project No. 2008AA06Z308)
and Ministry of Education of the People’s Republic
of China (20070141004) are much gratefully acknowledged.
Dv (cm3/g)
0.025
virgin carbon
DBD3
0.020
0.015
0.010
0.005
0.000
2
4
6
8
10 12 14 16 18 20
Pore width (angstrom)
Figure 8. Pore size distribution of virgin carbon
and three times adsorption vs DBD regeneration
cycle GAC. This figure is available in colour online
at www.apjChemEng.com.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
REFERENCES
[1] S. Yenisoy-Karakas, A. Aygün, M. Günes, E. Tahtasakal. Carbon, 2004; 42, 477–484.
[2] C.O. Ania, J.A. Menéndez, J.B. Parra, J.J. Pis. Carbon, 2004;
42, 1383–1387.
[3] M. Sheintuch, Y.I. Matatov-Meytal. Catal. Today, 1999; 53,
73–80.
[4] S. Kodama, H. Habaki, H. Sekiguchi, J. Kawasaki. Thin Solid
Films, 2002; 407, 151–155.
[5] D. Lee, S.H. Hong, K.H. Paek, W.T. Ju. Surf. Coat. Technol.,
2005; 200, 2277–2282.
[6] G.M. Walker, L.R. Weatherley. Chem. Eng. J., 2001; 83,
201–206.
[7] F. Salvador, J.C. Sánchez. Carbon, 1996; 34, 511–516.
[8] R.M. Narbaitz, J. Cen. Water Res., 1997; 31, 2532–2542.
Asia-Pac. J. Chem. Eng. 2009; 4: 649–653
DOI: 10.1002/apj
653
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