Journal of Chongqing University (English Edition) [ISSN 1671-8224]
Vol. 8 No. 3
September 2009
Article ID: 1671-8224(2009)03-0165-05
To cite this article: LIU Jian-hua, WANG Huai-jun. Treating dye wastewater by TiO2 coated on coal cinder [J]. J Chongqing Univ: Eng Ed [ISSN 1671-8224], 2009, 8(3): 165169.
Treating dye wastewater by TiO2 coated on coal cinder ∗
LIU Jian-hua 1,2,†, WANG Hai-jun 2
1
Key Laboratory of Yangtze River Water Environment, Ministry of Education, Yibin University, Sichuan 644000, P. R. China
2
Department of Chemical and Material Engineering, Jiangnan University, Jiangsu 214122, P. R. China
Received 21 April 2009; received in revised form 5 May 2009
Abstract: We investigated the photocatalytic degradation of dye wastewater by using titanium dioxide (TiO2) coated on a coal
cinder. The coal cinder was used as the carrier, with a thin film of TiO2 coated on it by using the sol-gel method. Using the
Congo red as the model pollutant for dye wastewater, we studied the decolorization efficiency, and effects of TiO2 film thickness
and roasting temperature on the efficiency. We also evaluated the recycling and regeneration of the immobilized TiO2
(TiO2/cinder). Results show that the decolorization rate of Congo red solution was more than 98% after 2 h treatment when we
used TiO2/cinder calcined at 500 °C for 2 h and coated four times as the photocatalyst. At the same time, the TiO2/cinder
remained high catalytic activity after being reused and regenerated for many times.
Keywords: titanium dioxide; coal cinder; Congo red; dye wastewater; photocatalysis
CLC number: O643.36
1
Document code: A
Introduction a
The majority of dyes used in the textile industry are
the azo dyes, accounting for more than 50% of all
commercial dyes. These dyes include azo groups (N=N)
mainly bound to substituted benzene or naphthalene
rings [1]. Dyes are visible even of small quantities
(≥0.005 mg), and the color of dyes can interfere with
transmission of sunlight into natural streams.
Furthermore, many of the azo dyes and their
intermediate products, such as aromatic amines, are
toxic to aquatic life, carcinogenic and mutagenic to
humans. Consequently, dyes have to be removed from
textile wastewater before discharge [2].
Dye wastewater is characterized by high
concentration, complicated component, deep color, and
difficult to biodegrade. Treatment of wastewater by
conventional chemical coagulation and biological
†
∗
LIU Jian-hua (刘建华): [email protected].
Funded by the Youth Fund Project of Yibin University (No. QJ0528).
methods to reduce biological oxygen demand (BOD),
chemical oxygen demand (COD), and suspended solid
are generally satisfactory, except the removal of dye
color [3]. Therefore, the decolorizing treatment of dye
wastewater is a difficult problem to solve. Many
researchers have studied on the semiconductor as a
photocatalyst in decolorization of dye wastewater [4].
Among a variety of photo catalysts, titanium dioxide
(TiO2) is the most preferable due to its non-toxic,
insoluble, stability, high photoactivity and inexpensive
nature [5-6]. However, the technology of
photocatalysis has not been industrially applied to
treating wastewater. The main problem of the
application is the separation and reuse of powder
photocatalyst. Such difficulty is more serious with
nanometer-scale ultrafine powder [7-8].
We used the coal cinder as a photocatalyst carrier to
solve this problem. Its main compositions are SiO2,
Al2O3, Fe2O3, and CaO [9-10]. TiO2 attached on it can
act as a composite photocatalyst, which shows higher
catalytic activity for facilitating the separation of
electron and hole during the reaction of photocatalysis.
The coal cinder has high adsorbability because it has
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J. H. Liu, et al.
Dye wastewater treatment
many pores on the surface and inner parts. Therefore,
when used as the photocatalyst carrier, it improves the
effect of photodegradation of wastewater and changes
from solid waste into valuables. At present, although
many researchers have studied on the wastewater
treatment using coal fly ash and scrap ion as
photocatalyst carriers, relatively less research has been
carried out on treating wastewater by using
photocatalyst fixed on cinder.
Congo red, which is a stable basic azo dye, is a
compound that contains azo groups (−N=N−). Due to
its high stability, Congo red is commonly used as a
titration indicator and a staining agent [11]. Moreover,
as a typical direct azo dye of biphenyl amine, Congo
red is commonly used in textile industries. Therefore,
we chose it as a model pollutant in this study.
different temperature for 2 h and cooling it at room
temperature. Then the TiO2 films on the cinder of
different layers and roasting temperature were formed.
2
X = ( A0 − A)/A0 × 100% ,
Experimental details
2.1 Preparation of TiO2 on cinder
2.2 Photocatalysis experiments
The reactor was a glass beaker (250 mL), in which
4.5 g cinder coated with TiO2 film were settled near the
surface of solution (about 1 cm). Then, 200 mL
solution of Congo red (20 mg/L) was added into the
beaker and stirred by a magnetic stirrer. For indoor
experiments, the light source was a lamp (Philips TLD
15 W) emitting between 300 nm and 450 nm. During
the reaction, the solution was sampled every 30 min.
The decolorization rate (X) was determined by
measuring the absorbance at 504 nm using an UVvisible spectrophotometer.
where A0 and A were the initial absorbance and the
reacting absorbance of Congo red, respectively.
2.1.1 Pretreatment of cinder
3
Coal cinder was pretreated to remove the chlorine
ion and other organic compounds on it. Loose and
porous cinder from the stokehold of Yibin University
was used, which was broken into granule with the
diameter of about 1.0 cm. The granules were soaked in
a solution of hydrochloric acid (H2O:HCl＝ 1:1) for
24 h, then taken out and washed for three times with
distilled water and anhydrous ethanol, respectively.
After dried in an oven, the cinder was put into muffle
to be calcined at 500 °C for 2 h, and cooled off for
further use.
2.2.2 Preparation of TiO2 film on cinder
The TiO2 film on cinder was prepared by the sol-gel
method. In 100 mL anhydrous ethanol, 20 mL butyl
titanate and 16 mL acetic acid were dissolved under
vigorous stirring for 1 h (solution A). Under stirring, a
mixture of 2 mL triethanolamine, 60 mL ethanol, and
16 mL water was dropwise added into the solution A.
Then, 0.2 mL polyglycol was added in the solution and
stirred for 30 min to form TiO2-sol. The resulting
alkoxide solution was remained in the dark for 16 h to
produce TiO2-gel. The TiO2 film was coated on the
cinder as follows: 1) dipping the cinder in the TiO2-sol
for 30 min and drying at room temperature; 2)
repeating the operation for different times; 3) heating to
166
Results and discussion
3.1 Influence of preparing condition of TiO2 film on
decolorizing efficiency
3.1.1 Calcination temperature of TiO2
We performed a set of tests to study the influence of the
TiO2/cinder calcination temperature on the decolorization
of Congo red. In these studies, we only changed the
calcination temperature of TiO2/cinder while keeping the
coating times of TiO2 on cinder for four times. The total
decolorizing reaction time was 2 h.
The calcination temperature was an important factor
affecting the photocatalysis efficiency. Fig. 1 shows
that the decolorization rate increased as the calcination
temperature of TiO2-cinder increasing. It reached the
peak at 500 °C.
It has been proved that the content of anatase in TiO2
arrays increases with increasing heat treatment
temperature. Near 500 °C, the rutile phase emerges,
and the content of rutile increases as the temperature
increasing [12]. It is widely accepted that the anatase
phase of titania is a relatively ideal photocatalytic
material among its three crystalline phases [13]. In
addition, the anatase with a small fraction of rutile
shows enhanced photocatalytic activity compared to
the pure anatase due to the electron and hole transfer
J. Chongqing Univ. Eng. Ed. [ISSN 1671-8224], 2009, 8(3): 165-169
J. H. Liu, et al.
Dye wastewater treatment
between the two phases [14]. Beckley et al. [15]
presented that the mixed crystal has higher catalytic
activity compared to the pure anatase and rutile. The
catalyst treated at 500 °C has better photocatalytic
efficiency. The reason is that the mixed crystal can
facilitate the separation and prevent the recombination
of cavities with electrons.
(UV) irradiation only; 2) using the cinder only; 3)
using TiO2 under UV irradiation; 4) using TiO2/cinder
under UV irradiation.
Fig. 2
Relationship between coating times and the
decolorization rate
Fig. 1 Influence of calcination temperature of TiO2-cinder on
decolorization rate
3.1.2 Coating times of TiO2 on cinder
We kept the calcination temperature of TiO2/cinder
at 450 °C, and changed the coating times only to study
the influence of the coating times of TiO2 on cinder.
The total reaction time of decolorization was 2 h.
The coating times of TiO2 on cinder was an
important factor affecting the efficiency of the
photocatalysis. Fig. 2 shows that the decolorization rate
increased as more coating times, and it reached the
maximum value at four times. When the coating times
were zero, the decolorization rate was 42%. This means
that the cinder has high adsorbability, and it can partly
decolorize the Congo red without TiO2 coating because
it has many pores on its surface and inner parts.
Increasing the number of the catalyst layers, the
amount of TiO2 coated on the cinder is increased and
the activity of photocatalysis reaction is improved.
However, the cinder can not be coated for too many
times. Otherwise, the coating film would be uneven,
and will be easily crazed and separated from the cinder
surface [16-17].
3.2 Contrast experiment
We decolorized the Congo red solution under the
following conditions respectively: 1) using ultraviolet
Fig. 3 shows that the Congo red could be
decolorized partly using UV irradiation or cinder
without any catalyst. However, the decolorization
efficiency was significantly improved by using
TiO2/cinder as the photocatalyst under UV irradiation,
suggesting that TiO2 could catalyze the decomposition
of Congo red with the exposure of UV radiation.
Recently, Strataki et al. [18] investigated the
photodegradation mechanism of several dyes under UV
irradiation. The photocatalytic degradation of organic
dyes in wastewater using TiO2 (an n-type
semiconductor) is initiated by light of wavelength
λ ≤ 390 nm (3.2 eV). Then, electrons are excited from
the valence band to the conduction band, generating
positive holes and free electrons. The produced
electron-hole pairs can recombine or interact with other
organic substrates on the surface of TiO2 particles via
oxidation and reduction reactions. In an aqueous
solution, positive holes are scavenged by surface
hydroxyl groups to produce very reactive oxidizing
hydroxyl radicals (•OH), which promotes the
degradation process and subsequently leads to the total
mineralization of the organic substrate [6,19].
The first step of organic oxidative decomposition is
the oxidation of organic molecules with hydroxyl
radicals produced on the photocatalyst surface.
Therefore, the adsorption process of organic pollutant
on photocatalyst is very important for heterogeneous
photocatalytic reactions [20]. Coating TiO2 on cinder,
which has strong adsorption capacity, can improve the
decolorization efficiency of the Congo red. Moreover,
J. Chongqing Univ. Eng. Ed. [ISSN 1671-8224], 2009, 8(3): 165-169
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J. H. Liu, et al.
Dye wastewater treatment
it can also solve the problem of TiO2 particles recovery.
Fig. 3 Decolorization rate of Congo red under different
4
Conclusion
We presented a new method combining the effect of
adsorption and photodegradation in treating dye
wastewater. Results show that the coating times of
TiO2 on cinder and the calcination temperature of
TiO2/cinder have considerable impact on the
decolorization efficiency. Coated for four times and
calcined at 500 °C for 2 h, TiO2/cinder had the best
photocatalytic efficiency. The TiO2 coated on cinder
can be recycled and the method of regeneration may be
used to maintain high efficiency of the photocatalyst.
The TiO2 coated on cinder can solve the problem of
separating TiO2 powder from water, and improve the
decolorization efficiency of dye wastewater. However,
full-scale experiments in future research are needed to
ensure the application of this method. The degradation
mechanism should be researched by studying the end
products and intermediates produced during the
photodegradation process of the dye wastewater.
3.3 Recycling use of the TiO2/cinder film
Acknowledgements
We compared the results of reusing TiO2/cinder to
decolorize the Congo red for eight times directly and
ultrasonicating the TiO2/cinder in distilled water for 30
minutes after each reaction (Fig. 4).
Fig. 4 shows that the decrease of decolorization
efficiency was inconspicuous after the TiO2 film was
used for eight times directly. The decrease may be
caused by the long-term or multiple use of TiO2 film,
because possible adsorption of intermediates on TiO2
active sites might make the catalyst site unusable for
degradation of fresh dye molecules [21-22]. The
TiO2/cinder can be recovered simply by ultrasonicating
it in distilled water for 30 min.
We greatly acknowledge the Youth Fund Project of
Yibin University (No. QJ05-28).
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Edited by XUE Jing-yuan
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