Pergamon
PII:
Solar Energy Vol. 73, No. 4, pp. 281–285, 2002
 2003 Elsevier Science Ltd
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SUNLIGHT / ZnO-MEDIATED PHOTOCATALYTIC DEGRADATION OF
REACTIVE RED 22 USING THIN FILM FLAT BED FLOW PHOTOREACTOR
L. SELVA ROSELIN † , G. R. RAJARAJESWARI, ROSILDA SELVIN, V. SADASIVAM,
B. SIVASANKAR and K. RENGARAJ
Department of Chemistry, Anna University, Chennai 600 025, India
Accepted 2 August 2002
Abstract—Photocatalytic degradation of one of the most widely used cotton dyes, namely reactive red 22 (RR
22), was investigated in the presence of a thin film of ZnO photocatalyst using a thin film flat bed flow
photoreactor under solar radiation. The effects of reaction parameters such as pH, amount of ZnO coating, flow
rate and concentration of the dye solution on the percentage removal of dye were examined. In a single pass
mode at 30 ml / min flow rate, 52.7% decrease in concentration was achieved for 200mM dye solution (pH 10).
It has been demonstrated that in continuous circulation mode the time required to decompose half the
concentration of the dye in 500 ml of 200 mM was 15.8 min. Complete removal of 200 mM dye solution (pH
10) was achieved at about 100 min.
 2003 Elsevier Science Ltd. All rights reserved.
the ground level, solar irradiation starts at about
300 nm. In addition, the | 5% UV component of
sunlight is also useful in the excitation process.
Most of the previous reports dealt with photocatalytic reactions induced by irradiation of semiconductor particles suspended in aqueous solution
(Muszkat et al., 1992, 1995; Reeves et al., 1992;
Li et al., 2001). The powder catalysts, however,
suffer from two drawbacks. In the first place the
turbidity created by the catalyst when kept as a
suspension will considerably decrease the amount
of solar light interacting with the catalyst. Secondly, the problem associated with the separation of
the finely divided photocatalyst from the treated
water. Fixing the photocatalyst onto the stationary
inert support is preferable to avoid these complications. The catalyst can be immobilized onto the
surface of various polymeric materials in different
reactor configurations. Krysova et al. (1998) have
studied the photocatalytic degradation of diuron in
aqueous solution on a TiO 2 layer of a batch mode
plate reactor irradiated with UV sun-bed tubes. It
was estimated that the total power required for
diuron to degrade from 1 3 10 24 M to 1 3
10 26 M is 271 Wh / dm 3 . Therefore the artificial
UV light source tends to be somewhat expensive.
When the sun is exploited as a source of light,
capital cost and recurring expenses for artificial
light sources can be saved in the detoxification
plant. Muradov (1994) has suggested immobilized
titania on a plate for the degradation of nitroglycerine and Rhodamine B under solar irradiation. In the present investigation, solar energy-
1. INTRODUCTION
Effluent streams, especially those from textile
dyeing industries, are highly coloured and toxic.
Conventional treatment methods are ineffective in
completely removing the pollutants, thereby
stimulating further research and development
activity in the field. Recent studies have shown
that the photocatalytic method resulted in complete degradation of organics (Ahmed and Ollis,
1984; Poulios et al., 1998). In this technique, a
semiconductor on irradiation with a photon of
sufficient energy, greater than or equal to band
gap energy, promotes an electron from the valence band to conduction band. Once promoted the
electron leaves a vacancy behind in the valence
band called a ‘hole’. The positive holes can
directly oxidize the organic material by removal
of an electron or react with water or OH 2 forming
a powerful oxidant such as a hydroxyl radical.
The conduction band electrons take part in the
reduction reaction with dissolved oxygen, producing superoxide anion radical. Superoxide anion
radical is highly reactive and can oxidize any
organic compound. The minimum energy required
for excitation of an electron for the commonly
used photocatalysts such as TiO 2 and ZnO is 3.2
eV, which corresponds to 387.5 nm. These semiconductors can be activated by sunlight, since at
†
Author to whom correspondence should be addressed. Tel.:
191-442-351-126x3142; fax: 191-442-350-397; e-mail:
[email protected]
281
282
L. S. Roselin et al.
mediated photocatalytic degradation of one of the
widely used cotton dyes, namely reactive red 22
(RR 22), was studied by using a simple plate
photoreactor in a batch mode in which the liquid
film was allowed to flow over the thin layer of
ZnO fixed on a glass plate. The reaction parameters such as flow rate, amount of catalyst used for
coating, pH and concentration of the dye were
optimized.
2. EXPERIMENTAL
2.1. Materials
The dye used in the present study is Remazol
red B (C.I. Reactive red 22 (RR 22), 14282),
2-(3-amino-4-methoxy phenyl sulphonyl) ethanol
sulphate ester → 1-naphthyl-5-sulphonic acid,
widely used in dyeing cotton and was obtained
from Vanavil Dye House, India. The dye was used
as such without any further purification. The dye
shows an absorption maximum at 530 nm (´ 5
9800 l mol 21 cm 21 ). Zinc oxide (99.8% purity)
used as photocatalyst was obtained from Merck.
The surface area and particle size for zinc oxide
are 10 m 2 / g and 110 nm, respectively. All other
chemicals and reagents used for analysis and
experimental work were of analytical grade obtained from Central Drug House (CDH) and used
as such.
2.2. Photoreactor
The thin film flat bed flow reactor consisted of
a glass plate (27.3 3 22.7 cm 2 ) fixed with an
inclination to facilitate the flow of solution. The
glass plate was coated with a thin film (1–2 mm)
of the photocatalyst using ZnO slurry and a thin
layer chromatography applicator. The dye solution
was pumped through a perforated glass tube so as
to allow the flow as a thin film over the entire
surface of the coated glass plate along the long
axis of the support plate and collected into a
receiving flask. The flow rate of the solution was
maintained with the help of a peristaltic pump.
The dye solution was exposed to direct sunlight as
it flowed over the photocatalyst-coated glass plate.
The intensity of the sunlight during the reaction
time was in the range 808–1070 W/ m 2 . The
progress of the reaction was followed by withdrawing 3 ml of the photolysed water from the
receiving flask after each cycle and measuring the
absorbance at 530 nm in a Systronics spectrophotometer (Model 118). Total mineralization
of the dye was confirmed by estimation of inorganic minerals such as sulphate, nitrate and
ammonium ions by standard methods of analysis
(APHA, 1989).
3. RESULTS AND DISCUSSION
3.1. Effect of flow rate
In order to optimize the flow rate of the dye
solution on the ZnO-coated glass plate for the
degradation of the dye, a series of experiments
were carried out at different flow rates in the
range from 15 to 35 ml / min using the same
concentration of the dye solution (75 mM) and the
same amount of catalyst coating (1 g). The thickness of the liquid film and the residence time of
the solution on the catalyst layer at all flow rates
were theoretically calculated as 0.11, 0.14, 0.18,
0.22, 0.26 mm and 4.0, 3.0, 2.4, 2.0, 1.7 s / cm for
flow rates 15, 20, 25, 30 and 35 ml / min, respectively. The % removal of the dye in single-pass
mode was observed for varying flow rate. The
results demonstrate that the % removal of dye
decreases with increasing flow rate and the decrease slows down after a flow rate of about
30 ml / min (Fig. 1). Therefore, a flow rate of
30 ml / min has been fixed for further studies.
3.2. Effect of the amount of ZnO coating
The minimum amount of ZnO required to form
a uniform thin film coating to cover the entire
surface of the chosen glass plate was found to be
about 1 g. The thickness of the ZnO coating can
be optimized for the photocatalytic removal of RR
22. Photocatalytic experiments were performed
Fig. 1. Effect of flow rate on the % removal of the dye ([RR
22], 75 mM; volume, 500 ml; pH 6.8; weight of the catalyst, 1
g (1.57 mg / cm 2 ); solar radiation intensity, 808–1070 W/ m 2 ).
Sunlight / ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor
with various amounts of ZnO coating in the range
1–5 g. The % removal of the dye in single-pass
mode was observed for varying amount of catalyst coating. It is seen from Fig. 2 that the %
removal of the dye increases as the ZnO coating
thickness increases. With increasing amount of
ZnO the availability of semiconductor particles
for absorption of photons increases, thereby
producing a greater number of oxidizing sites,
which consequently increase the rate of the reaction. The increase is not pronounced beyond 3 g
ZnO coating. This may be due to the number of
photons that are absorbed by the active site of the
catalyst which is exposed to sunlight becomes
constant beyond 3 g of ZnO coating since additional photocatalyst is not illuminated. Also the
light intensity profile within the layer is, essentially, decreasing above the saturation level (Corboz
et al., 2000). Therefore the optimum amount of
ZnO for the experimental set-up is to be fixed as 3
g (i.e. 4.7 mg / cm 2 ) having a thickness of 1.4 mm.
3.3. Effect of pH
The pH of the solution can be one of the most
important parameters for the photocatalytic process, since the particle size, surface charge and
band edge positions of the semiconductor oxide
are strongly influenced by pH (Fox and Dulay,
1993). The photocatalytic experiment was carried
out at various pH values in the range 4–12 to find
out the optimum pH. As can be seen from Fig. 3,
when the initial pH of the dye solution varied
from 4 to 12, the % removal of the dye reached a
maximum at 10 followed by a decrease on
reaching pH 12. The pH effect on the photocatalytic degradation of the dye with ZnO was
283
Fig. 3. Effect of pH on the % removal of dye ([RR 22], 75
mM; volume, 500 ml; weight of the catalyst, 3 g (4.7 mg /
cm 2 ); flow rate, 30 ml / min; solar radiation intensity, 808–
1070 W/ m 2 ).
explained on the basis of adsorption of the dye on
the catalyst surface. It was found that with
increasing pH the amount of dye adsorbed is
decreased. The strong adsorption leads to a drastic
decrease in the active centre on the catalyst
surface which results in decrease in the absorption
of the irradiation surface and consequently to a
lowering of the reaction rate. A similar influence
of pH on the photocatalytic degradation rate of
the reactive black 5 dye was reported by Poulios
and Tsachinis (1999). Furthermore, the increase
in the removal of dye with pH may be explained
on the basis that hydroxyl radicals are involved in
the photocatalytic reaction. With increasing pH,
the hydroxyl ion concentration increases and so
the hydroxyl radical formation increases. In alkaline solution the formation of the OHE radical
is much easier than in neutral and acidic solution
(Shourong et al., 1997). In a high pH value (i.e.
pH 12), a drastic decrease in the reaction rate
could be due to the formation of Zn(OH) 2 . In
addition, ZnO undergoes photo-corrosion as given
in Eq. (1) (Domenech and Prieto, 1986)
1
ZnO 1 2h 1 → Zn 21 1 ] O 2
2
(1)
3.4. Effect of initial dye concentration
Fig. 2. Effect of catalyst weight on the % removal of the dye
([RR 22], 75 mM; volume, 500 ml; pH 6.8; flow rate, 30 ml /
min; solar radiation intensity, 808–1070 W/ m 2 ).
The effect of concentration of RR 22 on the %
removal of the dye was investigated in the
concentration range 25–400 mM. It is seen from
Fig. 4 that the % removal of the dye increases
steeply as the concentration of the dye increases
to 100 mM and thereafter there is only a marginal
increase and at 400 mM it decreases. The increase
was due to the increase in the availability of dye
284
L. S. Roselin et al.
was achieved at about 100 min. In single pass
mode at 30 ml / min flow rate, 52.7% decrease in
concentration was achieved for 200 mM dye
solution (pH 10). The finally treated water was
tested for complete mineralization by estimation
of inorganic ions such as sulphate, nitrate and
ammonium ions. Results showed that the solar
photolysis in the presence of immobilized ZnO
has destroyed the RR 22 dye effectively (destruction efficiency reached 98.3%).
4. CONCLUSIONS
Fig. 4. Effect of initial dye concentration on the % removal of
dye (volume, 500 ml; pH 10.0; weight of the catalyst, 3 g
(4.7 mg / cm 2 ); flow rate, 30 ml / min; solar radiation intensity,
808–1070 W/ m 2 ).
molecules for oxidation. The decrease at higher
concentration may be explained as follows: as the
dye solution becomes dense it obstructs the
penetration of light and hence a smaller number
of oxidative species are generated. In addition, at
higher concentrations, a greater number of dye
molecules are adsorbed virtually masking the
surface of the catalyst particle and preventing the
photons interacting with the catalyst. Mengyue et
al. (1995) have observed a similar decrease in the
rate for the degradation of organophosphorus
pesticides using thin films of TiO 2 under UV
radiation.
3.5. Total removal of the dye
Under the optimized conditions using 3 g ZnO
coating, 500 ml of 200 mM RR 22 dye solution
(pH 10) were allowed to flow on the thin film
coated with ZnO at a flow rate of 30 ml / min. The
dye solution was recirculated for complete decolourisation. Table 1 shows the % removal of
dye with number of circulations and time. Complete removal of 200 mM dye solution (pH 10)
Table 1. Photocatalytic removal of RR 22 dye a
No. of circulations
Time (min)
% Removal of RR 22
0
1
2
3
4
5
6
0.0
16.7
33.3
50.0
66.6
83.1
99.5
0.0
52.7
67.3
78.9
89.0
93.8
98.3
a
[RR 22], 200 mM; volume, 500 ml; flow rate, 30 ml / min;
pH 10; catalyst weight, 3 g.
The feasibility of the photocatalytic decomposition of RR 22 dye in aqueous solution using a
simple and inexpensive bench-scale photoreactor
with immobilized ZnO under solar radiation has
been shown. The experimental results demonstrated that the RR 22 dye in the textile effluents
could be completely removed by using solar
energy in conjunction with semiconductor photocatalysts.
Acknowledgements—L. Selva Roselin wishes to thank the
Council of Scientific and Industrial Research (CSIR), New
Delhi, India for financial support through a Senior Research
Fellowship.
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