Int. J. Chem. Sci.: 8(4), 2010, 2763-2774
________________________________________
*
Author for correspondence; E-mail: abohasan_hilla@yahoo.com
PHOTODECOLORIZATION OF BISMARCK BROWN R IN
THE PRESENCE OF AQUEOUS ZINC OXIDE SUSPENSION
FALAH H. HUSSEIN*
, MOHAMMED H. OBIES and
ABASS A-ALI DREA
Department of Chemistry, College of Science, Babylon University, HILLA (Iraq)
ABSTRACT
Bismarck brown R, (4-[5-C2, 4-diamino-5-methylphenyl) diazenyl-2-methylphenyl] diazenyl-6-
methylbenzene-1, 3-diamine dihydrochloride, an anionic azo dye, was degraded photocatalytically under
UV irradiation using zinc oxide aqueous suspension. The effects of various parameters, such as
photocatalyst mass, pH of aqueous solution, initial dye concentration, light intensity, the type of current
gas and temperature on photocatalytic degradation were investigated. The percentage of decolorization
was calculated from the residual concentration by spectrophotometer.
The results in this study show that the change in temperature was the fewer factors that effect on
the rate of photocatalytic decolorization. The results indicated that the apparent decolorization efficiency
of bismarck brown R rate was increased slightly with increasing temperature. The activation energy of
photocatalytic decolorization was calculated and found to be equal to 24 ± 1 kJ mol-1.
The results indicate that the rate of decolorization was faster than the total mineralization. The
complete decolorization was achieved in less than 60 minutes of irradiation. However, the decrease of
total organic carbon (TOC) was about 88% after the same period of irradiation.
Decolorization and mineralization of bismarck brown R in the absence of light and/or catalyst
were performed to demonstrate that the presence of light and catalyst are essential for the decolorization of
this dye.
Key words: Photocatalytic reactions, Bismarck brown R, Zinc oxide, Decolorization efficiency,
Mineralization.
INTRODUCTION
Azo dyes are used extensively in various industries, such as textile, pharmaceutical,
food, cosmetic and printing industries. Thousands of these dyes are used currently and about
2764 F. H. Hussein et al.: Photodecolorization of Bismarck….
half million tons are produced annually worldwide1
.
Grzechulska and Morawski2
reported that the removal of color from wastewaters is
more important than the removal of other organic colorless chemicals. Decolorization of
dyeing factory effluent was regarded very important because of aesthetic and environmental
concerns3
.
The illumination of suspended semiconductor in an aqueous solution of dye with
unfiltered light (polychromatic light) lead to the possibility of the existence of two
pathways4,5.
(i) In the first pathway, the part of light with energy equal to or more than the band
gap of the illuminated semiconductor will cause a promotion of an electron to conduction
band of the semiconductor and as a result, a positive hole will be created in the valence band.
The formed photoholes and photoelectrons can move to the surface of the semiconductor in
presence of light energy. The positive hole will react with adsorbed water molecules on the
surface of semiconductor producing •
OH radicals and the electron will react with adsorbed
oxygen on the surface. Moreover, they can react with deliquescent oxygen and water in
suspended liquid and produce perhydroxyl radicals (HO•
2) with high chemical activity6
. The
processes in this pathway could be summarized in the following equations:
Semiconductor + h? ? h+ + e?
…(1)
h+
+ OH? ? •
OH …(2)
h+
+ H2O ? H+
+ •
OH …(3)
e? + O2 ? O2
?•
…(4)
O2
?• + H+ ? HO2
•
…(5)
(ii) In the second pathway, the other part of light with energy less than the band gap
of the illuminated semiconductor will be absorbed by the adsorbed dye molecules. Dye
molecules will be decolorized by a photosensitization process. The photocatalytic
decolorization of dyes, which is described as a photosensitization processes are
characterized also by a free radical mechanism. In this process, the adsorbed dyes
molecules(s) on the surface of the semiconductor could absorb a radiation in the visible
range in addition to the radiation with a short wavelengths7-9. The excited colored dye (S*)
(in the singlet or triplet state) will inject an electron to the conduction band of the
semiconductor10. The processes in this path way could be summarized in the following
equations:
Int. J. Chem. Sci.: 8(4), 2010 2765
S + h? (in the visible or UV regions) ? S*
…(6)
S*
+ Semiconductor ? S+•
+ e? (to the conduction band of semiconductor) …(7)
e?
+ O2 ? O2
?• …(8)
O2
?•
+ H2O ? OH?
+ HO•
2 …(9)
S+•
+ OH? ? •
OH + S …(10)
Oliveira et al.11 concluded that the ZnO can be used in the degradation of dyes as an
alternative to TiO2. They observed that ZnO has higher decolorization velocity than TiO2.
Complete decolorization of dyes was achieved after 25 minutes, when ZnO was used while
90 minutes are needed to reach the same result, when TiO2 was used. Sakthivela et al.12 also
found that ZnO can absorb wider spectrum of light than TiO2 can do, when dealing with azo
dye.
The present work, aims to study the photocatalytic decolorization of aqueous
solution of bismarck brown R using ZnO as a photocatalyst. Bismarck brown R, whose
structure is shown in Fig. 1, is a certified biological stain, for microscopy, histology, and
cytology, and also used in textile industries.
•2HCl
H N2
H C3
NH2
N
N
CH3
N
N
H N2 NH2
CH3
Fig. 1: Structure of bismarck brown R
EXPERIMENTAL
Zinc oxide with 99.5% purity was supplied by Merck, E. Merck, Darmstadt.
Bismarck brown R (standard Fluka for microscopy) was purchased from Fluka (product of
U.S.A.) and used without further purification. Solutions were prepared using distilled water.
Photocatalytic decolorization and mineralization processes were carried out in an
experimental setup containing the photoreactor and a gas supply. The gas stream (oxygen or
nitrogen) was continuously flowed through the photoreactor. The radiation source was a
Philips mercury lamp (Germany). The radiation source was positioned perpendicularly
2766 F. H. Hussein et al.: Photodecolorization of Bismarck….
above the reaction vessel. The suspension of ZnO in 100 mL of aqueous solution of
bismarck brown R was illuminated with UV (A) irradiation at intensity ranging from 1.41 to
3.52 mW cm-2. The mean wavelength of ? = 350 nm.
In all experiments, the required amount of the ZnO was suspended in 100 mL of
aqueous solution of Bismarck brown R using a magnetic stirrer. At predetermined times; 2
mL of reaction mixture was collected and centrifuged for 15 minutes. The supernatant was
carefully removed by a syringe with a long pliable needle and centrifuged again at same
speed and for the same period of time. This second centrifugation was found necessary to
remove fine particles of ZnO. After the second centrifugation, the absorbance at certain
wavelengths of the supernatants was determined using ultraviolet visible spectrophotometer;
type Cary 100 Bio UV-visible spectrophotometer Shimadzu (Varian). The
photodecolorization percentage of bismarck brown R was followed spectrophotometrically
by a comparison of the absorbance, at specified interval times, with a calibration curve
accomplished by measuring the absorbance, at 230 and 459 nm, with different
concentrations of the dye solution as shown in Fig. 2. Abs.
Wavelength (nm)
2.0
1.5
1.0
0.5
0.0
200 300 400 500 600 700 800
Fig. 2: UV-Visible spectra of different concentrations of bismarck brown R
Mineralization of bismarck brown R was assessed by following total organic carbon
(TOC) and total inorganic carbon (TIC) at different times of irradiation by using TOC
5000A Shimadzu analyzer.
pH of the solutions was adjusted with 1 M HCI or 1 M NaOH.
Performance efficiency was obtained by using the following equations:
Int. J. Chem. Sci.: 8(4), 2010 2767
% Degradation efficiency =
C C o t ?
Co
x 100 …(11)
% TOC degradation =
TOC TOC o t ?
TOCo
x 100 …(12)
where, Co and Ct are the initial and final concentration of dye for time t of irradiation
and TOCo and TOCt are initial and final total organic carbon of dye for time t of irradiation
RESULTS AND DISCUSSION
Effect of catalyst concentration
Fig. 3 shows the effect of catalyst concentrations on the decolorization of bismarck
brown R. The rate of decolorization increased with the increasing catalyst concentrations
from 1 g L-1 to 3.75 g L-1. Thereafter the rate of decolorization remains constant and then it
decreased with increasing catalyst concentration. These results strongly agreed with our
previous findings13,14. This behavior could be explained due to increasing total active surface
area with the increasing catalyst concentration and hence, more active sites on catalyst
surface will be available.
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Decolorization efficiency
Amount of catalyst (mg\100 mL)
Fig. 3: Effect of mass of ZnO on decolorization %
The increase in catalyst concentration above a maximum level will increase the
number of particles suspended in the aqueous solution of dye (increasing the turbidity of the
suspension) and as a result, there will be decrease in penetration of irradiation and hence,
photoactivated volume of suspension decreases15,16.
2768 F. H. Hussein et al.: Photodecolorization of Bismarck….
Effect of pH
The effect of pH on the efficiency of decolorization of 46 ppm of bismarck brown R
was carried out at different pH ranging between 2-12. The results are given in Fig. 4.
Decolorization efficiency was found to depend strongly on pH of solution because the
reaction take place on the surface of semiconductor. Fig. 4 shows that the decolorization
efficiency of bismarck brown R increased with increasing pH, exhibiting maximum
decolorization efficiency at pH 9. This behavior could be explained on the basis of zero
point charge (ZPC)16. The ZPC of ZnO is 9 and with the increase in pH of solution, the
surface of ZnO will become negatively charged by adsorbed hydroxyl ions; However in pH
lower than ZPC, the hydroxyl ions adsorbed on the surface will be decreased and as a result,
the formation of hydroxyl radicals, which is mainly effective in decolorization process, will
decrease. Fig. 4 shows that the rate of decolorization was decreased dramatically in strong
acid media (pH = 2.1). This could be explained due to photocorrosion of ZnO17.
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70
pH=2.1
pH=4.5
pH=6.7
pH=9.0
pH=12.0
C/Co
Time (min)
Fig. 4: Effect of pH on decolorization %
Effect of dye concentration
The effect of initial dye concentration on the photocatalytic degradation of bismarck
brown R was studied at different concentrations of dye in the range of (0.2-1.0) x 10-4 M. Fig.
5 shows the percent degradation at various initial dye concentrations. It was observed that
the percent degradation gradually increased with the decreasing initial dye concentration.
Percentage of decolorization was found to be 96.2, 92.8, 87.5, 76.9, and 67.5 at (0.2, 0.4, 0.6,
0.8 and 1.0) x 10-4 M initial concentrations of dye, respectively.
Int. J. Chem. Sci.: 8(4), 2010 2769
This behavior may be due to the decrease in the concentration OH?
adsorbed on
catalyst surface with the increasing dye concentration. The competitions between OH?
ions
to adsorb on active sites of the catalyst will be in the favor of dye ions, when the
concentration of dye was increased13,18. As a result, •
OH formation rate decreased and then
the rate of decolorization also decreases. The inverse proportionality of rate of
decolorization with dye concentration may be also due to increase of reduction of light
intensity reach the catalyst surface and consequently, photon absorption on surface of
catalyst is also reduced with the increasing dye concentration13,18.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80
0.0001M
0.00008M
0.00006M
0.00004M
0.00002M
C/Co
Time (min)
Fig. 5: Effect of initial concentration of bismarck brown R on
photodegeradation efficiency
Effect of temperature
Reaction was followed at different temperatures in the range 285.15- 301.15 K using
350 mg of zinc oxide. The results indicate that the decolorization efficiency of bismarck
brown R with time increases with increasing temperature. Fig. 6 shows that the rate of
decolorization increases with time at four different temperatures.
The acceleration of rate of photocatalytic decolorization of bismarck brown R by a
rise in temperature may be related to the promotion of the production of free radicals with
the increasing temperature19,20. However, the results indicated that the variation in
temperature within the range of 285.15 to 301.15 K does not significantly affect the
photocatalytic degradation of bismarck brown R. These results confirm those presented by
previous authors21-24, where the effect of temperature was explained as the variable with the
smallest effect, especially for values near 323.15 K, where the limiting stage is the
2770 F. H. Hussein et al.: Photodecolorization of Bismarck….
adsorption of the dye on the surface of catalyst, but at low temperature, the desorption of the
products formed limits the reaction because it is slower than the degradation on the surface
and the adsorption of the reactants on the surface of catalyst23.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50
T= 285.15K
T= 290.15K
T=295.15K
T=301.15 K
C/Co
Time (min)
Fig. 6: Effect of temperature on the photodegradation efficiency of bismarck brown R
The activation energy of 24 ± 1 kJ mol-1 for photocatalytic decolorization efficiency
of bismarck R brown was calculated from Fig. 7. The low value of activation energy in this LnK
3.3 3.35 3.4 3.45 3.5 3.55
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
1000/T
Fig. 7: Arrhenius plot
work is similar to our previous findings24-27 for photocatalytic oxidation of different types of
alcohols on anatase and metallized anatase. Kim and Lee23 explained that the very small
Int. J. Chem. Sci.: 8(4), 2010 2771
activation energy in photocatalytic reactions is the apparent activation energy Ea, whereas
the true activation energy Et is nil. These types of reactions are operating at room
temperature. The apparent activation energy tends to the heat of adsorption of the product
whereas desorption of the final product from the surface of catalyst is the limiting step.
Effect of light intensity
The results listed in Table 1 indicates that the photocatalytic decolorization
efficiency of bismarck R brown increases with increase in light intensity, attaining 100% at
2.93 mW cm-2.
Table 1: Effect of light intensity on photocatalytic decolorization efficiency
Light intensity(I) (mWcm-2) P.D.E. %
0.55 97.7
1.05 99.2
1.41 99.6
1.97 99.92
2.93 100
3.52 100
These results are in good agreement with the findings of Lim and Kim28. They
reported that at light intensity more than one sun equivalent (1-2 mWcm-2, the increase of
rate of reaction is proportional to the square root of light intensity. However, at light
intensity less than one sun equivalent, the increase of rate of reaction is directly proportional
to the light intensity.
Mineralization of bismarck brown R
The results shown in Fig. 8 indicate that photocatalytic decolorization of bismarck R
brown was faster than the decrease of total organic carbon (TOC). The results show that the
complete decolorization was achieved in less than 60 minutes of irradiation, while the
decrease of total organic carbon (TOC) was about 88% in the same period of irradiation.
These findings are in good agreement with those reported before19-20,29. This may be related
to the formation of some by products, which resist the photocatalytic degradation.
2772 F. H. Hussein et al.: Photodecolorization of Bismarck…. TOC (%)
Time (min)
Fig. 8: Mineralization of bismarck brown R
CONCLUSIONS
(i) Control experiments indicated that the presence of UV light, oxygen and zinc
oxide were essential for the effective destruction of dye.
(ii) The photocatalytic decolorization of bismarck brown R using zinc oxide as
photocatalyst strongly depends on the amount of catalyst, concentration of dye,
pH, and light intensity.
(iii) The temperature is the factor with the smallest effect on the photocatalytic
decolorization of bismarck brown R.
(iv) Photocatalytic decolorization of bismarck R brown was faster than the decrease
of total organic carbon (TOC).
(v) The photocatalytic decolorization process can expressed by both; the pseudo
first order reaction kinetics and the Langmuir-Hinshelwood kinetic model.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge to Prof. Dr. Detlef Bahnemann, "Photocatalysis
and Nanotechnology" (Head), Institut fuer Technische Chemie, Gottfried Wilhelm Leibniz
Universitaet, Hannover (Germany) for providing necessary laboratory facilities.
Int. J. Chem. Sci.: 8(4), 2010 2773
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Revised : 24.11.2010 Accepted : 25.11.2010