Abstract—In this study, a composite photocatalyst of TiO2/silica
was prepared, analyzed and used in a batch reactor for the
photocatalytic degradation of synthetic textile (methyl orange)
wastewater. The successive photocatalyst was characterized by
Scanning Electron Microscopy and Energy Dispersive Spectroscopy
(SEM- EDS) and zeta potential (ZP) analyses. Operating parameters
such as loading, initial pollutant concentration and pH were
optimized. The performance of the photocatalyst reactor was
evaluated on the basis of color removal and degradation kinetics. The equilibrium data was analyzed using the Langmuir and
Freundlich isotherm models. Experimental data was best
represented by the Langmuir isotherm model. The maximum
adsorption capacity of dye onto composite photocatalyst was found
to be 20.24 mg/g. The pseudo second order kinetic model adequately
described the kinetic data.
Keywords—Composite photocatalyst, kinetics, methyl orange,
photocatalysis
I. INTRODUCTION
HE accelerated growth of textile industry and the
increasing worldwide concern for environmental
conservation have revealed the great problem of
environmental pollution by azo dyes. Due to the stability and
toxicity of azo dyes and the presence of residual surfactants,
azo dye textile wastewaters are resistant to the conventional
biological treatment [1].
With this growing demand, heterogeneous photocatalysis employing TiO2 semiconductor catalyst is one of the few
attractive alternatives to resolve this problem. A number of
important features for the heterogeneous photocatalysis have
extended their feasible applications in water treatment, such
as operating at ambient temperature and pressure, low
operative costs and complete mineralization of parents and
Kwena Pete was with the Faculty of Technology, Lappeenranta University of
Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland. She is now with the
Centre for Renewable and Water, Vaal University of Technology, Private Bag
X021 Vandebijlpark 1900, South Africa (*Corresponding author. Email address:
[email protected]; Tel. +27-73-446-2988; Fax: +27-16-950-9796).
Mika M Sillanpää is with the Faculty of Technology, Lappeenranta
University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland
Maurice S. Onyango is with the Department of Chemical and Metallurgical
Engineering, Tshwane University of Technology, Pretorai,0001
Ochieng Aoyi is with the Centre for Renewable and Water, Vaal University
of Technology, Private Bag X021 Vandebijlpark 1900, South Africa
their intermediate compounds without secondary pollution
[2]. The post-separation of the semiconductor TiO2 catalyst
after water treatment remains as the major problem towards
the practicality at an industrial process. The fine particle size
of the TiO2, together with their large surface area-to-volume
ratio and surface energy creates a strong tendency for catalyst
agglomeration during the process. Such particles
agglomeration is highly disadvantageous in views of surface-
area reduction and its reusable lifespan. To solve this
problem, TiO2 catalyst powder is loaded onto supporting
silica material in such a way as to provide high surface area
and accessibility of the catalyst.
The main aim of this work was to investigate the
heterogeneous photocatalytic degradation of methyl orange
dye using TiO2/silica composite photocatalyst. The effect of
operating parameters such as the composition of the
composite particle size, catalyst loading, initial pollutant
concentration and solution pH were determined.
II. MATERIALS AND METHODS
A. Materials
Chemicals used were of analytical reagent grade, used as
received and purchased from Sigma Aldrich. Methyl orange
(MO, 99 %,), an azo dye pollutant in wastewater. The TiO2
was used as photocatalyst with Ludox HS-30 colloidal silica,
as the supporting material.
B. Synthesis of Composite photocatalyst
The TiO2 supported-silica samples were prepared by
adding Ludox HS-30 colloidal silica solution to TiO2 with
proper mixing to ensure a homogeneous TiO2 supporting onto
silica. After mixing, the mixture was dried at 110 °C and
screened to different particle sizes (particle sizes 0-38 75-150,
150-250 µm). The coated particles were then dispersed in
Mill Q-plus water, till the pH of the suspension was close to
6.5. By this procedure, a sample of 3-20 wt% of TiO2 onto
silica particles was prepared.
C. Photocatalytic experiment
Photocatalytic degradation experiments were carried out in
100 ml semi batch reactor at 25 ± 3°C. Milli Q-plus water
(resistance = 18.2 M.Ω) was used for all experimental work.
Analysis of Kinetic Models in Heterogeneous
Catalysis of Methyl Orange Using TiO2/Silica
Composite Photocatalyst
Kwena Y. Pete, Mika M Sillanpää, Maurice S. Onyango, and Ochieng Aoyi
T
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
169
The UV lamp (low-pressure mercury lamp, wave length, 254
nm (Pen-Ray), with an intensity of 5.5 mW/m2), protected by
a quartz sleeve was used. For all experiments, the solution
was left to equilibrate for 30 min in the dark before the lamp
was switched on. This was sufficient to reach an equilibrated
adsorption as deduced from the steady-state concentrations.
Samples were taken every 30 minutes and immediately
filtered through a polypropylene syringe filter (0.45 µm) in
order to remove the photocatalyst.
D. Characterization
The surface morphology of the the successive composite
photocatalyst was characterised by SEM equipped with EDS
(Hitachi S-4800 Ultra-High Resolution Field Emission
Scanning Electron Microscope). Zeta sizer Nano Series model
ZEN 3600 (Malvern, the UK) was employed to measure the
isoelectric points before the degradation tests.
E. Chemical analysis
The changes in the intensity of dye colour were observed
from its characteristics absorption band, at 466 nm using UV-
vis spectrophotometer (Perkin-Elmer Lambda-45
spectrophotometer).
III. RESULTS AND DISCUSSION
A. Characterization
Prior to photodegradation experiments, material
characterization was done using SEM-EDS and ZP analyses
techniques. These techniques helped to interpret the
photocatalysis process under TiO2/silica composite
photocatalyst composition.
1. SEM-EDS analysis
The morphology of the successive photocatalysts with 75-
150 µm particle size was characterized by SEM technique, its
images and EDS spectra are shown on Fig. 1 and Fig. 2. The
prepared TiO2/silica photocatalyst exhibited a regular
morphology which is related to the dispersion of TiO2 on
silica surface. The EDS spectras revealed that the Ti was
detected, which clearly shows that crystallites of TiO2 is well
dispersed on the surface of the silica supporting material.
Fig. 1 SEM images of 15 % TiO2/silica composite photocatalyst
Fig. 2 EDS spectra of 15% TiO2/silica composite photocatalyst
2. Isoelectric point measurements (IEP)
The point of zero charge (PZC) of TiO2/silica composite
was determined by zeta potential measurements (Fig. 3).
The IEP for the particles was at pH 2.0. This results are
consistent with the findings revealed by Papirer [3] and
Persello [4] where the silica’s point of zero charge ranges
between 2 and 3. In addition, the surface charges were
changed from positive to negative as a function of pH due
to the unprotonization of the surface groups. At a pH less
than the IEP, the surface has a positive charge due to the
formation of Si-OH2+. At a pH above the IEP, the surface of
the silica has a negative charge due to the deprotonation of
the silanol group resulting in Si-O-.
Fig. 3 Zeta potential of TiO2/silica as a function of pH
B. Photocatalytic degradation of methyl orange
Control experiment was conducted on two different
conditions, dark adsorption over composite photocatalysts and
photolysis under UV light only (Error! Reference source
not found.4). No significant change was observed during
adsorption test after 180 minutes. This elucidates the
attainment of adsorption equilibrium and 30 min was used in
all the experiments throughout. In direct photolysis, 58%
colour removal was achieved during 210 min by the
assistance of light. To recognize the role of support material
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
170
during the photocatalytic degradation of methyl orange, the
15 wt% TiO2/silica composite was compared with
unsupported TiO2 amount equivalent to the one available on
both photocatalyst for the degradation efficiency. It can be
clearly seen from Fig. 4 that the complete degradation of
methyl orange was observed over supported system, whereas
the degradation over TiO2 itself did not reach complete
degradation even at 210 min. This can be explained in terms
of the interaction between TiO2 and supporting material
(silica), as well as the different structure from bulk TiO2.
Taking into account the surface analysis result, the modified
photocatalyst TiO2 samples having a larger surface area could
allow a larger amount of surface adsorbed species [5].
Fig. 4 Adsorption of methyl orange over silica loading ( );
photocatalytic degradation of methyl orange over UV ( ); UV-
TiO2/silica ( ); UV-TiO2 ( )
C. Effect of composite photocatalyst loading
To find the optimum photocatalyst loading, the effect of
photocatalyst was investigated by varying the loading of the
photocatalyst in 25 mg/L methyl orange solution. It is
apparent from Fig. 5 that above 0. 1 g photocatalyst loading
the reaction rate aggravate and becomes independent of the
photocatalyst loading.
Fig. 5 Effect of TiO2/silica photocatalyst loading 0.1 g ( ); 0.5 g
( ); 1 g ( )
This can be ascribed in terms of availability of active sites
on TiO2/silica surface and the light penetration of
photoactivating light into the suspension. The availability of
active sites increases with the suspension of photocatalyst
loading. Additionally, the decline in the percentage of
degradation at higher catalyst loading may be due to
deactivation of activated molecules by collision with ground
state molecules [6].
D. Effect of dye initial concentration
The characteristic of organic dye concentrations in
wastewater from textile industry is usually in the range of 10-
50 mgL-1 [7]. Therefore, methyl orange solutions were varied
in the range 5-50 mgL-1 during the photocatalytic degradation
at solution pH 6.3. The degradation rate was found to
increase up to an initial concentration of 35 mgL-1and then
decreased (Error! Reference source not found.6). The
inadequate number of surface sites on silica/ TiO2 particles
may affect the photodegradation efficiency. As the
concentration of methyl orange pollutant increase, more
molecules of the compound get adsorbed on the surface of the
photocatalyst. As a result, the reactive species (·OH and ·O2-)
needed for the degradation of dye also increases. However,
the formation of ·OH and ·O2- on the composite photocatalyst
surface remains constant for a 5.5 mW/m2 light intensity and
0.1 g catalyst loading for 210 minutes duration of irradiation.
Hence, the available OH radicals are inadequate to attack the
methyl orange molecules at higher concentrations due to
constant reaction conditions.
Fig. 6 Effect of initial concentration on TiO2/silica photocatalyst 5
mgL-1 ( ); 15 mgL-1 ( ); 25 mgL-1 ( ); 35 mgL-1 ( ); 50 mgL-1 ( )
E. Effect of solution pH
To study the effects of H+ concentrations on dye
degradation, comparative experiments were performed at
three pH values: 3, 7 and 9 (Fig. 11). In the acidic pH,
minimization of electron–hole recombination is a key factor
for the enhanced degradation of methyl orange. At pH 3, the
surface of silica/TiO2 is positively charged and thus attracting
the largest amount of negatively charged anions of methyl
orange, hence there is a substantial degradation. Zeta
potential (ZP) measurements indicate that the surface charge
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
171
of the TiO2/silica composite photocatalyst decreased as the pH
increased signifying repulsive forces between photocatalyst
surface and dye in alkaline media. Furthermore, the limiting
behavior in alkaline medium is due to the negative charge of
a composite photocatalyst and therefore methyl orange
transformation products can be repelled away from the
silica/TiO2 surface which opposes the adsorption of substrate
molecules on the surface of the composite photocatalyst. As a
result, methyl orange degradation declines in alkaline
medium
F. Photodegradation isotherms and kinetics of methyl
orange
The isotherm modelling basically reflects the interaction
between substrate and photocatalyst until the state of
equilibrium is reached. In order to optimize the effectiveness
of the TiO2/silica on the photodegradation of methyl orange,
linear form of Langmuir and Freundlich models were applied
for the TiO2/silica composite photocatalyst. For brevity, the
linear form of the Langmuir and Freundlich isotherm model
are described by (1) and (2), [8, 9]. The correlation
coefficients (R2) along with other parameters for two different
models were calculated and listed in Table I. Clear deviations
were observed in the Langmuir and Freundlich isotherm
models. Fig. 7 indicates a straight line with a correlation
coefficient (R2) of 0.995 signifying that the degradation of
methyl orange onto the composite photocatalyst fits the
Langmuir isotherm reasonably well. The Freundlich
constants, Kf and n, calculated from this investigation, are
2.3*10-3and 0.399, respectively. This implies poor fitting for
the Freundlich isotherm model. Comparison of the correlation
coefficients of both models suggests that the Langmuir model
is suitable. The fact that the Langmuir isotherm fits the
experimental data well can be due to homogeneous
distribution of active sites on the TiO2/silica surface.
(1)
where, qm (mgL-1) and KL (Lmg-1) are the Langmuir
constants related to adsorption capacity and rate of
adsorption, respectively, qe is dye concentration at
equilibrium onto adsorbent ((mgL-1), Ce is dye concentration
at equilibrium in solution (mgL-1).
(2)
where Kf is the Freundlich constant related to adsorption
capacity, nf is measure of the surface heterogeneity, ranging
between 0 and 1. For linearization of the data, the Freundlich
equation is written in logarithmic form:
(3)
Photocatalysis is time dependent process and it is very
imperative to determine the rate of photocatalysis for
designing and evaluating the photocatalyst in removing the
pollutants from waste water. The data for the
photodegradation of methyl orange on the composite
photocatalyst was applied to pseudo first and second order
kinetic model, and the results are presented in Table II. Fig. 9
and Fig. 10 shows the plot of pseudo first order and pseudo
second order kinetic model, respectively. The correlation
coefficient for the second order kinetic model (0.999) is
greater than that of the first order kinetic model (0.936).
Therefore the dye photocatalytic system by TiO2 /silica is a
second order reaction. The Pseudo-first-order and second
order model equation are given in (4) and (5), [10]:
(4)
where qt and qe are the adsorption capacity (mmol/g) at
time t and at equilibrium respectively, while k1 represents the
pseudo first order rate constant (min -1). The pseudo first
order model was generalized to two-site-occupancy
adsorption to form a pseudo-second-order equation:
(5)
where k2 is the pseudo second order rate constant (g/mmol
min). It has been distinguished that this model is able to
estimate experimental qe values quite well and is not very
sensitive for the influence of the random errors [10]
Fig. 6 Effect of solution pH on TiO2/silica photocatalyst pH 3 ( );
pH 7 ( ); pH 9 ( )
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
172
Fig. 7 Linear transform of Langmuir isotherm
TABLE I
ISOTHERMS CONSTANTS AND CORRELATION COEFFICIENTS FOR THE
DEGRADATION OF METHYL ORANGE ONTO COMPOSITE PHOTOCATALYST
Langmuir
isotherm
Freundlich
isotherm
qm (mg/g)
k (min -1
)
R2
n Kf R2
20.24 6.5 0.999 0.399 2.3*10-3 0.5987
Fig. 8 Linear transform of Freundlich isotherm
Fig. 8 Pseudo first order plots for the photocatalytic degradation of
methyl orange dye onto TiO2/silica
Fig. 9 Pseudo second order plots for the photocatalytic degradation
TABLE II
KINETIC PARAMETERS FOR DEGRADATION OF METHYL ORANGE
Pseudo first order model k (min -1
)
0.122
qe (mg/g)
0.95
R2
0.936
Pseudo second order model k (min -1
)
0.118
qe (mg/g)
0.844
R2
0.999
IV. CONCLUSION
The characterization of TiO2 supported silica revealed the
well dispersion of TiO2 on the surface of silica. The optimum
conditions for the degradation of methyl orange at pH 3
include an initial concentration of 25 mg/L and 0.1 g of
TiO2/silica composite photocatalyst loading. The equilibrium
studies showed that the Langmuir model was most accurate
and the methyl orange dye photocatalytic system by TiO2
/silica composite photocatalyst is a second order reaction.
ACKNOWLEDGMENT
The This research was funded by the European Union and
the city of Mikkeli, Finland and the Water Research
Commission, South Africa.
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