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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


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


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 ( )


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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