POTENTIAL APPLICATION OF A LOCALLY SOURCED PHOTOCATALYST FOR THE PHOTOCATALYTIC ...jestec.taylors.edu.my/Vol 10 Issue 12 December 2015... · 2015. 12. 4. · 2.3. Photocatalytic degradation

Journal of Engineering Science and Technology Vol. 10, No.12 (2015) 1641 - 1653 © School of Engineering, Taylor’s University

1641

POTENTIAL APPLICATION OF A LOCALLY SOURCED PHOTOCATALYST FOR THE PHOTOCATALYTIC

DECOLOURISATION OF METHYL ORANGE IN AQUEOUS SOLUTION

F. A. AISIEN, N. A. AMENAGHAWON*, O. I. URHOBOTIE

Department of Chemical Engineering, Faculty of Engineering, University of Benin,

PMB 1154, Ugbowo, Benin City, Edo State, Nigeria

*Corresponding Author: [email protected]

Abstract

Periwinkle shell ash (PSA) was investigated for its photocatalytic properties in decolourising methyl orange in aqueous solution. The effects of irradiation

time, initial dye concentration and PSA dosage on the decolourisation process

were investigated. The optimum values of the process variables were:

irradiation time, 50 minutes; initial dye concentration, 20 mg/L and PSA

dosage, 3 g/L. The kinetics of the process was well described by the pseudo

first order and Langmuir-Hinshelwood kinetic models while the adsorption

equilibrium was well elucidated by both the Langmuir isotherm and Freundlich

isotherm equations with high R2 values.

Keywords: Periwinkle shell ash, Photocatalysis, Kinetics, Adsorption equilibrium.

1. Introduction

The textile, paper, food and beverage and other related industries generate large

quantities of wastewater as a result of the high water requirements of the operations

typically encountered in these industries [1]. The waste waters from these industries

typically contain residual dyes which are not biodegradable with typical examples

being methyl orange, acid yellow, methylene blue, remazol red, etc. [2-4].

Konstantinou and Albanis [5] reported that textile dyes and other industrial

dyestuffs make up one of the largest groups of organic compounds that represent an

increasing environmental threat as a result of their release in coloured wastewater.

They contribute to non-aesthetic pollution of the environment and can result in the

generation of dangerous by products through oxidation [6].

1642 F. A. Aisien et al.

Journal of Engineering Science and Technology December 2015, Vol. 10(12)

Nomenclatures

b Langmuir isotherm parameter, L/mg

Ce Equilibrium dye concentration, mg/L

Co Initial dye concentration, mg/L

Ct Instantaneous dye concentration, mg/L

K Langmuir-Hinshelwood model parameter, L/mg

Kf Freundlich isotherm parameter, mg/g

k1 Pseudo first order model parameter, 1/min

k2 Pseudo second order model parameter, g/min.min

kr Langmuir-Hinshelwood model parameter, mg/L.min

n Freundlich isotherm parameter

qe Equilibrium adsorption capacity, mg/g

qo Maximum adsorption capacity, mg/g

qt Instantaneous adsorption capacity, mg/g

R2 Correlation coefficient

ro Initial rate of reaction, mg/L.min

t Time, minutes

Vs Volume of solution, L

W Catalyst dosage, g/L

Abbreviations

BET Brunauer Emmet Teller

PSA Periwinkle Shell Ash

TiO2 Titanium dioxide

UV Ultra violet

XRD X ray diffraction

XRF X ray fluorescence

ZnO Zinc oxide

Coagulation through the use of chemical reagents and adsorption are two

common methods of treating dye containing wastewater. Nevertheless, these are

mere phase transfer methods in that the dyes are transferred from the liquid to the

solid phase thereby requiring further treatment [7]. Photocatalysis has been

recognised as a promising technique for removing dyes from wastewater. It is an

advanced oxidation process which utilises semiconductor photocatalysts such as

titanium dioxide (TiO2) and zinc oxide (ZnO) for the mineralisation of pollutants

in solution into more stable and less harmful forms [8]. It is faster and cheaper

than most bioprocesses and radiation based processes as it can be carried out

under direct sunlight and it can be applied for treating pollutants that are not

readily amenable to some conventional treatment methods [9, 10]. Akyol et al.

[11] reported that TiO2 and ZnO possess good photocatalytic properties and

nominated both to be promising catalysts for photocatalytic degradation of

coloured wastewater. Irrespective of their obvious advantages, the use of TiO2 and

ZnO as photocatalyst is limited by the recovery potential of the catalysts and

economic viability of the process with respect to the efficiency in the use of

radiation. Hence there is the need to source for efficient substitute catalysts that

have comparable catalytic efficiencies but with very little of the demerits

observed with the use of TiO2 and ZnO.

Potential Application of a Locally Sourced Photocatalyst for the . . . . 1643


The focus of this work is to investigate the potential use of locally sourced

periwinkle shell ash as photocatalysts for the photocatalytic decolourisation of

methyl orange in aqueous solution. Periwinkle shells are waste products generated

from the consumption of periwinkle. The shells are typically disposed of after

consuming the edible parts as sea food thereby contributing to environmental

problems. Some of the uses of periwinkle shells include coarse aggregate in

concrete, manufacture of brake pads, paving of water logged areas, etc.

Nevertheless a large amount of these shells are still disposed annually. The reuse

capacity of these shells can be improved by utilising them for the development of

value added products such as photocatalysts. In the process, the environmental

nuisance constituted by their inappropriate disposal is also eliminated. In

investigating the photocatalytic capacity of periwinkle shell ash, the effects of

factors such as irradiation time, initial dye concentration and catalyst dosage on the

degradation process were studied. The photocatalytic degradation of methyl orange

dye was further evaluated by carrying out kinetic and isotherm studies.

2. Materials and Methods

2.1. Preparation and characterisation of photocatalyst

Periwinkle shells were sourced locally from Benin City in Edo State of Southern

Nigeria. The shells were washed and dried to constant mass in an oven at 110oC.

The dried shells were crushed, calcined at 600oC in a muffle furnace in such a

way that neither the fuel for heating not the exhaust gases came in contact with

the material that was being calcined. The calcined material was then sieved to

obtain fine particles (< 350µm) of periwinkle shell ash (PSA). The PSA was

characterised by determining the composition of oxides and elements using X-

Ray Fluorescence (XRF) and X-ray diffraction (XRD) analysis respectively [12].

The surface structure of the PSA was evaluated by nitrogen adsorption method at

-196ºC. The surface area of the PSA was determined using the standard BET

equation [13]. Other properties such as bulk density and porosity were determined

using standard methods.

2.2. Preparation of dye solution

Methyl orange, an azo dye with molecular formula C14H14N3NaO3S and molecular

weight 327.33 g/mol was obtained from Stanvac Laboratories in Benin City, Edo

State, Nigeria. The commercially obtained sample of the dye was used without

further purification. A stock solution of methyl orange was prepared by dissolving

an appropriate amount of the dye in 1000 mL of deionised water. Working solutions

with different concentrations of dye were prepared by appropriate dilutions of the

stock solution with deionised water before each experiment.

2.3. Photocatalytic degradation studies

Photocatalytic degradation of methyl orange was carried out in a photoreactor.

Radiation was provided by four UV lamps (40W, TUV G6T5, λmax = 254 nm,

manufactured by Philips, Holland) which surrounded the reactor so as to ensure a

homogenous radiation field inside the reactor. The light intensity at the centre of



photoreactor was measured by Lux-UV-IR meter (Leybold Co.). The mixture of

the photocatalyst and dye solution to be decolourised was held in a completely

stirred 10 L capacity holding tank. Silicone tubing was connected to both ends of

the reactor with one end connected to an easy load Masterflex peristaltic pump

which served to convey the photocatalyst-dye mixture to the reactor. Samples

were withdrawn at regular intervals and were immediately centrifuged at 8000

rpm for 10 min to remove suspended catalyst particles. The samples were

analysed for undegraded dye using a UV-Visible spectrophotometer (Shimadzu

UV 2101 PC) at a wavelength of 520 nm. A calibration curve based on Beer-

Lambert’s law was established by relating the absorbance to the concentration of

dye. The effect of irradiation time, initial dye concentration and photocatalyst

dosage on the decolourisation efficiency was investigated. The percentage

photocatalytic decolourisation of the dye was calculated using Eq. (1):

100o t

o

C CDecolourisation efficiency

C

−= × (1)

The amount of dye adsorbed at time t, (qt) and at equilibrium (qe) were

calculated using Eqs. (2) and (3).

( )s o tt

V C Cq

W

−= (2)

( )s o ee

V C Cq

W

−= (3)

where Co, Ce and Ct are the initial, equilibrium and instantaneous dye

concentrations respectively. Vs is the volume of the aqueous solution and W is the

amount of catalyst.

3. Results and Discussion

3.1. Characterisation of photocatalysts

The results of the chemical characterisation of the periwinkle shell ash

photocatalysts carried out using X-Ray Fluorescence (XRF) to determining

oxides present in the ashes and the complete mineralogical analysis to

determine the ultimate elemental composition of the catalysts carried out by X-

ray diffraction (XRD) have been previously reported by Aisien et al. [1]. Other

properties such as the surface area, bulk density and porosity of the

photocatalysts were also reported in the work. The major constituents of the

PSA used were calcium oxide (CaO), silica (SiO2) and aluminium oxide (Al2O3)

which accounted for 41.3, 33.2 and 9.2% of the weight of PSA characterised.

Ultimate elemental analysis showed that the major elements in PSA were iron

(19.2%) and zinc (16.5%). The surface area, bulk density and porosity of the

PSA were obtained as 400 m2/g, 2940 kg/m

3 and 0.004 respectively. These

results are similar to those reported in the literature [14, 15]. Some of the

elements and oxides present in PSA are semiconductors and oxides of the

semiconductors respectively and they have been reported to possess

photocatalytic properties thus supporting the choice of PSA for this study.



3.2. Effect of irradiation time

The effect of irradiation time on the photocatalytic decolourisation of methyl

orange is shown in Fig. 1. Photocatalytic decolourisation of methyl orange

increased with increase in irradiation time up to about 50 minutes. Beyond 50

minutes of irradiation, there was no observable increase in the decolourisation

efficiency indicating that a state of equilibrium had been reached. The initial

increase in the decolourisation efficiency could be attributed to the abundant

active sites available on the photocatalyst surface at the initial stages of the

process. At equilibrium, the active sites on the photocatalyst surface are occupied

by the dye molecules thus resulting in the insignificant change in the

decolourisation efficiency observed beyond 50 minutes [16]. Similar results have

been reported by other researchers [3, 11, 17].

Fig. 1. Effect of irradiation time on the photocatalytic

decolourisation of methyl orange (PSA dose, 3 g/L; initial

concentration, 20 mg/L; temperature, 20oC).

3.3. Effect of initial concentration

The effect of initial dye concentration on the photocatalytic decolourisation

process is shown in Fig. 2. The decolourisation efficiency increased from about

25 to about 63% when the initial dye concentration was increased from 10 to 20

mg/L. This observation may be attributed to the fact that more methyl orange

molecules were adsorbed onto the surface of the photocatalyst. Konstantinou and

Albanis [5] reported that the rate of dye decolourisation generally increases with

the increase in dye concentration up to a certain level beyond which any further

increase in dye concentration does not positively influence the degradation

process. Similar observations have been reported by other researchers [18]. The

rate of the degradation reaction is dependent on the rate of formation of the

hydroxyl radicals and the probability of the radicals reacting with the dye

molecules. As the concentration of dye is increased, there will be more of the dye

molecules in solution and this will increase the probability of the radicals reacting

with the dye molecules consequently resulting in an enhancement of the rate of

the degradation of the dye [5]. Increasing the initial dye concentration beyond 20

mg/L did not favour the decolourisation process as a result of the formation of



several layers of adsorbed dye molecules on the photocatalyst surface. The

adsorbed layer of dye molecules prevents further interaction between fresh dye

molecules and the surface of the photocatalyst. Hence the photocatalytic reaction

is inhibited because fresh incoming dye molecules have limited access to the

hydroxyl (•OH) and oxygen free radicals (O2•) that would have been generated by

the photocatalyst [19, 20]. Furthermore, as a consequence of the Beer-Lambert’s

law, increasing the initial dye concentration beyond 20 mg/L reduces the path

length of photons entering the solution which leads to lower penetration of

radiation to the catalyst particles and consequently lowers the photocatalytic

reaction rate [11].

Fig. 2. Effect of initial concentration on the photocatalytic

decolourisation of methyl orange (PSA dose, 3 g/L; irradiation time,

50 minutes; temperature, 20oC).

3.4. Effect of catalyst dosage

Figure 3 shows the effect of PSA dosage on the photocatalytic decolourisation

process. The decolourisation efficiency showed an initial increase from about

15% to a maximum of about 54% when the PSA dosage was increased from 1

to 3 g/L. Increasing the PSA dosage beyond 3 g/L did not favour the

decolourisation process as seen from the decreasing trend shown in Fig. 3. The

initial increase in decolourisation efficiency might be attributed to the increase

in the number of active sites on the photocatalyst surface which in turn

increases the number of hydroxyl and superoxide radicals being generated in

solution [21]. When the amount of the catalyst was increased beyond the

optimum value, the rate of the decolourisation reaction decreased because the

increase in turbidity of the solution as a result of increased catalyst dosage

resulted in the interception of light by the suspension [22]. Sun et al. [23]

reported that the interception of light by the suspension results in a decrease in

the generation of hydroxyl radicals which are the primary oxidants in the

photocatalytic reaction system.



Fig. 3. Effect of PSA dosage on the photocatalytic decolourisation

of methyl orange (irradiation time, 50 minutes; initial concentration,

20 mg/L; temperature, 20oC).

3.5. Kinetics of photodegradation

The kinetics of the photodegradation process was studied using the pseudo first

order, pseudo second order and the Langmuir-Hinshelwood kinetic models.

3.5.1. Pseudo first order model

The pseudo first order equation is expressed in its integrated form as follows [24]:

1ln( ) lne t eq q q k t− = − (4)

The plot of ln(qe - qt) as a function of t as presented in Fig. 4 resulted in a linear

relationship from which qe and k2 were calculated. The pseudo first order rate

constants calculated from the plot at different initial dye concentrations are given in

Table 1. The straight line plots as well as the high R2 values obtained for the range of

concentration investigated shows that the pseudo first order equation was able to

describe the kinetics of the process.

Fig. 4. Pseudo first order kinetic plot for the

photocatalytic decolourisation of methyl orange.



3.5.2. Pseudo second order model

The pseudo second order equation is expressed in its integrated linear form as follows:

2

2

1 1

t e e

tt

q k q q= + (5)

Figure 5 shows a plot of t/qt versus time at different initial dye concentrations. The

results of the regression analysis are shown in Table 1. It was observed from the

relatively low R2 values that the pseudo second order model did not significantly fit

the experimental data. This suggests that the model might not be able to sufficiently

explain the kinetics of the photocatalytic degradation process.

Fig. 5. Pseudo second order kinetic plot for the


3.5.3. Langmuir-Hinshelwood model

The Langmuir-Hinshelwood kinetic equation is expressed as follows [25]:

1

r eq

o

eq

k KCdcr

dt KC= − =

+

(6)

ro is the initial rate of reaction in mg/L.min, kr is the rate constant for photocatalysis in

mg/L min, K is the rate constant for adsorption in L/mg, Ceq is the concentration of

bulk solution in mg/L at adsorption equilibrium, c is the concentration of bulk solution

at any time t. The linearised form of Eq. (6) is given as follows:

1 1 1 1

o r eq rr k K C k= + (7)

The plot of 1/ro versus 1/Ceq resulted in a straight line as shown in Fig. 6. The

values of the kinetic parameters calculated from the plot are given in Table 1. The

high R2 value obtained shows that the model was able to describe the kinetics of

the process.



Fig. 6. Langmuir-Hinshelwood kinetic plot for the


Table 1. Kinetic constant parameter values

for the photocatalytic degradation of methyl orange.

Adsorption

Kinetic Model Parameters

Initial dye concentration (mg/L)

10 20 30 40

Pseudo first

order

k1 (min-1) 0.036 0.022 0.020 0.018

qe (mg/g) 1.153 4.396 5.752 6.625

R2 0.993 0.944 0.967 0.999

Pseudo second

order

k2 (gmg-1min

-1) 0.00172 0.00162 0.00117 0.00311

qe (mg/g) 4.048 6.452 11.364 5.571

R2 0.812 0.603 0.879 0.580

Langmuir-

Hinshelwood

kr 250

K 0.0455

R2 1.000

3.6. Adsorption isotherm studies

Adsorption isotherms were used to elucidate the equilibrium between the

concentration of methyl orange in the aqueous phase and that in the solid

(catalyst) phase. The Langmuir and Freundlich isotherm equations were utilised

for this study.

3.6.1. Langmuir isotherm

The Langmuir isotherm equation is applied for monolayer adsorption onto an

adsorptive surface containing homogeneously distributed active sites [26]. The linear

form of the Langmuir equation is given as:

1e e

e o o

C C

q bq q= + (8)



qo is the maximum sorption capacity (mg/g) of the adsorbent while b is the sorption

constant (L/mg) at a given temperature. A linear plot of Ce/qe against Ce as shown in

Fig. 7 was employed to obtain the values of qo and b from the slope and intercept of

the plot respectively. The values of the Langmuir isotherm parameters as well as the

correlation coefficient (R2) of the Langmuir equation are given in Table 2.

Fig. 7. Langmuir isotherm plot for the


Table 2. Kinetic parameters for Langmuir isotherm.

qo (mg/g) b (L/mg) R2

0.531 5.157 0.999

The separation factor (RL) can be used to determine whether or not the adsorption

process will be favourable [27]. The parameter predicts that the type of isotherm could

to be irreversible (RL=0), favourable (0<RL<1) or unfavourable (RL > 1).

1

(1 )L

o

RbC

=

+

(9)

For the present study, the values of RL obtained at different initial dye

concentrations are presented in Table 3. These values are between zero and one

indicating that the adsorption was favourable.

Table 3. RL values and type of isotherm.

Initial concentration

(mg/L) RL Value

10 0.019

20 0.010

30 0.006

40 0.005

3.6.2. Freundlich isotherm

The Freundlich isotherm is an empirical equation employed to describe heterogeneous

systems. It is expressed in its linear form as:



lnq ln 1/ lne f eK n C= + (10)

Kf and n are the Freundlich constants related to the adsorption capacity and

adsorption intensity respectively. Their values were determined respectively from

the intercept and slope of the linear plot of ln qe versus ln Ce as shown in Fig. 8.

The values of the Freundlich isotherm parameters as well as the correlation

coefficient (R2) of the Freundlich equation are given in Table 4. The value of n

was obtained as 1.561. According to Agarry et al. [28], values of n between 1 and

10 represent favourable adsorption.

Table 4. Kinetic parameters for Freundlich isotherm.

Kf (mg/g) n R2

0.393 1.561 0.959

Even though a comparison of the results obtained show that the Langmuir

equation yielded a higher R2 value than the Freundlich equation, the closeness of both

R2 values to unity suggests that both equations could readily be used to represent the

condition of equilibrium of the system.

Fig. 8. Freundlich isotherm plot for the


4. Conclusions

The photocatalytic decolourisation of methyl orange using periwinkle shell ash

was investigated in this study. The photodegradation process was affected by

factors such as irradiation time, initial dye concentration and PSA dosage. The

optimum adsorption conditions are as follows: irradiation time, 50 minutes;

initial dye concentration, 20 mg/L; PSA dosage, 3 g/L. The kinetics of the

decolourisation process was well described by the pseudo first order and

Langmuir-Hinshelwood kinetic models with high correlation coefficient values

while the adsorption equilibrium was well elucidated by both the Langmuir and

Freundlich isotherm equations.



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