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Adsorption and Solar Photocatalytic Degradation of

Diclofenac in Wastewater John Akach, Maurice S. Onyango, and Aoyi Ochieng

1Abstract---In this study, the semiconductor TiO2 was attached

onto activated carbon (AC) and bound by silica xerogel to form the activated carbon-TiO2-silica xerogel (CTS) composite catalyst. The CTS composite was then used to degrade diclofenac (DCF) in three phase fluidised bed reactors. Adsorption and photodegradation experiments were carried out on sunny days to test the effect of various operating parameters. The best mass of the CTS composite

catalyst was found to be 1.5 g/l while the optimum superficial air velocity was 0.007 m/s. It was also found that decreasing the initial concentration of DCF reduced the treatment time. The adsorption of DCF onto the CTS composite followed Langmuir isotherm. Using the optimal composite composition resulted in over 90% removal of the DCF.The fast removal of diclofenac in ppm concentration shows great potential in applying this process to remove real pharmaceuticals in wastewater which are normally at lower

concentrations.

Keywords---Adsorption, Diclofenac, Fluidised bed reactor, Pharmaceutical wastewater, Photocatalysis

I. INTRODUCTION

HE presence and persistence of pharmaceuticals in

wastewaters are growing concerns. Pharmaceutical

products such the diclofenac (DCF) in wastewater and

receiving waters have been found to be toxic to birds and

aquatic life [1]. Therefore, it is important to remove DCF

from wastewater. However, the elimination of

pharmaceuticals such as DCF in conventional wastewater

treatment plants is often incomplete owing to their resistance

to biodegradation [1]. Therefore, alternative treatment

methods have been investigated for their effectiveness

towards the removal of DCF.

These alternative treatment methods for DCF include

adsorption [2], photo-Fenton [3], ozonation [4], sonication [5]

and TiO2 photocatalysis [6]. Among the advanced

technologies, TiO2 photocatalysis has emerged as one of the

most promising method. This is mainly due to its low cost

especially when used with sunlight, low chemical input and

its destruction of pollutants instead of transferring them to

another phase [7, 8].

John Akach is with the Centre for Renewable Energy and Water, Vaal

University of Technology, Private Bag X021, Vanderbijlpark, 1900, South

Africa (corresponding author: fax:+27-16-950-9796; email: [email protected])

Maurice S. Onyango is with the Department of Chemical and

Metallurgical Engineering, Tshwane University of Technology, Pretoria,

South Africa (email: [email protected]).

Ochieng Aoyi is with the Centre for Renewable Energy and Water, Vaal

University of Technology, Private Bag X021, Vanderbijlpark, 1900, South

Africa (email: [email protected])

Several studies have shown that photocatalysis can be

improved by combining it with adsorption [9]. This is due to

the fact the adsorbent brings the substrates in close proximity

to the TiO2 thus increasing the rate of photocatalysis [10].

Several adsorbents such as chitosan [11], silica [12], zeolites

[13] and activated carbon (AC) [10] have been used. One of

the best adsorbents for combining with TiO2 has been AC due

to its strong affinity for a wide range of pollutants. In order to

combine adsorption and photocatalysis, a composite catalyst

of TiO2 and AC needs to be developed. One problem facing

such composites has been the detachment of TiO2 from AC.

In order to solve this problem, binders such as silica have

been used to bond the TiO2 and the adsorbent together [12].

A lot of work has been carried out on the photocatalytic

degradation of DCF using TiO2 [14-17]. Also, the use of AC

for the removal of DCF from wastewater has been

investigated [2]. However, there are very few studies into the

use of a combined AC and TiO2 for the removal of DCF from

wastewater [18]. Of the studies that have employed a

composite of AC and TiO2 to treat DCF, none has used silica

to bond TiO2 and AC. Also, none of the studies utilizing TiO2

and AC have utilized sunlight to irradiate the TiO2. In this

work, a composite catalyst of TiO2 and AC bound by silica

xerogel and activated by sunlight was used to adsorb and

photodegrade DCF. The aim of the study was to determine the

optimum operating parameters for the combined adsorption

and photocatalysis system.

II. METHODOLOGY

A. Equipment

The experiments were carried out in three phase fluidised

bed reactors (FBPR). The FBPR were made of borosilicate

glass which has a high transmittance of the solar UV. The

distributor plate of the FBPR was made of sintered glass (pore

size 10 – 16 µm). During operation, air was bubbled through

the FBPR from under the distributor to induce fluidization of

the composite catalyst. The FBPR were mounted on a rooftop

where there was no obstructions to sunlight.

B. Materials

Aeroxide P25 TiO2 was purchased from Acros Organics.

Ludox HS-30 colloidal silica and diclofenac sodium salt

(DCF) were obtained from Sigma-Aldrich (South Africa).

HCl (32%) and commercial powdered activated charcoal

(AC) were purchased from Labchem (South Africa). All

T

Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg

192

experiments were carried out with deionized water

(Resistivity 18.2 MΩcm) from a Millipore Direct Q reverse

osmosis unit.

C. Preparation of AC-TiO2-Silica Xerogel Composite

Catalyst

Known quantities of AC, TiO2 and colloidal silica were

stirred magnetically in a capped bottle until the mixture was

homogeneous. Then 15 ml of the slurry in the bottle was

spread on 30 × 30 cm glass plates to make thin layers. The

glass plates were then put in an oven at 90°C in order to gel

the colloidal silica. The resulting dry flakes of AC and TiO2

bound by silica xerogel on the glass plates were then crushed

and sieved to a size range of 38 – 75 µm. This composite

catalyst powder was then shaken in a 0.05 M HCl solution

followed by several washes with deionised water. Finally, the

composite catalysts were dried at 50°C in the oven to obtain

the AC-TiO2-silica xerogel (CTS) composite.

D. Adsorption and Photodegradation Experiments

Adsorption and photodegradation experiments were carried

out between 8 am and 3 pm on sunny days in the FBPR. Dark

adsorption experiments were carried out first by covering the

FBPR with black polythene plastic to keep off sunlight. Then

a known mass of the CTS composite was added into 450 ml

of DCF solution in the FBPR to start adsorption. Air sparging

was then started and air flow regulated and maintained at the

appropriate air velocity. After reaching the adsorption

equilibrium, the black polythene plastic covering the FBPR

was removed to allow sunlight to irradiate the FBPR in order

to start photodegradation. During adsorption and

photodegradation, 3 ml aliquots of the DCF solutions were

sampled periodically and filtered with 0.45µm GHP syringe

filters (Pall). The samples were then analysed at a λmax of 276

nm in a UV-vis spectrophotometer (PG instruments T60). All

adsorption and photodegradation experiments were performed

in duplicates.

III. RESULTS AND DISCUSSION

A. Effect of the CTS Composite Mass

Experiments were carried out to determine the effect of the

CTS composite mass on the adsorption and photodegradation

of DCF. The results (Fig 1) show an increase in the adsorption

and photodegradation of the DCF with increase in mass of the

CTS composite. At 0 g/l there was no catalyst to carry out

adsorption and photodegradation. Although there was no

adsorption and photodegradation, some DCF was removed

through photolysis.

Increasing the CTS mass lead to an increase in the amount

of AC and TiO2 active sites for adsorption and photocatalysis,

respectively. However, increasing the composite mass beyond

1.5 g/l did not show any significant increase in the

photodegradation of the DCF. This was due to an increase in

the shielding of some the CTS catalyst particles by others

when the CTS mass was increased beyond 1.5 g/l [19]. The

shielded TiO2 particles were blocked from sunlight and thus

could not be activated. This shielding thus reduced the

increase in photodegradation with increase in the mass of

CTS composite.

Fig. 1: Effect of mass of the CTS composite (g/l) on the adsorption and photodegradation of DCF; C0 = 10 mg/l, superficial air velocity

= 0.014 m/s

When shielding occurs in solution, the shielded catalysts

become inactive. It would be cost effective not to have any

inactive catalyst particles at any time during

photodegradation. The optimum mass of the CTS composite

was therefore taken as that which would ensure the highest

number of catalyst particles in solution without some catalyst

particles shielding others. Therefore, 1.5 g/l was selected as

the optimum mass of the CTS composite.

B. Effect of the Superficial Air Velocity

The effect of the superficial air velocity on the adsorption

and photocatalytic degradation of DCF was investigated using

1.5 g/l of CTS composite. The results (Fig. 2) show that the

adsorption and photodegradation of DCF increased with

increasing superficial air velocity up to an optimum of 0.007

m/s beyond which no further increase was observed. When

the reactors were not sparged with air (0 m/s), some

adsorption occurred during the turbulent mixing of CTS

composite and the DCF solution at the start of the experiment.

However, by the time photocatalysis started, almost all the

CTS composite particles had settled onto the distributor. Only

those CTS particles at the very top of the distributor were

exposed to sunlight and could therefore photodegrade the

DCF molecules. Photocatalysis using these few CTS

composite particles resulted in the little degradation of DCF

observed at a superficial air velocity of 0 m/s.

At a superficial air velocity of 0.003 m/s, some fluidization

of the catalyst was observed. There was also considerable

settling of the CTS catalyst on the wall and distributor of the

reactor. Due to the settling of the CTS composite, only a

portion of the optimum amount of catalyst was fluidized and

was therefore available for adsorption and photodegradation.

At a superficial air velocity of 0.007 m/s, settling of the CTS

on the reactor walls and distributor reduced markedly. Almost

all the CTS composite particles remained suspended in

0

0.2

0.4

0.6

0.8

1

0 30 60 90 120 150 180 210 240 270 300 330 360 390

C/C

0

Time (min)

0 0.5 1 1.5 2

Adsorption Photodegradation


193

solution. The fluidized CTS composite particles had enough

surface area exposed to the DCF molecules and sunlight for

increased adsorption and photodegradation.

Increasing the superficial air velocity above 0.007 m/s,

resulted in further reduction in the settling of the catalyst on

the reactor walls and the distributor, but did not increase the

adsorption and photodegradation. This was due to the fact that

the maximum surface area of the catalyst had been exposed at

air velocity of 0.007 m/s. Similar results were reported by

Matsumura and co-workers [20]. They found an optimum

superficial air velocity of 0.006 m/s which is comparable to

the optimum superficial velocity observed in this work (0.007

m/s).

Fluidization is costly due to the electric power needed to

run the compressor. From an economic point of view, the

lowest superficial air velocity resulting in the maximum

adsorption and photodegradation was chosen as the optimum

air velocity. A superficial air velocity of 0.007 m/s was

therefore chosen as the optimum.

Fig. 2: Effect of superficial air velocity (m/s) on the rate of adsorption and photocatalysis of DCF. CTS loading = 1.5 g/l, C0 =

10 mg/l.

C. Effect of the Initial Concentration of DCF

The effect of the initial substrate concentration on the

adsorption and photodegradation of the substrates was

determined using 1.5 g/l of the CTS composite catalyst. The

results (Fig 3) show a decrease in the time of removal of DCF

with a decrease in the initial substrate concentration. There

was a fixed number of active sites on the CTS composite for

adsorption and photodegradation of DCF molecules. Only a

certain number of the DCF molecules could be adsorbed by

the CTS active sites. Increasing the number of DCF molecules

in the solution beyond the number of available adsorbent

active meant that some DCF molecules would remain in the

solution. Therefore, an increase in the initial concentration of

DCF left more DCF molecules to be photodegraded. Only a

fixed number of active sites were available for

photodegradation of the substrates at any one time. Therefore,

it would take longer to photodegrade the solution with more

substrate molecules. Fig. 3: Effect of initial concentration of substrate (mg/l) on the

adsorption and photodegradation of DCF. CTS loading = 1.5 g/l,

superficial air velocity = 0.014 m/s.

The typical concentration of pharmaceutical pollutants in

wastewater has been reported to be in the order of ng/l to low

µg/l [21]. The experiments in this work were carried out at

higher concentrations of mg/l. However, the results of this

work could still be applicable to the pharmaceutical

contaminants at lower concentrations. This is due to the fact

that the increase in the removal of substrates with decreasing

initial concentration from 15 to 2 mg/l shows that the

substrate removal would keep increasing at substrate

concentrations below 2 mg/l. However, the optimum mass of

the CTS composite used will have to be reduced when

removing lower concentrations of substrates.

D. Adsorption Isotherms

The data in Fig. 3 was fitted to the Langmuir and

Freundlich isotherms. The results (Table 1) show a good fit

for the Langmuir isotherm for DCF adsorption. The

adsorption of DCF also showed a good fit for Freundlich

isotherm. However, the Langmuir isotherm showed a better fit

than the Freundlich isotherm. This shows that the adsorption

of DCF onto the CTS composite was monolayer. This is due

to the fact that Langmuir isotherm models a monolayer

coverage of the adsorbent surface by the substrate while the

Freundlich isotherm models a multilayer adsorbent coverage

[22].

TABLE I

LANGMUIR AND FREUNDLICH ISOTHERMS OF DCF ADSORPTION ONTO THE CTS COMPOSITE

Langmuir isotherm Freundlich isotherm

K R2 n R2

DCF 172.4 0.204 0.990 1.539 29.208 0.988

0

0.2

0.4

0.6

0.8

1

0 30 60 90 120 150 180 210 240 270 300 330 360

C/C

0

Time (min)

0 0.003 0.007


0

2

4

6

8

10

12

14

16

0 30 60 90 120 150 180 210 240 270 300 330 360 390

Init

ial

conce

ntr

atio

n (

mg/l

)

Time (min)

10 20 30 40



194

IV. CONCLUSION

In this work, TiO2 was attached onto activated carbon (AC)

using silica xerogel to make the CTS composite. The CTS

composite was used to adsorb and photodegrade diclofenac

(DCF) in fluidised bed reactors using sunlight as the energy

source. The best mass of the CTS composite was 1.5 g/l while

the optimum superficial air velocity was 0.007 m/s. A

decrease in the initial concentration of DCF led to a decrease

in the time of removal of DCF. Adsorption isotherm studies

showed that there was a monolayer adsorption of DCF onto

the CTS composite. These results show the great potential of

the combined adsorption and photodegradation system for the

removal of pharmaceuticals in water streams.

ACKNOWLEDGMENTS

This work was supported by the Water Research

Commission of South Africa (Project K5/2105).

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John Akach has a BTech degree in Chemical and Process Engineering.

He is currently pursuing an MTech degree in Chemical Engineering at the

Vaal University of Technology. His research interests include the use of solar

photocatalysis, adsorption, composite catalysts, and fluidized bed reactors to

treat pharmaceutical pollutants.


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