Effect of Various Additives on Photocatalytic Degradation ...downloads.hindawi.com/journals/chem/2009/248548.pdf · Harbin Institute of Technology Shenzhen Graduate School, Shenzhen

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2009, 6(S1), S422-S428

Effect of Various Additives on Photocatalytic

Degradation of 4-Nitrophenol

KASHIF NAEEM and FENG OUYANG*

Environmental Science and Engineering Research Center,

Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China.

[email protected]

Received 28 April 2009; Accepted 15 June 2009

Abstract: The photocatalytic degradation of 4-nitrophenol (4-NP) assisted by

titanium dioxide (TiO2) was investigated in aqueous suspension under

irradiation by UV light. The effect of different supporting materials mixed

physically with TiO2 on the photocatalytic degradation of 4-NP has been

studied. TiO2 with all supports exhibits good degradation efficiency of 4-NP

and was better than TiO2 alone. The addition of SiO2 and ZSM-5 only caused a

little change in 4-NP degradation. However, degradation of 4-NP was

improved from 34.89% to 60.53% within 120 min photocatalysis in the

presence of optimal amount of AC. The degradation was also fairly enhanced

in the presence of cheaper rice husk and the activity was closed to AC.

Keywords: 4-Nitrophenol, Degradation, Rice husk, TiO2

Introduction

Nitrophenols are some of the most refractory pollutants, which can be present in industrial

wastewater. Among them, 4-nitrophenol (4-NP) is environmentally important for severa1

reasons. Owing to high toxicity and carcinogenic character, 4-NP is characterized as

environmentally hazardous material. This toxic pollutant is used in the production of

pesticides, insecticides and herbicides1 and many synthetic dyes

2. Therefore, 4-NP and its

derivatives are common pollutants in many natural water and wastewater systems. It is

reported as potential toxic pollutant by United States Environmental Protection Agency

(USEPA) and its maximum allowable concentrations in water ranged from 1 to 20 ppb2.

The removal of pollutants from wastewater is of great concern, because their complete

biodegradation requires several days or weeks. Advanced oxidation processes (AOPs) are

efficient treatment methods owing to their ability of complete degradation of wide range of

organic pollutants. Titanium dioxide (TiO2) assisted photocatalysis is a well known emerging

Effect of Various additives on Photocatalytic Degradation S423

AOP for the removal of organic pollutants in water and air3-5

. TiO2 is of great interest due to

its non-toxic nature, photochemical stability and low cost, particularly when sunlight is used

as the source of irradiation6,7

. However, shortcoming of using TiO2 in photocatalytic

processes is its rapid aggregation in a suspension resulting in decrease of effective surface

area in addition to recombination of generated electron-hole pairs. This disadvantage of

TiO2 results in low catalytic efficiency.

Various methods were documented to improve photocatalytic efficiency of TiO28-11

.

Another effective method that can increase the photocatalytic efficiency of TiO2 is to add an

additive or support material such as silica (SiO2), alumina (Al2O3), zeolite (ZSM-5) or clay

and activated carbon (AC)12-20

. TiO2 with supports or additives offers high specific surface

area which helps in more effective adsorption than TiO2 alone12,17,19,21,22

. The synergy

between TiO2 particle and the support enhances the degradation which is attributed to

reduction in the electron–hole recombination reaction on the surface23

. Over the years, the

opportunity of producing activated carbon from cheaper and readily available source like

rice husk (RH) for water purification is interesting. Rice is one of the main crops, which is

widely cultivated in Asian countries. The RH is an agricultural waste produced in the

milling process when the grain is separated from the outer covering (husk).

Earlier, we have reported the photocatalytic degradation of phenol under the influence

of TiO224

and iron-doped TiO2 nanoparticles25

. In the present work, the influence of the

supports such as activated carbon, silica and zeolite (ZSM-5) on the photoactivity of pure

TiO2 has been examined for the degradation of 4-NP. Thus the prime objective of the

present work was to improve the efficiency of photocatalytic process using supports. Results

were also compared utilizing cheap material RH as an alternative source of AC.

Experimental

TiO2 powder of P25 was the product of Degussa Co., Germany. It contains about 80%

anatase and 20% rutile form with an average particle size of 21 nm and BET surface area of

50±15 m2g

-1. Analytical reagent grade phenol and hydrochloric acid were obtained from

Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 4-nitrophenol (4-NP) was

purchased from Aladdin Chemical Reagent Co., China. AC (SSA 950 ±10 m2/g) was

obtained from Ningxia Coal Co., Ltd. (China). ZSM-5 (Si/Al = 50, 5-6 µm, 320 m2/g) was

obtained from Nankai University (Tianjin) and SiO2 (125-425 µm, 460 m2/g) was purchased

from Qingdao Haiyang Chemical Co., Ltd. All chemicals were used as such without further

purification. The rice husk obtained from a farmer near Chang Chun (China) was washed

with distilled deionized water to remove all dirt and then oven-dried at 80 °C till constant

weight. The dried RH was sieved through 80 mesh. The dried RH was stored in a polythene

bag kept in desiccator and was used as such without any physical or chemical treatment.

Water used for chemical solutions was purified using a Milli-Q system (Millipore

Corporation).

Photocatalytic measurement

Photocatalytic degradation of 4-NP in TiO2 suspension was performed in an open Pyrex-

glass cell with 500 mL capacity. 0.05 g of TiO2 was suspended in 200 mL of 4-NP aqueous

solution (1 × 10-4

M) using appropriate amount of support or RH at pH 5. Air was

continuously bubbled through the suspension. The suspension was magnetically stirred in

the dark for at least 15 min to ensure the establishment of an adsorption/desorption

equilibrium. Then light was turned on and it was treated as the starting point (t = 0) of the

DE

, %

S424 FENG OUYANG et al.

reaction. UV irradiation was carried out using a 125 W bulb from Leijian Special Light

Source Co., Ltd. (Shenzhen, China) with a wavelength of 365 nm. The pH of suspension

was measured using PH-035 digital pH meter from Changlilai Technology Co., Ltd.

(Shenzhen, China) by adding HCl (0.1 M). After a regular interval of time, the samples of

7mL volume were collected and immediately centrifuged at 3000 rpm for 10 min, and then

filtered through a 0.45 µm Millipore filter to remove the TiO2 particles. The filtered samples

were stored at 4 °C prior to analysis. The quantitative determination of 4-NP was performed

by measuring its absorbance at 315 nm with a Helios Gamma UV-vis spectrophotometer.

The degradation efficiency (DE) of each sample was computed using following equation:

DE 100%tA A

A

−= ×

o

o

(1)

where, Ao and At is the absorbance at time zero and time t, respectively.

Results and Discussion

Photocatalytic degradation of 4-NP

Aqueous solution of 4-NP was irradiation in the presence or absence of TiO2-P25 by 125 W

UV-lamp was carried out. The role of photocatalytic degradation and the effect of direct

photolysis on the degradation of 4-NP were studied. Figure 1 shows the change in

degradation efficiency versus irradiation time of aqueous solution of 4-NP. Control

experiment was performed by employing UV-irradiated blank solution. The degradation

efficiency of 4-NP was negligible when the aqueous solution was irradiated without TiO2.

Degradation of 4-NP was only 4% within 120 min in the direct photolysis indicating that the

photocatalysed degradation in the presence of TiO2/UV is particularly recognized to the

photocatalytic reaction of the TiO2 particles, followed by the formation of an electron–hole

)( +− − vbcb he pair on the surface of catalyst:

- +

2 2 cb vbTiO TiO +e + hh+ ν → (2)

Figure 1. Effect of UV light and TiO2 particles on photocatalytic degradation of 4-NP.

Irradiation time, min


Very reactive hydroxyl radicals (•OH) can be formed either by the decomposition of

water or by the reaction of the +

vbh with OH-.

+ +

vb 2h + H O OH + H•→ (3)

+ -

vbh OH OH•+ → (4)

The •OH is exclusively strong, non-selective oxidant, which conducts the partial or

complete mineralization of organic pollutants26

. Electrons (ecb-) in the conduction bond are

also responsible for the generation of •OH, which have been indicated as the primary source

of pollutant degradation27

. - -

cb 2 2e + O O•

→ (5)

-

2 2 22O + 2H O 2 OH + 2OH + O•− •

→ (6)

OH pollutant degradation products• + → (7)

The whole mechanism of photoactivity of TiO2 particle is depicted in scheme 1.

Scheme 1. Mechanism of TiO2 under UV light photoexcitation.

Effect of support concentration

In order to examine the effect of support on the degradation of 4-NP, a series of

experimentation was accomplished by the different types of supports, namely AC, SiO2 and

ZSM-5. Figures 2-4 show results for the degradation of 4-NP from aqueous solution

employing different supports loading varying from 25 to 100 mg. It can be seen that

photocatalytic degradation of 4-NP was found to increase then decrease with the increase in

support concentration. This trend is probable for the reason that as the number of support

particles surrounding the 4-NP increases, more 4-NP is adsorbed by these particles28

. It was

found that for all supports, a concentration of 50 mg with TiO2 produced the best

photocatalytic degradation of 4-NP.

From Figures 2-4, it can be concluded that all the supports when mingle with TiO2 enhanced

the photocatalytic degradation of 4-NP, with the order AC > ZSM-5 ≅ SiO2. TiO2 aided with

AC has the highest degradation efficiency for 4-NP under the experimental conditions. The

results indicate that the effective surface area and adsorption capacity of the supported TiO2

were much higher than that of TiO2 alone, which favor rapid degradation of 4-NP.

At/A

At/A

At/A


Figure 2. Effect of TiO2/AC on absorption intensity of 4-NP.

Figure 3. Effect of TiO2/ZSM-5 on absorption intensity of 4-NP.

Figure 4. Effect of TiO2/SiO2 on absorption intensity of 4-NP.




At/A


Comparison with rice husk

Figure 5 shows the effect of RH on the absorption intensity of 4-NP versus irradiation time. It

is easily seen from Figure 5 that relative to 4-NP degradation over TiO2 alone (Figure 1), the

degradation of 4-NP is fairly increased in the presence of RH. The inset of Figure 5 shows that

the synergistic result of TiO2 and RH is in comparison with TiO2 and AC system. This

suggests that RH is an alternative and cheap material and can be used as support material.

Figure 5. Change in absorption intensity of 4-NP with TiO2/RH. The inset shows the DE of

4-NP assisted by TiO2 with RH and AC.

Kinetics of photocatalytic degradation of 4-NP

From engineering point of view, it is useful to find out a simple and user-friendly rate

equation that fits the experimental rate data. It can be seen from Figures 2-4 that the

dependence of 4-NP concentration on the irradiation time was fitted to an exponential

function suggesting that the photocatalytic degradation of 4-NP established pseudo first-

order kinetics with respect to the 4-NP concentration:

[ ][ ]app

d 4-NP4-NP

dk

t− = (8)

The integration of equation (8) under the restriction [4-NP]t = [4-NP]o at the start of

irradiation (t = 0), where [4-NP]o is the initial concentration, yields equation (9):

[ ]

[ ]t

app

ο

4-NPln

4-NPk t

− =

(9)

where appk is the apparent first order rate constant (min

-1). The computed kapp values from

equation (9) at optimal support concentration are listed in Table 1. It can be seen that kapp

values for TiO2/AC and TiO2/RH are in good agreement demonstrating that RH can be used

as alternative supporting material to AC.



Table 1. Apparent rate constant of 4-NP under different conditions.

TiO2 TiO2/AC TiO2/ZSM-5 TiO2/SiO2 TiO2/RH

kapp, min-1

0.0036 0.0078 0.0040 0.0039 0.0069

Conclusions

The photocatalytic degradation of 4-nitrophenol (4-NP) was investigated in aqueous

suspension of TiO2 irradiated by UV light. Various supporting material were shown to

enhance the photocatalytic degradation of 4-NP. All supports show improved degradation of

4-NP than TiO2 alone within 120 min of photocatalysis. A comparable result was obtained

when an optimal amount of rice husk was used as a supporting material. Therefore, rice husk

could be used as an alternative to AC.

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