Decomposition of 2-chlorophenol in aqueous solution by UV irradiation with the presence of titanium dioxide

Pergamon PII: S0043-1354(96)00147-9

War. Res. Vol. 30, No. I I, pp. 2569-2578, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0043-1354/96 $15.00 + 0.00

DECOMPOSITION OF 2-CHLOROPHENOL IN AQUEOUS SOLUTION BY UV IRRADIATION WITH THE PRESENCE OF TITANIUM DIOXIDE Y O U N G KU*®, R E N - M I N G LEU and K U E N - C H Y R LEE Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan, Republic of China (First received September 1995; accepted in revised form April 1996)

Abstract--The decomposition of 2-chloropbenol in aqueous solution by UV/TiO2 oxidation process was studied under various solution pH values, light intensities and types of TiO2. The removal of 2-chlorophenol and organic intermediates was found to be more effective for acidic solutions. The decomposition of 2-chlorophenol can be fitted well by a pseudo first-order kinetics. Experimental results indicated that the distribution of non-chlorinated and chlorinated intermediates on the photocatalytic decomposition rate of 2-chlorophenol were highly dependent on solution pH. At the same light intensity, the adequate dosage of TiO2 was found to be less for alkaline solutions than that for acidic solutions. Increasing the light intensity would significantly increase the decomposition rate of 2-chlorophenol at pH 3, but not at pH 11. Both rutile and anatase forms of TiO2 were used in this study. The destruction rate of 2-chlorophenoi of unit weight of rutile was less than that of anatase. However, the decomposition rates of 2-chlorophenol of unit surface area for both were almost the same in this research. Thus, the difference of the decomposition rates of 2-chlorophenol might be attributed to the difference of the surface area of various forms of TiO2, not to the difference of the crystal properties. Copyright © 1996 Elsevier Science Ltd Key words--UV/Ti02 process, 2-chlorophenol, pH, UV light intensity, titanium dioxide dosage

INTRODUCTION The presence of halogenated aromatics in aquatic bodies has been known to cause severe pollution problems because of their carcinogenicity. The major sources of halogenated aromatics include the improper discharge of industrial wastewater and the chlorination of naturally occurring aromatic matters during water purification (Bellar et al., 1974). Public concern over the contamination of drinking water supplies and the aquatic environment has stimulated investigations involving the development of various treatment technologies to remove organics from water. For instance, several photocatalytic semiconductors such as TiO2, CdS, ZnS, etc., were illuminated with near-UV light to generate highly oxidative holes on the solid surface, initiate the oxidation of organic pollutants in waters by converting dissolved oxygen, water or hydroxyl ions to hydroxyl and other free radicals. Semiconductor photocatalysis may be more effective than the conventional chemical oxidation methods because semiconductors are inexpensive and capable of mineralizing various refractory organic compounds. The photocatalytic activity and stability are the major *Author to whom all correspondence should be addressed.

concerns regarding the selection of semiconductors as a photocatalyst. The decomposition rates of organics by photocatalytic reactions are influenced by the active site and the photon absorption of the catalyst used. Several reports have evaluated the effect of photocatalyst dosage and light intensity on the photocatalytic reactions. Auguliaro et al. (1991) indicated that at low photocatalyst loadings, the removal of organic compounds increased linearly with the catalyst loading; however, the presence of excess photocatalyst in the aqueous solutions could cause a shielding effect on the penetration of light. AI-Sayyed et al. (1991) reported that the decomposition rate of 4-chlorophenol increased linearly with the amount of TiO2 up to a level of 3--4 g/1 at solution pH 3. Tseng and Huang (1991) demonstrated that the oxidation rate of chlorophenols increased with the concentration of TiO2, reached a maximum level at 3 g/1 then decreased to a constant value upon further increase in TiO2 at neutral conditions. Ku and Hsieh (1992) depicted that the removal increased with increasing TiO2 amount, thereby approaching a limiting value at a loading of about 1.4 g/l for a solution containing about 3 x 10 -4 M 2,4dichlorophenol at initial solution pH 6.5. Thus on the basis of previous findings, suitable amounts of TiO2

2569

2570

Y. Ku et al.

for the photocatalytic reaction were about 1 4 g/l loadings at acidic solutions and were assumed to be dependent on experimental conditions. However, the light intensity also significantly affected the rate of organic compounds photodecomposition. The photolytic decomposition rate of phenols was found in several investigations to be between half- and first-order to light intensity (D'Olivera et al., 1990; Okamoto et al., 1985). In this study, a detailed investigation of the photocatalytic decomposition of 2-chiorophenol in the presence of suspended TiO2 particles is discussed under various solution pH values, light intensities and amounts of photocatalyst

3

/5

and the path way of decomposition was suggested by establishing elemental balance. EXPERIMENTAL Both the anatase and rutile forms of TiO2 particles employed in this study were approximately spherical and non-porous with greater than 99.9% purity supplied by CERAC Inc. Most experimental tests were carried out with the anatase form of TiO2, the rutile form was occasionally used for comparison purpose. A specific BET surface area of 10.0m2/g was determined for the anatase form of TiO2 with Micromeritic ASAP 2400, and the rutile form of TiO2 had a BET area of 3.3 m2/g. The pHzpc of TiO2 particles was determined

[6 5

r

"

i'l

10

ll

12

7

8

8

13

2. Water Bath

7. Radio Sensor

3. pH Meter

8. Magnetic Stirrer

1 I. Variable Voltage Transformer 12. Automatic Voltage Regulator 13, Power Supply

4. pH Electrode

9. Stir Bar

14. UV Lamp

1. Photolytic Reactor

5. Lamp Stand

6. Radio Meter

10. Thermocirculator

15. Black Box

Fig. 1. Schematic diagram of batch experimental unit.

2-Chlorophenol decomposition by UV/TiO~ Table I. Amounts of 2-chlorophenol adsorbed by TiO2 particles at various solution pH. Initial concentration of 2-chlorophenol = 7.78 x 10-5 M; TiO2 loading = 2,0 g/l

Percent of 2-CP removal due to TiO2 adsorption (%)

3.0 4.8

5.0 4.0

pH 7.0 3.7

9.0 2.3

I 1.0 2.0

5.3

4.4

4.1

2.5

2.2

Amounts of 2-CP adsorbed by TiO2 (rag/g)

2571

adjusting and were monitored continuously. The solution was agitated with a magnetic agitator for about 30 rain in order to evaluate the amounts of the 2-chloropbenol removal due to the adsorption by TiO2 particles. The dissolved oxygen levels of reaction solutions were measured and maintained at around 8 mg/l with the air flow rate of 0.02 l/min at l atm. An aliquot of solution was then sampled for ahsorbance measurement. After the light source was turned on, a small portion of the reaction solution was withdrawn at intermittent periods of reaction time and centrifuged to remove TiO2 at 500 rpm for 10min and analyzed for 2-chlorophenol, TOC and chloride concentrations. The concentration of 2-chlorophenol were analyzed by direct injection into a SpectraPhysics high pressure liquid chromatograph equipped with UV/visible detector. The UV light absorbance of solution containing TiO2 suspensions was determined by Shimadzu model UV-160A UV/visible spectrophotometer. The total organic carbon content was analyzed by O.I.C model 700 TOC analyzer. The chloride concentration was analyzed by Dionex model DX-100 ionic chromatograph.

with Leza-600 laser electrophoresis zeta potential analyzer. The 2-chlorophenol and other chemicals used for analysis were reagent grade, as purchased from Merck, All experimental solutions were prepared with double distilled water. The photoreactors used in this study were shown in Fig. 1 contained two capped batch annular photoreactors made entirely of fused silica each with an effective volume of 2 I. The reactors were water-jacketed to maintain the solution temperature at 25°C for all runs. The 365 nm black-blue fluorescent UV lamp with approximately 15W maximum output was used as a light source and located within the inner tube. The light intensity of the UV lamp was adjusted by the variable voltage transformer and detected by Spectroline model DRC-100X digital radiometer combined with DIX-365 radiation sensors. The 2-chlorophenol solution was added to the reactor along with predetermined amount of TiO2 particles. The solution pH values were maintained constant at desired levels with NaOH and H~SO4 solutions by manually

RESULTS AND DISCUSSIONS A series of experiments was carried o u t to study the a d s o r p t i o n b e h a v i o r of TiO2 particles as s h o w n in Table 1. T h e disappearance o f 2-chlorophenol c o n c e n t r a t i o n was f o u n d to be below 5 % implying t h a t removal due to TiO2 a d s o r p t i o n could be neglected. However, the a m o u n t s o f 2-chlorophenol a d s o r b e d by TiO2 particles decreased markedly with

2.5 2.0

C)

slope=16.4

°~ TiO2(Anatase)/HSystem 20

1.5

0

/~ /

-~ 1.0

0.5

f o o

I

0.0

/

ooooo pH= 11.o_+o.I,[2-CP]o=O

" |

I

I

.

aoaaa pH= 11.0+0. l~[2-CPJof7.7fl*l 0-'M p H - - 3 0 + 0 1 , 1 2 CPJo 0

slope=5.1 I

I

I

I

[

0.1

|

I

I

I

I

I

I

I

[TiOa](g/1)

I

[

0.2

I

I

I

' i

I

|

I

|

|

0.3

Fig. 2. The effect of solution pH on the 365 UV nm light absorbance for solution containing TiO2 particles.

2572

Y. Ku et al. 50.0

0

,~ 4 0 . 0 g}

UV/TiO~(Anatase )/2 - C.P .System UV Intensity=22.5 W/m-_TiOz Do s a g e = 2 . 0 ~ / 1 [2-CPJo=7.78"10~ hi Stirring Speed=500 r.p.m. Temp.=24:~l C pH=3.0±O.1

30.0 • F,,,I

/0.]=8.0-t-0.1 rag/1

o 20.0

:::::

[Z-CP k _ [Interme.Jc

tct-h

o

10.0 o 0.0 0.0

200,0

400.0

800.0

Time(rain.)

800.0

1000.0

Fig. 3. Species distribution of the photocatalytic decomposition of 2-chlorophenol in the presence of TiO2 at pH 3.

increasing the solution pH over the range of solution pH studied. A possible explanation is that for neutral and acidic solutions, the undissociated species (C6H+CIOH) is the predominant species and has a higher affinity with TiO2 particles (pHzPc about 6.3 as determined in this study) than the dissociated species (C6H4C10-) predominated in alkaline solutions for a pH value greater than 9.55. After 10 h of aeration, experimental results indicated that less than 4% of 2-chlorophenol was removed by aeration at solution pH 3. Thus, the disappearance of 2-chlorophenol by aeration as compared with that decomposition by photocatalysis could be neglected. As shown in Fig. 2, the monochromatic absorption coefficient at 365 nm of solution containing TiOz particles at pH 3 was significantly less than that at pH 11. The difference of the absorbance of TiO2 suspensions at various solution pH might be attributed to the variation of surface charge of TiO2 which is highly dependent on solution pH, and not due to the species distributions of 2-chlorophenol because the absorbance of TiO2 particles was almost unaffected by the addition of 2-chlorophenol as shown in Fig. 2. Based on the experimental results described above, the removal of 2-chlorophenol due to TiOz adsorption and aeration could be neglected. Therefore, the concentrations of chlorinated, non-chlorinated intermediates and carbon dioxide after 2-chlorophenol

decomposition were determined by establishing elemental balances of chlorine and carbon as described below: mass balance of carbon: CTotat

=

(Chlorophenol)c + (Interme)c + (CO2)c (1)

(Interme)c.T = (Interme)c~ct + (Interme)c-o. + (Interme)c~c

(2)

mass balance of chlorine: Clrota~ = (Chlorophenol)cl + (Interme)c-c~ + CI- (3) where Croft, is the total initial amount of chlorophenols as carbon, (Chlorophenol)c is the amount of chlorophenols as carbon, (Interme)c is the amount of intermediates as carbon, (CO2)c is the amount of CO2 as carbon, Clrotat is the total initial amount of chlorophenols as chlorine, (Chlorophenol)ct is the amount of chlorophenols as chlorine, (Interme)c_oH is the amount of non-chlorinated organic intermediates produced as carbon, (Interme)c~:~ is the amount of chlorinated organic intermediates produced as carbon or as chlorine, (Interme)c-c is the amount of intermediate over the time that 99% of (Interme)c-o, and (Interme)c~c~ disappeared as carbon, (Interme)c,T

2-chlorophenol decomposition by UV/TiO2 is the amount of intermediate including (Interme)c~., (Interme)c~c~ and (Interme)c~ as carbon, and CI- is the amount of chloride ion. The decompositions of 2-chlorophenol in aqueous solution by UV/TiO2 process were discussed under various solution pH values, TiO2 dosages, light intensities to investigate the removals of reactants and organic intermediates. The temporal variations of reactants and organic intermediates were extensively monitored in order to determine the completeness of decomposition and the reaction kinetics.

chloride rose continuously. The structure of organic intermediates were analyzed qualitatively by HPLC and IC analyzer in this research indicating the presence of catechol, hydroquinone and various organic acids. But these qualitative results are too complicated to establish a detailed kinetic analysis. Thus, owing to the complexity of the decomposition schemes of chlorophenols by advanced oxidation processes, the simplified two steps consecutive kinetic model developed previously by the authors (Ku and Hsieh, 1992) was used to describe the temporal behaviors of reacting carbon-containing species during the reaction. Each step of the reaction was assumed to be first order and irreversible reaction. Thus, the profile of species could be derived by chemical kinetic calculation. The model is schematically shown as below:

pH effect on the decomposition rates of 2-chlorophenol The temporal distribution of reacting chemical species for 2-chlorophenol decomposition by UV/ TiO2 at solution pH 3 is shown in Fig. 3. This figure revealed that the concentration of 2-chlorophenol , (Interme)c~. + C1 -

\ /

kl

(2-chlorophenol)

2573

,

(Interme)c~c

, C02 + H20

(4)

, (Interme)c_o

kiz

CI--

decreased exponentially, the concentration of organic intermediates increased to a maximum before gradually decreasing, and the concentration of

80.0

where k~ is the global pseudo first-order decomposition rate constants of 2-chlorophenol, kf, is the global pseudo first-order generation rate constants of

UV/TiOs(Anstase )/Z - CP 2System

UV Intensity=22.5 .W/m TiOs Dosage=2.0 sg/1 [2-CPJo=7.76*lO- M

70.0

Stirring Speed=500 r.p.m.

Temp.=24+! C pH=7.0±O. 1 [Os]=a.0+.0.! mg/l

o 60.0

ooooo [~-cPk

aaaaa Interme )¢_~+ (Imterme)c~lc AAAAA [~In_ternae )¢-ctJc

M

" ' " " ~2]~

G

iiinniii

50.0

•

•

•

•

0

a

0

•

•

•

•

•

•

I

I

"~ 40.0

3o.o ~ 20.0

•

•

a

vo I0.0 o

0.0

r,

0.0

i

i

i

i

i

i

i

i

t

200.0

!

i

i

i

i

i

~

i

J

i

T

I

"1" I

400.0

I

"t/" I

I

T

i

600.0

i

"?" I

I

"r

i

I

T

800.0

Time(min.) Fig. 4. The kinetic behaviors described by the two consecutive kinetic model for the decomposition of 2-chlorophenol at pH 7 by UV/TiO: process.

2574

Y. Ku et al. 50.0 UV/TiOz(Anatase)/2-Cp zSystem UV I n t e n s i t y = 2 2 . 5 ~ / m Ti02 D o s a g e = 2 . 0 ag/1 [2-CP]o=7.78"10-M Stirring Speed=500 r.p.m. Temp.=24+l C . [Oz]=8.0+O.1 mg/1

40.0

~- 30.0 ','I

@ ,

..~,20.0 eee=o pH=3.0+0.1 =: --'=" pH=5.O±O.1 pH=7.0+0.1 ¢*"¢,¢*"pH=9.0:t:0.1 :'---:: pH= 1l.O+O.t

C.) I I

¢,2

I

10.0

0.0 0.0

200.0

400.0

600.0

Wime(min.)

800.0

1000.0

Fig. 5. The effect of solution pH on the photocatalytic decomposition of 2-chlorophenol in the presence of TiO2. non-chlorinated organic intermediates, k~2 is the global pseudo first-order generation rate constants of chlorinated organic intermediates, and k~ is the global pseudo first-order rate constants for decomposition of Intermec~c. The modelling result of the mixed series-parallel kinetic model at solution pH 7 is shown in Fig. 4. Good agreement between experimental results and calculated values were obtained in most experimental runs indicating that the proposed degradation path and kinetic models were suitable and rational to describe the temporal behaviors of reacting species, thus the mechanism of consecutive chlorine elimination of the upper path way in equation (4) could be verified. The temporal variation of 2-chlorophenol concentration for experiments conducted at different solution pH values in the presence of TiO2 particles are depicted in Fig. 5. This figure indicated that the

removal efficiency decreased significantly with solution pH. The complete disappearance of 2chlorophenol could be achieved within 3.5 h at pH 3, but required more than 10 h at pH 11. The global pseudo first-order photocatalytic rate constants of 2-chlorophenol decomposition for aqueous solution of various pHs are listed in Table 2. The distribution of 2-chlorophenol species (dissociated and undissociated) was reported to vary with solution pH condition (Shen et al., 1995), and probably influence the amount and species of 2-chlorophenol adsorbed on the TiO2 surface. The higher removals of 2-chlorophenol by photocatalysis at acidic conditions were possibly attributed to the increased amounts of undissociated 2-chlorophenol species adsorbed on the TiO2 surface as shown in Table 1, similar results were reported by Davis and Huang (1990). Various organic intermediates were reported to be produced from the decomposition of pollutants by

Table 2. The globalpseudo first-orderphotolyticrate constant and the ratio of kfi and kh to kf of 2-chlorophenolunder varioussolution pH in the presence of TiO:. Initial concentration of 2-chlorophenol= 7.78 x 10-SM; TiO~ loading= 2.0g/l; surface light intensity= 22.5 W/ms pH 3.0 5.0 7.0 9.0 ll.0 k~ (rain-L) 0.0270 0.0100 0.0082 0.0058 0.0047 kfi (min-~) 0.0081 0.0038 0.0045 0.0042 0.0035 kh (rain-t) 0.0189 0.0062 0.0037 0.0016 0.0012 k(t/kf 0.30 0.38 0.55 0.73 0.74 k~2/k~ 0.70 0.62 0.45 0.27 0.25 k~ (min-t) 0.0062 0.0056 0.0016 0.0005 0.0003

2-chlorophenol decompositionby UV/TiO2

2575

50.0

UV/TiOt(Anatase )/2- qe .System

•

40.0

~

UV .Intensity=22.5 W/m _TiOz D o s a g e = 2 . 0 ~ / 1 [2-CPJe=7.78*IO~ M _Temp=.=82.O 4++IO Stir-r1inSgp[oeezdj=5OOmg/1C r.p.m.

/

20.0

~10.0 Ill/

3U

-'1~[ /

/ eeeee p_H=5oO+O.l %. / ****- i,H_=~.o.ioa \ ~HHHH~pH=9~0~0.1 ~

\

0.0 0.0

........

2 0 0'. 0. . . . . .

.' "o

~'d6

.......

6 0 0' . 0. . . . . . . . .

8 0 0'. 0. . . . . . . . .

1000.0

Wime(min.)

Fig. 6. The effect of solution pH on the distribution of organic intermediates.

0.015

UV/TiOz( Rutile )/ 2-CP System pH=3.0±O.1 5 [2-CP]o=7.78"10- M Stirring Speed=500 r.p.m. Temp.=24±l C D.O.=8.0iO.1 rag/1 0.010 ----....-Jl

I -P..I

:~'o

oo5

~/

s

~ ~ ~aAA

.2,r:=0.96)

.~/,_~;l~z-z_.r,-_-°.~}

1----~.~ w / I n

le,........

0.000 0.0

~K--g.o,r --O.~lJ

1.0*lo-S*I*[TiOs]

, ......... 2.0 [TiOe]

1 +2.2"[Ti0~]

, ........ 4.0

.,, ........ , ....... 6 0. 8.0

.. 10 .0

(g/l)

Fig. 7. The effect of TiO2 loading on the global pseudo first-order photolytic rate constants, k(, of 2-chlorophenol at pH 3 under various light intensities.

2576

Y. Ku et al.

advanced oxidation processes in aqueous solution (Kormann et al., 1991; Pruden and Ollis, 1983; Turchi and Ollis, 1989). Some intermediates might be more toxic than the original pollutants (Rice, 1980), therefore, making the completeness of mineralization of pollutants should be of primary concern. In this study, the total concentration of all organic intermediates symbolized as (Interme)c~c in solution is expressed as total organic carbon (TOC). The effect of solution pH on the decomposition of organic intermediates is shown in Fig. 6. In this study, the organic intermediates formed during the decomposition of 2-chlorophenol by irradiation in the presence of photocatalyst are subdivided into two categories. One is the non-chlorinated intermediates symbolized as (Interme)c-oH including catechol among others. Another is the chlorinated intermediates symbolized as (Interme)c-o including namely chlorohydroquinone among others (D'Oliveira et al., 1990; Sehi et al., 1989). As indicated in Table 2, the global formation rate constants of non-chlorinated organic intermediates, kf~, were smaller than those of chlorinated organic intermediates, k(2, in acidic solutions, but were larger than those in alkaline solutions. The contribution of non-chlorinated intermediates to the decomposition of 2-chlorophenol in the early stages of reaction was found to be increased with increasing solution pH. The difference of negative charge density on the benzene ring of 2-chlorophenol species adsorbed on the TiO2 surface

might alter OH" electrophilic attack and further influence the formation of non-chlorinated and chlorinated organic intermediates under various solution pH. The global mimealization rate constants of organic intermediates, k~, are summarized in Table 2. The k~ values were fairly constant in acidic conditions, but apparently decreased for the solution pH values above 7. The effect of Ti02 dosage and light intensity on 2-chlorophenol decomposition Experimental results shown in Fig. 7 indicate that the global rate constant of 2-chlorophenol decomposition, k~ at pH 3 increased with an increasing TiO: dosage, subsequently approaching a limiting value for dosages above 2.0 g/1. Meanwhile, k~ values at pH 11 decreased with increasing TiO2 amount for loadings above 0.1 g/1 as shown in Fig. 8. The difference of adequate dosages for various solution pH might be due to the fact that the UV light absorbance of TiO2 suspensions at solution pH 11 was higher than that at solution pH 3 as shown in Fig. 2. The adequate loading of TiO2 shown in Fig. 7 increased with the light intensity possibly because of the increased distance of light penetration. The observed gradual decrease of k¢ with TiO2 at higher dosages was similar to the results reported by other investigations (D'Oliveira et al., 1990; Matthews, 1986) and are possibly attributed to the decrease of light penetration by suspensions of photocatalyst.

UV/Ti0z(Anatase)/2-CP

System

pH=ll.O+0.1

0.012

[ 2 - C P ] o = 7 . 7 8 " 1 0 -~ M Stirring Speed=500 r.p.m. Temp.=24+ 1 C D.0.=8.0+_0.1 m g / 1

-,'-'0.008 ,

o - ----~ L i g h t

t

0.004

r ~.

0.000

l

0.0

Intensity=f6.2

o " : : Light Intensity=19.2 ~ ~ -" ~ ~ L i g h t I n t e n s i t y = 2 2 . 5

~

i

W/m~

W/.m 2 ~/m

_---~

I

[TiOz] (g/l)

Fig. 8. The effect of TiO2 loading on the global pseudo first-order photolytic rate constants, k[, of 2-chlorophenol at pH 11 under various light intensities.

2-chlorophenol decomposition by UV/TiO2

2577

0.04

0.03 Q J

J

O

I .~.~

0.02

UY/TiO,(Anatase)/2-CP System pH=3.0+0.1 [ 2 - C P ] 0 = 7 . 7 8 . 1 0 - Id S t i r r i n g Speed=500 r.p.m Temp.=24+l C . D . O . = 8 . 0 + O . I mg/l

1=18.z

ooooo

W/.m:(<=0.98)

aoaaa I=19.2 W/mz (r,=0.99)

0.01

Aa*AA I=22.5 Y/m

k~-

(r =0.98)

2.13" 10-'*I* [Ti 0,] l+l.3"[ViO,]

o.oo - ~ 0.0

2.0

4.0

6.0

[Ti02] (g/l)

8.0

10.0

12.0

Fig. 9. T h e effect o f TiO2 l o a d i n g w i t h rutile f o r m o n the g l o b a l p s e u d o f i r s t - o r d e r p h o t o l y t i c r a t e

constants, k;, of 2-chlorophenol at pH 3 under various light intensities. Figure 7 depicts the effect of light intensity on the global pseudo first-order kinetic constant, kf, of 2-chlorophenol decomposition at pH 3. The light intensity used in the experiment were in the range about 16.2-22.5 W / m 2 which were lower than those of previous investigations (AI-Sayyed et al., 1991; Turchi and Ollis, 1989). Furthermore, the global rate constant, kf, was found to have increased linearly with increasing light intensity for TiO2 dosages between 0.01 and 2 g/l indicating that the decomposition rate of 2-chlorophenol at pH 3 is primarily determined by the generation of OH' radicals. The light intensities were found to have no obvious effect on k~ at pH 11 under various concentrations of TiO2 particles as shown in Figure 8. This can be explained by the effect that the rate is possibly influenced by the amount of 2-chlorophenol adsorbed on the TiO2 which were low for alkaline solutions as shown in Table 1. Proper selection of photocatalyst loading and light intensity should consider solution pH, light

penetration, species distribution and concentration of the pollutant to be degraded.

Table 3. The global psuedo first-order photolytic rate constant of 2-chlorophenol decompostion per unit weight at various solution pH in the presence of TiO2. Initial concentration of 2-chlorophenol = 7.78 x 10 -5 M; surface light intesity = 22.5 W/m 2

Table 4. The global psuedo first-order photolytic rate constant of 2-chlorophenol decomposition per unit surface area at various solution pH in the presence of TiO2. Initial concentration of 2-chlorophenol = 7.78 x 10-5 M; surface light intesity = 22,5 W/m 2

Rate constant per unit weight ( m i n - ' . g -~) for various forms of TiO2

Rate per unit surface area (min-~.(m 2) ') for various forms of TiO2 0.1

Anatase Rutile

0.1

Dosage of TiO2 (g/l) 0,5 2.0 5.0

8.0

0.0640 0.0340 0.0135 0,0060 0.0043 0,0240 0.0115 0.0042 0,0018 0,0013

The effect of 7702 form on the decomposition of 2-chlorophenol

Several researchers studied the effect of crystals properties on the organic compounds decomposition. The anatase form of TiO2 was reported to possess a higher activity than that of rutile form (Fox and Dulay, 1993). The photocatalysis is a heterogeneous solid-liquid catalytic reaction in essence. A number of experimental studies (A1-Sayyed et al., 1991; D'Oliveira et al., 1990; Hsiao et al., 1983; Ku and Hsieh, 1992; Okamoto et al., 1985) have shown that the photocatalytic decomposition of many organic pollutant follows the Langmuir kinetics. In this study, the apparent pseudo first-order photolytic rate constants, k., and the adsorption equilibrium constant, K, of 2-chlorophenol decomposed by ultraviolet irradiation with various loadings of

Anatase Rutile

0.5

Dosage of TiO2 (g/l) 2.0 5.0 8.0

0.0064 0.0034 0.0014 0.0006 0.0004 0.0072 0.0035 0.0013 0.0006 0.0004

2578

Y. Ku et al.

anatase and rutile form of TiO2 particles at pH 3 can be fitted adequately under various TiO2 dosages and light intensities by the following Langmuir-type equations and are shown in Figs 7 and 9:

Acknowledgements--This research was supported by grant

from National Science Council, Republic of China, under contract number NSC 84-2621-P011-003.

for anatase form k;=

2.13 x 10 -3 × I0 x [TiO2] 1 + 1.3 x [TiO2]

REFERENCES

K -- 1.3 l/g k, = 1.64 x 10-3 l/g.min t.m2/W

(5)

for rutile form

k~=

1.0 x 10 -3 x /o x [TiO2] 1 + 2.2 x [TiOz]

K = 2.2 1/g k~ = 0.45 × 10 -3 l/g.min -~.m2/w

(6)

The global destruction rates of 2-chlorophenol per unit weight of rutile were found to be less than those using anatase as shown in Table 3. However, taking into account the fact that the specific surface area of anatase (10 m2g) was determined to be approximately three times that of rutile (3.3m2g), the global destruction rates of 2-chlorophenoi per unit surface area for both forms of TiO2 were almost the same as shown in Table 4. Based on the observation of this study, the differences between the global photocatalytic destruction rates of 2-chlorophenol by various forms of TiO: particles might be attributed to the difference of the surface area of TiO2, not to the difference of the crystal properties in this study. CONCLUSIONS The photocatalytic degradation process with TiO2 particles has been shown in this study to be feasible for achieving high degrees of 2-chlorophenol removal. Almost complete disappearance was observed in only a few hours of illumination time. However, the demineralization of reaction intermediates requires a longer time. Because the distribution of 2-chlorophenol species varies with solution pH conditions, the higher removals at acidic conditions were possibly attributed to the increased amounts of undissociated 2-chlorophenol species adsorbed on the TiO2 surface. The formation of non-chlorinated and chlorinated organic intermediates was highly influenced by solution pH. Two forms of TiO2 were used in this study, in which the difference between the global degradation rates of 2-chlorophenol might be affected by the difference of the surface area of various forms of TiO2 rather than the difference of the crystal properties.

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