2003 saquib titanium dioxide mediated photocatalyzed degradation of a textile dye derivative, acid...

8/10/2019 2003 Saquib Titanium Dioxide Mediated Photocatalyzed Degradation of a Textile Dye Derivative, Acid Orange 8, In Aqueous Suspensions

1/9

~ ~ r ~

E L S E V I E R

Desalinat ion 155 (2003) 255-263

DESALINATION

www.elsevier.com/locate/desal

T i ta n ium d i o x i de m e d i a te d ph o to c a ta ly z e d de g r a da ti on o f a

t e x t i le dy e de r iv a t iv e a c id o r a ng e 8 i n a que o us s us p e ns i o ns

M ohd Saquib, M ohd Muneer

Department o f Chemistry, Aligarh Mu slim University, Aligarh 202002, India

Tel. +91 571) 270-3515; Fa x +91 571) 270-2758 ; em ail: chtl2m m @ ainu.hie.in

Received 8 January 2002; accepted 9 Decemb er 2002

A b s t r a c t

Ti t a n i um d i ox i de m e d i a t e dpho t oc a t a l y s e dde g ra da t ion o f a texti le dye derivative, acid orang e 8 ( 1 , wa s invest ig ated

i n a que ou s s us pe ns i ons o f t i ta n i um d i ox i de by m ord t o t ing t he de p l e t ion o f t o t a l o rga n ic c a rbon (TO C ) c on t e n t a s a

func t ion of i r radia t ion t ime un der a va r ie ty o f condi tions. Th e deg rada t ion k ine t ic s w ere s tudied un der d i f fe ren t

cond i t ions such as pH , ca ta lys t concent ra t ion , subs tra te concent ra tion , d i f fe rent types o f TiO2 and in the pre sence o f

e l e c t ron a e c e p t o r s s uc h a s hyd roge n pe rox i de (1-I20 ) a nd po t a s s ium b rom a t e (KB rO3) be si de s m o l e c u l a r oxyge n . T he

degra da t ion ra te s were fo und to be s t rongly influenced by a l l the abo ve pa ramete rs. Th e photoca ta lys t De gussa P25 wa s

found t o be m o re e f f i c i e n t c om pa re d wi t h o t he r pho toc a ta ly s ts . The dye wa s found t o be a ds o rbe d on t he s u rfa ce o f t he

pho t oc a t a l y s t a t a c i d ic pH .

Keywords:

Photocata lys i s; Text i le dye ; Ac id orange 8 ; T i tanium dioxide

I I n t r o d u c t i o n

T h e e n v i r o n m e n t a l p r o b le m a s s oc i a te d w i t h

t e x t i l e d y e i n g a n d f i n i s h i n g a c t i v i t i e s m a i n l y

o r i g i n a t e f r o m t h e e x t e n s i v e u s e o f a w i d e v a r i e t y

o f o r g a n i c d y e s t u f f s , t h e c h e m i c a l c h a r a c t e r i st ic s

o f w h i c h v a r y g r e a t l y [ 1 , 2] . A z o d y e s , c h a ra c -

t e r i z e d b y t h e p r e s e n c e o f t h e N = N l in k a g e ,

c o m p r i s e a b o u t h a l f o f a ll t ex t il e d y e s u s e d t o d a y

[ 3] . O n e m a j o r s o u r c e o f t h e s e e f f l u e n t s is t h e

*Corresp onding author .

w a s t e a r i s i n g f r o m i n d u s t r i a l p r o c e s s e s w h i c h

u t i l i z e d y e s t o c o l o r p a p e r , p l a s t i c s a n d n a t u r a l

a n d a r t i f i c i a l f i b e r s [ 4 ] . A s u b s t a n t i a l a m o u n t o f

d y e s t u f f is l o s t d u r i n g t h e d y e i n g p r o c e s s i n th e

t e x ti le i n d u s t r y [4 ], w h i c h p o s e s a m a j o r p r o b l e m

f o r t h e i n d u s t r y a s w e l l a s a t h r e a t t o t h e e n v i r o n -

m e n t [ 4 - 8 ] . D e c o l o u r i z a t io n o f d y e e f f l u e n t s h a s

t h e r e f o r e a c q u i r e d i n c r e a s i n g a t t e n t i o n . D e c o l -

o u r i z a t io n o f d y e e f f l u e n t b y b i s u l f i t e - c a ta l y s e d

b o r o h y d r i d e r e d u c t i o n h a s a l s o b e e n r e p o r t e d [ 9] .

D u r i n g t h e p a s t t w o d e c a d e s , p h o t o c a t a l y t i c

p r o c e s s e s i n v o l v i n g TiO 2 s e m i c o n d u c t o r p a r t i c le s

001 1-9 164 /03/ - See fron t matter 200 3 Elsevier Science B.V. All rights reserved

PII: S0011-9164 03 )00303-5


2/9

256 M. Saquib, M. Muneer / Desalination 155 2003) 255-263

u n d e r UV l ig h t i ll u mi n a t io n h a v e b e e n s h o wn t o

be po ten t i a l ly advan tageous and use fu l in the

t rea tmen t o f was te w a te r po l lu tan t s . Ea r l i e r

s tud ies [ 10 -13] have sho wn tha t a wide range o f

o rgan ic subs t ra te s such as a lkanes , a lkenes ,

a romat ic s , su r fac tan t s , and pes t i c ides can be

c o m p l e t e l y p h o t o mi n e r a l iz e d i n t h e p r e s e n c e o f

T i O 2 a n d o x y g e n .

The re a re seve ra l s tud ies r e la ted to the use o f

semiconduc to r s in the pho tominera l i za t ion o f

pho tos tab le dyes [14-20] . Pho toexc i t a t ion o f

s e mi c o n d u c t o r s l e a d s t o t h e f o r ma t i o n o f a n

e lec t ron ho le pa i r , wh ich can even tua l ly b r ing

abou t a r edox reac t ion f rom organ ic subs t ra te s

d i s so lved in w a te r . Al te rna t ive ly , d i rec t absorp -

t ion o f l igh t by the dye , adsorbed on the semi -

cond uc to r su r faces , can l ead to cha rge in jec t ion

f rom the exc i t ed s t a te o f the dye to the con-

d u c t i o n b a n d o f th e s e mi c o n d u c t o r . It h a s b e e n

show n ea r l i e r [ 14 -20] tha t such p rocesses can be

u s e d f o r r e m o v i n g c o l o u r i n g ma t e r i a l f ro m d y e

ef f luen t s in the p resence o f v i s ib le l igh t.

N o ma j o r e f f o rt s h a v e b e e n ma d e t o s t u d y th e

degrada t ion k ine t i c s o f an azo dye de r iva t ive

such as ac id o range 8 . Thus , we have under -

t aken a de ta i l ed s tudy on the pho todegrada t ion o f

th i s t ex t i le dye de r iva t ive , sens i ti zed by T iO2 in

aqueous so lu t ion .

CH3

NaO S ~ N N

2 Exper imenta l

2.1. Reagents and chemicals

Ac i d o r a n g e 8 ( 1 ) wa s o b t a i n e d fr o m Al d r i c h

and used wi thou t any fu r the r pur i f i ca t ion . The

wa te r used in a l l the s tud ies was d oub le d i s t i ll ed .

Whi le the pho toca ta lys t t i t an ium d iox ide , P25

( De g u s s a AG) , wa s u s e d i n mo s t o f th e e x p e r i-

me n t s , o t h e r c a t a l y s t p o wd e r s - - n a me l y

H o m b i k a t U V 1 0 0 ( S a c ht le b e n c h e m i c G m b H ) ,

P C 5 0 0 ( Mi l e n i u m i n o r g a n ic c h e mi c a l s ) a n d T T P

( T r a v a n c o r e t i ta n i u m p r o d u c t s , I n d ia ) - - we r e

used fo r compara t ive s tud ies . P25 cons i s t s o f

75 ana tase and 25 ru t i le wi th a spec i f i c BET -

sur face a rea o f 50 m2g ~ and a p r im ary pa r t i c le

s i ze o f 20 n m [2 1] . Ho m b i k a t UV1 0 0 c o n s is t s o f

100 ana tase wi th a spec i f i c BE T-sur face a rea

>250 m2g ~ and a p r ima ry pa r t i c le s i ze o f 5 nm

[22]. T he pho toca ta lys t PC5 00 has a B ET-sur face

a rea o f 287 m2g ~ w i th 100 ana tase and a

pr imary pa r t i c l e s i ze o f 5 -10 nm [23] , whereas

TTP has a BE T-sur face a re a o f 9 .82 mEg 1 . The

o the r chemica l s used in th i s s tudy such as NaO H,

HNO3 , H 2 0 2 a n d KB r O 3, we r e o b t a i n e d f r o m

Me r c k .

2.2. Procedure

Stock so lu t ions o f the dye con ta in ing the

d e s i r ed c o n c e n t r a t i o n w e r e p r e p a r e d i n d o u b l e

d i s t i l l e d wa t e r . An i mme r s i o n we l l p h o t o -

c h e mi c a l r ea c t o r ma d e o f P y r e x g l a s s e q u i p p e d

wi th a m agne t i c s t i r ring ba r , a wa te r c i r cu la t ing

j a c k e t a n d a n o p e n i n g f o r s u p p l y o f mo l e c u l a r

oxygen was used . For i r r ad ia t ion exper imen ts

2 5 0 m L o f t h e s t o c k s o l u ti o n we r e t a k e n i n t o th e

p h o t o r e ac t o r a n d t h e r e q u i r e d a m o u n t o f p h o t o-

ca ta lys t wa s added ; the so lu t ion was s t ir r ed and

bubb led wi th molecu la r oxygen fo r a t l eas t

30 m in in the da rk to a l low eq u i l ib ra t ion o f the

s y s t e m s o t h a t t h e l o s s o f c o mp o u n d d u e t o

adsorp t ion cou ld be taken in to accoun t . T he ze ro

t ime read ing was ob ta ined f rom a b lank so lu t ion

kep t in the da rk bu t o th e rwise t r ea ted s im i la r ly to

the i r r ad ia ted so lu t ion . The suspens ions were

c o n t i n u o u s l y p u r g e d wi t h mo l e c u l a r o x y g e n

th roughou t each exper imen t . I r r ad ia t ions were

ca r r i ed ou t us ing a 125 W medium-pressu re

mercu ry l amp. IR- rad ia t ion and sho r t -wave leng th

UV - r a d ia t io n w e r e e l i mi n a t e d b y a wa t e r j a c k e t.

Samples (10 mL) were co l l ec ted be fo re and a t


3/9

M. Saquib, M. Munee r Desalination 155 (2003) 255 -263

257

r e g u l a r i n t e r v a l s d u r i n g th e i r r a d i a ti o n . T h e y

w e r e c e n t r i f u g e d b y a J a n e t z k i T - 2 4 l a b o r a t o r y

c e n t r i f u g e b e f o r e a n a l y si s .

2.3 . Analysis

T h e d e g r a d a t io n w a s m o n i t o r e d b y m e a s u r i n g

t h e to t a l o r g a n i c c a r b o n ( T O C ) c o n t e n t w i t h a

S h i m a d z u T O C 5 0 0 0 A a n a ly z e r b y d i re c t ly

i n j e c t in g t h e a q u e o u s s o l u ti o n . F o r e a c h e x p e r i-

m e n t t h e d e g r a d a t i o n r a te o f th e m o d e l p o l l u t a n t

w a s c a l c u l a t e d f r o m t h e i n it ia l s lo p e o b t a i n e d b y

l in e a r r e g r e s s i o n f r o m a p l o t o f t h e n a t u r a l

l o g a r it h m o f t h e T O C o f th e d y e a s a f u n c t io n o f

i r r a d i a t i o n t i m e , i . e . , f i r s t - o r d e r d e g r a d a t i o n

k i n e ti c s . I t w a s c a l c u l a t e d i n t e r m s o f [ m o l e L -

m i n - l ] .

3 R e s u l t s

3.1 . Photo lys is o f Ti02 suspensions containing

ac id orange 8

F i g . 1 s h o w s t h e d e p l e t i o n i n T O C o n i r ra d i-

a t io n o f a n a q u e o u s s o l u t i o n o f a c id o r a n g e 8

( 0 .2 5 m M ) i n t h e p r e s e n c e a n d a b s e n c e o f T iO 2

b y t h e P y r e x - f il t e r e d o u t p u t o f a 1 25 W m e d i u m -

1,3.

1

I , - O,4

0 2

i i I

o :3

- , , , , m - - . ~ r s o e o c a ~ d y a

i , 1 , , t I i I , ,

4 0 0 8 4

rra dia t io n t ime r a i n )

Fig. 1. Influence of photocatalysts o n the degradat ion of

acid orange 8. Experimental condit ions: dye concentra-

t ion (0 .25 mM ), V = 250 m L, imm ers ion wel l photo-

reactor, 125 W m edium-pressure Hg lam p, photocatalyst:

De gussa P25 (1 gL-~), cont . 02 p urgin g and st irring,

irradiat ion t ime = 1.5 h.

p r e s su r e m e r c u r y l a m p . I t w a s f o u n d t h a t 7 5

m i n e r a l iz a t io n o f t h e m o d e l c o m p o u n d t o o k p l a c e

a f t er 9 0 m i n o f il l u m i n a t i o n , w h e r e a s n o o b s e r v -

a b l e l o s s o f th e d y e o c c u r r e d i n th e a b s e n c e o f th e

p h o t o c a t a l y s t . T h e d e g r a d a t i o n c u r v e s c a n b e

f i t t e d r e a s o n a b l y w e l l b y a n e x p o n e n t i a l d e c a y

c u r v e , s u g g e s t i n g fi r s t - o r d e r k in e t i c s . T h e r e s u l t -

i n g f i r s t - o r d e r r a t e c o n s t a n t w a s u s e d i n a l l

s u b s e q u e n t p l o t s to c a l c u l a t e t h e d e g r a d a t i o n r a t e

u s i n g th e f o r m u l a b e l o w :

- d [ T O C ] / d t = kc

w h e r e k i s th e r a t e c o n s t a n t , c t h e c o n c e n t r a t i o n

o f t h e p o l lu t a n t , a n d n i s th e o r d e r o f r e a c t io n .

C o n t r o l e x p e r i m e n t s w e r e c a r r i e d o u t i n a l l

c a s e s u s i n g u n i r r a d i a t e d b l a n k s o l u t i o n s . I t h a s

b e e n f o u n d t h a t t h e d y e i s a d s o r b e d s i g n i f ic a n t l y

i n t h e d a r k a t p H 3 a n d 5 . 3 . T h e z e r o i r r a d i a t i o n

t i m e r e a d i n g s w e r e o b t a i n e d f r o m b l a n k s o l u t i o n s

k e p t i n t h e d a r k , b u t o t h e r w i s e t r e a t e d s i m i l a r l y t o

t h e i r r a d i a t e d s o l u t i o n s .

3 .2 . Compar i son o f d i f f e ren t pho toca ta lys t s

T h e i n f l u e n c e o f f o u r d i f f e r e n t p h o t o c a t a l y s t s

( D e g u s s a P 2 5 , H o m b i k a t U V 1 0 0 , P C 5 0 0 a n d

T T P ) o n t h e d e g r a d a t i o n k i n e t ic s o f a c i d o r a n g e

8 ( 1 ) i s s h o w n i n T a b l e 1 . I t w a s o b s e r v e d t h a t

Table 1

Com parison of degradat ion rate for the mineral izat ion o f

acid orange 8 und er different photocatalysts

Type of photocatalyst Degradat ion rate

(mo le L -1 min j 10 -3)

P25 0.00392

UV100 0 .00265

PC500 0 .00173

TTP 0 .00043

Experimental condit ions: dye concentrat ion (0.25 m M ),

V= 250 mL , P25 (1 gL-l) , Sachtleben Hom bikat UV 100

(1 gL-t), PC 500 (1 gL-t) , TT P (1 gL-~), mm ersion well

photoreactor, 125W medium -pressure Hg lamp, cont . 02

purging, i rradiation t ime

= 1 5 h


4/9

258

M. Saquib, M. M uneer Desalination 155 2003) 255 -263

4 0 0 JO O S

= n O ~

i

O n 0 4

f,

t m

0

, i i I I I I I

l I I I f I i

2 4 6 6

p i t

Fig. 2. Influence of pH o n the deg radation rate for the

mineralizationof acid orange 8. Experimentalconditions

as in F ig. 1.

t h e d e g r a d a t i o n o f d y e p r o c e e d s m u c h m o r e

r a p i d ly i n t h e p r e s e n c e o f D e g u s s a P 2 5 c o m p a r e d

w i t h o t h e r p h o t o c a t a l y s t s . In a l l o f th e f o l l o w i n g

e x p e r i m e n t s , D e g u s s a P 2 5 w a s u s e d a s t h e

p h o t o c a t a l y s t s i n c e t h i s m a t e r i a l e x h i b i t e d t h e

h i g h e s t o v e r a l l a c t i v i t y f o r t h e d e g r a d a t i o n o f t h e

m o d e l c o m p o u n d .

3 .3 . pH e f f ec t

U s i n g D e g u s s a P 2 5 a s th e p h o t o c a t a l y s t , t h e

p h o t o m i n e r a l i z a t i o n o f a c id o r a n g e 8 1 ) i n t h e

a q u e o u s s u s p e n s i o n s o f T i O 2 w a s s t u d ie d i n t h e

p H r a n g e b e t w e e n 3 t o 1 1. Th e d e g r a d a t i o n ra t e

f o r t h e m i n e r a l i z a t i o n o f a c i d o r a n g e 8 1 ) a s a

f u n c t i o n o f r e a c t i o n p H i s s h o w n i n F ig . 2 . Th e

d e g r a d a t i o n ra t e f o r t h e T O C d e p l e ti o n is f o u n d

t o g r a d u a l l y i n c r e a s e f r o m p H 3 to 9 f o l l o w e d b y

a s u d d e n d e c r e a s e a s t h e p H r i s e s t o p H 1 1 . A

s i g n i f i c a n t a d s o r p t i o n o f t h e d y e o n t h e s u r f a c e o f

t h e p h o t o c a t a l y s t w a s o b s e r v e d a t p H 3 a n d 5 .3 a s

s e e n b y t h e ty p i c a l T O C V a m o u n t o f T i O 2 at

d i f f e r e n t p H F i g . 3 ) . Th e a d s o r p t i o n s t u d y o f a z o

d y e 1 w a s i n v e s t i g a t e d b y s t i r r in g th e a q u e o u s

s o l u t i o n i n t h e d a r k f o r 2 4 h i n a r o u n d - b o t t o m e d

f l a s k c o n t a i n i n g v a r y i n g a m o u n t s o f T iO 2 s u c h a s

0 ,0 .5 , 1 , 2 and 5 gL - ] a t pH 3 , 5 .3 , 9 and 11 .

I t i s p e r t in e n t t o m e n t i o n h er e t h a t t h e p H o f

t h e s o l u t i o n w a s a d j u s t e d b e f o r e i r ra d i a t io n a n d

i t is n o t m a i n t a i n e d t h r o u g h o u t t h e r e a c t io n . A

~ 1 I I I I I I ,

1

o 2

\ - ~ p H - 5 . 3

- i

{} i i I I i , , i I , I , I I , I i i I I I I i , , ,

O 1 2 3 4 5

A m o u . t o f T iO 2 ( g L l )

Fig. 3. D epletion in TOC Vs amount o f the TiO at

different pH sho wing the adsorption of acid orange 8 in

the dark. Experimental conditions: dye concentration

0.25 m M), V = 250 m L, Photocatalyst: De gussa P25

0.5, 1, 2 and 5 gL-l), stirring in the dark for 24 h.

d e c r ea s e i n t h e p H o f t h e r e a c t io n m i x t u r e w a s

o b s e r v e d a t t h e e n d o f i l l u m i n a t i o n .

3.4. Ef fect o f substrate co ncentrat ion

I t is i m p o r t a n t b o t h f r o m a m e c h a n i s t i c a n d a n

a p p l ic a t io n p o i n t o f v i e w t o s t u d y th e d e p e n d e n c e

o f p h o t o c a t a l y t i c r e a c t i o n r at e s o n t h e s u b s t r a t e

c o n c e n t r a t i o n . H e n c e t h e e f f e c t o f s u b s t r a t e

c o n c e n t r a ti o n o n t h e d e g r a d a t io n o f a c i d o r a n g e

8 1 ) w a s s t u d i e d a t d i f f e r e n t c o n c e n t r a t i o n s :

0 .1 2 , 0 . 2 5 , 0 . 3 5 a n d 0 .5 m M . F i g . 4 s h o w s t h e

d e g r a d a t i o n r a t e f o r t h e m i n e r a l i z a t i o n o f a c i d

o r a n g e 8 a s a f u n c t i o n o f s u b s t r a t e c o n c e n t r a t i o n

u s i n g D e g u s s a P 2 5 a s t h e p h o t o c a t a l y s t . I t i s

i n t e r e s ti n g t o n o t e t h a t t h e d e g r a d a t i o n r a t e

i n c r e a s e d w i t h t h e i n c r e a s e i n s u b s t r a t e c o n -

c e n t r a t i o n f r o m 0 .1 2 t o 0 .2 5 m M . A f u r t h e r

i n c r e a se i n th e s u b s t r a t e c o n c e n t r a t i o n f r o m 0 .2 5

t o 0 .5 m M l e d t o a d e c r e a s e i n t h e d e g r a d a t i o n

rate.

3.5 . Ef fect o f catalys t concen trat ion

T h e e f f e ct o f p h o t c a ta l y s t c o n c e n tr a t i o n o n

t h e d e g r a d a t i o n k i n e t i c s o f a c i d o r a n g e 8 1 ) w a s

i n v e s t i g a t e d u s i n g d i f f e r e n t c o n c e n t r a t i o n s o f

D e g u s s a P 2 5 v a r y i n g f r o m 0 .5 t o 5 g L- L A s


5/9

M. Saquib, M. M uneer / Desalination 155 2003) 255-263

259

o

~ ao s

, + i I I , I i I i i I , i , , , , i , , , , i , k

O 1 O 2 O 3 ~ 1 O ,5 0 6

S u b s t r a t e C o n c e n t r a t i o n

( r a M )

Fig. 4. Inf luence of substrate concentration on the

degradation rate for the mineralization o f acid orange 8.

Experimental conditions: dye concentrations 0.12, 0.25,

0.35, and 0.5 mM ), V = 200 m L, Photocatalyst: D egussa

P25 1 gL-t) , imm ersion well photoreactor, 125 W

medium -pressure Hg lamp, con t. O5 purging an d stirring,

irradiation tim e = 1.5 h.

' ~ D

Din

| W

oom

~ o o a

0

, , i l l

1 2 3 4 $ 6

C a l a l y s t c o n c e n t ~ t ~ n ( g L 1 )

Fig. 5. Inf luence o f catalyst concentration on the degrada-

tion rate for the mineralization of acid orang e 8 at d if-

ferent photocatalyst concentrations. Experimental co nd i-

tions: dy e concentration 0.25 mM ), V= 250 mL . Photo-

catalyst: De gussa P25 0.5, 1, 2 and 5 gL-l) , immersion

well photoreactor, 125 W medium-pressure Hg lamp,

cont. O2 pu rging an d stirring, irradiation tim e =1.5 h .

expec ted , the degrada t ion ra te o f the ac id

orange 8 has been found to inc rease wi th the

inc rease in ca ta lys t concen t ra t ion f rom 0 .5 gL-

t o 2 g L - I A f u r th e r in c r e a s e i n t h e c at a ly s t

conce n t ra t ion f rom 2 to 5 gL- t l eads to a dec rease

in the deg rada t ion ra te F ig . 5 ) due to the adsorp -

t ion o f the d ye on the su r face o f the pho to -

ca ta lys t . Th i s obse rv a t ion ind ica tes tha t beyond

th i s op t imu m co ncen t ra t ion , o the r f ac to r s a f fec t

t h e d e g r a d a t i o n o f mo d e l c o mp o u n d . A t h i g h

TiO2 concen t ra t ions , pa r t i c le s aggrega te , wh ich

reduce s the in te r fac ia l a rea be twe en the reac tion

so lu t ion and the ca ta lys t ; thus , they dec rease the

number o f ac t ive s i t e s on the su r face . L igh t

sca t t e r ing by the pa r t i c le s and the inc rease in

opac i ty m ay be o the r r easons fo r the dec rease in

the deg rada t ion ra te .

m

|

_ t m

J

}

I ~ ~ T H : O z P 2 Y ~ ' K B r O j

P 2 5

E le c tr on a c c e p t o r s

Fig. 6. Degradation rate for the mineralization o f acid

orange 8 in the presence o f differen t electron acceptors.

Experimental conditions: dye concentration 0.25 mM),

V = 250 mL, P25 1 gL-]), electron acceptors: KB rO3

5 mM), HzO2 10 mM), immersion well photoreactor,

125 W medium-pressure Hg lamp, cont. 02 pu rging and

stirring, irradiation tim e = 1.5 h.

3 .6 . E f f e c t o f e l e c t r o n a c c e p t o r s

Since hydroxy l r ad ica l s appear to p lay an

impo r tan t ro le in pho toca ta lys i s , e l ec t ron accep-

t o r s s u c h a s h y d r o g e n p e r o x i d e a n d p o t as s iu m

b r o ma t e w e r e a d d e d t o t h e s o l u ti o n i n o r d e r t o

e n h a n c e t h e f o r ma t i o n o f h y d r o x y l r a d ic a ls a n d

a l so to inh ib i t the e - /h +) pa i r r ecom bina t ion . The

degrada t ion ra te s fo r the m inera l i za t ion o f ac id

orange 8 in the p resence o f these e lec t ron

accep to rs a re shown in F ig . 6 . Bo th the add i t ives

such as hydrogen pe rox ide and po tass ium

bromate had a b enef ic ia l e f fec t on the degrada-

t io n o f t h e m o d e l c o mp o u n d .


6/9

260

M Saquib , M. Mu neer / Desa l ina t ion 155 2003) 255 -263

4 D i s c u s s i o n

The pho toca ta lysed degrada t ion o f va r ious

organ ic sys tems us ing i r rad ia ted T iO2 i s we l l

documented [10] . The in i t ia l s tep in the TiO2

me dia ted pho toca ta lysed degrada t ion is p roposed

to invo lve the genera t ion o f the (e - /h ) pa i r

l ead ing to the fo rm at ion o f a hydroxy l r ad ica l and

superox ide rad ica l an ion [Eqs. (1 ) - (3 ) ] :

TiO 2 + ho --~ e~b+ h~b (1)

02 + e~b ~ 0 2- (2 )

H20 + h~b ~

OH

+ H

3 )

I t has been sugges ted tha t the hydroxy l

rad ica l s and superox ide rad ica l an ions a re the

pr imary ox id iz ing spec ies in the pho toca ta ly t i c

ox ida t ion p rocesses . These ox ida t ive reac t ions

resu l t in the b leac h ing o f the dy e , and the e f f i -

c i e n c y o f t h e d e g r a d a t i o n d e p e n d s u p o n t h e

o x y g e n c o n c e n t r a ti o n , w h i c h d e t e r mi n e s t h e e f fi -

c i e n c y w i t h w h i c h t h e c o n d u c t i o n b a n d e l e c tr o n s

a r e s c a v e n g e d a n d t h e

e - / h )

recombina t ion i s

p reven ted . A l te rna t ive ly , the e lec t ron in the con-

duc t ion band can be p icked up by the adsorbed

d y e mo l e c u l e s , l e a d in g t o t h e f o r ma t i o n o f a d y e

rad ica l an ion ; sub sequen t r eac t ion o f the rad ica l

an ion can l ead to degrada t ion o f the dye . In the

presen t case bo th m echan ism s can opera te in an

oxygen-sa tu ra ted so lu t ion .

The resu l t s on the pho todegrada t ion o f the

mo d e l c o m p o u n d u s i n g d i f f er e n t k i n d s o f T iO 2

pho toca ta lys t s wi th d i f fe ren t bu lk and su r face

proper t ies , i .e . , BET-surface , impuri t ies , la t t ice

mi s m a t c h e s o r d e n s i t y o f h y d r o x y l g ro u p s o n t h e

ca ta lys t ' s su r face , a re apparen t ly re spons ib le fo r

the pho toca ta ly t i c ac t iv i ty s ince they a f fec t the

adsorp t ion b ehav io r o f a po l lu tan t o r in te rmed ia te

mo l e c u l e a n d t h e l i fe t ime a n d r e c o mb i n a t io n r a te

o f e lec t ron-ho le pa i rs .

An ea r l i e r s tudy [24] has shown tha t Degussa

P25 ow es i t s h igh pho to reac t iv ity to a s low

r e c o mb i n a t i o n o f e l e c t r o n a n d h o l e s w h e r e a s

Sach t leben Ho m bika t UV 100 has a h igh pho to -

reac t iv i ty due to i t s f a s t in te r fac ia l e lec t ron

t rans fe r r a te . In these s tud ies i t was found tha t

Degussa P25 showed be t t e r pho toca ta ly t i c ac t i -

v i t y f o r t h e d e g r a d a t io n o f a m o d e l c o m p o u n d .

Ear l i e r s tud ies have shown tha t Deg ussa P25 wa s

found to show be t te r ac t iv i ty fo r the pho to -

c a t a l y t i c d e g r a d a t i o n o f a l a r g e n u mb e r o f

o r g a n ic c o m p o u n d s [ 2 5 - 2 8 ] . O n t h e o t h e r h a n d ,

L indn er e t a l . [29] show ed tha t Hom bika t UV 100

w a s a l mo s t f o u r t ime s mo r e e f f e c t iv e t h an P 2 5

w h e n d i c h l o r o a c e ti c a c i d w a s u s e d a s t h e mo d e l

po l lu tan t . The pho toca ta lys t H om bika t U V 100

w a s f o u n d t o b e b e t t e r f o r t h e d e g r a d a t i o n o f

benz id ine and 1 ,2 -d ipheny l hydraz ine as shown

in a recen t s tudy [30] .

These re su l t s ind ica te tha t the ac t iv i ty o f the

pho toca ta lys t a l so depends on the type o f the

mode l po l lu tan t . Ano the r r eason fo r the be t t e r

e f f i c ie n c y o f th e D e g u s s a P 2 5 p h o t o c a t a ly s t c a n

be exp la ined by the fac t tha t Degussa P25 i s a

mix tu re o f 25% ru t i le and 75% ana tase .

An impor tan t pa ram ete r in the pho toca ta ly t i c

reac t ions t ak ing p lace o n the pa r t i cu la te su r faces

i s the pH of the so lu t ion s ince i t d ic ta te s the

sur face cha rge p roper t i e s o f he pho toca ta lys t and

s ize o f aggrega tes i t fo rms . For T iO2 the w i th

Degussa P25 be ing the pho toca ta lys t , the ze ro

point o f charge (pH~pc) is a t pH 6.25. He nce , a t

more ac id ic pH va lues , the pa r t i c le su r face i s

pos i t ive ly cha rged , whi le a t pH va lues above

6.25, i t i s neg at ively cha rged [31 ] . In th is s tudy i t

was shown tha t the degrada t ion ra te fo r a mode l

com pou nd under inves t iga t ion i s s t rong ly in f lu -

enced by the reac t ion pH, which inc reases wi th

the inc rease in pH f rom 3 to 9 fo l lowed by a

sudde n decrea se as the pH r ises to 11.

D u e t o t h e l o w p K a v a l u e o f th e s u l p h o n i c

group , ac id o range 8 was fu l ly in the an ion ic

fo rm wi th in the pH range s tud ied . Wi th

inc reas ing pH, the nega t ive cha rges on T iO2 a re

expec ted to repe l the dye , and a dec rea se in the

e f f i c iency o f pho to -degrada t ion wi th inc reas ing


7/9

M. Saquib, M. Muneer / Desal inat ion 155 2003) 25 5-2 63

261

p H i s e x p e c te d . Ho w e v e r , i t wa s o b s e r v e d t ha t

the ra te inc reased wi th an inc rease in pH. S imi la r

resu l ts w ere repo r ted ea r l i e r fo r the ace ta te ion

and ac id b lue 40 [32 ,33] . Th i s e f fec t may be

a t t r ibu ted to more e f f i c i en t genera t ion o f

hydro xy l r ad ica l s by T iO 2 wi th an inc reas ing

concen t ra t ion o f OH - . A t the a lka l ine pH va lues ,

the hydroxy l r ad ica l s have to d i f fuse away and

degrade the d ye in the bu lk so lu t ion .

The dye was found to be s ign i f i can t ly

adsorbed in the da rk a t pH 3 and 5 .3 a s show n in

F ig . 3 . I t is in te res t ing to no te tha t a t pH 9 wh ere

t h e a d s o r p t io n o f t h e d y e o n t h e s u r fa c e o f t h e

pho toca ta lys t i s neg l ig ib le , the degrada t ion ra te

f o r th e m i n e r a l iz a t i o n o f t h e d y e i s ma x i mu m,

w hich sud den ly dec reases a t pH 11. Th is e f fec t

can be a t t r ibu ted to the fac t tha t the molecu le

may be undergo ing s t ruc tu ra l changes in the

doub le -bond cha rac te r l ead ing to the oxo fo rm,

wh i c h n o r ma l l y h ap p e n s a t h i g h e r p H v a l u e s.

The e f fec t o f subs t ra te concen t ra t ion on the

degrada t ion ra te fo r the minera l i za t ion o f 1

(Fig . 4) w as s tud ied, as i t i s im portant both f rom

a m echan i s t i c and the app l i ca t ion po in t o f v iew.

As ox ida t ion p roceeds , l e s s and l e s s o f the

sur face o f the T iO2 pa r t ic l e i s covered as the

po l lu tan t i s decomposed . Ev iden t ly , a t to ta l

decompos i t ion , the r a te o f degrada t ion i s ze ro

and a dec reased pho toca ta ly t i c r a te i s expec ted

wi th inc reas ing i l lumina t ion t ime . I t has been

agreed , wi th m inor va r ia t ions , tha t the express ion

fo r the ra te o f pho tominera l i za t ion o f o rgan ic

subs t ra te s wi th i r r ad ia ted T iO 2 fo l lows the

L a n g m u i r - H i n s h e l w o o d ( L - H ) l a w f o r t h e fo u r

possible s i tuat ions , i .e . , (a) the react ion takes

p lace be tween two adsorbed subs tances , (b ) the

reac t ion occurs be twe en a r ad ica l in so lu t ion and

an adsorbed subs t ra te m olecu le , ( c ) the r eac t ion

takes p lace be tween a r ad ica l l inked to the

sur face and a subs t ra te m olecu le in so lu t ion , and

(d) the reac t ion o ccurs w i th bo th spec ies be ing in

solut ion. In a l l cases , the expres s ion fo r the ra te

equa t ion i s s imi la r to tha t de r ived f rom the L-H

m ode l , w h ich has been use fu l in mode l ing the

process , a l though i t i s no t poss ib le to f ind ou t

wh e the r the p rocess t akes p lace on the su r face , in

the so lu t ion o r a t the in te r face .

Ou r re su l ts on the e f fec t o f the in i t i al concen -

t r at i on o n t h e d e g r a d a t i o n r a t e o f t h e mo d e l

com poun d , show n in F ig . 4 , ind ica te tha t the r a te

inc reases wi th the inc rease in the subs t ra te con-

cen t ra t ion f rom 0 .125 to 0 .25 mM. A fu r the r

inc rease in subs t ra te concen t ra t ion l eads to a

dec rease in the degrada t ion ra te . Th i s ma y be due

to the fac t tha t, a s the in i t i a l concen t ra t ions o f the

d y e i n c r e a s e , m o r e a n d m o r e d y e m o l e c u l e s a re

adsorbed on the su r face o f the ca ta lys t. H ence ,

the pene t ra t ion o f l igh t to the su r face o f the

ca ta lys t dec reases and the re la t ive amo unt o f OH

and 02- on the su r face o f the ca ta lys t do no t

inc rease a s the in tens i ty o f l igh t and i l lumina t ion

t ime i s cons tan t. C onverse ly , the i r concen t ra t ions

dec rease wi th the inc rease in concen t ra t ion o f the

dye as the l igh t pho tons a re l a rge ly absorbed and

preven ted f rom re ach ing the ca ta lys t su r face by

the dye m olecu les . Conse quen t ly , the degrada t ion

e f f i c ie n c y o f th e d y e d e c r e a s e s a s t h e d y e

concen t ra t ion inc reases .

One p rac t i ca l p rob lem in us ing T iO2 as a

pho tca ta lys t i s the undes i red e lec t ron /ho le r ecom -

b ina t ion , w hich , in the abse nce o f p roper e lec t ron

accep to r o r donor , i s ex t rem ely e f f i c i en t and thus

represen ts the m a jo r ene rgy-w as t ing s t ep , wh ich

l imi t s the ach ievab le quan tum y ie ld . One s t r a tegy

to inh ib it the e lec t ro n-ho le pa i r r ecomb ina t ion i s

to add o the r ( i r r eve r s ib le ) e l ec t ron accep to r s to

the reac t ion , wh ich cou ld have seve ra l d i f f e ren t

e f fec t s , i .e . , (1 ) to inc rease the num ber o f t rapped

e lec t rons and , consequen t ly , avo id recombina-

t ion , (2 ) to genera te more rad ica l s and o the r

ox id iz ing spec ies , (3 ) to inc rease the ox ida t ion

ra te o f in te rmed ia te compo unds and (4 ) to avo id

prob lems caused by low o xygen con cen t ra t ion . I t

i s pe r t inen t to m en t ion he re tha t in h igh ly tox ic

wa s t e wa t e r wh e r e t h e d e g r a d a t i o n o f o r g a n i c

po l lu tan t s i s the ma jo r conce rn , the add i t ion o f

inorgan ic ions to enhan ce the o rgan ic d egrada t ion

ra te may o f ten be jus t i fi ed . In th i s conn ec t ion , we


8/9

262 M. Saquib , M . M uneer / Desa l ina t ion 155 2003) 25 5-263

s t u d i e d t h e e f f e c t o f e l e c t r o n a c c e p t o r s s u c h a s

h y d r o g e n p e r o x i d e a n d b r o m a t e o n t h e p h o t o -

c a ta l y ti c d e g r a d a t io n o f th e m o d e l c o m p o u n d

u n d e r i n v e s t i g a t i o n . T h e s e a c c e p t o r s a r e k n o w n

t o g e n e r a t e h y d r o x y l r a d i c a l s a c c o r d i n g t o t h e

E q s . ( 4 ) - ( 6 ) :

H202 e cB ~ O H + O H -

( 4 )

B r O 3 + 2 H + + e c B

}

B r O 2 + H 2 0 ( 5 )

B r O ~ + 2 H + 6 e c a - ~ [ B r O 2, H O B r ] - ~

B Y + 3 H 2 0

( 6 )

A s e x p e c t e d , b o t h a d d it iv e s , h y d r o g e n p e r o x -

i d e a n d p o t a s s i u m b r o m a t e , s h o w e d a b e n e f ic i a l

e f f e c t o n th e p h o t o c a t a l y t i c d e g r a d a t i o n o f m o d e l

c o m p o u n d a s s h o w n i n F ig . 6 . T h e e n h a n c e d

d e g r a d a t i o n r a t e i n th e p r e s e n c e o f H 20 2 c a n b e

e x p l a i n e d b y s e v e r a l r e a s o n s . F i r s t l y , it i n c r e a s e s

t h e r a t e b y r e m o v i n g t h e s u r f a c e - tr a p p e d

e l e c t r o n s , th e r e b y l o w e r i n g t h e e l e c t r o n - h o le

r e c o m b i n a t i o n r a t e a n d i n c r e a s i n g t h e e f f i c ie n c y

o f h o l e u t i l i z a t i o n f o r r e a c t i o n s s u c h a s ( O H - + h +

-- , O H ) . S e c o n d l y ,

H202

m a y s p l it p h o t o l y t i c a ll y

t o p r o d u c e O H r a d i c a l s d i r e c t l y , a s c i t e d i n

s t u d ie s o f h o m o g e n e o u s p h o t o o x i d a t io n u s i n g

U V / ( H 2 0 2 + 0 2 ) [ 3 4] . T h i rd l y , t h e s o l u t io n p h a s e

m a y a t t i m e s b e o x y g e n s t a r v e d b e c a u s e o f e i th e r

o x y g e n c o n s u m p t i o n or s lo w o x y g e n m a s s

t r a n sf e r ; p e r o x i d e a d d i t i o n t h e r e b y i n c r e a se s t h e

r a te t o w a r d s w h a t i t w o u l d h a v e b e e n h a d a n

a d e q u a t e o x y g e n s u p p l y b e e n p r o v id e d .

5 C o n c l u s i o n s

T i O 2 c a n

e f f i c i e n t ly p h o t o c a t a l y s e t e x t il e d y e

d e r i v a t i v e s s u c h a s a c i d o r a n g e 8 u s i n g a r t i fi c i a l

r a d i a t i o n s o u r c e s . T h e o b s e r v a t i o n s o f t h e s e

i n v e s t i g a ti o n s c l e a r l y d e m o n s t r a t e th e i m p o r t a n c e

o f c h o o s i n g t h e o p t i m u m d e g r a d a t i o n p a r a m e t er s

t o o b t a i n a h i g h d e g r a d a t i o n r a t e , w h i c h i s

e s s e n t i a l f o r a n y p r a c t ic a l a p p l i c a t io n o f p h o t o -

c a t a l y ti c o x i d a t i o n p r o c e s s e s . T h e b e s t d e g r a d a -

t io n c o n d i t io n d e p e n d s s t r o n g l y o n t h e k i n d o f

p o l l u t a n t .

A c k n o w l e d g e m e n t s

F i n a n c ia l s u p p o r t b y t h e D e p a r t m e n t o f

S c ie n c e a n d T e c h n o l o g y ( D S T ), G o v e r n m e n t o f

I n d i a ( N e w D e l h i ) , a n d t h e T h i r d W o r l d

A c a d e m y o f S c i e n c e s ( T r i e s te , I t a ly ) i s g r a t e f u l l y

a c k n o w l e d g e d . T h e t o ta l o r g a n i c c a r b o n a n a l y z e r

u s e d f o r t h e a n a l y s e s o f th e s a m p l e s w a s a g i f t

f r o m t he A l e x a n d e r V o n H u m b o l d t F o u n d a t i o n

( G e r m a n y ) .

R e f e r e n c e s

[ 1 A. Reife and H.S. Freeman ,Environmen tal Chem istry

of Dyes and P igments, Wi ley , Ne w Y ork , 1996.

[2] C. O Neil l , F.R Haw kes, D .L Haw kes, N .D

Lourenco, H.M Pincheiro and W. Deice, J . Chem.

Technol . Bioteehnol ., 74 (1999) 1009.

[3] J .R Easton, Colour in Dye, ouse Effluent , P. Cooper,

ed. , SDC , Bradford, 1995, p. 9.

[4] H. Zoll inger, in: Colour Chem istry, H.F . Eblel and

C.D. Brenzinger,eds. , 1st ed. ,VCH ,NewY ork, 1987,

Chap. 16.

[5] C.E . Searle, Chem ical Carcinogenesis , AC S M ono-

graph, Am er. Chem . Sot . , W ashington, DC , 1976.

[6] C .T . He l m es , C .C . S igm an , Z .A . F und , M .K.

Thom pson, M .K. Voeltz, M . Makie, P.E. Klein and

J.B. Lent , J. Environ. Sci . Health A , 19 (1984) 97.

[7] M . Boeninger, Carcinogenicity and metabolism o f

azo dyes, especial ly those derived fro m be nzidine,

DHI-IS(NIOSH ), Publicat ionNo. 8 0-119, July, 1980.

[8] J .J . Roxon , A.J. Ryan and S.E. W right , Foo d Cosmet.

Toxicol . , 5 (196 7) 367.

[9] M .M . Cook, J .A. Ulman an d A. Irael idis , Abstracts of

Papers, 203 National M eeting of the Am er. Chem.

Soc. , San Francisco, 1992, and references therein.

[10] D .M. Blake, Bib l iography of work on the photo-

cata ly t ic removal of hazardous compounds f rom

water and air , National Renew al Energy Lab oratory,

US A, 1999.

[11] J .M . Herm ann, Catalysis Today, 53 (1999) 115-129.


9/9

M. Saqu ib, M. Mune er / Desa l ina t ion 155 2003) 255 -263 263

[12] M.I. Litter, Applied C atalysis B: Environm ental, 23

1999) 89-114.

[ 13] A. Fujishima, T.N. Rao an d D.A. Tryk, J. Photochem.

Photobiol. C: Photochem. Rev., 1 2000) 1-21.

[14] K. Vinodgo pal, I. Bedja, S. Hotechand ani and P. V.

Kam at, Langm uir, 10 1994) 1767-1771.

[15] K. Vinodgopal and P. V. Kamat, J. Photochem.

Photobiol. A: Chem ., 83 1994) 141-14 6.

[16] A. Mills, A. Belghazi, R.H. D avies, D. W orsley and

S. Morris, J. Photochem. Photobiol. A: Chem., 79

1994) 131-139.

[17] M. Vau tier, C. Guillard and J.M. Hermann, J. Catal.,

20 2001) 46-59.

[18] I. Arsalan, I.A. Balcioglu and D.W. Bahnemann,

Dyes and Pigments, 47 2000) 207-218.

[19] S. Sakthivel, B. Neppolian, B. Arabindo, M. Pala-

nicham y and Murugesan, Ind. J. Engineering Mat.

Sci., 7 2000) 87-93.

[20] J. Zhao, T. Wu, K. W U, K. Oikawa, H. Hidaka and

N. Serpone, Environ. Sci . Technol., 32 1998) 2394-

2400.

[21] R.I. Bickley, T.G. Carreno, J.S . Lees, L. Palmisano

and R.J.D. Tilley, J. Solid State Chem ., 92 1991)

178-190.

[22] M. Lindner, D.W. Bahnemann, B. Hirthe and W.D.

Griebler, J. Sol. Energy Eng., 119 1997) 1 20-125.

[23] S. Rauer, Untersunchung von kommerziell erhalt-

lichen Titandioxiden hinsichtlich ihrer photo-

katalytischen Aktivtat, Diplomarbeit, fachhochschule

Hannover, Fachbereich Maschinenbau Vertiefung

Umwelt-und Verfahrenstechnil, Hannover, 1998.

[24] S.T . Martin, H. Hermann, W. Choi an d M.R. Hoff-

mann, J. Chem. Soc. Faraday Trans., 90 1994) 33 15 -

3322.

[25] M. Mu neer, J. Theurich and D. Bahnem ann, J.

Photochem. Photobioi. A: Chem., 143 2001) 213 -

219.

[26] M. Mun eer and D. Bahnemann , W ater Sci. Technol.,

144 2001) 331-337.

[27] M. Muneer, J. Theurich and D. Bahnemann, Res.

Chem . Intermed., 25 1999) 667-683.

[28] M. Saquib and M. M uneer , Dyes and Pigments, 53

2002) 237-249.

[29] M. Lindner, D.W. Bahnemann, B. Hirthe and W.D.

Griebler, Novel TiO 2 powd ers as highly active

photocatalysts, in: So lar Water D etoxification; So lar

Engineering, W.B. Stine, T. Tanaka and D.E.

Clar idge, eds. ASM E, New York, 1995, pp. 339-408.

[30] M. Muneer , H.K. Singh and D. Bahnemann, Chemo-

sphere, 49 2002) 193-203.

[31] H. Hiegendorff, Untersuchung en zur Bedeutung der

Adsorption in de r Photokatalyse,PhD Thesis, Depart-

ment o f Chemistry, Universi ty ofH ann ove r , Hanno-

ver, Germany, 1996.

[32] C. Koval, Proc. Symp. on Photoelectrochemistry,

Electrochemical Society, Pennington, N J, 1991.

[33] M. M uneer, R. Philip and S. D as, Res. Chem.

Intermed., 23 1997) 233-246.

[34] G.R. Peyton and W.H. Glaze, Environ. Sci. Technol.,

22 1988) 761-767.

2003 saquib titanium dioxide mediated photocatalyzed degradation of a textile dye derivative, acid...

Documents