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Transcript of 2003 Saquib Titanium Dioxide Mediated Photocatalyzed Degradation of a Textile Dye Derivative, Acid...
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8/10/2019 2003 Saquib Titanium Dioxide Mediated Photocatalyzed Degradation of a Textile Dye Derivative, Acid Orange 8, In Aqueous Suspensions
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~ ~ 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
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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
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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
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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
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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 .
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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
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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
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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 ) .
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[4] H. Zoll inger, in: Colour Chem istry, H.F . Eblel and
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[5] C.E . Searle, Chem ical Carcinogenesis , AC S M ono-
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