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Photodecomposition of several chloroaromatics using a commercial prototype reactor
Chemosphere, Vol.17, No.lO, pD 1971-1976, Printed in Great Britain
1988
0045--6535/88 $3.00 + .OO Pergamon Press plc
PHOTODECOMPOSITION OF SEVERAL CHLOROAROMATICS USING A COMMERCIAL PROTOTYPE REACTOR Yi Ming Xu*, P i e r r e E. Mdnassa**, a n d C o o p e r H. L a n g f o r d * Department of Chemistry Concordia University 1455 de M a i s o n n e u v e Blvd. West Montrdal, Qudbec Canada
H3G 1M8
* P r e s e n t a d d r e s s : Dept. of C h e m i s t r y , Y a n g z h o u T e a c h e r ' s College, Y a n g z h o u , J i a n g s u , P.R. C h i n a +*Present a d d r e s s :
C - I - L , I n c . , E x p l o s i v e s T e c h n i c a l C e n t r e , McMasterville, Qudbec J3G 1T9
Abstract: We r e p o r t o u r r e s u l t s o n t h e s t u d y o f a p h o t o a s s l a t e d c a t a l y t i c d e c h l o r i n a t i o n of t h e o r t h o , meta a n d p a r a - d i c h l o r o b e n z e n e s (o-DCB, m-DCB, p-DCB) a n d 2 ' , 3 , 4 - t r i c h l o r o b i p h e n y l (TCB) b y m e a n s of a p r o t o t y p e commercial r e a c t o r u s i n g t h e s e m i c o n d u c t o r a n a t a s e a n c h o r e d o n a f i b e r g l a s s mesh.
1971
1972
Introduction Chlorinated hydrocarbons hazardous
are classified by the Environmental Protection Agency as
w a s t e , toxic p o l l u t a n t s a n d c a r c i n o g e n s
and the environment.
(1).
Yet t h e y a r e w i d e l y u s e d i n i n d u s t r y
C h l o r o b e n z e n e s a r e o f t e n u s e d in t h e m a n u f a c t u r e o f m a n y p e s t i c i d e s .
They are also by-products
formed from the d e g r a d a t i o n of o t h e r c h l o r i n a t e d h y d r o c a r b o n s
P o l y c h l o r i n a t e d b i p h e n y l s a r e r e m a r k a b l y s t a b l e c o m p o u n d s (2). u s e d in i n d u s t r y etc...).
(1).
They have been intensively
( e . g . d i e l a c t r i c f l u i d s in c a p a c i t o r s a n d t r a n s f o r m e r s ,
dedusting
agents
H o w e v e r , t h e d i s c o v e r y o n e d e c a d e a g o i n E u r o p e (3) a n d N o r t h A m e r i c a {4) o f
chlorinated hydrocarbons environmental problem.
in both n a t u r a l a n d d r i n k i n g w a t e r s h a s f o c u s e d a t t e n t i o n on the T h i s p r o m p t e d d i f f e r e n t s t u d i e s o n m e t h o d s to t r e a t , d e g r a d e a n d
d i s p o s e of t h e s e chemical p o l l u t a n t s . hydrocarbons
It i s k n o w n t h a t t h e t o x i c i t y o f c h l o r i n a t e d
d e c r e a s e s w i t h d e c r e a s i n g c h l o r i n e c o n t e n t (5), t h e r e f o r e e v e n a p a r t i a l
d e c h l o r i n a t i o n m a y h e c o n s i d e r e d a s a p a r t i a l d e t o x i f i c a t i o n t h a t m a y l e a d to t o t a l microbial degradation
{6).
E l e c t r o n s (e-) a n d h o l e s (h*) p h o t o g e n e r a t e d
by b a n d g a p excitation of s e m i c o n d u c t o r
p a r t i c l e s a r e k n o w n to i n d u c e r e d o x r e a c t i o n s t h a t c a n be u s e d to p h o t o d e g r a d e
chlorinated
e n v i r o n m e n t a l p o l l u t a n t s (7).
substantial
In an earlier study
C a r e y a n d O l i v e r (8) r e p o r t e d
r e d u c t i o n in t h e t o x i c i t y o f PCB to a l g a e in w a s t e w a t e r b y u l t r a v i o l e t (UV) i r r a d i a t i o n with anatase powders.
R e c e n t l y we s h o w e d (9) t h a t it w a s p o s s i b l e to e x t e n d a n d g e n e r a l i z e
t h i s d e t o x i f i c a t i o n to mixed p h a s e s anatase. {10).
(hexanee/water, paraffin/water)
H e r e we u s e a p r o t o t y p e of a n e w r e a c t o r f a b r i c a t e d
It c o n s i s t s of a n e a r UV l a m p s u r r o u n d e d
which a t h i n a n a t a s e l a y e r is firmly bonded.
u s i n g a s u r f a c e modified
by Nu-tech, London, Ontario
coaxially by a porous fiberglass
m e s h to
We e x a m i n e d t h e p h o t o a s s i s t e d c a t a l y t i c
d e c h l o r i n a t i o n o f o-DCB, m-DCB a n d TCB i n a q u e o u s s o l u t i o n . Experimental Chemicals P h e n a n t r o l i n e a n d o-DCB {gold l a b e l ) , m-DCB (98~) a n d p-DCB (99+~) w e r e p u r c h a s e d Aldrich.
2'3,4-trichlorobiphenyl
from
(TCB) w a s o b t a i n e d f r o m U l t r a S c i e n t i f i c a n d f o u n d to h a v e
a s i n g l e peak in its g a s c h r o m a t o g r a m .
The h e x a n e (Caledone) u s e d was s p e c t r o s c o p i c g r a d e .
Ferric chloride (Anachemia) and potassium oxaiate {Fisher) were laboratory reagent grade, All t h e c h e m i c a l s w e r e u s e d w i t h o u t f u r t h e r was used throughout
purification.
Doubly distilled deionized water
this study.
Photochemical reactor T h e N u - l i t e cell (10) c o m p r i s e d a j a c k e t , a lamp a n d a p h o t o c a t a l y t i c s l e e v e .
The lamp
{ P h i l l i p s E l e c t r o n i c s "TL"K40W/09N) e m i t s l i g h t i n t h e 300-430 nm r a n g e w i t h a p e a k a t 350 nm.
It is m o u n t e d
coaxially within the jacket.
A r o u n d t h e l a m p l i e s a s l e e v e f o r m e d of a
f i b e r g l a s s m e s h which was coated with a firmly b o n d e d l a y e r of a n a t a s e . w a t e r c i r c u l a t e d i n t h e r e a c t o r w a s 1000 ml w i t h a d e a d v o l u m e of 100 ml. passes through
As t h e w a t e r
t h e s l e e v e , t h e o p e n p o r e c o n f i g u r a t i o n a n d t h e l a r g e s u r f a c e a r e a of t h e
mesh creates a turbulent anatase.
T h e t o t a l a m o u n t of
mixing that ensured
contact between the organic pollutants and the
W a t e r w a s r e c y c l e d b y m e a n s o f a p e r i s t a l t i c p u m p { M a s t e r f l e x 7563-10, Cole
P a l m e r ) w i t h a flow r a t e o f 100 m l / m i n .
T h e flow r a t e w a s n o t a c r i t i c a l p a r a m e t e r .
Analyses Solutions of d i c h l o r o b e n z e n e s were p r e p a r e d
b y d i s s o l v i n g i n w a t e r 240 m g o f DCB's i n
1973
12 litre batches.
The trichlorobiphenyl solution was prepared by first dissolving T C B in a
12 litre batch to effect water dissolution. Stirring was carried out overnight, the prepared solutions contained 20 p p m of DCB's and 10 p p b of TCB.
After irradiation 25 ml of
the aqueous solution was extracted for 50 minutes with 100 ml of hexane. efficiency compared to a standard was found to be above 94~.
Extraction
Degradation percentages were
calculated as a function of the loss of the parent peak:
Deg ~ = Bi - Ci Bi
Where Ci is the ith irradiated sample and Bi its corresponding dark run. All the extracted photolysed and control samples were analyzed chromatographically by a Perkin-Elmer Model Sigma 4B gas chromatograph equipped with an electron capture detector. The packed column used was 2Z O V + 3X QF.
Carrier gas in all the analyses was 6~ methane +
95Z argon at a flow rate of 60 ml per minute. (DCB's analyses) or 200eC (TCB analyses).
Column temperature was set at either 1000C
With these conditions the parent peaks appeared
at ca 5.5 min (o-DCB), 4.9 min (m-DCB), 5.1 min (p-DCB) and at ca 3.9 min for TCB.
No
intermediate peaks were observed. Results and Discussions First, the intensity of the lamp in the reactor's configuration was estimated b y means of ferric oxalate actinometry (11,12). It was found to be 3.05 x 10 n quanta per second.
With
such an intensity one would expect all the light to be absorbed by the anchored anatase and the photoreaction to proceed in a reasonable time. Solutions of o-DCB, m-DCB, p - D C B and T C B were irradiated for different amounts of time ranging from 0.Shr to 3 hr.
The percentage degradation was calculated as mentioned earlier.
Figure I shows this percentage as a function of time.
In o, and m - D C B cases the reaction
starts with zero order kinetics, o-DCB and m - D C B percentage degradation is almost equal even after 3 hours,
p - D C B photodegradation was m u c h less, even after 6 hours.
In the case
of p-DCB, there is deviation from linearity. This suggests that the reaction approaches first-order kinetics. Experimental difficulties prevented us from examining longer irradiation time.
In the case of TCB, 50~ of the starting material was transformed after 3
hours and the kinetics of the reaction resemble those of p-DCB. It is not excluded that due to the hydrophobic nature of the pollutants some of the T C B or DCB's would absorb on the fiberglass surface. until saturation. hexane.
This adsorption would then increase with time
In order to avoid this, between each run the reactor was flushed with
A n adsorption/desorption test showed that within experimental error all the D C B
adsorbed could be desorbed b y running hexanes through the reactor for 3 hours. In order to account for a deviation in linearity found in the p - D C B kinetic curve, an attempt to measure the adsorption isotherms of o-DCB, m - D C B and p - D C B was made. do not imply significant D C B coverage on Ti02 under reaction conditions.
The results
Therefore it is
probable that the photogenerated reactive species desorbs from the Ti02 surface into solution and then reacts with the pollutant.
It is not k n o w n at the present experimental
stage which reactive species is involved in the photodegradation of DCB'e and TCB.
It is
1974
h o w e v e r k n o w n t h a t it is g e n e r a t e d b y Ti0z b a n d - g a p e x c i t a t i o n s i n c e no d e g r a d a t i o n was o b s e r v e d in a similar r e a c t o r b u t w i t h o u t Ti0z.
As well, b a t c h e x p e r i m e n t s u s i n g a wide
b a n d lamp s h o w e d t h a t a 320 nm c u t - o f f f i l t e r s t o p p e d d i r e c t p h o t o l y s i s in t h e a b s e n c e of Ti02. H y d r o x y l r a d i c a l s h a v e b e e n c o n s i d e r e d to h e t h e r e a c t i v e s p e c i e s i n v o l v e d in t h 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 of m a n y o r g a n i c c o m p o u n d s (13-16).
It is a l s o k n o w n t h a t
s u p e r o x i d e ion (02-) h a s n u c l a o p h i l i c (17,181 a n d o x i d a t i v e (19,20) c h a r a c t e r i s t i c s in a v a r i e t y of system s. d e c h l o r i n a t e DDT.
D u r e j a e t al (21) s h o w e d t h a t s u p e r o x i d e g e n e r a t e d in DMF c a n More r e c e n t l y S u g i m o t o e t al (22) s h o w e d t h a t in a n a p r o t i c medium s u c h
a s DMF o r DMSO it w a s p o s s i b l e to c o m p l e t e l y m i n e r a l i z e p o l y c h l o r o a r o m a t i c s , b y a s u p e r o x i d e ion i n i t i a t e d p a t h w a y b u t t h a t DCB's w e r e n o t a t t a c h e d .
60
W h a t e v e r may b e t h e r e a c t i v e
[]
O
R
[]
40
20
0
120
0
240
360
Time (min) F i g u r e 1: P e r c e n t a g e d e g r a d a t i o n a s a f u n c t i o n o f time ( (
) p-DCB, (
) TCB.
) o-DCB, (
I n i t i a l c o n c e n t r a t i o n s in w a t e r :
DCBs = 20 oonm TCB = 10 p p b
) m-DCB,
1975
species
that
is photogenerated,
of reactive
species
the reactor
under
are
produced
(3 h o u r s )
kinetics.
the
time (Figure
kinetics
to b e a p p l i c a b l e
o f Ti0t m u s t b e c o n s t a n t .
since the light intensity
In the case of o-DCB and the reaction
By plotting
irradiation
zero-order
on the surface
consideration
kept constant.
period
for
kinetics
natural
and
m-DCB and
This is the case for
the amounts
within
concentration
o f Ti0~ a n d
the available
follows with a good approximation
log of normalized
the concentration
water
experimental
zero order
of p-DCB and
TCB v~s
2) i s o n l y l i n e a r i n t h e c a s e o f p - D C B .
0 9 ~ "--'lnh- 0.5
-o
_J
0.5
o
'
2- o
90
3do
Time (rain)
Figure
2:
I n Bi a s a f u n c t i o n
of time (
) p-DCB, (
) TCB.
I n i t i a l concentrations in
Ci water:
This suggests may not. species
p - D C B = 20 p p m , TCB = 10 p p b .
that
is constant
DCB's are different have
the degradation
From the preceding
of p-DCB follows first-order
discussion
one can conclude from those
to a w a i t f u r t h e r
and
that
o f TCB.
kinetics
since the photogeneration
reaction
pathways
A full explanation
involving
w h i l e t h a t o f TCB of the reactive
the dechlorination
of each of these
pathways
will
experiments.
Conclusions Our study
of the Nulite reactor
showed
that
it could
be used
for a partial
degradation
of
1976
of h i g h l y r e f r a c t o r y
chlorinated hydrocarbons
trichlorobiphenyls.
Our experimental data showed different percentages
a f u n c t i o n of time.
o-DCB a n d m-DCB b e i n g d e g r a d e d
adsorbed
such as dichlorobenzenes and of d e t o x i f i c a t i o n a s
f a s t e r t h a n p-DCB.
No e v i d e n c e f o r
DCB's o n t h e Ti0t s u r f a c e c o u l d b e o b t a i n e d from a d s o r p t i o n / d e s o r p t i o n
measurements.
This suggests
a d e s o r p t i o n f r o m t h e Ti02 s u r f a c e of t h e p h o t o g e n e r a t e d
r e a c t i v e s p e c i e s r e s p o n s i b l e f o r t h e d e c h l o r i n a t i o n o f DCB's a n d TCB. t h e f o r m e r f o l l o w i n g a z e r o to f i r s t - o r d e r
T h e d e g r a d a t i o n of
k i n e t i c s while t h a t of t h e l a t t e r b e i n g more
complex. Acknowledgements The authors
w i s h to t h a n k C o n c o r d i a E c o t o x i c o l o g y L a b o r a t o r i e s :
Dr. P e r r y A n d e r s o n , f o r
t h e u s e of t h e e q u i p m e n t a n d Mr. P r a s a d Aysola f o r t e c h n i c a l a s s i s t a n c e . supported
T h e w o r k was
b y a n NSERC S t r a t e g i c G r a n t a n d FCAR (Quebec) f u n d s .
References (1)
M. S i t t i n g " H a n d b o o k of Toxic a n d H a z a r d o u s C h e m i c a l s " , Noyes P u b l i c a t i o n s :
Park
Ridge, NJ (1981). (2)
O. H u t z i n g e r , S. Safe, a n d V. Zitko.
" T h e C h e m i s t r y of PCB's".
CRC P r e s s I n c . ,
(3)
R.R. Rook, J. Amer. Water Works Assoc. 68, 106 (1976).
(4)
P.D. Foley, G.A. M i s s i n g h a m , ibid, 68, 106 (1976).
(5)
N.I. Sax in "Dangerous Properties of Industrial Materials", 3rd ed., p. 550, Von-
C l e v e l a n d , Ohio (1974).
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D. Ghosal, I.S. You, D.K. C h a t t e r j e e a n d A.M. C h a k r a b a r t y ,
(7)
M. B a r b e n i , E. P r a m a u r a , E. P e l i z e t t i , E. B o r g a r e l l o , N. S e r p o n e a n d M.A. J a m i e s o n ,
S c i e n c e , 228, 135 (1985).
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J.H. C a r e y , a n d B.G. Oliver, Water PolL, Res. J. of C a n a d a , New S e r i e s :
Vol. 15,
No. 2, 157 (1980). (9)
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(10)
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(11)
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(12)
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(13)
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(15)
M. F u j i h i r a , Y. S a t o h , a n d T. Osa, Bull. Chem. Soc. J p n , 55 666 (1982).
(16)
I. Izumi, F.F. F r a n , a n d A.J. B a r d , J. P h y s . Chem., 85, 218 (1981).
(17)
E.J. C o r e y , K.C. Nicolaou, M. S h i b a s a k , Y. M a s h i d a a n d C.S. S h i n e r , T e t r a h e d r o n
3207, (1984).
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(19)
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(21)
P. D u r e j a , J.E. C a s i d a a n d L.O. Ruzo, T e t r a h e d r o n
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(Received
in
Germany
4 April
1988;
accepted
L e t t . , 23, 5003 (1982).
28 J u l y
1988)
1988
0045--6535/88 $3.00 + .OO Pergamon Press plc
PHOTODECOMPOSITION OF SEVERAL CHLOROAROMATICS USING A COMMERCIAL PROTOTYPE REACTOR Yi Ming Xu*, P i e r r e E. Mdnassa**, a n d C o o p e r H. L a n g f o r d * Department of Chemistry Concordia University 1455 de M a i s o n n e u v e Blvd. West Montrdal, Qudbec Canada
H3G 1M8
* P r e s e n t a d d r e s s : Dept. of C h e m i s t r y , Y a n g z h o u T e a c h e r ' s College, Y a n g z h o u , J i a n g s u , P.R. C h i n a +*Present a d d r e s s :
C - I - L , I n c . , E x p l o s i v e s T e c h n i c a l C e n t r e , McMasterville, Qudbec J3G 1T9
Abstract: We r e p o r t o u r r e s u l t s o n t h e s t u d y o f a p h o t o a s s l a t e d c a t a l y t i c d e c h l o r i n a t i o n of t h e o r t h o , meta a n d p a r a - d i c h l o r o b e n z e n e s (o-DCB, m-DCB, p-DCB) a n d 2 ' , 3 , 4 - t r i c h l o r o b i p h e n y l (TCB) b y m e a n s of a p r o t o t y p e commercial r e a c t o r u s i n g t h e s e m i c o n d u c t o r a n a t a s e a n c h o r e d o n a f i b e r g l a s s mesh.
1971
1972
Introduction Chlorinated hydrocarbons hazardous
are classified by the Environmental Protection Agency as
w a s t e , toxic p o l l u t a n t s a n d c a r c i n o g e n s
and the environment.
(1).
Yet t h e y a r e w i d e l y u s e d i n i n d u s t r y
C h l o r o b e n z e n e s a r e o f t e n u s e d in t h e m a n u f a c t u r e o f m a n y p e s t i c i d e s .
They are also by-products
formed from the d e g r a d a t i o n of o t h e r c h l o r i n a t e d h y d r o c a r b o n s
P o l y c h l o r i n a t e d b i p h e n y l s a r e r e m a r k a b l y s t a b l e c o m p o u n d s (2). u s e d in i n d u s t r y etc...).
(1).
They have been intensively
( e . g . d i e l a c t r i c f l u i d s in c a p a c i t o r s a n d t r a n s f o r m e r s ,
dedusting
agents
H o w e v e r , t h e d i s c o v e r y o n e d e c a d e a g o i n E u r o p e (3) a n d N o r t h A m e r i c a {4) o f
chlorinated hydrocarbons environmental problem.
in both n a t u r a l a n d d r i n k i n g w a t e r s h a s f o c u s e d a t t e n t i o n on the T h i s p r o m p t e d d i f f e r e n t s t u d i e s o n m e t h o d s to t r e a t , d e g r a d e a n d
d i s p o s e of t h e s e chemical p o l l u t a n t s . hydrocarbons
It i s k n o w n t h a t t h e t o x i c i t y o f c h l o r i n a t e d
d e c r e a s e s w i t h d e c r e a s i n g c h l o r i n e c o n t e n t (5), t h e r e f o r e e v e n a p a r t i a l
d e c h l o r i n a t i o n m a y h e c o n s i d e r e d a s a p a r t i a l d e t o x i f i c a t i o n t h a t m a y l e a d to t o t a l microbial degradation
{6).
E l e c t r o n s (e-) a n d h o l e s (h*) p h o t o g e n e r a t e d
by b a n d g a p excitation of s e m i c o n d u c t o r
p a r t i c l e s a r e k n o w n to i n d u c e r e d o x r e a c t i o n s t h a t c a n be u s e d to p h o t o d e g r a d e
chlorinated
e n v i r o n m e n t a l p o l l u t a n t s (7).
substantial
In an earlier study
C a r e y a n d O l i v e r (8) r e p o r t e d
r e d u c t i o n in t h e t o x i c i t y o f PCB to a l g a e in w a s t e w a t e r b y u l t r a v i o l e t (UV) i r r a d i a t i o n with anatase powders.
R e c e n t l y we s h o w e d (9) t h a t it w a s p o s s i b l e to e x t e n d a n d g e n e r a l i z e
t h i s d e t o x i f i c a t i o n to mixed p h a s e s anatase. {10).
(hexanee/water, paraffin/water)
H e r e we u s e a p r o t o t y p e of a n e w r e a c t o r f a b r i c a t e d
It c o n s i s t s of a n e a r UV l a m p s u r r o u n d e d
which a t h i n a n a t a s e l a y e r is firmly bonded.
u s i n g a s u r f a c e modified
by Nu-tech, London, Ontario
coaxially by a porous fiberglass
m e s h to
We e x a m i n e d t h e p h o t o a s s i s t e d c a t a l y t i c
d e c h l o r i n a t i o n o f o-DCB, m-DCB a n d TCB i n a q u e o u s s o l u t i o n . Experimental Chemicals P h e n a n t r o l i n e a n d o-DCB {gold l a b e l ) , m-DCB (98~) a n d p-DCB (99+~) w e r e p u r c h a s e d Aldrich.
2'3,4-trichlorobiphenyl
from
(TCB) w a s o b t a i n e d f r o m U l t r a S c i e n t i f i c a n d f o u n d to h a v e
a s i n g l e peak in its g a s c h r o m a t o g r a m .
The h e x a n e (Caledone) u s e d was s p e c t r o s c o p i c g r a d e .
Ferric chloride (Anachemia) and potassium oxaiate {Fisher) were laboratory reagent grade, All t h e c h e m i c a l s w e r e u s e d w i t h o u t f u r t h e r was used throughout
purification.
Doubly distilled deionized water
this study.
Photochemical reactor T h e N u - l i t e cell (10) c o m p r i s e d a j a c k e t , a lamp a n d a p h o t o c a t a l y t i c s l e e v e .
The lamp
{ P h i l l i p s E l e c t r o n i c s "TL"K40W/09N) e m i t s l i g h t i n t h e 300-430 nm r a n g e w i t h a p e a k a t 350 nm.
It is m o u n t e d
coaxially within the jacket.
A r o u n d t h e l a m p l i e s a s l e e v e f o r m e d of a
f i b e r g l a s s m e s h which was coated with a firmly b o n d e d l a y e r of a n a t a s e . w a t e r c i r c u l a t e d i n t h e r e a c t o r w a s 1000 ml w i t h a d e a d v o l u m e of 100 ml. passes through
As t h e w a t e r
t h e s l e e v e , t h e o p e n p o r e c o n f i g u r a t i o n a n d t h e l a r g e s u r f a c e a r e a of t h e
mesh creates a turbulent anatase.
T h e t o t a l a m o u n t of
mixing that ensured
contact between the organic pollutants and the
W a t e r w a s r e c y c l e d b y m e a n s o f a p e r i s t a l t i c p u m p { M a s t e r f l e x 7563-10, Cole
P a l m e r ) w i t h a flow r a t e o f 100 m l / m i n .
T h e flow r a t e w a s n o t a c r i t i c a l p a r a m e t e r .
Analyses Solutions of d i c h l o r o b e n z e n e s were p r e p a r e d
b y d i s s o l v i n g i n w a t e r 240 m g o f DCB's i n
1973
12 litre batches.
The trichlorobiphenyl solution was prepared by first dissolving T C B in a
12 litre batch to effect water dissolution. Stirring was carried out overnight, the prepared solutions contained 20 p p m of DCB's and 10 p p b of TCB.
After irradiation 25 ml of
the aqueous solution was extracted for 50 minutes with 100 ml of hexane. efficiency compared to a standard was found to be above 94~.
Extraction
Degradation percentages were
calculated as a function of the loss of the parent peak:
Deg ~ = Bi - Ci Bi
Where Ci is the ith irradiated sample and Bi its corresponding dark run. All the extracted photolysed and control samples were analyzed chromatographically by a Perkin-Elmer Model Sigma 4B gas chromatograph equipped with an electron capture detector. The packed column used was 2Z O V + 3X QF.
Carrier gas in all the analyses was 6~ methane +
95Z argon at a flow rate of 60 ml per minute. (DCB's analyses) or 200eC (TCB analyses).
Column temperature was set at either 1000C
With these conditions the parent peaks appeared
at ca 5.5 min (o-DCB), 4.9 min (m-DCB), 5.1 min (p-DCB) and at ca 3.9 min for TCB.
No
intermediate peaks were observed. Results and Discussions First, the intensity of the lamp in the reactor's configuration was estimated b y means of ferric oxalate actinometry (11,12). It was found to be 3.05 x 10 n quanta per second.
With
such an intensity one would expect all the light to be absorbed by the anchored anatase and the photoreaction to proceed in a reasonable time. Solutions of o-DCB, m-DCB, p - D C B and T C B were irradiated for different amounts of time ranging from 0.Shr to 3 hr.
The percentage degradation was calculated as mentioned earlier.
Figure I shows this percentage as a function of time.
In o, and m - D C B cases the reaction
starts with zero order kinetics, o-DCB and m - D C B percentage degradation is almost equal even after 3 hours,
p - D C B photodegradation was m u c h less, even after 6 hours.
In the case
of p-DCB, there is deviation from linearity. This suggests that the reaction approaches first-order kinetics. Experimental difficulties prevented us from examining longer irradiation time.
In the case of TCB, 50~ of the starting material was transformed after 3
hours and the kinetics of the reaction resemble those of p-DCB. It is not excluded that due to the hydrophobic nature of the pollutants some of the T C B or DCB's would absorb on the fiberglass surface. until saturation. hexane.
This adsorption would then increase with time
In order to avoid this, between each run the reactor was flushed with
A n adsorption/desorption test showed that within experimental error all the D C B
adsorbed could be desorbed b y running hexanes through the reactor for 3 hours. In order to account for a deviation in linearity found in the p - D C B kinetic curve, an attempt to measure the adsorption isotherms of o-DCB, m - D C B and p - D C B was made. do not imply significant D C B coverage on Ti02 under reaction conditions.
The results
Therefore it is
probable that the photogenerated reactive species desorbs from the Ti02 surface into solution and then reacts with the pollutant.
It is not k n o w n at the present experimental
stage which reactive species is involved in the photodegradation of DCB'e and TCB.
It is
1974
h o w e v e r k n o w n t h a t it is g e n e r a t e d b y Ti0z b a n d - g a p e x c i t a t i o n s i n c e no d e g r a d a t i o n was o b s e r v e d in a similar r e a c t o r b u t w i t h o u t Ti0z.
As well, b a t c h e x p e r i m e n t s u s i n g a wide
b a n d lamp s h o w e d t h a t a 320 nm c u t - o f f f i l t e r s t o p p e d d i r e c t p h o t o l y s i s in t h e a b s e n c e of Ti02. H y d r o x y l r a d i c a l s h a v e b e e n c o n s i d e r e d to h e t h e r e a c t i v e s p e c i e s i n v o l v e d in t h 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 of m a n y o r g a n i c c o m p o u n d s (13-16).
It is a l s o k n o w n t h a t
s u p e r o x i d e ion (02-) h a s n u c l a o p h i l i c (17,181 a n d o x i d a t i v e (19,20) c h a r a c t e r i s t i c s in a v a r i e t y of system s. d e c h l o r i n a t e DDT.
D u r e j a e t al (21) s h o w e d t h a t s u p e r o x i d e g e n e r a t e d in DMF c a n More r e c e n t l y S u g i m o t o e t al (22) s h o w e d t h a t in a n a p r o t i c medium s u c h
a s DMF o r DMSO it w a s p o s s i b l e to c o m p l e t e l y m i n e r a l i z e p o l y c h l o r o a r o m a t i c s , b y a s u p e r o x i d e ion i n i t i a t e d p a t h w a y b u t t h a t DCB's w e r e n o t a t t a c h e d .
60
W h a t e v e r may b e t h e r e a c t i v e
[]
O
R
[]
40
20
0
120
0
240
360
Time (min) F i g u r e 1: P e r c e n t a g e d e g r a d a t i o n a s a f u n c t i o n o f time ( (
) p-DCB, (
) TCB.
) o-DCB, (
I n i t i a l c o n c e n t r a t i o n s in w a t e r :
DCBs = 20 oonm TCB = 10 p p b
) m-DCB,
1975
species
that
is photogenerated,
of reactive
species
the reactor
under
are
produced
(3 h o u r s )
kinetics.
the
time (Figure
kinetics
to b e a p p l i c a b l e
o f Ti0t m u s t b e c o n s t a n t .
since the light intensity
In the case of o-DCB and the reaction
By plotting
irradiation
zero-order
on the surface
consideration
kept constant.
period
for
kinetics
natural
and
m-DCB and
This is the case for
the amounts
within
concentration
o f Ti0~ a n d
the available
follows with a good approximation
log of normalized
the concentration
water
experimental
zero order
of p-DCB and
TCB v~s
2) i s o n l y l i n e a r i n t h e c a s e o f p - D C B .
0 9 ~ "--'lnh- 0.5
-o
_J
0.5
o
'
2- o
90
3do
Time (rain)
Figure
2:
I n Bi a s a f u n c t i o n
of time (
) p-DCB, (
) TCB.
I n i t i a l concentrations in
Ci water:
This suggests may not. species
p - D C B = 20 p p m , TCB = 10 p p b .
that
is constant
DCB's are different have
the degradation
From the preceding
of p-DCB follows first-order
discussion
one can conclude from those
to a w a i t f u r t h e r
and
that
o f TCB.
kinetics
since the photogeneration
reaction
pathways
A full explanation
involving
w h i l e t h a t o f TCB of the reactive
the dechlorination
of each of these
pathways
will
experiments.
Conclusions Our study
of the Nulite reactor
showed
that
it could
be used
for a partial
degradation
of
1976
of h i g h l y r e f r a c t o r y
chlorinated hydrocarbons
trichlorobiphenyls.
Our experimental data showed different percentages
a f u n c t i o n of time.
o-DCB a n d m-DCB b e i n g d e g r a d e d
adsorbed
such as dichlorobenzenes and of d e t o x i f i c a t i o n a s
f a s t e r t h a n p-DCB.
No e v i d e n c e f o r
DCB's o n t h e Ti0t s u r f a c e c o u l d b e o b t a i n e d from a d s o r p t i o n / d e s o r p t i o n
measurements.
This suggests
a d e s o r p t i o n f r o m t h e Ti02 s u r f a c e of t h e p h o t o g e n e r a t e d
r e a c t i v e s p e c i e s r e s p o n s i b l e f o r t h e d e c h l o r i n a t i o n o f DCB's a n d TCB. t h e f o r m e r f o l l o w i n g a z e r o to f i r s t - o r d e r
T h e d e g r a d a t i o n of
k i n e t i c s while t h a t of t h e l a t t e r b e i n g more
complex. Acknowledgements The authors
w i s h to t h a n k C o n c o r d i a E c o t o x i c o l o g y L a b o r a t o r i e s :
Dr. P e r r y A n d e r s o n , f o r
t h e u s e of t h e e q u i p m e n t a n d Mr. P r a s a d Aysola f o r t e c h n i c a l a s s i s t a n c e . supported
T h e w o r k was
b y a n NSERC S t r a t e g i c G r a n t a n d FCAR (Quebec) f u n d s .
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M. S i t t i n g " H a n d b o o k of Toxic a n d H a z a r d o u s C h e m i c a l s " , Noyes P u b l i c a t i o n s :
Park
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E.J. C o r e y , K.C. Nicolaou, M. S h i b a s a k , Y. M a s h i d a a n d C.S. S h i n e r , T e t r a h e d r o n
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(Received
in
Germany
4 April
1988;
accepted
L e t t . , 23, 5003 (1982).
28 J u l y
1988)