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15 Photocatalytic Degradation of 4-Chlorophenol in TiO Aqueous Suspensions Downloaded by UNIV OF ALBERTA on November 9, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0244.ch015

2

Chengdi Dong and Chin-Pao Huang* Department of Civil Engineering, University of Delaware, Newark, DE 19716

Photocatalytic degradation of 4-chlorophenol in TiO aqueous suspen­ 2

sions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated prod­ uct, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields

hydroxyhydroquinone,

which can readily be oxidized and mineralized to carbon dioxide. Com­ plete dechlorination and mineralization of 4-chlorophenol

can be

achieved. In contrast, direct photolysis of 4-chlorophenol produces hy­ droquinone and p-benzoquinone as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiO , is gener­ 2

ated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring.

ΤΉΕ

I N I T I A L STEPS O F P H O T O C A T A L Y T I C O X I D A T I O N of aromatic compounds

in aqueous suspensions of T i 0 or other semiconductors have been described in terms of several mechanisms (1-13). The most important and widely ac­ cepted of these mechanisms is the generation from water decomposition of hydroxyl radicals that attack the aromatic ring. 2

Corresponding author

0065-2393/95/0244-0291$08.54/0 © 1995 American Chemical Society

In Aquatic Chemistry; Huang, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1995.

292

A Q U A T I C

Ti0

2

+ hv

H 0 + /i 9

+

h

+

+

(1)

e-

ΌΗ + H

4

C H E M I S T R Y

+

ΌΗ

(2)

(3)

Downloaded by UNIV OF ALBERTA on November 9, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0244.ch015

OH A similar degradation mechanism was proposed for homogeneous oxida­ tion reactions involving hydroxyl radicals (14-20). Therefore, one would i n ­ tuitively expect to find similar réaction intermediates in heterogeneous oxi­ dation systems. However, the degradation pathways of chloro-substituted phenols are far from conclusive. For example, Barbeni et al. (16) proposed that the initial steps of the photocatalytic degradation of 4-chlorophenol in the presence of T i 0 would form hydroquinone through an attack by hydroxyl radicals. 2

OH

φ

+

OH — i p j

+ cr

(4)

CI

In contrast, Lipczynska-Kochany and Bolton (19) reported that in photodegradation of 4-chlorophenol mediated by hydrogen peroxide the primary product was 4-chlorocatechol and hydroquinone was only a minor product.

(5)

The formation of hydroquinone implies that the initial hydroxylation step takes place with dechlorination at the para position. In contrast, the formation of 4-chlorocatechol implies that the initial hydroxylation reaction occurs without dechlorination at the ortho position. Because of the different physical and chemical properties of these two intermediates, the overall reaction kinetics and mechanisms can be much different. Information about the degradation pathway and oxidation products is i m ­ portant from the viewpoint of water quality control. The objective of this study was to determine the intermediate products during the photocatalytic oxida-

In Aquatic Chemistry; Huang, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1995.

15.

D O N G

&

293

Photocatalytic Degradation

H U A N G

tion of 4-chlorophenol in T i 0 aqueous suspensions. To verify the extent of 4-chlorophenol detoxification, the formation of carbon dioxide and chloride ions was also measured. O n the basis of the results obtained, we proposed a reaction pathway for the photocatalytic oxidation of 4-chlorophenol. 2

Downloaded by UNIV OF ALBERTA on November 9, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0244.ch015

Materials and Methods Materials. 4-Chlorophenol, 4-chlororesorcinol, hydroquinone, and hydroxyhydroquinone were purchased from Aldrich Chemical Co., Milwaukee, W I ; 4-chlorocatechol was purchased from T C I American, Portland, OR. Acetonitrile, hexane, methylene chloride, methanol, and pyridine were obtained from the Fisher Scientific Co., Fulton, C A . The photocatalyst was Degussa P-25 titanium dioxide [mainly anatase form; surface area 50 m / g ; p H ^ (pH at zero point of charge) 6.3; contains some impurities such as A l 0 (