Contaminant degradation in water Heterogeneous photocatalysis degrades halogenated hydrocarbon contaminants
L David E OLlis Chemical Engineering Deparhnent North Carolina State Universiry Raleigh, N.C. 27695 Heterogeneous photocatalysis uses a continuously illuminated, photoexcitable solid catalyst to convert reactants absorbed on the photocatalyst surface. Before 1980, there were a number of claims of heterogeneously photocatalyzed chemical conversions. These conversions included several reactions of environmental interest, such as the oxidation of carbon monoxide (CO) to carbon dioxide (C&) ( I , 2); the oxidation of cyanide (CN-) (3,4);the oxidation of sulfite (4); and the decarboxylation of acetic, proponic, and butyric acids (5, a). However, the surface areas of a number of the powdered solid phases (Ti&, ZnO, Fq03) used in these studies left unanswered the question of whether the solids were functioning as photoactivated reagenrs or catalysts. A 1980 paper that summarized the previous results suggested that if conversion of more than 100 moled e s per surface reaction site had been demonstrated-without changing the catalyst-then the observed process could reasonably be regarded as catalytic (7). Photocatalyzed dehalogenations have been suggested previously in the literalure. A report in 1976 noted the release of chloride ion upon illumination of a semiconductor oxide, titanium dioxide (Ti&), in the presence of chlorinated 48Q Emiron. Sci.Technol., MI. 19, NO. 6,1985
biphenyls (8). This intriguing result corresponded to the generation of only monolayer equivalents of product on the solid surface, leaving open the photoreagent-vs.photocatalyst question. Ano!.her paper reported disappearance of p-dichlorobenzene in water with illuminated TiOZ; again, chlorine mass balance and product identification were not reported (9). Other processes mentioned include chloride transfer to ethane from carbon tetrachloride (CCL), fluorotrichloromethane (CFCl,), and difluorodichloromethane (CF2C12)(IO);and dechlorination of CF2C12and CFC1, on illumi-
nated zinc oxide (ZnO) (11). The suggestion of dechlorination activity, and the general oxidation results for solutes led us to examine heterogeneous photocatalysis as a possible tool for water purification.
phot~talyticdechlorination The demonstration of catalysis and the measurement of reaction rates vs. reactant concentration are conveniently realized in a recirculating, differential conversion photoreactor (12). The quartz reactor wall allows entry of all near-W light of 300 nm to CHCI, > CHzBr2 > CHzCIz >> Ccb.
chlorobydroearboa degradation Contamination of drinking water often is the result of one of two industrial solvents, trichloroethylene (TCE) and perchloroethylene W E ) (21). Dichloroethanes have bcen found in drinking water (21, 22), as have chloroacetic acids (23). Complete photocatalyzed Emimn. Sci.Techml.,MI. 18, No. 6 . l W 481
FIGURE 2
Reciprocal initial rata M. reclpmcal initiil concentration tor dlchlommethane (CH,Q)
4-
1.6 4.4 0.18
4.1 6.2 30.0 6.8
3
1.1
2
CCWH
3.9 2.2 5.5 8.5 -0
0.02 0.003 0.005 0.02 0.01 0.01 0.02 0.01 0.02 0.02 0.002 0.
1
miming (catalyst)
--7
0
lb
40
minerdizatioIls of 1,24ChlOroethane, TCE, WE, and mono- and dichlomacetic acids have been observed (2426). However, t r i c h l o r d c acid is
unreactive.
FIGURE 3
Redpmcal rate vs. reciprocalmactanl concentration 6
/
0.16
482 Envim. Sc).Technol..Vci. 10. No. 6.1985
Reciprocal rate vs. reciprocal concentration plots are l m m for all of the reactantsthat are mineralized. Figure 3 presents data for 1,2-dichloroethane, two chloroacetic acids, and perchloroethylene. The corresponding rate constants and bindingconstants are preIntermdates were noted in two instances. The conversion of TCE yielded an unstable intermediate identified hy gas chromatography-mass spearmwey (GC/MS) as dichlomacetaldehyde. In turn, this intermediate completely decomposed to HCI and cq. ultimately, appximately 100% of the chlorine was recovered as chloride ion (24). Degradation of 1 3 4 chlomhne yielded trace vinyl chloride as an unstable intermediate that was rapidly degraded to HCI and C@ (26). Ethylene dibromide EDB (1,24bmrnoethane, or ethylene dibromide) has long been used as a scavenger for lead in gasoline, as a component of pesticides, and as a soil and crop fumigant. Recently,traces of EDB have been found in drinking-water wells in California and Florida (27). Because pesticide application was the suspe*ed source of EDB contamination in water wells, the federal government has banned the use of EDB-containing pesticides. Neither EDB nor its 1,l- isomer is converted by light or catalyst alone. In the presence of near-Wlight and illuminated Ti@, EDB dehalogenates r a p
idly and eventually m i n e d i s completely. Prcduction and consumption of a trace organobromine intermediate have been observed (Figure 4). This unstable intermediate was identified as vinyl bromide by GCIMS. A 100% recovery of bromide ion was demonstrated when EDB and vinyl bromide levels became undetectable (28). Chlorobenzene conversions In contrast to the above results demonstrating complete minediations of a number of chlorinated alkanes and alkenes to C& and HCl, the conversion of monochlorobenzene in dilute aqueous solutions produced no C@ (25). A characteristic result is shown in Figure 5. No degradation of the ring carbons was observed, and the results are presented as fractions of reactant and products found. The products formed correspond to the following sequence:
- chlorophenol - quinone (5) (1)
chlorobenzene hydroquinone
FIGURE4
Photocataly~4degradation of EDB'
i
1.0
Step 1 is, and step 2 almost certainly is, photocatalyzed. The hydroquinone-tcquinone oxidation, step 3, has both a homogeneous photochemical and heterogeneous photocatalytic rate wmponent. Unusually long illumination times also produce trace amounts of condensed products such as 4,4'-dichloro1,l'-biphenyl and 4,5'-dichloro-l.l'biphenyl, which were identified using a G U M S library. These products suggest a slow, homogeneous photochemical pathway, paralleling the heterogeneous photocatalysis results in Figure 5. These data indicate that the photocatalytic treatment of chlorobenzenesproduces a range of complex products. The products are relatively stable and include undesirable products, such as chlorophenols, that are easily tasted in water. However, a recent French report indicates that at higher dissolved oxygen levels, complete m i n e d i t i o n of chlorobenzenesis possible (29).
.,....I__ .. ..;, i::,::
0 0
(2)
(3)
..3
I
1
100
200
-
-
,
,,
I
300
403
f "
m
Time (min)
.EDB = 1,2dibmrmmmhane: BT bmmide ion Reptinled w l h fmm Referem 28.Cwtight 1984. American Chemicd SocKIiy
prmarion
FIGURE 5
Monochlorobenzene and signldng products vs. time
I
II
0.77
0
//
0.2ChlorObenzene
-
0.1
and pbenzoquinones
\
Ochlorophena,
Reaction product inhibition 0 Characteristic products of these min0 30 e d i t i o n reactions are HCI and HBr. Each was established as a reaction inhibitor. On the other hand, C& has been shown not to inhibit the rate of conversion. The competition by the inhibiting component for an adsorption site on a Langmuir surface leads to a which is linear in inhibitor wncentration: modification of Equation 3: 1 1 _ = _ KC e. = r k (6) 1 + KC + KICl 1 Substituting Equation 6 into Equation 2 + c,] ()' results in a new reciprocal rate form,
-&
Figure 6 shows that HC1 inhibition is adequately described by Equation 7 for conversions of perchloroethylene and dichloroacetic acid. Similarly, HBr inhibition is adequately described by Equation 7, up to a pH of 3.5 (18). For more acidic systems, a stronger inhibition becomes evident. These results Envlmn. Sci.Technol..Vol. IS. No. 6,1985 403
( 5 ) Krauetler, 8.; Bard, A. 1. 1. Am. Chem. Soc. 1978,100, 2239. (6) Krauetler, B.; Bard, A. 1. J. Am. Chem. SOC. 1978, IW,5985. (7.) Childs, L. F!; Ollis, D. E 1. Card. 1980,
FIGURE6
bhibitlon by production ofHCL*
-M-,-2x1 __ .
(8) COrey, J. H.;Lawrence,I.; Tosine, H.M. Buil. Environ. Conlam. Tmicol. 1976, 16. 6.0’ 1 I . I ,
acid
(9) Oliver, B. 0.;Cosgrove, E.G.; Carey, 1. H.Environ. Sci. Technol. 1979.13, 1075. (IO) Ausloos, F!; Rebkrl, R. F.; Glasgow, L. 1. Res. Not. Bur Stand. 1977,82. 1. (11) Filby, W.G.; Mintos, M.; Gusten, H. Ber: Bumenges. Phys. Chem. 1981,85. 189. f12) Childs. L. F! Ph.D. Dissertation. Plince~~~~. ’ ton Univ&sitv, Princeton. N.J.. 1981. (13) Herrman,i. M.; Pichat, F! 3. Chem. Soc. Eng. Pam. 1980,176, 1138. (14) 1.1. Worm Treat. Exom. 1974.23, ..Rwk, . 234. (IS) Bellar, T. A.; Lichtenkrg, 1.1.; Kroner, R. D. 1. Am. Hhrer Rbrkr ASSOC.1974, 66, 703. (16) Oliver, B. G.; Lawrence, 1. 1. Am. Ware, U b r b Assoc. 1979, 71. 161. (17) “Annual Repart an Carcinogenesis Bioassay of Chloroform”; National Cancer InstiN e Eahesda. Md.. 1976. (18) Pruden, A.L.; dIIis, D. E E ~ V ~ P Osci. ~ . Technol. 1983. 17, 628. (19) Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. E 1. Carol. 1983.82.418. (20) NKuyen.’T.:~Ollis. D. F.. submitted for publi&on in 1. Cotal. (21) Brass, H.1. 1. Am. Worer Works ASSOC. 1982,74,107. (22) Svmons. 1. M. et al. J. Am. Hhter Works &s&. 1975, 67,634. (23) Nonvood, D. L. etal. Environ. Sei. Tech~
0.6
5
n
-n
r
~
M I 1980. 14. 187. ~~~~
(24) Prude% A. L.: Ollis. D. E 1. Coral. 1983,82,404.
(27) Chcm. k k . Sem. 7.1983. D. 13 I
Barkni, M. et al. Nouv. 1. Chim. 198
~,547.
suggest chloride ion (C1-) or bromide ion (Br-) inhibition at neutral to slightly acidic pH values and a stmnger inhibition by HC at pH values near the pK. of Ti% surfaces, that is, pH values from 3.5 to 4.5 (30).
Solar applications The ground-level solar spectrum contains approximately 1% near-W photons of sufficient energy to photoexcite the Ti@ catalyst. The solardriven, photocatalyzed decomposition of trichloromethane and ~chlorcethylene using Ti% catalysts has been demonstrated (31). An aqueous solution of 50 ppm halocarbon can be completely mineralized in two or three hours of flumination. An interesting result of this solar study was the fmding that the reaction rate is nearly constant over much of the day, appanmtly indicating only a slight variation in near-UV illumination. The scattered solar “blue” flux varies only about 10% over this perid (32). potential for purilyiag water
Drinking water standards typically concern halocarbon levels in the 1-50404 Emiron. Sci.Technol., hl.19, No. e. 1985
ppb range. At these low concentrations, the rate equation becomes pseudc-fiitorder, with an apparent fzst-order rate constant, k‘, a prcduct of kK.In other words, rate = kKC. The value of the panvneters in Table 1 indicates that the order of ease of conversion is chloroolefins > chloroparafhs > chlw d c acids. Thus, the common water contaminants, trichloroethylene and perchloroethylene, appear to be the strongest candidates for degradation by a potential photoMtalytic pmcess.
(30) Krylov, 0. “Catalysis by Non-Metals”; Academic Press: New York, N.Y., 1972. (31) Ahmed, S.; Ollis, D. E Sol. Energy 1984,32. 1. (32) Meinel, A. B.; Meinel, M. P. “Applied Solar Energy”; Addison Wesley: Reading, Mass., 1976: p. 63.
Acknowledgment Before publication, this article was reviewed for suitability as an ES&T feature by B I U ~E. Rittmann, JJeparhnent of Civil Engineering, University of Illinois, Urbana, Ill. 61801 and R. Rhodes Trussell,
James M.Montgomery Engineering, h a dena, Calif.91100.
References W.R.; Veerkamp, R. F.; Leland, T. W.1. Cud. 1976,43, 304. (2) Iulliet, E et al. Faraday Symp. Chem. Soc. 1973. 7. 57. (3) F&k; S. W.;Bard, A. 1.Am. Chem. Soc. 1977,99, 303. (4) Frank, S. W.;Bard, A. J . Phys. Chem. 1977,81, 1484. (1) Murphys,
&vid E OUis, distinguished professor of
chemical engineering ar North Carolina
Skue University, has research inreresis in
phaiocaralysis and biochemical engineering. Following his f o r m 1 education in chemical engineering (B. S., Calrech, 1963; M.S., Northwestern, 1964; Ph.D., Sianford, 1969), Ollis taught for 11 years ai Princeion and for four ai the Davis campus of the Universiry of Chlifornia. His inieresi in phoiocatalysis was sparked by a sabbatical visit with Pmfessors S. Eichner and J . E. Germain of rhe Universird Claude B e m a d , near ihe Beaujolais area of Frame.
