dechlorination and hydrolysis) through the COOH radical,
which in turn again produces HCOOH, or the HCO radical
finally yielding both HCOOH and HCHO. The observed
amounts of these products obviously depend on the relative
rates of the redox pathways.
(8) Nickelsen, M. G.; Cooper, W. J.; Kurucz, C. N.; Waite, T. D. Environ.
Sci. Technol. 1992, 26, 144.
(
9) Cooper, W. J.; Nickelsen, M. G.; Meacham, D. E.; Cadavid, E. M.;
Waite, T. D.; Kurucz, C. N. J. Environ. Sci. Health 1992, A27, 219.
(
10) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28,
045.
2
•
•
(
e) Dichloromethyl ( CHCl
2
) and monochloromethyl ( CH
2
-
(11) Hooker, P. D.; Klabunde, K. J. Environ. Sci. Technol. 1994, 28,
1243.
(12) Koper, O. B.; Wovchko, E. A.; Glass, J. A., Jr.; Yates, J. T., Jr.;
Klabunde, K. J. Langmuir 1995, 11, 2054.
13) Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1991, 25, 973.
14) Chin, P. C.; Reinhard, M. Environ. Sci. Technol. 1995, 29, 595.
15) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. Environ.
Sci. Technol. 1995, 29, 439.
Cl) radicals have been reported in pulse radiolysis experiments
•
(
30). CHCl
2
can originate reductively from CHCl
Cl . Their possible reaction pathways are
reported in Scheme 1.
f) Methanol. Methanol is not detected under aerobic
photocatalytic degradation of CH Cl , but is formed under
3
or oxi-
datively from CH
2
2
(
(
(
(
2
2
the present conditions. Following the sequence depicted for
(16) Khindaria, A.; Grover, T. A.; Aust, S. D. Environ. Sci. Technol.
1995, 29, 719.
•
in Scheme 1, methanol can be formed from CH
2
Cl (route A)
(
(
(
17) Asmus, K. D.; Bahnemann, D. W.; Krisher, K.; Lal, M.; Honig, J.
Life Chem. Rep. 1985, 3, 1.
18) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized
Metal Electrodes; Plenum: New York, 1980.
or through CH
reduction of CH
2
(OH)Cl intermediate (route B). Further
Cl will produce CH Cl. Since the thermal
•
2
3
hydrolysis of methyl chloride is a slow reaction (k ) 1.5 ×
-
4
-1
-2
-1 -1
-
1
1
0
s
with water and k ) 2.4 × 10
M
s
with OH at
19) Pruden, A. L.; Ollis, D. F. Environ. Sci. Technol. 1983, 17, 628.
00 °C) (46, 47), the low amount of methyl chloride detected
(20) Hsiao, C. Y.; Lee, C. L.; Ollis, D. F. J. Catal. 1983, 82, 418.
(21) Hisanaga, T.; Harada, K.; Tanaka, K. J. Photochem. Photobiol. A:
Chem. 1990, 54, 113.
suggests that either reductive dechlorination is very fast as
compared to reduction followed by protonation or that the
(
22) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci.
Technol. 1991, 25, 494.
alternative pathway through CH
favored. Route B that would be followed because of the
species CH (OH)Cl, concurrently to methanol formation,
2
(OH)Cl (route B) is kinetically
(
23) Sabin, F.; Turk, T.; Vogler, A. J. Photochem. Photobiol. A: Chem.
2
1
992, 63, 99.
could also generate formaldehyde, which is the main product
observed (see Figure 3b).
(24) (a) Bahnemann, D. W.; Fisher, C. H.; Hoffmann, M. R.; Hong,
A. P.; Moning, J.; Kormann, C. Am. Chem. Soc. Environ. Div.
1
987, 27 (2), 528. (b) Hilgendorff, M.; Hilgendorff, M.; Bahne-
(
g) Condensation products (21, 22). As evidenced by C
2
Cl
formation, radical-radical reactions also occur.
4
mann, D. W. J. Adv. Oxid. Technol. 1996, 1, 35.
and C Cl
2
6
(
(
(
25) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1995, 29, 1646.
26) Choi, W.; Hoffmann, M. R. J. Phys. Chem. 1996, 100, 2161.
27) Kuhler, R. J.; Sauto, G. A.; Caudill, T. R.; Betterton, E. A.; Arnold,
R. G. Environ. Sci. Technol. 1993, 27, 2104.
Other possible reactions include radical to molecule reaction,
•
such as CHCl
process.
2
3
+ CHCl , followed by a subsequent redox
In conclusion, in the absence of oxygen under photo-
catalytic conditions, halomethanes are efficiently hydrolysed,
interconverted, or disproportionated through the pathways
described in Scheme 1.
(28) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991,
5, 1522.
2
(
29) Bahnemann, D. W.; Cunningham, J.; Fox, M. A.; Pelizzetti, E.;
Pichat, P.; Serpone, N. In Aquatic and Surface Chemistry; Helz,
G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis: Boca Raton, 1994;
p 261.
After this work was submitted, a paper on photocatalytic
degradation of CHCl
3
and CHBr
3
also giving additional
(30) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
information on CCl was published (48). The two sets of data
4
Chem. Rev. 1995, 95, 69.
31) Minero, C.; Catozzo, F.; Pelizzetti, E. Langmuir 1992, 8, 481.
32) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95,
(
(
are consistent and agree on the proposed photocatalytically
enhanced hydrolysis mechanism, although the two experi-
mental works have been carried out under quite different
conditions. The effect of light intensity and concentration
of substrate, catalyst, and electron scavengers was rationalized
5
166.
(
33) Pelizzetti, E.; Minero, C.; Maurino, V.; Sclafani, A.; Hidaka, H.;
Serpone, N. Environ. Sci. Technol. 1989, 23, 1380.
(34) Minero, C.; Pelizzetti, E.; Malato, S.; Blanco, J. Chemosphere 1993,
26, 2103.
(
49). Due to the saturative behavior of the rate versus
concentration, it is likely that the rate or the photonic
efficiency (Table 1) observed under our conditions is slightly
less than the rate of chloride evolution given in Figure 2 of
(35) Piccinini, P.; Calza, P.; Minero, C.; Pelizzetti, E. New J. Chem.
1
996, 20, 1159.
(
(
(
(
36) Augustynski, J. Struct. Bonding (Berlin) 1988, 69, 1.
37) von Stackelberg, M.; Stracke, A. B. Z. Elektrochem. 1949, 53, 118.
38) Eberson, L. Acta Chem. Scand. 1982, B36, 533.
39) Dutoit, E. C.; Cardon, F.; Gomes, W. P. Ber. Bunsen-Ges. Phys.
Chem. 1976, 80, 477.
ref 48. The different amount of condensation products (C
2
products) is consistent with the different initial concentrations
in agreement with a previous report (50).
The transformation frame here proposed is suitable to
describe in a qualitative way all the observed results,
highlighting also the effect of hole and electron scavengers
recently reported (48).
(40) Gr a¨ tzel, M. In Fine Particles Technology. From Micro to Nano-
particles; Pelizzetti, E., Eds.; Lewis: Boca Raton, 1994; p 719.
(
41) Mertens, R.; von Sonntag, C.; Lind, J.; Merenyi, G. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1259.
(
(
42) Wawzonek, S.; Duty, R. C. J. Electrochem. Soc. 1961, 108, 1135.
43) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M.
Environmental Organic Chemistry; Wiley: New York, 1993.
44) Chemseddine, A.; Boehm, H. P. J. Mol. Cat. 1990, 60, 295.
45) Robinson, E. A. J. Chem. Soc. 1961, 1663.
46) Fells, I.; Moelwyn-Hughes, E. A. J. Chem. Soc. 1958, 1326.
47) Fells, I.; Moelwyn-Hughes, E. A. J. Chem. Soc. 1959, 398.
48) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31, 89.
49) (a) Minero, C.; Pelizzetti, E.; Malato, S.; Blanco, J. Sol. Energy
Acknowledgments
Financial support of CNR, MURST, and PNRA Progetto
Antartide Contaminazione Ambientale is kindly appreciated.
(
(
(
(
(
(
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(
(
(
(
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Received for review July 30, 1996. Revised manuscript re-
X
ceived March 14, 1997. Accepted March 24, 1997.
(
(
(
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ES960660X
X
Abstract published in Advance ACS Abstracts, May 15, 1997.
VOL. 31, NO. 8, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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