1440
A. Mallakin et al. / Chemosphere 40 (2000) 1435±1441
ANT goes to the excited triplet-state (3ANT) via inter-
system crossing from the excited singlet-state 1ANT. The
experimental solution was open to the air, so the main
quencher of 3ANT would be ground triplet-state oxygen
References
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3
3O2. This is because the concentration of ANT would
Cook, R.H., Pierce, R.C., Eaton, P.B., Lao, R.C., Onuska, F.I.,
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be low in the aqueous medium relative to O2 concen-
trations (0.25 mM) (Krylov et al., 1997; Robinson and
Cooper, 1970). This means the rate of reaction of 3ANT
with 3O2 should be diusion limited with an average
6
interval of less than 10 s between collisions (Krylov et
al., 1997). Because the ANT excited triplet state is rel-
Foote, C.S., 1979. Mechanisms of photooxidation. In: Wass-
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3
atively long lived (10 6±10 s), the majority of the ex-
3
cited ANT should be quenched by oxygen, generating
1
Huang, X.-D., Dixon, D.G., Greenberg, B.M., 1993. Impacts of
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singlet-state oxygen O2 (Lakowics, 1983). These singlet
1O3 molecules could then react with ANT forming the
endo-peroxide. In type I photosensitization reactions, as
in type II reactions, after absorbance of a photon, ANT
Huang, X.-D., McConkey, B.J., Babu, T.S., Greenberg, B.M.,
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3
goes to ANT via intersystem crossing from the excited
singlet state. The 3ANT does not react with 3O2, but
instead with another PAH or solvent to generate a PAH
free radical or solvent free radical. These PAH radicals
would also be reactive with O2, forming oxidation
products. Assuming the 9,10-endoperoxide of ANT is
the initial oxidation reaction, a type II photosensitiza-
tion mechanism is more likely.
Katz, M., Chan, C., Tosine, H., Sakuma, T., 1979. Relative
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Our studies show that the initial 1O2 attack is ex-
clusively at the 9,10 position in the central ring of ANT.
The electron density of this ANT central ring is very low
(Mezey et al., 1998), explaining why the initial reaction
is so speci®c to this position. This low electron density of
the central ANT ring is also a key factor in the very
rapid rate of photooxidation.
Krylov, S.N., Huang, X.-D., Zeiler, L.F., Dixon, D.G.,
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The initial ANT concentration (5 lg/ml) in water
used in this study is well above its solubility limit. This
was necessary so we would have sucient material for
analysis. Interestingly, the ANT did not precipitate, but
rather formed an emulsi®ed solution. Further, the ki-
netics of photooxidation were pseudo ®rst-order,
mono-phasic and rapid. From this we conclude that
PAHs do not have to be fully solubilized to be subject
to photomodi®cation. This has environmental impli-
cations, as PAHs in aquatic environments are often
bound to suspended particulate matter. Our data sug-
gest that under these conditions photomodi®cation can
occur.
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Acknowledgements
The authors acknowledge members of the Greenberg
and Dixon laboratories for fruitful discussions. This
research was supported by a Canadian Network of
Toxicology Centres grant, a CRESTech grant, and
NSERC Research and Strategic grants to B.M.G and
D.G.D.
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