Environmental Engineering Reference
In-Depth Information
hibited a half-life of only a few hours (
t
1
/
2
= 4-150 h). Walker et al. [29]
reported degradation half-lives of between 1.8 and 8 days in natural estuarine
water and sediment-water test systems. In general biodegradation is lower
than photolytic processes [29].
Chlorothalonil can undergo photodegradation in the water column
demonstrating a half-life of few hours [30, 31], while faster degradation has
been witnessed in ground water under simulated solar irradiation with a half-
life less than an hour [32]. Millet et al. [33] calculated the half-lives in the
aquatic environment of chlorothalonil photodegradation using the program
GC-Solar as 22-206 days (from summer to winter). Chlorothalonil photode-
composition was carried out in different natural waters [31, 34] and it was
observed that the degradation rate was significantly enhanced compared to
distilled water, with the exception of seawater in both natural and simulated
solar irradiation. More than 99% of chlorothalonil degrades within 60 h in
lake and river water while 33%and28% still remain in distilled and seawa-
ter, respectively, under natural solar irradiation. Simulated solar irradiation
for 10 hresultedin63%and59% decline of chlorothalonil concentration
in distilled water and seawater, respectively, while more than 99%wascon-
sumed in both river and lake waters. The degradation rates have shown
that the presence of DOM in natural waters enhances the photodegradation
rate of chlorothalonil. The enhanced photodegradation in both outdoor and
laboratory experiments is attributed to the presence of naturally occurring
photosensitizers in natural waters such as dissolved organic matter, nitrates,
and carbonate-bicarbonate ions.
As far as the formation of photoproducts is concerned, six compounds
could be detected as possible degradation intermediates, however, only five
have been identified including one pair of isomers. The main degrada-
tion products were benzamide (1), chloro-1,3-dicyanobenzene (2), dichloro-
1,3-dicyanobenzene (3), trichloro-1,3-dicyanobenzene (4) [32]. A tentative
degradation pathway is proposed for chlorothalonil photodegradation in nat-
ural waters (Fig. 2).
Chromatographic data indicate the presence of one additional product,
compound 5. This compound involves a benzyl and methyl group in the
molecule, while the absence of isotopic pattern of chlorine atoms supports
the homolytic cleavage of the C - Cl bond. No nitrogen atoms seem to be
present in the molecule, demonstrating the lack of nitrile groups that could
be photohydrolyzed to the corresponding benzoic acids through benzamide
intermediates. However, the data were insufficient to propose a structure.
Compounds 1 (benzamide) and 5 (unidentified) have been detected only
during the photodegradation of chlorothalonil in natural and humic wa-
ters [31, 34]. It was the first time that benzamide had been observed during
chlorothalonil photodegradation in natural waters indicating that dissolved
organic matter afforded both an increase in photosensitization and in
•
OH
processes.
