Environmental Engineering Reference
In-Depth Information
context. Aqueous photolysis of 3,4-dichloroaniline occurs through sunlight ir-
radiation [71]. In general, for halogenated anilines this transformation rate
is highly dependent on the position of the halogen on the ring. For 3,4-DCA,
the faster rate of photodegradation is consistent with its high absorption band
(
>
300 nm) and the relatively high quantum yield (
Φ
(313 nm)
= 0.052) [72].
In winter the transformation rate was shown to be completed in 20 days,
ten times slower than that in summer. The half-life for direct phototransfor-
mation in water was determined at 0.4 h, while natural sunlight resulted in
half-lives of 6
3.6 h at the water surface [73]. Dichloroanilines are reported
to be transformed first into phenolic compounds and then into aminophe-
noxazones but no toxicological data exist for these substances.
Loss and oxidation of the alkyl chains are the dominant processes dur-
ing the phototransformation of ureas [74], as depicted at Fig. 6. On the
other hand, substitution of a halogen atom by a hydroxyl-group, and hy-
droxylation of the aromatic ring are considered to be the secondary pro-
cesses. It was observed that diuron photodegradation consisted of two phases
of different but almost constant rates. The degradation started at a high
rate and when approximately 50% of the original substance degraded the
rate of degradation fell to a twentieth of the first phase. The complete
process of the degradation could be best described by a cubic function
(
y
= -10
-4
x
3
+ 0.030
x
2
- 3.0255
x
+ 100; R
2
= 0.952) [74].
Absorption of light may indeed induce a phototransformation of diuron
with a possible formation of more toxic intermediate photoproducts. In en-
±
Fig. 6
Photolytic degradation pathways of diuron in aqueous environment based on re-
ported identified phototransformation products [68-70]
