Environmental Engineering Reference
In-Depth Information
vironmental conditions photolysis is usually slow and photoproducts do not
accumulate appreciably. Microtox was used for the evaluation of the toxic-
ity of the irradiated solutions of diuron [68]. The observed toxicity is due
to minor photoproducts, however, it does not disappear so rapidly. Hence
the decrease of herbicide concentration is not necessarily associated with
a lower toxicity of the solution. The toxicity also increased with irradiation
time, but it remained almost stable between 50%and98%conversion.It
was not possible to evaluate the EC
50
of pure diuron since it is higher than
its solubility in water [68]. Bonnemoy et al. [68] deduced that at least one
of the photochemical intermediates is more toxic than diuron, while syner-
gistic effects among degradation products cannot be excluded. It has been
reported [68] that when diuron is irradiated in aqueous solution photohy-
drolysis products in
meta
and in
para
positions are the main photoproducts
(compounds 1 and 2). However, the small amounts of 3-(3,4-dichlorophenyl)-
1-formyl-1-methylurea (3) and 3-(3,4-dichlorophenyl)-1-methylurea (4), re-
sulting from the oxidation or elimination of a methyl group, may be present
since such products are formed by irradiation in the absence of water [68].
It was reported by Tixier et al. [75] that products (1) and (2) have al-
mostthesametoxicityto
Vibrio fischeri
as diuron, but that compounds (3)
and (4) are significantly more toxic. Consequently, the observed increase in
toxicity may be mainly due to minor photoproducts. It is most likely that
during diuron photolysis the toxic products formed will be photoconverted
in a further reaction step, but this conversion is less rapid and the com-
plete disappearance of the herbicide is not associated with a decrease of
toxicity.
3.6
Zinc and Copper Pyrithiones
Zinc pyrithione (bis(2-pyridylthio)zinc 1,1
-dioxide ) (ZnPT) has a short half-
life in natural seawater (
t
1
/
2
≤
4 h; [76]) and low aqueous solubility. Zinc
pyrithione photolysis and biodegradation are both very rapid procedures
with photodegradation being the most effective process whilst hydrolysis
half-lives range from 96 to 120 days. Zinc pyrithione is known to undergo
trans
-chelation, with copper and possibly manganese replacing the zinc ion,
depending on their relative concentrations in the water or sediment. It has
been suggested that in waters where UV degradation of zinc pyrithione is
poor, it may accumulate in sediments as these complexes, prior to biodegra-
dation and
or hydrolysis [39, 77].
Photolytic half-lives ranging between 2-18 min have been reported in
a variety of irradiation conditions [76, 78] (filtered Xe arc lamp and natu-
ral sunlight), while less toxic photoproducts are formed [79]. Degradation
of pyrithione is mainly caused by the UV component of the sunlight spec-
trum, and Neihof et al. [78] reports that wavelengths of 320-355 nm are
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