Environmental Engineering Reference
In-Depth Information
produced upon the photolysis of cDOM in sunlit natural waters and scav-
enged by nitrate [25]. All these reactive transients are considered to play
a significant role in limiting the persistence of many chemical pollutants.
The ability of each reactive agent to contribute toward the phototransforma-
tion of the contaminant under natural solar light is likely to be affected by
the composition of natural water, particularly on the concentrations of ni-
trate, cDOM, and bicarbonate
carbonate ions. Humic and fulvic substances
in natural waters were found to be present at concentrations ranging from
0.3 to 30 mg L
-1
, while nitrate and bicarbonate (also considered principal
components of surface water) are present with concentrations ranging from
3to323
/
Mand0.4to4.4 mM, respectively. An important advantage of
photosensitized photodegradation is the possibility of using light of wave-
lengths longer than those corresponding to the absorption characteristics of
the pollutants.
On the other hand, a decrease in a contaminant photodegradation rate in
natural waters could be observed. DOM present in natural waters absorbs
most of the available photons emitted (since it is one of the most import-
ant sunlight-absorbing components of the aquatic environment [26]) thereby
slowing down direct photochemical reaction (optical filter effect). Another
reason for the observed filter effect may be the binding of micropollutants
to DOM by hydrophobic partitioning or weak van der Waal forces, thus af-
fecting photodegradation. The retarded photodegradation rate, especially in
seawater, is also consistent with
•
OH scavenging by chloride ions [27].
As a result, phototransformation of pollutants under natural conditions
may be a complex process. In order to evaluate the persistence of pollutants
such as antifouling biocides, both direct and photosensitized transformation
experiments under relevant laboratory and field conditions should be per-
formed using natural waters of different composition.
More detailed information of the photoreactivity of each biocide under
natural and
µ
/
or simulated solar irradiation is given in the following sections.
3
Kinetics, Photoproducts, and Reaction Pathways
of Antifouling Booster Biocides
3.1
Chlorothalonil
Chlorothalonil (2,4,5,6-tetrachloroisophtalonitrile) has been found to de-
grade after 4 weeks in natural sea water and even faster in water supple-
mented by cultured marine bacteria. Research by Davies [28] found degrada-
tion still occurred when the biocide was present in low concentrations. Even
faster degradation has been witnessed in freshwater where chlorothalonil ex-
