Environmental Engineering Reference
In-Depth Information
releasing an intermediate, DPT [130]. The optimum pH for the activity was
pH 7.5 to 8, and the relative activities were reduced to approximately 40-50%
at pHs 5.5 and 9.5 [126]. These observations suggest that the chelating residue
of pyoverdin may participate in a nucleophile attack at the tin atom and dur-
ing the protonation of the phenyl group of TPT, to form more stable DPT- and
MPT-pyoverdin as shown in Fig. 6 [131].
The half-lives of TBT in an estuarine river along the Georgia coast (at the
low concentration of TBT (
<
5 ng L
-1
) range from 3 to 13 days. Evidence sug-
gesting that microalgae play an important role in TBT degradation in sunlit
coastal water included the following:
1. TBT degradation rates were high in light compared to that in dark
degradation and there was no evidence of TBT photolysis because the TBT
degradation does not practically proceed in filtered estuarine water in sun-
light as shown in Fig. 7.
2. (Hydroxybutyl)tins and dibutyltin were the major degradation products
in the light and by the cultures of diatoms and dinoflagellates, while only
dibutyltin was observed in the dark.
3. The TBT degradation increased in sunlight when nitrate was added as
shown in Fig. 7 [97].
The degradation rates were always higher in sunlight-incubated samples
than those in the dark (Fig. 7). The TBT degradation was considered to in-
crease in the light because it is metabolized by the algae. Further support
for this conclusion was that the TBT degradation rates increased signifi-
cantly when nitrate was added to water before the samples were exposed to
sunlight. The half-life of TBT was 1-2 days in nitrate-supplemented water
(Fig. 7). The dominant algae in the normal and nitrate-supplemented estuar-
inewaterwerethediatoms
Skeletonema costatum
or
Skeletonema tropicum
(summer months). Phytoplankton that can degrade TBT, included diatoms
(
Skeletonema costatum
and
Chaetoceros curvisetus
), and a dinoflagellate (
Pro-
centrum triestium
). Green algae (
Dunaliella tertiolecta
)andchrysophytes
(
Isochrysis galbana
and
Cricosphaera ricoco
) showed a very limited ability to
degrade TBT (Table 13 [97]). The stimulation of algal growth may be of use in
enclosed aquatic areas with the organotins, since dense algal cultures may be
able to rapidly degrade these organometallic compounds [97].
Tsang et al. [135] investigated two microalgal species,
Chlorella vulgaris
and
Chlorella
sp. for their capabilities in degrading TBT at its sublethal con-
centration. The biosorption of TBT by the algal cell wall was the major
mechanism in reducing 40% of the initial TBT from the medium in the first
2 days. The half-life of TBT incubated with
C. vulgaris
was 60 h while that
with
Chlorella
sp. was 80 h. At the end of the experimental period of 14 days,
27 and 41% of the original TBT were recovered as DBT and MBT in the cul-
tures of
C. vulgaris
, respectively [135]. The capability of such debutylating
processes therefore accounts for the higher tolerant ability of
C. vulgaris
than
that of
Chlorella
sp. [135].
