Environmental Engineering Reference
In-Depth Information
marine environment. Of particular concern is the stability of TBT in con-
taminated sediment where biodegradation processes may be inhibited [47].
Because of the long persistence of TBT in sediment, sediment may be a big
reservoir for TBT in some locations [67].
The concentrations of TBT in seawater, sediment, fish, etc., are shown in
Fig. 5 [99]. The concentrations are different (10
3
-10
5
times) between the sea-
water or freshwater and sediments or fish. These data show that TBT is stable
in the sediment, sludge and living organisms such as algae and fish. Bioaccu-
mulation factors for TBT, range from 400 to 50
10
4
for fishes, 1500 to 2.2
×
×
10
4
for molluscs and were
>
3
10
5
for algae [67].
The main ingredient of the sediment is oxygen in the form of metal
oxides, and those of fish and shellfish are proteins containing nitrogen elem-
ents as amino acids. The organotin compounds easily form a five- or six-
coordination state by which the tin element is coordinated with electronega-
tive atoms such as nitrogen, oxygen, sulfur or phosphorus [16]. In particular,
the organotin compounds very easily form the five- or six-coordination state
by the coordination of three sites with alkyl or phenyl groups and two or
three sites with the oxygen or nitrogen atoms of metal oxides or proteins
(R
3
SnL
1
L
2
or R
3
SnL
1
L
2
L
3
). Therefore, the organotin compounds show high
affinities to those substances containing nitrogen or oxygen atoms. This is
the first characteristic property of the organotin compounds described in
Sect. 2.1.
×
5
Biomethylation of Organotin Compounds
Biomethylation proceeds through microorganisms such as bacteria hav-
ing a methylating capability [100-105]. The principal naturally occur-
ring methylating agents are: (a) methylcobalamin (vitamin B
12
); (b)
S
-
adenosylmethionine (methyl-group donor, active methionine); and (c)
methyl iodide (probably formed by the methylation of the iodide ion by
S
-
adenosylmethionine) [100]. Biomethylation is known in the methylation of
metals such as mercury, arsenic, lead, chromium, tin, palladium and thallium.
For example, methyl mercury compounds are produced from inorganic mer-
cury in sediment by anaerobic bacteria through the action of the methlylating
agent, methylcobalamine [100].
The main methylating agent for tin compounds is believed to be methyl-
cobalamin (CH
3
CoB
12
), although the other methyl donors may be involved in
extracellular reactions, e.g. methyl iodide, CH
3
I [32].
The first laboratory studies on inorganic tin compounds were conducted
in 1974 by Huey et al. [106] using pure cultures of tin-resistant
Pseudomonas
bacteria from Chesapeake Bay. Incubation with SnCl
4
·
5H
2
Oledtothepro-
duction of what was believed to be a dimethyltin species [24]. The aquatic
