Geology Reference
In-Depth Information
have been attributed to shipping. Global concern was sufficiently great
that in 2001 the International Maritime Organisation (IMO) success-
fully negotiated the International Convention on the Control of Harmful
Anti-fouling Systems on Ships. The Convention aims to ban the appli-
cation of organotin-based anti-fouling paints on ships from 2003 and
their presence on ship hulls from 2008, but does require ratification of 25
countries comprising 25% of the world's merchant shipping tonnage
before coming into effect.
TBT exists in solution as a large univalent cation and forms a neutral
complex with Cl
or OH
. It is extremely surface active and so is readily
adsorbed onto suspended particulate material. Such adsorption and
deposition to the sediments limits its lifetime in the water column.
Degradation, via photochemical reactions or microbially mediated
pathways, obeys first-order kinetics. Several marine organisms, as di-
verse as phytoplankton to starfish, debutylate TBT. Stepwise debutylat-
ion produces di- and mono-butyltin, which are much less toxic in the
marine environment than is TBT. As degradation rates in the water
column are on the order of days to weeks, they are slow relative to
sedimentation. TBT accumulates in the sediments where degradation
rates are much slower, with the half-life being on the order of years.
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Furthermore, concentrations are highest in those areas, such as marinas
and harbours, which are most likely to undergo dredging. The intrinsic
toxicity of TBT, its persistency in the sediments and its periodic remo-
bilisation by anthropogenic activity are likely to retard the long-term
recovery of the marine ecosystem.
Although the unrestricted use of TBT should end, significant chal-
lenges remain in many parts of the world. For the most part, the coastal
tropical ecosystems remain unprotected and the sensitivity of its indig-
enous organisms relatively poorly evaluated. TBT endures in sediments
globally, with concentrations usually greatest in environments most
likely to be perturbed, such as ports and marinas. The widespread
introduction of TBT into seawater will continue from vessels not yet
subject to legislation until at least 2008, depending on when the IMO
Convention comes into force. However, the paramount lesson learned
from TBT should be that potential replacement compounds must be
properly investigated prior to their introduction in order to avoid
another global pollution experiment.
QUESTIONS
(i) What are the main sources and sinks for dissolved and partic-
ulate metals entering the ocean?
