Download Organometallic compounds: TBT

Environmental Geosciences
Human Interactions with the
Environment
Organometallic
Compounds
Andrea Koschinsky
Toxic effects of environmental compounds
Toxic effects of environmental compounds
Organometallic compounds
Definition of organometallic
compounds:
constituting an organic compound
containing a metal, especially
a compound in which a metal
atom is bonded directly to a
carbon atom.
Artificially produced organometallic
compounds
Biotransformation of arsenic in
marine ecosystems: natural
production of organometallic
compounds
Organometallic compounds
Organometallic compounds: Organotin
Organometallic compounds: Organotin
Organometallic compounds: Organotin
Organometallic compounds: TBT
Tributyltin compounds (TBT’s) have entered the marine environment as a result of
their use in anti-fouling paints on boats and aquaculture equipment. They are
manufactured compounds that have no counterpart in nature. The friction caused
between the hull of a ship and the water causes ‘drag’ which can affect fuel
consumption. This drag effect is increased by the growth of marine organisms and
plants on the ship’s hull known as ‘fouling’. As a ship typically only enters dry dock
every two to five years for cleaning, an alternative approach to reduce fouling is the
application of anti-fouling paints. These coatings inhibit the growth of marine organisms
through the controlled release of biocides.
The most common and effective chemical used to date has
been tributyl tin. They came into use in all classes of
shipping in the 1970’s and were subsequently used to treat
net enclosures of mariculture installations and wooden
lobster pots. Due to its broad-spectrum toxicity, TBT is also
used as a fungicide, bactericide and as an insecticide on
textiles, paper, leather and electrical equipment. TBT
compounds were initially used as they were more effective
and longer-lasting than mercury or copper-based antifouling
paints. The organic form of the metal is more toxic than the
inorganic form due to the greater ease of uptake by
organisms.
Organometallic compounds: TBT
Organometallic compounds: TBT
Organometallic compounds: TBT
The fate of TBT in the environment
Organometallic compounds: TBT
The fate of TBT in the environment
Organometallic compounds: TBT
The fate of TBT in the environment
Organometallic compounds: TBT
The fate of TBT in the environment
TBT is bioaccumulated by fish, crabs and micro-organisms. The chemical half-life of TBT
in harbour water is approximately one week and is thought to be controlled by biological
uptake and subsequent degradation.
Degradation rates are slower
in the sediment (months to
years) than in the water
column (days to weeks)
although concentrations of
TBT are higher in marine
sediment. It is suggested that
this may be due to the
inhibitory effect of higher TBT
concentrations on bacterial
activity. TBT is generally
degraded by bacteria into
dibutyltin (DBT) and
monobutyltin (MBT) which are
less toxic forms of their parent
compound.
Organometallic compounds: TBT
Organometallic compounds: TBT
Organometallic compounds: TBT
Organometallic compounds: TBT
Endocrine Effects Of Tributyltin Compounds
Since the early eighties the development of male sexual characteristics (the so-called
imposex condition) has been reported for females of some marine neogastropod
snails. This phenomenon has been primarily attributed to the contamination of
coastal areas with tributyltin (TBT). Species inhabiting rocky shores, as well as soft
bottoms of peripheral seas, are reported to be affected.
The mechanism of toxicity in snails is described as a result of increased testosterone
level in treated female snails. Some scientists postulated TBT inhibition of
Cytochrome-P450-aromatase, which normally metabolizes testosterone to 17 bbestradiol in females.
An alternative hypothesis is neurotoxic effects of TBT resulting in increased
testosterone production in female snails. Based on the published studies, a hormonal
(androgen) effect of TBT is assumed in this group of marine snails. The effect is
definitively concentration dependent. On the basis of an extensive two-generation
study with snails , it was be demonstrated that imposex is induced in adult, female N.
lapillus at TBT concentrations below 2 ng/L.
Organometallic compounds: TBT
Organometallic compounds: TBT
The Severity of the Pollutant and its Effects
TBT compounds have been described by scientists as being amongst the most toxic
compounds ever produced and are therefore lethal to a wide variety of planktonic
organisms. TBT compounds are known to have affected at least 72 species of gastropod
mollusc world-wide. They affect planktonic mollusc larvae which are 10-1000 times more
sensitive than the adult molluscs. Initially, carnivorous neogastropods (such as whelks)
were found to be affected but more recently other herbivorous gastropods (whelks on
rocky shores) have been affected.
Example - Pacific Oyster
TBT also has sublethal effects which are
detrimental to local shellfisheries. These
compounds may cause reduced growth
rates and other effects to young Pacific
oysters (Crassostrea gigas ). TBT
causes a thickening of the oyster shells
and a large reduction in meat content.
The effect is due to an enzyme
disfunction in shell deposition and results
in making the oysters unmarketable.
Oysters affected in this way are often
referred to as “golf-ball oyster”.
A) Normal oyster, about 8
years old B) Oyster
with first 5 years of
growth affected by
TBT, followed by 3
years after TBT
ban; C) Oyster,
exposed to high
levels of TBT
throughout 8 years
of life
A
B
C
Organometallic compounds: TBT
The ecotoxicological impact of TBT on fish, birds seals and other marine mammals has
been less well studied. TBT has however been shown to have an endocrine disruptive
effect in fish, birds and mammals. As organotins (e.g. TBT) are bioaccumulative in
certain marine species, some of which are consumed by humans, there is justifiable
concern over the level of exposure to humans. Butyltin compounds including TBT, DBT
and MBT have been detected in almost all marine mammal liver samples from all
around the world. This illustrates the world-wide distribution of organotins in the
oceans. Concentrations of TBT are greatest in coastal waters especially around
developed countries.
It is thought that TBT is moderately to slightly toxic to mammals. Some mammals such
as the sea lion have the ability to degrade or expel butyltins from their body, whilst
others, such as dolphins, exhibit age-dependent biomagnification of butyltins. High
doses of organotins have been shown to damage the central nervous system and
reproductive mechanisms in mammals. It is therefore likely that organotins are being
passed to humans consuming marine mammals as a significant proportion of dietary
protein and fat intake. The specific effects of TBT on humans, however, are still not that
clear.
Organometallic
compounds: TBT
Organometallic compounds
Organometallic compounds: TBT
As the environmental effects of TBT became known in the 1980’s, governments
around the world introduced legislation to tackle the problem. In July 1987, the UK
Government banned the use of TBT-based antifouling paints for vessels under
25m in length and also its use in aquaculture. The use of TBT on ocean-going
vessels is still permitted on the grounds that these vessels do not sit for long
periods of time in near-shore waters and are therefore unlikely to affect local
shellfisheries. However, through the International Maritime Organisation, a ban on
the application of TBT came into force on the 1st January 2003, with a complete
global prohibition by 1st January 2008.
Other compounds can be used in anti-fouling paints
as an alternative to TBT e.g. copper- and mercurybased compounds however these are not as
effective or long-lasting as paints with associated
TBT compounds present. In addition, the impact on
the environment of these alternative anti-fouling
paints may not be fully understood. Other
alternatives exist such as polysiloxane silicone
polymers and epoxy urethane coatings. These
methods create a slippery surface from which
micro-organisms are washed off when water flow
rates exceed approximately 2m/s.
Organometallic compounds: TBT
Organometallic compounds: TBT