environmental geosciences

Environmental Geosciences

Human Interactions with the Environment

Andrea KoschinskyOrganometallic Compounds

Toxic effects of environmental compounds

Organometallic compoundsDefinition of organometallic compounds:constituting an organic compound containing a metal, especially a compound in which a metalatom is bonded directly to a

carbon atom.

Biotransformation of arsenic in marine ecosystems: natural

production of organometallic compounds

Artificially produced organometallic compounds

Organometallic compounds

Organometallic compounds: Organotin

Organometallic compounds: TBTTributyltin 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: TBTThe fate of TBT in the environment


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.


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 bb-estradiol 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: TBTThe 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.

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

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

C

B


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


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 mercury-based 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.

environmental geosciences

Documents

tbt compounds

tbtorganometallic compounds

manufactured compounds

tbttributyltin compounds

tbtthe fate of tbt

concentrations of tbt

chemical halflife of

marine sediment