TBT - a risk to the environment

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TBT - a risk to the environment

Toxicology
The importance of organotin compounds (OZV) for the environment is due to their high ecological damage potential. OZVs even have a hormone-like effect on numerous marine organisms, so that these species are particularly endangered. The concentrations at which chronic effects are indicated are already at a few ng/L (0.000 001 mg/L).

At the beginning of the 1980s, a direct link between population decline and TBT concentration in the water was demonstrated in an oyster farm in southern France. The oysters showed pronounced shell malformations and were inhibited in their growth. A particularly drastic form of TBT poisoning is the so-called imposex behaviour of marine snails. The development of male sexual characteristics in female snails prevents reproduction of this species. The average TBT concentrations measured in German rivers are all above the effect concentrations. TBT is blamed for the worldwide observed decline in the populations and species diversity of these marine snail species.

Fields of application
The inputs of organostannic compounds into the environment can be very diverse. They result from the manifold applications, which include both biocidal and non-biocidal applications. OZVs are divided into four groups according to their degree of alkylation: mono-, di-, tri-, tetra-organotin compounds.

Applications of organotin compounds:
The mono- and dialkylated compounds (MonoOZV, DiOZV) form the main part of the total production of organotin compounds. The applications include stabilisers in PVC materials, catalysts and precursors. The biocidal applications of the trialkyled compounds range from additives for antifouling paints and pesticides to impregnating agents.

TetraOZV
Initial product

TriOZV

Antifouling paints

biocide
griculture
fungicide
Preventive food products
acaricide

Wood preservation
Stone protection
Disinfection
Textiles

DiOZV, MonoOZV

PVC

heat-, light stabilizer
Homogeneous catalysts
Silicones, Urethanes
Esterifications

Prekurs

SnO2 on glass
Worming agent for poultry
Textiles

Pathways into the environment

By using the described products, OZVs can enter the environment in various ways.

In addition, unwanted emissions also lead to environmental pollution. Starting from emission sources, OZVs end up in soil and water. In water, the compounds accumulate solid particles and then sediment. Through transformation reactions and hydrodynamic processes, the species can again enter the water phase (remobilisation). A path that must be critically evaluated is the accumulation of pollutants from organisms living in the aquatic environment, such as fish and mussels.

Analysis and quality assurance

In order to determine the content of OZV in environmental samples, great demands are made on sample preparation and instrumental technology. The different polarities of the tin compounds require an additional derivatisation step in addition to extraction.

Results of the certification of the CRM477

For separation and detection, coupled techniques must be used which are highly selective and at the same time very strong in detection.

This results in high demands on the quality assurance measures. GALAB participates in European interlaboratory tests for the certification of reference materials for organotin compounds. The result of such a special round robin test.

Behaviour in the environment
In the following, the environmental behaviour of tin compounds will be described. Transformation reactions, which include methylation and degradation reactions, are particularly important for the environmental behaviour of tin compounds.

Chemical and physical properties: Transformation reactions

Methylation, i.e. the formation of toxic OZV from inorganic tin can take place biologically as well as chemically (transmethylation). Degradation reactions are typical for this class of compounds and can be chemically/physically as well as biologically induced. In any case a degradation process takes place by successive splitting off of alkyl groups up to inorganic tin. The degradation rate depends largely on the prevailing environmental conditions. With the introduction of TBT-containing antifouling paints, degradation tests were carried out. It was shown that the TBT was decomposed within a few hours in a beaker. However, this biocide, which appeared to be ideal, subsequently proved to be extremely persistent in the environment. In anaerobic sediment, the half-lives are several years.

Chemical and physical properties: Stability of TBT

The organotin compounds have a high affinity for attaching themselves to solids, so that sediments act as ìsinksî for these pollutants. Sorption occurs mainly on the organic matrix, such as biofilms, and less on mineral surfaces. This behaviour means that no particle size effect can be observed. The individual tin compounds can be bound to both small and coarse particles, depending on their organic carbon content.

Grain size distribution of butyltin compounds

Monitoring
To classify the concentrations of pollutants in sediments, a quality classification scheme was introduced by the Wassergütestelle Elbe. It consists of seven classes based on the concentrations measured in the middle and lower part of the Elbe at the time of introduction.

Today, the classification is used for the evaluation of the Hamburg dredged material. Class IV (>250 µg Sn/kg) is set as the maximum value for the dumping of dredged material. Sediments from ports dumped in the Wadden Sea are assessed according to Class II. Organotin monitoring was started by the Wassergütesstelle Elbe in the early 1990s. Monthly mixed samples were collected and analysed at selected permanent measuring stations. The results of the investigations from 1994-1998 at two measuring stations are shown in the figures for the parameters TBT and TTBT (data from the Elbe Water Quality Station).

Bioaccumulation of butyltin species in fish

The station in the middle section of the Elbe near Bitterfeld shows the influence of an industrial emitter, the second station shows the influence of the port of Hamburg with superimpositions from the upstream current. The importance of bioaccumulation has already been considered. The figure shows the contents of tin compounds as found in different fish compartments. To evaluate the concentrations, the maximum allowable concentration in fish tissue, which is calculated from the ADI value, was shown as a dashed line.