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Ecotoxicology and Environmental Safety 56 (2003) 180189

Case study: bioavailability of tin and tin compounds

Heinz Ru del

Fraunhofer Institute for Molecular Biology and Applied Ecology (Fraunhofer IME), 57377 Schmallenberg, Germany

Received 20 March 2003; accepted 20 March 2003

Abstract

This article reviews the literature related to the bioavailability of tin, inorganic tin compounds, and organotin compounds. On the

one hand, the toxicity of metallic tin and inorganic tin compounds is low. In aqueous systems, the potential bioavailability of tin

seems to depend on the concentration of the truly dissolved ion species. Some studies suggest that tin is an essential trace element forhumans. However, organotin compounds have been proven to be of toxicological relevance. Triorganotin compounds are

particularly toxic explaining their wide use as biocides (e.g., in antifouling paints or pesticides). Persistence of organotin compounds

is governed by moderate to fast aerobic biotic degradation processes, slow anaerobic biotic degradation, slow abiotic degradation by

photolysis, and fast, but reversible, adsorption/desorption processes. Organotin compounds are ubiquitously distributed in aquatic

organisms. Bioconcentration in organisms and ecotoxicity are dependent on the bioavailable fraction. The bioavailability is highest

at neutral and slightly alkaline pH and is reduced in the presence of dissolved organic carbon. The biomagnification of organotin

compounds via the food chain is of minor importance compared with the bioconcentration from the water phase.

r 2003 Elsevier Inc. All rights reserved.

Keywords: Availability; Bioavailability; Bioconcentration; Ecotoxicity; Monitoring; Organotin compounds; Tin; Toxicity; Tributyltin (TBT)

1. Introduction

Human beings have used tin since the Bronze Age.

For thousands of years, tin and tin alloys were used for

production of such consumer products as tin dishes or

drinking mugs. Starting with the Industrial Revolution,

inorganic tin compounds were produced for various

purposes. Around 1940 the industrial production of

organotin compounds started. The latter are currently

technically and economically important, for example,

as biocides and plastic stabilizers.

The primary intention of this article is to discuss the

bioavailability of tin and its compounds in the environ-

ment. Emphasis is placed on the organotin compounds,

which are of high toxicological relevance and for which

the database is most extensive. Depending on environ-

mental conditions, organotin compounds exist as

neutral ion pairs and complexes or as cations in aquatic

systems. Therefore, different concepts are applicable

for evaluation of their bioavailability.

This review uses the definitions for the terms

availability and bioavailability of metal ions in aquatic

systems given by Di Toro et al. (2001). They define

availability as the fraction of the total metal in the

water column or sediment compartments that is un-

bound, free, or available for uptake by an organism.

Bioavailability refers to the fraction of the total metal

that is taken up by an organism and subsequently

transported to a site of action/receptor, or target organ.

However, other authors (e.g., Hare, 1992; Mackay and

Fraser, 2000) use the term bioavailability in a broader

sense, comparable to the availability definition ofDi

Toro et al. (2001). Availability may also be designated

as potential bioavailability.

A definition of bioavailability of pollutants in soils

and sediments was given byPeijnenburg et al. (1997a, b

and elsewhere in this issue). These authors suggest

bioavailability to be a dynamic process with the

distinction of two different phases: a physicochemically

driven desorption and a physiologically driven uptake

process. Although this definition was developed for soils

and sediments, it may be applicable to the aquatic phase

as well, because most compounds are adsorbed to

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particles in the water phase (e.g., to dissolved organic

matter, DOM) and are not truly dissolved. For such

truly dissolved compounds (free ions, nonadsorbed

organic compounds), only the second step would be

relevant. The concept of the two subsequent phases has

some similarities with the availability/bioavailability

approach ofDi Toro et al. (2001).From the cited definitions it is obvious that bioavail-

ability is not a scaled property, specific to a certain

compound. It can only be estimated by comparison

of definite experimental situations or in relation to the

bioavailability of other compounds. For example,

bioavailability of a compound in a compartment may

be determined in relation to an ecotoxicological end-

point or to bioaccumulation in an organism. Because

of the high experimental expenditure for these types

of investigations, in many studies a pragmatic approach

is taken to estimate bioavailable fractions. Because a

number of aquatic studies for different compounds

had shown that a correlation exists between the

concentration of freely dissolved compounds in the

water phase and bioconcentration and effects in

organisms, it is assumed that the fraction of compounds

that is freely dissolved is potentially bioavailable

(as discussed for organic chemicals in Haitzer et al.,

1998, or for metal compounds in Di Toro et al., 2001).

However, the presence of DOM and other factors

may reduce the fraction of the compounds that is

freely dissolved (Haitzer et al., 1998; Di Toro et al.,

2001).

The dissolved fraction in aquatic studies is often

determined operationally as the fraction of a compoundthat passes through a 0.45-mm membrane filter. This

approach is also used in international standards for the

determination of metals in water (e.g.,ISO 11885, 1996)

or for the quantification of dissolved organic carbon

in water (EN 1484, 1997). The exact measurement of

the truly dissolved fraction is possible only with high

experimental expenditure or for some compounds

with special techniques (e.g., free copper ions with an

ion-selective electrode). For soils and sediments, bioa-

vailable fractions are often correlated with certain

extraction procedures (with aqueous solutions or

organic solvents). Therefore, extractability is sometimes

used synonymously for bioavailability, or attempts are

made to correlate bioavailability for an organism with

an extraction procedure with a certain solvent (e.g.,Reid

et al., 2000).

This article discusses in separate sections the bioavail-

ability of metallic tin, inorganic tin compounds, and

organotin compounds. In case of the organotin com-

pounds most data are available for tributyltin (TBT).

This literature review is mainly based on information

compiled byBulten and Meinema (1991)for tin and tin

compounds in general, and byFent (1996)andMaguire

(1996) for organotin compounds.

2. Analysis of tin and organotin compounds

A prerequisite for bioavailability studies is a sensitive

and precise analytical method. In case of tin speciation

analyses are necessary to distinguish among the several

tin species. After dissolution of tin and tin alloys in acids

and acid digestion of inorganic tin compounds inenvironmental or biological samples, the analysis may

make use of atomic absorption spectrometry (AAS).

Other methods are inductively coupled plasmaoptical

emission spectrometry (ICPOES), inductively coupled

plasmamass spectrometry (ICPMS), or at low con-

centration levels by hydride generation coupled to AAS

or to ICPMS. Solid samples may be analyzed directly

by X-ray fluorescence spectrometry. Organotin com-

pounds are mostly analyzed after extraction or digestion

applying either the Grignard method (e.g., derivatiza-

tion with pentylmagnesium bromide), or the ethylborate

method. The resulting derivatives from both methods

are volatile and can be analyzed by gas chromatographic

(GC) methods and detection with atomic emission

detection (GC-AED) or mass spectrometry (GC-MS).

A comprehensive technical report of the IUPAC

Commission on Microchemical Techniques and Trace

Analysis on the determination of tin species in the

environment was published recently (Leroy et al., 1998).

Other articles on organotin analysis includeDirkx et al.

(1995), Morabito et al. (2000), and Pellegrino et al.

(2000). Quevauviller et al. (2000) reported on measure-

ments of organotin compounds in environmental

reference materials (mussels, sediments). A number of

certified reference materials for organotin compoundshave been produced (e.g., by BCR or NIST) that allow

the validation of laboratory methods and internal

quality control.

3. Bioavailability of metallic tin and tin alloys

The world smelting output of tin in 1990 was

B225,000 metric tons per year (Graf, 1996). Tin is

generally considered to be nontoxic in its metallic form.

Cases of poisoning with tin are almost unknown,

because ingested tin is poorly absorbed by organisms.

However, massive inhalation of tin by exposed indus-

trial workers may lead to irritation of the respiratory

tract (Graf, 1996).

Although the author knows of no studies on the

bioavailability of metallic tin, it can be assumed that the

bioavailability is low. Only small amounts of tin may be

(chemically) dissolved from tin materials or tin alloys in

the environment (e.g., by acid waters). The resulting tin

ions may then be bioavailable (refer to the next section).

Small amounts of tin also may be dissolved in food in

contact with tin or tin-plated materials and become

potentially bioavailable.

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4. Bioavailability of inorganic tin compounds

A large number of inorganic tin compounds are

known. In these tin compounds the tin atom is either

divalent (stannous) or tetravalent (stannic). Examples

for industrially important compounds are tin(II) chlor-

ide and tin(II) sulfate, which are used for the plating ofsteel; tin(II) fluoride as a compound in some toothpastes

(to prevent tooth decay); tin(IV) oxide in combination

with other pigments as a ceramic colorant; and tin(IV)

chloride as a basic compound in organotin syntheses

(Graf, 1996; Bulten and Meinema, 1991). The world

consumption of inorganic tin is assumed to be o40,000

metric tons per year (Graf, 1996).

Concentrations of inorganic tin in air, soil, and waters

are usually low (except in areas with minerals containing

high levels of tin or in the surroundings of tin processing

industries). Tin toxicities of natural origin in plants,

animals, or humans have not been reported (Bulten and

Meinema, 1991). In aqueous solutions tin(IV) is more

stable than tin(II), which can be oxidized to tin(IV).

Evaluation of inorganic salts of tin revealed only low

toxicities to organisms. Ingested amounts of inorganic

tin salts are only poorly adsorbed (low bioavailability).

For example, only 2.85% of tin(II) and 0.64% of tin(IV)

were absorbed in experiments. This small part entering

the organism was later completely eliminated via the

urine (Bulten and Meinema, 1991).

Only few ecotoxicological studies with inorganic tin

compounds are published. Generally, the toxicity seems

to be low. With an EC50 (i.e., the effect concentration,

where 50% of tested organisms show an effect) of22 mg/L, only a low toxicity was observed with daphnids

when investigating the immobilization (B200 times

higher concentrations of Sn2+ were necessary in

comparison with the EC50 of Cu2+; Khangarot and

Ray, 1989). A study byPawlik-Skowronska et al. (1997)

found that tin(II) and tin(IV) salts inhibited the growth

of a planktonic cyanobacterium. Toxicity grew with

increasing tin concentrations, augmenting both pH

values and test duration; tin(II) seemed to be more

toxic than tin(IV). The presence of humic acids reduced

the toxicity of tin. At high pH values, anionic tin species

like SnO3H, SnO

3

2, or Sn(OH)6

2 exist, while at

neutral or acidic pH values cationic or neutral tin

species like Sn(OH)+, Sn(OH)22+, Sn(OH)2, or SnO are

present (Pawlik-Skowronska et al., 1997). The free Sn2+

ion is not stable at the high pH values tested (stability

occurs only below BpH 4;Pettine et al., 1981).

It is not clear if the toxicity of tin(II) and tin(IV) ions

can also be described using the free-ion activity model

(FIAM;Morel, 1983; Peijnenburg, 2003 this issue). This

model is used to understand the bioavailability of metal

ions and is based on the assumption that the toxicity of

metal ions in water is correlated with the concentration

of the free metal ions and not to the concentration of the

total metal ion fraction, which also includes ions

adsorbed to or complexed by particulate matter or

DOM. In general, the actual concentration of the free

metal ion is mainly dependent on the water parameters

pH, hardness, and DOM. This was demonstrated for

copper byErickson et al. (1996), although exceptions to

this model are known (Campbell, 1995). Thus it is clearthat the bioavailable fraction of tin as of any other metal

ions is not a constant but is multifactorially influenced.

Some studies suggest that tin is an essential trace

element for humans (possibly as an ionic constituent of

gastrine, a stomach-stimulating peptide hormone).

Natural foods contain trace amounts of tin. It is

assumed that the average daily intake is in the range

0.21 mg (Bulten and Meinema, 1991). In feeding

experiments levels of 0.52 ppm of tin in the diet

improved growth of rats by 2560%. Chloride, sulfate,

and orthophosphate salts of tin had no toxic effects in

rats after feeding diets with 450650 ppm tin for 13

weeks. Inorganic tin does not induce teratogenic or

carcinogenic effects (Bulten and Meinema, 1991).

It is assumed that under certain environmental

conditions methylation of inorganic tin by microorgan-

isms takes place (Gadd, 2000). The occurrence of

methyltin compounds in estuarine and coastal environ-

ments was monitored byAmouroux et al. (2000). In the

past some authors doubted that natural methylation of

tin by microorganisms occurs (Bulten and Meinema,

1991).

5. Bioavailability of organotin compounds

5.1. Properties

Organotin compounds are chemicals that possess

at least one tincarbon bond. The tin atom is tetravalent

in all organotin compounds produced industrially. The

general formula for these organotin compounds is

R(4n)SnXn with n 023: The organic groups R arealkyl or aryl groups that are bound covalently with the

central tin atom. X represents such anions as OH,SH, OSnR3, or OR

0. Important chemicals are the

mono-, di- and trisubstituted butyltin and phenyltin

compounds (MBT, DBT, TBT, MPT, DPT, TPT).

Because of the hydrocarbon substituents, organotin

compounds are hydrophobic. The extent of hydropho-

bicity depends on the degree of alkylation/arylation at

the central tin atom (number of groups, length of alkyl

chain).

The water solubilities of most organotin compounds

are low and dependent on pH, ionic strength, and

temperature. Data for TBT-Cl are in the range from 5

to 50 mg/L, whereas the water solubility of DBT-Cl2 is

higher, up to 92 mg/L (Reincke et al., 1999). Depending

on environmental conditions, organotin compounds

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in antifouling systems on ships by January 2003, and a

complete prohibition by January 2008.

Organotin compounds are released through several

routes into the environment (Fent, 1996). The major

input of triorganotin compounds into aquatic systems

derives from their use in antifouling paints. TBT is used

mostly in so-called self-polishing copolymers thatrelease TBT continuously. Harbor areas are especially

affected by TBT contamination. In harbor sediments,

flakes of antifouling paints from the removal of old

coatings may be present and may serve as reservoirs that

cause locally high concentrations of TBT. For other

compounds such as triphenyltin, the input via their use

as pesticides in agriculture is more important. Waters

may be contaminated with organotin compounds by

effluents from industrial plants. Further inputs to the

environment result from the large-scale use of polyvinyl

chloride (PVC), which contains mono- and diorganotin

compounds as stabilizers. Leachate from landfills where

organotin-containing wastes are dumped may contain

organotin residues, as well as municipal wastewater and

sewage sludge. However, in view of the low water

solubility of most industrially produced organotin

compounds and their strong tendency to adsorb to

sediments, substantial widespread surface water con-

tamination from these sources is unlikely (Bulten and

Meinema, 1991). Further, as a result of the similar

strong adsorption to soil, leaching from and transport in

soil do not take place to a measurable extent (Bulten and

Meinema, 1991). From the organotin compounds only

the methyl derivatives are considerable volatile (e.g.,

tetramethyltin). Further, for bis(tributyl)tin oxide(TBTO), a co-distillation with water may occur (Bulten

and Meinema, 1991). Releases into the environment

from waste incineration seem to be of minor importance

(emission products probably are inorganic tin com-

pounds). In general, the contamination of the atmo-

sphere with organotin compounds is assumed to be

low. A more detailed description of releases into the

environment is given in Fent (1996).

Concentrations of organotin compounds in organisms

are particularly high near sources such as commercial

ports, pleasure-boat marinas, shipyards, much-traveled

shipping routes, as well as industrial manufacturers and

processing plants of organotin compounds. In areas free

from sources the loads are lower, with TBT concentra-

tions o10 mg/kg wet wt. (Tanabe et al., 1998). In coastal

areas as well as deep sea, organotin compounds in

organisms are detectable.

As stated above, only a few studies on the bioavail-

ability of organotin compounds are available. However,

numerous monitoring studies demonstrate that organo-

tin compounds are bioavailable in marine and limnic

systems (refer to the data compilation by Maguire,

1996). As an example, data from a monitoring study

using samples from the German Environmental Speci-

men Bank (ESB; Ru del et al., 1999) are presented in

Table 1. ESB samples are taken on a regular basis from

representative ecosystems in Germany following stan-

dard operating procedures (BMU, 2000;UBA, 1996).

The TBT concentrations in marine biota from the

ESB sampling sites of the North Sea remained nearly

constant between 1985 and 1998. In the rivers a time-dependent decrease was obvious, which probably is a

result of the 1990 ban on TBT-containing antifouling

paints for small boats. For TPT a clear decrease was

observed in the marine samples between 1985 and 1998.

This decrease is correlated to the cessation of use of TPT

as a co-toxicant in antifouling paints in 1985. Such a

decrease was not observed in the Rhine and Elbe rivers.

Here the entry of TPT seems to be correlated to the use

of TPT as fungicide (e.g., for application on potatoes).

The concentration data from different trophic levels

of the North Sea suggest that there seems to be no

biomagnification of organotin compounds in this

ecological system. Sta b et al. (1996) have reported

similar results for a limnic ecosystem; see later.

5.4. Bioaccumulation

There are various pathways by which an organism

may take up organotin compounds. The uptake from

the water or sediment phase via the body surface is

referred to as bioconcentration. Uptake via the food

chain is designated as biomagnification. Accumulation,

the result of both pathways, is often proportional to the

concentration of the compound in the environment. The

extent of bioaccumulation is further influenced bybiodegradation/excretion mechanisms of the respective

organism. Bioconcentration factors (BCFs) for organo-

tin compounds vary considerably, most likely as a result

of different environmental conditions and different

taxonomic groups. BCFs for TBT range from o1 up

to 152,000 (range of data as cited inRu del et al., 1999).

Highest BCFs were observed when very low concentra-

tions of TBT were applied in the test systems. DBT and

MBT showed a lower tendency to bioaccumulation.

Biomagnification of organotin compounds over the

food web has also been examined. For the crab

Rhitropanopeus harrisii, the enrichment of TBTO in

the organism was investigated after exposure via water

or via food (as cited in Alzieu, 1996). After 4 days of

feeding with contaminated food, the biomagnification

factor amounted to 4400 in the hepatopancreas and

between 500 and 1300 in other tissues. When dosing via

water, the BCFs were lower by factors of 1030. For the

common mussel Mytilus edulis, a BCF of 5000 was

observed with TBT uptake via the water phase, whereas

a biomagnification factor of only 2 was calculated upon

feeding with contaminated algae (Laughlin et al., 1986).

Mensink et al. (1997) calculated that the North Sea

whelkBuccinum undatumenriches TPT in relation to the

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mussel Mytilus edulis by a factor of B8. In contrast,

TBT concentrations were lower in the snail by a factor

ofB7 as compared with mussels (no biomagnification).

Sta b et al. (1996) analyzed butyltin and phenyltin

compounds in the food web of Netherlands limnic

waters. In the examined waterbirds (top predators), they

found lower TBT and TPT concentrations than in the

species from lower levels of the trophic system such as

fish, mussels, and crustaceans. Thus, there seemed to be

no biomagnification in this ecological system. In general

the availability/bioavailability of organotin compounds

via the food chain seems to be of minor importance for

TBT and TPT as compared with uptake via the water

phase.

5.5. Ecotoxicity

In comparison with inorganic tin compounds, some

organotin compounds are highly toxic. The toxicity of

the different organotin compounds is related to ex-

posure concentration and duration, bioavailability, and

the sensitivity of the organisms. The endocrine disrup-

tion properties of TBT and TPT in certain aquatic

organisms are of major concern. Observed endocrine

effects are pathomorphological transformations of the

genital organs (designated as superimposed sex or

imposex). Endocrine effects were observed at levels of

B1ng/L TBT (Gibbs and Bryan, 1996). TPT is

suspected to have a similar potential disruptive endo-

crine effect (Horiguchi et al., 1997;UBA, 2000).

The chronic toxicity of TBT is also high. The German

Federal Environmental Agency (Umweltbundesamt;

UBA, 2000) uses the following data for their assessment

of TBT and TPT. For a 90-day fish test the no

observed effect concentration (NOEC) was reported to

be 10 ng/L (freshwater fish Poecilia reticulata). Studies

with rainbow trout showed NOECs of 24mg/L after 28

days. TPT toxicity seems to be similarly high. Effect

concentrations were 3.9 mg/L (LC50, i.e., lethal concen-

tration for 50% of exposedPimephales promelaslarvae)

and 0.15mg/L (early life stage test). Effects on plankton

or oysters were observed for TBT at ng/L concentration

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Table 1

Concentration ranges for different organism groups and different sampling sites

Biota/origina Minimum concentration

(mg/kg ww)

Maximum concentration

(mg/kg ww)

Years Comment

TBT

Brown algae/North Sean 16 2 6 19851996 No trend

Common mussels/North Sean 18 4 21 19851996 No trendEel pout/North Sean 5 11 22 19941998 No trendSeagull eggs/North Sea n 3 o1b 4 19941998 No trendZebra mussel/Rhinen 4 4 14 1996 Concentrations increased

downstream

Bream (muscle)/Rhinen 8 11 37 19961998 Concentrations increaseddownstream and decreased

with time

Zebra mussel/Elben 1 940 1996 Site near harborBream (muscle)/Elben 17 25 470 19931998 Concentrations increased

downstream and decreased

with time

TPT

Brown algae/North Sean 16 o5b 14 19851996 Concentrations decreased

with timeCommon mussels/North Sean 18 o5b 98 19851996 Concentrations decreased

with time

Eel pout/North Sean 5 27 60 19941998 Concentrations decreasedwith time

Seagull eggs/North Sea n 3 o5b o5a 19941998 No trendZebra mussel/Rhinen 4 o5b 10 1996 Concentrations decreased

downstream

Bream (muscle)/Rhinen 8 o5b 53 19961998 Concentrations increaseddownstream and with time

Zebra mussel/Elben 1 15 1996 Site near harborBream (muscle)/Elben 17 o5b 253 19931998 Concentrations increased

downstream

Data are given as mg/kg of the respective organotin cation and refer to wet weight (ww).

Source: Environmental specimen bank (Ru

del et al., 1999).an=number of analyses.bBelow the respective limit of determination.


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levels (Alzieu, 1996). For a more detailed discussion of

effects and compilations of data refer to Alzieu (1996)

and Fent (1996).

5.6. Toxicity

The lowest documented toxicological endpoint ofbutyltin compounds is a depression of the immune

system of the thyroid gland by TBTO. The World

Health Organization (WHO) gives a lowest observed

adverse effect level (LOAEL) of 0.25 mg/kg body

weight per day (WHO, 1999). With a safety factor of

10, a no observed adverse effect level (NOAEL) value

of 0.025 mg/kg body weight per day was calculated by

the German Federal Institute for Health Protection of

Consumers and Veterinary Medicine (BGVV, 2000).

With a safety factor of 100, a tolerable daily intake of

0.25mg/kg body weight was stated. Because for DBT no

complete assessment is possible, the authority uses

preliminarily the same total daily intake value as for

TBT until a better data basis becomes available (BGVV,

2000).

Data on the toxicity of organotin compounds to

humans are available from accidental exposures. The

methyl compounds are particularly toxic (Bulten and

Meinema, 1991). In a recent Chinese report of deadly

poisonings after meals prepared from methyltin-con-

taminated food (Gui-Bin et al., 2000), analyses of the

inner organs of one victim yielded high concentration

levels of methyltins in liver (1.93 mg/g DMT, 1.42 mg/g

TMT), kidney (1.05mg/g DMT, 0.47 mg/g TMT), sto-

mach (0.104mg/g DMT, 0.304mg/g TMT), and heart(0.1 mg/g DMT, 1.48mg/g TMT). The respective con-

centrations for a control person were below the limit of

detection. Concentrations of methyltin compounds in

the contaminated food were not given.

Kannan et al. (1999) analyzed the concentrations of

organotin compounds in the blood of 32 Americans

of different origin and age. The mean concentrations

detected were 8 ng/mL MBT (present in 53% of the

samples), 5 ng/mL DBT (81%), and 8 ng/mL TBT

(71%). Levels ranged from concentrations below the

limit of determination up to 101 ng/mL of total butyltins

in blood. The authors assume that the residues are due

to exposures of humans to organotin compounds as

stabilizers or as biocides in household articles. The

toxicological relevance of the observed contamination

levels are unknown.

A similar human monitoring study was recently

conducted with blood samples from the Environmental

Specimen Bank/Human Specimen Bank in Germany.

Only low levels of organotin compounds were detected

(Ru del and Steinhanses, 2001). The investigation

comprised blood samples from student collectives from

two German cities. From the 30 samples analyzed, only

one sample showed a concentration above the limit of

determination for DBT (1.6 ng/mL; limit of determina-

tion, 0.4 ng/mL blood). In 5 samples MBT was found

at levels above the limit of determination (range: 0.7

1.4 ng/mL; limit of determination, 0.3ng/mL blood).

Concentrations of the other compounds analyzed were

below the respective limits of determination

(TBTo0.3 ng/mL; TPTo0.4 ng/mL; DPTo0.3 ng/mL;MPTo 1 ng/mL).

5.7. Factors determining bioavailability

Generally, the parameters that significantly influence

the bioavailability of metals in natural waters and

sediments are hardness, alkalinity, pH, temperature,

oxidation/reduction potential, composition and concen-

tration of other ions, particulate matter, and organic

carbon content. Cation exchange capacity also might

influence the bioavailability in soils and sediments. For

metal ions the pH is the most important factor

controlling partitioning (Di Toro et al., 2001). The most

important phases for interactions with metal ions are the

organic carbon and the metal oxides of the sediment or

soil. For organic compounds the most decisive para-

meter is the organic carbon content in the respective

compartment. In case of polar compounds the pH value

may also be important. For organotin compounds only

a few systematic studies are available on the influences

of pH values and organic matter content on bioavail-

ability.

Fent (1996) and Looser et al. (1998) presented data

on the influence of the pH value on the bioavailability

of organotin compounds in aquatic test systems. Theystudied the uptake and bioconcentration of TBT-Cl and

TPT-Cl in Daphnia magna, fish larvae of Thymallus

thymallus, and the sediment organism Chironomus

riparius. BCFs were B2000 for the fish larvae, 680 for

the sediment organism, and 220 for the daphnids. A

correlation to the lipid content of the organisms was not

found. An important result of the study was that the

BCFs of TBT were higher at pH 8 as compared with pH

5. This correlates with the stability of the neutral TBT

complexes or ion pairs such as TBT-OH or TBT-Cl. The

difference in BCF was expected to be larger when

assuming the octanol-water partition coefficients deter-

mined for the compounds at those pH values (Arnold

et al., 1997). Therefore, it can be assumed that besides

the neutral TBT-Cl/TPT-Cl the TBT/TPT cations were

also taken up by the organisms. However, the neutral,

nondissociated molecules seemed to be more bioavail-

able than the respective organotin cations. Fent (1996)

assumed that the neutral TBT-OH can penetrate

biomembranes more easily than the charged hydrophilic

cations. Another study with fish yielded similar results.

Tsuda et al. (1990) found that the bioaccumulation of

TBT and TPT in carp was higher at more alkaline pH

values as compared with pH values o7.

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As discussed earlier, organotin compounds such as

TBT and TPT exist as neutral nondissociated molecules

at pH values above their pKavalue of 56.5. Because the

pH value of many limnic waters (e.g., in Germany) is

47 and of seawater 48, it is assumed that large

fractions of TBT and TPT in aquatic systems are in the

nondissociated form and therefore are potentiallybioavailable.

In addition to pH, another important variable

influencing bioavailability that was investigated in detail

for organotin compounds is the presence of organic

matter. For organic chemicals,Haitzer et al. (1998)have

compiled a comprehensive review on the influence of

DOM on the bioconcentration. At higher concentra-

tions of DOM, decreases of bioconcentration of 298%

in relation to the controls were found. In contrast, at

low levels of DOM some studies found inexplicable

increases in bioconcentration. Looser et al. (1998) and

Fent (1996) presented evidence that increasing humic

acids concentrations also caused a decrease in biocon-

centration of TBT and TPT. They found that low

concentrations of dissolved humic acids led to small

reductions in the bioconcentration of TPT in daphnids

and fish larvae. At concentration levels of410 mg/L

dissolved organic carbon, the reduction of bioconcen-

tration was significant. For T. thymallus larvae, for

example, the BCF was reduced by nearly 50% by

increasing the organic carbon content from 2 to 13 mg/

L. Further, the uptake times increased, resulting in

longer periods until equilibration was achieved. Fent

(1996) interpreted the results in the way that TBT

interacts with the humic acids, which affects its chemicalspeciation and partitioning. TBT and TPT may form

complexes with carboxyl, amino, or thiol groups of the

organic matter. The observed behavior of the organotin

compounds is similar to the general finding that the

binding of an organic chemical to the organic carbon

phase depends mainly on its hydrophobicity.

An important aspect of the study of Fent (1996) is

that because the applied organic carbon contents (DOM

levels) are those usual in ambient waters, it can be

assumed that only a portion of TBT/TPT is freely

dissolved. The other fraction is bound in organic

complexes and therefore assumed to be not available

for uptake via epithelial surfaces of gills or skin of biota.

The complexes may be too large or too polar to cross

cell membranes (Fent, 1996). Therefore, similar to metal

ions, only the freely dissolved fraction of the organotin

compounds seems to be potentially bioavailable.

6. Conclusion

Tin exists mainly in the oxidation states Sn(0), Sn(II),

and Sn(IV). The toxicity of metallic tin and inorganic tin

compounds on the one hand is low. Some studies even

suggest that tin is an essential trace element for humans.

Organotin compounds, on the other hand, are of high

toxicological relevance. The triorganotin compounds

are especially toxic, which is the reason for their wide

use in antifouling paints or as fungicides. Persistence of

organotin compounds is governed by moderate to fast

biotic degradation processes under aerobic conditions,slow anaerobic biotic degradation, slow abiotic degra-

dation especially by photolysis, and fast but reversible

adsorption/desorption processes. Bioconcentration in

organisms and ecotoxicity of the compounds are related

to the bioavailable fraction of the organotin com-

pounds. Generally, bioavailability is influenced by such

actual environmental conditions as ion composition of

the aqueous environment, pH, dissolved organic carbon

content, and presence of competing compounds. The

bioavailability of organotin compounds is influenced

mainly by the pH value and by the presence of humic

acids/dissolved organic carbon. Bioconcentration fac-

tors of both TBT and TPT were higher at alkaline pH

values as compared with acidic pH values, and increas-

ing humic acids concentrations caused a decrease in

bioconcentration of TBT and TPT in aquatic organisms.

Effect data are available for a large number of

organisms and organotin compounds. Some organisms

including certain marine snails show sensitive endocrine

effects (imposex) at ng/L concentration levels of

triorganotin compounds such as TBT and TPT.

Monitoring data reveal that organotin compounds are

ubiquitous in aquatic organisms, proving the bioavail-

ability of organotin compounds in the environment.

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