Tributyltin (TBT) compounds are extremely poisonous, an attribute that has seen them utilized as the active ingredient in marine anti-fouling paint formulations. The potency of TBT ensures good fuel efficiencies during ship operation and long lifetimes between repainting for both boats and structures used in mariculture. However, its extreme toxicity has resulted in numerous adverse biological effects on non-target organisms, of particular importance being the shell deformations observed in oysters and the masculinization of female marine snails. The environmental persistence of TBT ensures that such problems are likely to continue for some time to come. In the first instance, this book describes the manufacture and industrial applications of TBT compounds, reviews their distribution and behaviour in the environment, and summarises their deleterious effects on organisms and aquatic ecosystems.

The widespread use of these anti-fouling paints has meant that TBT has become a contaminant of global concern. Many countries have taken diverse steps to regulate the use of TBT-based products in order to protect coastal and fresh water ecosystems, together with their resources. The political response to this pollutant has been notably faster and more widespread globally than that shown towards other pollutants in the environment, such as lead, for example, despite the fact that public health was never threatened. The steps taken have not been without controversy, considering the economic advantages to shipping on the one hand and the ecological damage on the other hand.

Introduction

Earlier chapters have outlined the extensive usage of tributyltin (TBT) in marine antifouling paints which commenced in Europe around 1960, almost a decade after its biocidal properties were first reported (Bennett, 1983). Almost 30 years later it has been banned from use on small craft, to protect shellfish, such as oysters, mussels and scallops, and gastropods, from its impacts in confined waterways. The legacy of the unknown quantities of TBT that have leached from many square kilometres of painted surfaces in waters worldwide, has been that sediments in the vicinity of marinas, dockyards, wharves and other areas of high boating activity now contain measurable concentrations of TBT. This derives not only from paint scrapings, but from the preferential partitioning of dissolved TBT onto colloidal and particulate surfaces, which ultimately accumulate in bottom sediments. Once resident here, the concern is for its availability to sediment-dwelling biota, and its potential remobilisation to overlying waters.

There has been extensive research into the fate of TBT in the marine environment, and this has been reviewed by a number of authors (e.g. Laughlin & Linden, 1985; Clark, Sterritt & Lester, 1988; Muller, Renberg & Rippen, 1989; Stewart & de Mora, 1990). This includes degradation and removal processes in the water column and in sediments, as well as biotic transformations. Reactions will involve adsorption, photolysis, and both microbial and abiotic decomposition processes.

Introduction

The chemistry and toxicity of tin and organotin compounds have been extensively reviewed (e.g., Blunden and Evans, 1990; Maguire, 1987, 1991; World Health Organization, 1990; and references therein). The major uses of organotin compounds are as poly(vinyl chloride) (PVC) stabilizers, industrial catalysts, industrial and agricultural biocides, and wood preserving and antifouling agents. From an environmental point of view, tributyltin compounds used as antifouling agents are of most concern because of their extremely high toxicity to aquatic organisms. This chapter reviews the occurrence, persistence and toxicity of tributyltin, and its degradation products dibutyltin and monobutyltin, in fresh water environments.

Sources and environmental occurrence

The main use of tributyltin compounds, with direct introduction to water, is as antifouling agents. However, there are other pesticidal uses. In Canada, for example some tributyltin-containing pesticides are also registered for use as material preservatives, joinery wood preservatives, remedial wood preservatives, wood preservative stains, and as slimicides. In some parts of Africa, tributyltin compounds have been applied directly to water in the control of schistosomiasis, and they are used agriculturally against cotton-boll worms (Aboul Dahab, El-Sabrouti and Halim, 1990).

Butyltin species are not produced biologically, and therefore their environmental presence is due to anthropogenic input. In general the occurrence of butyltin species in the environment is a result of the use of tributyltin as an antifouling agent. However, the finding of tributyltin in a power plant discharge in Italy (Gabrielides et al., 1990), and in sewage treatment plant influents and effluents in Switzerland (Fent and Muller, 1991) and Canada (Chau, Zhang and Maguire, 1992) probably reflects slimicidal uses of tributyltin.

Introduction

The pesticide tributyltin (TBT) is part of a family of organotin compounds, the biocidal properties of which were first recognised in the early 1950s. TBT is extremely harmful to marine organisms. This character has been exploited with TBT used as the active ingredient in many formulations of marine antifouling paints. These paints are applied to boat hulls and other submerged structures, such as wharves, buoys, and fishpens to prevent fouling, that is, the attachment and growth of marine organisms such as barnacles, mussels, and algae. These fouling organisms contribute to corrosion and to the floating weight of vessels. They also produce turbulent flow and increase drag across the hulls of ships, thus decreasing speed and increasing fuel consumption. The antifouling paints work in the following way. Once the biocide is released from the paint film, a thin envelope of highly concentrated TBT is formed around the vessel hull. The larvae of fouling or nuisance organisms are killed or repelled when they encounter this layer, thus protecting the vessel.

Because of their extreme toxicity, and their longer useful lifespan – seven years as compared to the two years of copper alternatives – TBT compounds are the most effective antifouling agents so far devised. Their application can save vessel operators hundreds of millions of dollars annually in fuel and maintenance costs. For example, the Environmental Protection Agency (EPA) of the United States has found that the United States Navy, by using TBT copolymer antifoulants, could save an estimated US$130 million a year over the standard copper formula.

The origin and fate of butyltin compounds in the environment have been extensively studied in the past few years [1–6]. As described in Chapters 6 and 7 of this book, the toxicological impact of tributyltin (TBT) on marine organisms has been the major reason for a multitude of studies elucidating the biogeochemical pathway of this contaminant, together with its degradation products mono- and dibutyltin (MBT and DBT), in the environment. This interest and concern for the determination of butyltin compounds led to the development in the early 1980s of a wide variety of techniques which involve, in most cases, several different analytical steps: extraction, clean-up, derivatization, separation and final detection [7]. It is clear that accurate results for butyltin determinations is a prerequisite for good comparability of data and related studies on the toxicity and biogeochemical processes. However, the proliferation of multi-step techniques, although very sensitive, was not accompanied with the necessary awareness for the quality control of analysis. Only in the past five years has the availability of certified reference materials (CRMs), certified for their butyltin concentrations, allowed an improvement in the quality assurance of butyltin determinations in marine sediments. The overall quality of butyltin determinations is, however, far from being under control. Many techniques still need to be validated, some analytical steps (e.g. hydride generation) are often badly understood, and the number of CRMs is not sufficient to ensure a wide coverage of environmental and biological matrices.

A number of marine ecotoxicology studies devoted to TBT were initiated in the early 1980s following the harmful effects observed in oyster-growing areas, initially in France and later in Great Britain. This background implies that most of the data relate to mollusc shellfish, regarded as target species.

Bioaccumulation and metabolism

The presence of alkyl groups on the tin atom of TBT leads to an increase in its lipophilic properties, and thereby in an increased capacity to become bioaccumulated in living organisms. Theoretically, the octanol/water partition coefficient (Kow) provides the possibility of calculating the bioconcentration factor (BCF).

In the case of TBTO, Laughlin, French & Guard (1986) have shown that for salinity values ranging from 0 to 25 psu, Kow values vary between 7000 and 6000 respectively, while for higher salinity they range between 5000 and 6300. The above values are markedly higher than the value of 1550 found by Maguire, Carey & Hale (1983) in distilled water. Such disparity between Kow values leads to considerable uncertainties regarding the bioconcentration factor (BCF) calculated on the basis of various correlation equations. Thus, the equation of Veith, Defoe & Bergstedt (1979) (log BCF = −0.70 + 0.85 log Kow) would lead to a TBTO bioconcentration in freshwater fish ranging from approximately 100 (Kow = 1550) to 370 (Kow = 7000). Based on the equation of Mackay (1982), the BCF would range between 400 and 500.

Introduction

Prosobranch gastropods exhibit all types of sexuality but in most the sexes are separate and unchanged throughout the life of the individual (Fretter & Graham, 1962). Before the late 1960s, distinguishing males from females was a routine matter involving a simple examination to determine whether or not a specimen possessed a penis. The situation was radically changed around 1970 when surveys of populations of four neogastropod species in different parts of the world (both coasts of the United States and Atlantic Europe) revealed that some females were now penis-bearing (see Gibbs, Pascoe & Bryan, 1991c). In his short note on one of these species, the nassariid Ilyanassa obsoleta, Smith (1971) coined the term ‘imposex’ to describe ‘a superimposition of male characters onto unparasitized and parasitized females’; his subsequent work (Smith, 1980, 1981a–d) established the primary cause of female masculinization (development of a penis and vas deferens (sperm duct)) to be exposure to the leachates of marine antifouling preparations containing tributyltin (TBT) compounds. In the case of I. obsoleta, no major effects on population ecology and reproduction were detectable, but later studies of other species demonstrated that imposex resulting from exposure to remarkably low TBT concentrations can, in fact, profoundly disrupt breeding activity and lead to population extinctions on a broad scale.

Marine antifouling paints containing tributyltin (TBT) compounds were introduced in the mid-1960s and rapidly gained popularity because of both their effectiveness and ease of maintenance.

Organotin compounds

Although organotin compounds were first identified in the mid nineteenth century, industrial development of these chemicals did not occur until nearly a century later. Over the past 40 years world production and usage of organotin compounds has grown to a level that is currently estimated to be at around 50 000 tonnes per annum. The major industrial application of organotin compounds is with the mono- and di- derivatives, which are used as heat and light stabilizers in PVC processing. Mono/dibutyltin derivatives find widespread application in pipe, sheeting and other rigid PVC manufacture. Octyltin thioglycolate derivatives are approved worldwide as stabilizers for PVC used in food packaging materials and in bottles used for potable liquids. Thio-organotin stabilizers have the unique advantage of producing crystal clear PVC.

Triorganotin compounds are biologically active. They represent a smaller tonnage market – between 15 and 20% of total organotin world manufacture. Triorganotin compounds have an important place in the agrochemical field as fungicides and miticides, with the production and use of about 5000 tonnes per annum. Triphenyltin compounds (the acetate and hydroxide) are of particular importance in potato crop blight control. There was usage of triphenyltin derivatives in anti-fouling paint manufacture during the period 1960–85. Such usage has virtually disappeared from all world markets. Trineophyltin and tricyclohexyltin derivatives are used as miticides/acaricides. Tributyltin compounds were first examined for their biological properties in the early 1950s at the Institute for Organic Chemistry TNO, Utrecht, Holland (van der Kerk & Luijten, 1954).

Introduction

Tributyltin is not a compound in its own right, but only a constituent part of molecules in this class of organotin substances. It comprises three n-butyl chains attached to a single tin atom via covalent Sn–C bonds. As the tin exists in the form Sn(IV), the moiety displays an univalent positive charge. Commercial products are typically available in the form of bis(tributyltin) oxide (known as TBTO), acetate (TBT–OAc), halides (TBTF, TBTCI) and as the co-polymer with methylmethacrylate (TBTM). Industrial applications of TBT compounds followed the recognition of its biocidal properties, first noted in the early 1950s (Bennett, 1983). Although present applications include molluscicides, stone preservation, and disinfectants, the most important usage remains in wood preservatives and anti-fouling paints (Blunden & Evans, 1990). The production and use of TBT compounds in these two contexts are reviewed here in Chapter 2. Several other organotin compounds are routinely utilised in agricultural biocides, catalysts and as heat and light stabilisers for PVC. Reviews of environmental behaviour of such substances can be found elsewhere (Blunden, Hobbs & Smith, 1984; Maguire, 1991).

Because TBT can act as a broad spectrum biocide, it is useful in the preservation of wood, which is subject to decomposition via bacteria, fungi and insects. Unprotected soft woods degrade relatively quickly giving useful lifetimes of only one to two years. TBT treatment can extend this time to decades. TBT formulations have also proved useful for the preservation of marine timbers (Hill & Killmeyer, 1988).

Introduction

The tri(n-butyl)tin (TBT) cation, the active ingredient in some marine antifouling paints, is perhaps the most toxic substance ever deliberately introduced into natural waters (Goldberg, 1986). At seawater concentrations of only a few nanograms per litre, TBT disrupts reproduction in some molluscs, notably neogastropod whelks; and at levels of 10–100ng l−1 is toxic to many embryonic and larval organisms (Bryan and Gibbs, 1991). Throughout the 1980s, concentrations of TBT exceeded these levels in inshore waters at sites throughout the world, and in recognition of the accelerating ecosystem damage caused by TBT contamination, several countries legislated controls on its use as an antifouling biocide.

The hazard posed by a toxic substance is a function not only of environmental concentrations and its intrinsic toxicity, but also of its persistence. The resistance of a compound to environmental degradation processes increases the potential for long-range transport and uptake by non-target organisms.

TBT can now be considered to have exceeded initial estimates of its persistence and mobility in the marine environment, as its influence in offshore waters has been noted in several recent studies. Seas around the United Kingdom are perhaps the most intensively studied. TBT has been detected in the open surface waters of the North Sea (Coghlan, 1990). Although a survey of contaminants in seawater around England and Wales (Law et al., 1994) found TBT in only two out of six offshore samples, both sites were located offshore from major shipping rivers.

Atoms and atomic structure

The atom

For many years it was speculated that the ultimate state of subdivision of matter was discrete indivisible particles termed atoms. In the early nineteenth century John Dalton gave quantitative support to these ideas by assigning a self-consistent set of masses to atoms of elements. Elements are substances which cannot be decomposed by ordinary types of chemical change, or made by chemical union. However, subsequent work by Faraday, Crookes and Goldstein pointed out the electrical nature of atoms and that they are divisible. The simplest picture of an atom now shows a central nucleus surrounded by a cloud of electrons.

Electrons

When a neutral atom is supplied with sufficient energy it will ionise yielding positively and negatively charged fragments. The former, the positive ion, is characteristic of the particular parent atom, whilst the negative fragment, the electron, is the same irrespective of the parent atom. The charge and mass of electrons can be determined by studying their behaviour in electrical and magnetic fields. Early experiments revealed the ratio of charge to mass, e/m, which for all electrons is 1.7588 × 108 coulombs per gram (Cg−1).

To evaluate the mass of an electron, experiments were carried out to determine the applied potential required to prevent oil droplets carrying negative charges from settling due to gravity.

Natural abundance of the elements

Our knowledge about the elemental composition of the earth is restricted to the composition of the crust, atmosphere and oceans. Although the mantle and core represent over 99% of the earth's mass, their elemental compositions are not accurately known. Hence natural abundances of elements quoted in Table 3.1 refer only to the crust. Since the crust (35 km average thickness under continents and 10km under oceans) is not homogeneous and different parts of the earth contain different minerals, these values are averages of a large number of estimations. The relative homogeneity of ocean waters (Tables 6.3 and 6.4) and the atmosphere (Table 3.2) renders their compositions far less variable for major components.

Oxygen is the major element in the crust, with silicon coming second in abundance. Formation processes may have enriched the crust with certain elements in comparison with the mantle and core. The water and air masses are very different in composition from the crust, and, although elemental abundances may vary slightly with water depth or atmospheric height, the overall relative proportions of the major elements remain almost constant in the oceans and in the lower and middle atmosphere.

Although many problems in environmental chemistry are related to the abundance of elements in the environment, the chemical behaviour and properties of elements are largely independent of their abundances.

All four authors of this book are involved in teaching and/or research in environmental science and ecology. In the course of this work, we have found no shortage of advanced books in specialised aspects of our subject, and the availability of teaching texts on specific aspects of environmental chemistry has improved markedly in recent years. There does, however, in our view continue to be a need for a basic book covering the chemistry necessary to comprehend more specialised books on chemical aspects of environmental science and ecology. This book is intended to fulfil that need.

In preparing the book, we have been struck by the enormous range of aspects of chemistry which are involved in studying the environment; all major branches of the subject are involved to some degree. This has made our task more difficult and explains the involvement of such a large band of authors, needed to give expert coverage to all the topics included. Although specific individuals have prepared first drafts of whole or part chapters, these have been reviewed by all other authors, and the finished work is a group effort.

The book is not aimed at specialist chemists, although we hope that they may find some of the more applied material useful. Rather, it is aimed at students specialising in environmental science or ecology, and requiring a grounding in basic chemical concepts to make chemical aspects of their studies accessible.

General principles of analytical chemistry

Introduction

Analytical chemistry encompasses the science of characterising a sample. Two broad categories can be considered. Qualitative analysis deals with the question of what elements or compounds are present. In the simplest case, this may involve only sample identification. For instance, is an unknown water sample fresh or saline? Several screening tests may rely on determining whether or not a particular substance is present or absent. This has applications in quality control during food processing where the analysis might serve to distinguish foreign bodies. Urine samples from athletes and racehorses are routinely examined for the presence of prohibited drugs. Qualitative analyses may also be used to trace the source of a sample. Pollen analysis of honey indicates the country of origin. Comparing the ‘fingerprint’ of substances in an oil slick with that in the bilge tanks of ships may enable the responsible vessel to be identified. Quantitative analysis involves determining how much of an element or molecule is present. Several types of quantitative analyses can be contemplated. These are exemplified below using the analysis of petrol (gasoline). A complete analysis would quantify the amount of each constituent in the sample. This would involve the determination of hexane, octane, toluene, tetraethyllead, etc. Such analyses are generally impossible and for environmental samples usually unnecessary. For an ultimate analysis the amount of each element is quantified.