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Copper in Antifouling
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Copper in Antifouling

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1. Introduction

The situation with regards to the use of tributyltin (TBT) in antifoulings for yachts and pleasure craft is well known.  What is not, however, is the situation regarding the continued use of copper in antifoulings.  This situation has been compounded of late by numerous sources of confusing and misleading information on the subject.  The purpose of this article is to put the "copper in antifoulings" situation straight.

Copper has been used as a major ingredient in antifoulings for hundreds of years.  Copper is ideal for use as an antifouling active substance as it is a naturally-occurring material and is an essential element required for normal growth by all plants and animals at low concentrations.  As such, it is a normal and essential constituent in the eco-system in soil, sediment and water.

In all the years that copper has been used as an active substance in antifoulings on yachts and pleasure craft, no negative environmental impact on the marine environment has ever been conclusively attributed to copper.

The use of copper-based antifoulings provides clear advantages to the protection of the environment.  Aside from the advantages of copper antifoulings over organotin antifoulings on marine life, the ability to provide continuously fouling-free surfaces means substantial fuel savings and consequently reduced emissions of green house gases.

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2. Background

Because of copper's biocidal properties at a boat's surface, copper and copper compounds have been used for centuries for the protection of the underwater hulls of numerous types of marine craft.  Over the ages two different purposes have been served in copper-based marine hull protection.  In early wooden vessels, a sheathing of metallic copper was used to protect the timbers from being invaded by ship worms (worms that bore into wood and weaken the structure).  Simultaneously, copper-sheathed boats protected from the growth of barnacles and other marine fouling species which, if present, increased the frictional resistance of the water against the hull and slowed the speed of the boat.

In modern steel, aluminium and grp hulls, ship worms are not a problem; however, the growth of friction-causing marine organisms is.  Fouling both reduces the speed of the vessel and increases its fuel consumption. It also increases wear on propulsion systems, can damage corrosive protection coatings and can expose the hull accelerating corrosion. 

Therefore, today, whether the vessel is a bulk carrier, containership or tanker, a racing yacht, or a weekend sports boat, antifoulings have become a necessity. 

The copper compounds that are commonly approved and used in antifouling paints for underwater hull protection are: copper oxide, copper thiocyanate and powdered or flake copper metal.

It is generally agreed that one particular form of copper, the cupric ion (Cu++), is most responsible for copper toxicity.  Numerous studies have shown that most of the copper present in marine environment is not present as cupric ion.  In fact, the concentration of the cupric ion is at least an order of magnitude lower than the rest of the copper in the environment.  Most of the dissolved copper is complexed with organic and particulate material in marine and freshwater environments.  This reduces or completely removes its ability to be taken up by organisms (ie its bioavailability) and therefore toxicity to marine organisms. 

Once in water, copper prefers to bind into muds on the bottom which removes particulate copper complexes from the water column and introduces it into sediment where it becomes tightly bound to organic matter and inorganic compounds.

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3. The Basis of Antifouling Coatings

Biofouling is most commonly prevented by the use of antifouling paints or coatings.  These are of two types:

This article concerns the second of the above types. 

The goal in the design of biocidal antifoulings is to provide a surface that has active biocides present but which either do not leave the surface and/or are deactivated shortly after leaving the surface.  Thus, biocides which have a short active half life either by biodegradation or by complexation / deactivation (detoxification) are required. 

There are over 4000 fouling organisms present in the waters of the world.  These are classified into hard organisms (such as barnacles) and soft organisms (grasses and algae). Thus, an effective antifouling must have a broad spectrum of activity and this activity must be continuously available to protect against biofouling. 

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4. Copper in Antifouling Paints

Copper, in various chemical compositions, has always been one of the primary active substances used in antifouling paints to control hard fouling on the boat's surface on either commercial shipping or pleasure craft.  With the development of Self Polishing Copolymer (SPC) coatings, tributyltin (TBT) was introduced as another major active ingredient in the 1970's and later as a paint binder.  TBT/copper- based coatings became the industry standard for antifoulings used on ship underwater hulls, as well as pilings, levee and dock structures.  In pleasure craft, TBT-based paints were used extensively in the 1980's.  These paints in some cases did not contain copper as part of their formulation. Since the reporting of observed adverse impacts due to TBT, and subsequent restrictions on the use of TBT, copper-based paints have regained dominance in the market.

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5. Environmental Impact and Regulatory Activity on the Use of Copper-Containing Antifouling Paints

In all the years that copper has been used as an active substance in antifouling paints on commercial shipping and pleasure craft, no negative environmental impact on the marine environment has ever been conclusively attributed to copper. 

This can be shown in European waters where the use of copper antifouling paints in pleasure craft was replaced in the 1980's with TBT-containing paints, then re-introduced after the TBT ban in 1987.  In the commercial seafood areas around Arcachon Bay in France, the aquatic life and thriving seafood populations were severely affected by TBT pollution.  However, extensive monitoring programmes have shown the recovery of commercially sensitive species of aquatic invertebrates over the past 10 years as TBT-based yacht paints have been replaced once again with copper-based paints.

Some EU authorities have raised concern over copper compounds used in antifouling paints used in pleasure craft, leading to restrictions and/or bans of copper-based anti-fouling paints in Sweden and the Netherlands.  These bans or restrictions have come about through perceived risks to unique environmentally sensitive local environments.

In Sweden, the Baltic Sea is considered an environmentally sensitive environment as it is a relatively young, developing aquatic environment with harsh salinity gradients that could stress important aquatic species.  Due to this harsh environment, the Baltic has relatively small aquatic species diversity and it is therefore considered important to protect these species, e.g. the blue mussel and bladderwrack (seaweed). 

Therefore, the Swedish authorities applied the Precautionary Principle and placed severe restrictions on the use of copper anti-fouling paints in the Baltic Coast of Sweden to protect these aquatic species.  Although the importance of protecting the structure of the aquatic environment in the Baltic is recognised, adverse effects from copper have not actually been observed in this highly environmentally-sensitive environment. 

In the Netherlands, it has long been recognised that the copper loading in the inland aquatic environment is uniquely high due to the amount of pollution coming from river sources in other parts of Europe.  The levels found around inland waters clearly cannot be directly related or significantly affected by the use of copper-containing antifouling paints. 

However, the Dutch pesticide authorities decided to ban the use of copper-containing antifouling paints on all pleasure craft, especially in inland waters.  Again this is considered to be a uniquely sensitive environmental situation, where although copper levels are high through general pollution in other EU countries, adverse effects from copper have not actually been observed. Indeed, in a recent review, the EU's leading scientists have made a recommendation against the ban, because the scientific evidence did not support the conclusion that copper was negatively impacting the environment.

The global regulatory status of antifouling paints in pleasure craft is outlined in the Regulatory Map and the few bans and restrictions of copper can be put into perspective.  From the map it can be seen that copper is a well recognised and acceptable active substance throughout the world. 

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6. The Way Forward

Historically, copper has been regulated on an exceedance basis - if the copper concentration exceeded a certain level the presumption was there was risk of harm. In recent years there has been increasing recognition that this is over-simplistic, and that any assessment of copper impacts should of necessity include an assessment of bioavailability. This is because it is now recognised that water characteristics control copper speciation, and speciation controls toxicity.

The development of the freshwater Biotic Ligand Model (BLM) and the marine Dissolved Organic Carbon (DOC) correlation provide new tools to enable this bioavailability assessment to be achieved, and these methods are gaining regulatory recognition and acceptance around the world.

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