Chromated copper arsenate
Encyclopedia
Chromated copper arsenate (CCA) is a wood preservative used for timber treatment since the mid-1930s. It is a mix of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 and arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 (as Copper(II) arsenate
Copper(II) arsenate
Copper arsenate , also called copper orthoarsenate or tricopper arsenate or tricopper orthoarsenate, is a blue or bluish-green powder insoluble in water and alcohol and soluble in aqueous ammonium and dilute acids. Its CAS number is or .-Uses:Copper arsenate is an insecticide used in agriculture...

) formulated as oxides or salts. It preserves the wood from decay fungi, wood attacking insects, including termites, and marine borers. It also improves the weather-resistance of treated timber and may assist paint adherence in the long term.

CCA is known by many trade names, including the worldwide brands "Tanalith" "SupaTimber" and "Celcure". The chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 acts as a chemical fixing agent and has little or no preserving properties; it helps the other chemicals to fix in the timber, binding them through chemical complexes to the wood's cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 and lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

. The copper acts primarily to protect the wood against decay fungi and bacteria, while the arsenic is the main insecticidal component of CCA.

CCA is widely used around the world as a heavy duty preservative, often as an alternative to creosote
Creosote
Creosote is the portion of chemical products obtained by the distillation of a tar that remains heavier than water, notably useful for its anti-septic and preservative properties...

, and pentachlorophenol
Pentachlorophenol
Pentachlorophenol is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names...

. Other water-borne preservatives like CCA include alkaline copper quaternary compounds (ACQ
Alkaline copper quaternary
Alkaline Copper Quaternary is a water based wood preservative method. The treatment is made up of copper, a fungicide, and a quaternary ammonium compound , an insecticide which also augments the fungicidal treatment is a wood preservative that has come into wide use in the USA, Europe, Japan and...

), copper azole (CuAz), ammoniacal copper zinc arsenate (ACZA), copper citrate, and copper HDO (CuHDO)

Recognized for the greenish tint it imparts to timber, CCA is a preservative that has been extremely common for many decades. Over time small amounts of the CCA chemicals, mainly the arsenic, may leach out of the treated timber. This is particularly the case in acidic environments. The chemicals may leach from the wood into surrounding soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, resulting in concentrations higher than naturally occurring background levels. A study found that 12–13 percent of the CCA leached from treated wood buried in compost
Compost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...

 during a 12-month period. On the other hand there have been many other studies in less aggressive soil types that show leaching to be as low as 0.5 ppm (red pine poles in service,) or up to 14 ppm (treated pine in garden beds). Soil contamination
Soil contamination
Soil contamination or soil pollution is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment....

 due to the presence of CCA-treated wood after 45 years is minimal.

Should any chemicals leach from the wood they are likely to bind to soil particles, especially in soils with clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 or soils that are more alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

ne than neutral.

A number of countries have reviewed CCA during recent years and have looked at limiting the public exposure to CCA-treated timber by restricting its application in residential situations. These reviews have resulted from increasing public pressures and perceptions that arsenic-containing timber poses a health hazard. In response to these pressures the preservation industry in the USA and Canada volunteered not to use CCA for the treatment of residential timber, and on 1 January 2004 the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 (EPA) began restricting the use of CCA for such purposes. Exceptions were allowed, including the treatment of shakes and shingles, permanent wood foundations
Foundation (architecture)
A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...

, and certain commercial applications. It should be emphasised however that the regulatory agencies advised that CCA-treated timber products already in use pose no significant threat to health. Indeed CCA will continue to be used in North America in a wide variety of commercial and industrial applications such as poles, piling, retaining structures and many others.

Following the USA and Canada actions in restricting CCA, similar actions have been taken in other parts of the world, including the EU and Australia. In New Zealand the Environmental Risk Management Authority
Environmental Risk Management Authority
The Environmental Risk Management Authority is a New Zealand government agency which controls the introduction of hazardous substances and new organisms....

, reviewing the same data that prompted the actions elsewhere, concluded that there was no reason to restrict CCA use for any applications.

CCA timber is still in widespread use in many countries and remains an economical option for conferring durability to perishable timbers such as plantation grown pine.

Although widespread restrictions (see above) followed the publication of studies which showed low-level leaching from in-situ timbers (such as children's playground
Playground
A playground or play area is a place with a specific design for children be able to play there. It may be indoors but is typically outdoors...

 equipment) into surrounding soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, a more serious risk is presented if CCA-treated timber is burnt in confined spaces such as a domestic fire or barbecue. Scrap CCA construction timber continues to be widely burnt through ignorance, in both commercial, and domestic fires.

Notwithstanding this, disposal by burning i.e. in approved incinerators, is an acceptable option. It is particularly attractive if there is some energy captured in the process.

Disposal of large quantities of CCA-treated wastes or spent timber at the end of its lifecycle has been traditionally through controlled landfill sites. Such sites are lined to make them impervious in order to prevent losses to the water table and they are covered to prevent rainfall washing out any contained potential toxicants. These controlled sites handle a range of waste materials potentially more noxious than that posed by CCA-treated timber, e.g. paint-stuffs, car batteries, etc. Today, landfill sites are becoming scarcer and disposal of waste materials is becoming economically unattractive. The wood preservation and timber industries are therefore researching better ways of dealing with waste treated timber, including CCA-treated material.

See also

  • Timber treatment
  • Paris Green
    Paris Green
    Paris Green is an inorganic compound more precisely known as copper acetoarsenite. It is a highly toxic emerald-green crystalline powder that has been used as a rodenticide and insecticide, and also as a pigment, despite its toxicity. It is also used as a blue colorant for fireworks...

  • Scheele's Green
    Scheele's Green
    Scheele's Green, also called Schloss Green, is chemically a cupric hydrogen arsenite , CuHAsO3. It is a compound similar to Paris Green...

  • National Pesticide Information Center
    National Pesticide Information Center
    The National Pesticide Information Center is a collaboration between Oregon State University and the United States Environmental Protection Agency to provide objective, science-based information about a wide variety of pesticide-related topics, including: pesticide product information, information...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK