Chromated copper arsenate

Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to protect them from attack by microbes and insects. Like other copper-based wood preservatives, it imparts a greenish tint to treated timber. Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to protect them from attack by microbes and insects. Like other copper-based wood preservatives, it imparts a greenish tint to treated timber. CCA was invented in 1933 by Indian chemist Sonti Kamesam, and patented in Britain in 1934. It has been used for timber treatment since the mid-1930s, and is marketed under many trade names. Concerns over the possible release of chromium and arsenic to the environment have resulted in it being banned for timber destined for residential use in the early 2000s. Acute intoxication due to mishandling of treated products, e.g. by burning, is also a serious concern. Nevertheless, CCA remains a popular and economical option to make perishable timbers, such as plantation-grown pine, viable for applications like poles, piling, retaining structures, etc. The composition of CCA products is usually described in terms of the mass percentages of chromium trioxide or 'chromic acid' CrO3, arsenic pentoxide As2O5, and copper(II) oxide CuO. The preservative is applied as a water-based mixture containing 0.6–6.0% (by weight) of chromic acid, copper oxide, and arsenic acid (USDA, 1980), with pH 1.6–2.5. The mixture is infused into wood at high pressure. In the treated wood, arsenic is believed to be in the form of chromium (III) arsenate CrAsO4 and/or copper(II) arsenate Cu3(AsO4)2, or fairly stable chromium dimer-arsenic clusters. The chromium 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 and lignin. The copper acts primarily to protect the wood against decay, fungi, and bacteria, while the arsenic is the main insecticidal component, providing protection from 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. Alternative heavy-duty preservatives include creosote and pentachlorophenol. Similar water-borne preservatives include alkaline copper quaternary (ACQ) compounds, copper azole (CuAz), ammoniacal copper zinc arsenate (ACZA), copper citrate, and copper HDO (CuHDO). Concerns over the safety of CCA have focused on its chromium and arsenic contents.