The changing role of arsenic in the wood-preserving industry

    The toxicity of arsenic is well known, and the element features regularly in crime novels as a poison. A lethal dose is of the order of ≈130 mg. Despite this hazard, arsenic was used in agricultural pesticides until replaced by effective organic compounds in the second half of the twentieth century. While this use of arsenic declined, its application in the form of chromated copper arsenate (CCA) in wood preservatives increased from the 1970s to 2000 (see the graph and chart below). Wood for a wide range of construction purposes has been treated under high pressure with CCA, resulting in a product with a higher resistance to decay caused by insect and larvae infestation. Typically, 1m³ of pressure-treated wood contains approximately 0.8 kg of arsenic, and therefore the total quantities used in the construction and garden landscape businesses pose a major environmental risk. Once pressure-treated wood is destroyed by burning, the residual ash contains high concentrations of arsenic. Wood left to rot releases arsenic into the ground. Added to this, the chromium waste from the wood preservative is also toxic.

  The 2002 US Presidential Green Chemistry Challenge Awards recognized the development of a copper-based ‘environmentally advanced wood preservative’ as a replacement for chromated copper arsenate. The new preservative contains a copper(II) complex and a quaternary ammonium salt. Its introduction into the market coincides with a change of policy within the wood-preserving industry:  arsenic-based products should have been eliminated by the end of 2003.