Book Description

Preservative-treated wood is an economical, durable, and aesthetically pleasing building material; therefore, it is a natural choice for construction projects in our National Forests, National Parks, and other public lands. However, we need to ensure that the chemicals used in treated wood do not pose a threat to people or the environment. The purpose of this report is to provide a summary of the pertinent literature on leaching of wood preservative components and their mobility in the environment. The waterborne wood preservatives chromated copper arsenate (CCA), ammoniacal copper zinc arsenate (ACZA), ammoniacal copper quat (ACQ), copper dimethyldithiocarbamate (CDDC), and ammoniacal copper citrate (CC) resist leaching during service because of complex chemical reactions that take place within the treated wood. The effectiveness of these reactions in preventing leaching is dependent on treating factors, such as preservative formulation, preservative retention, and processing techniques, as well as post-treatment conditioning factors, such as temperature, humidity, and air flow. Copper naphthenate, an oilborne wood preservative, resists leaching because it is relatively insoluble in water. Little information is available on the inservice leaching rates of any of these wood preservatives, although CCA has been studied more extensively than the other preservatives. Past studies report widely varying leaching rates, but generally agree that the most rapid leaching occurs within the first months of service and is greatest in products with high retention levels and high proportions of exposed surface area, especially end-grain. Leaching is also increased by exposing the wood to high water flow, low pH, and water-soluble organic acids. Movement of leached chromium and copper appears quite limited in soil, because the metals are tightly bound to organic soil constituents. Arsenic is slightly more mobile in soil, but binds to iron, aluminum, and manganese. Generally, the preservative components are least mobile in organic soils, slightly more mobile in clay soils, and most mobile in sandy soils. In aquatic applications, the leached preservative components form complexes with the organic sediment fines, and accordingly are either deposited or moved downstream with the sediments. Additional research is needed to characterize typical in-service leaching rates of these preservatives and to determine how processing parameters affect long-term leaching rates. Keywords: wood preservatives, leaching, in service, CCA, ACZA, ACQ, CDDC, CC, copper napthenate.