Effect of weathering on chromated copper arsenate (CCA) treated wood : leaching of metal salts and change in water repellency

Abstract

Wood pressure-treated with chromated copper arsenate (CCA) wood preservative is commonly used for outdoor construction. Oxides of arsenic, copper, and chromium are bound in the wood by a complex series of chemical reactions, but a small percentage of these compounds are gradually released by leaching and weathering. Recent studies suggest that the release of these compounds can be minimized by applying finish to the surface of the treated wood, but little is known about how or to what degree finish properties affect release. This study evaluated the effects of water repellents in clear penetrating finishes on the amount of arsenic, copper, and chromium released from CCA-treated wood through leaching and ultraviolet (UV) radiation. Endmatched specimens were cut from CCA-treated deck boards and either left unfinished or dipped in a finish prepared with 1%, 3%, or 5% concentration of water repellent. All specimens were exposed to leaching from simulated rainfall and a subset of specimens was also exposed to UV radiation. Water from the simulated rain was collected and analyzed for arsenic, copper, and chromium. The water repellent greatly decreased the amounts of these elements in the runoff, but the water repellent concentration did not make a difference for the short duration of this study. It is possible that water repellent content would have a greater effect over a longer exposure period. Exposure to UV radiation caused a large increase in leaching from both finished and unfinished specimens. This effect may be a result of increased surface area via checking, as well as loss of fibers from the wood surface caused by UV-induced surface erosion. The results indicate that UV radiation plays an important role in the release of preservative components from CCA-treated wood and that the effect of UV exposure must be considered when developing methods to assess the ability of finishes to minimize preservative release. In a matching study to investigate the effects of UV radiation on water repellency, small wafers of CCA-treated wood were treated with the same water repellent. Water repellency increased slightly for specimens exposed to both simulated rain and exposure to UV radiation compared with specimens exposed only to simulated rain.

Citation

Williams, R. Sam; Lebow, Stan; Lebow, Patricia. 2003. Effect of weathering on chromated copper arsenate (CCA) treated wood : leaching of metal salts and change in water repellency. Proceedings : ninety-ninth annual meeting of the American Wood-Preservers'' Association ... Boston, Massachusetts, April 27-30, 2003 : Volume 99. Selma, Ala. : American Wood-Preservers'' Association: Pages 125-141