A cooperative research team composed of scientists from the LSU AgCenter and USDA Forest Service, Southern Research Station have developed an innovative method to rapidly recover metals from pressure-treated wood. The wood, commonly known as chromated copper arsenate or CCA, had been used to pressure treat lumber since the 1940s.
CCA is a chemical wood preservative containing chromium, copper and arsenic. CCA is used in pressure-treated wood to protect wood from rotting due to insects and microbial agents Since the 1970s, the majority of the wood used in outdoor residential settings has been CCA-treated. The EPA and industry voluntarily agreed they will phase out CCA-treated wood for residential uses by the end of 2003. Large volumes of CCA-treated residential wood remain in service, and CCA-treated industrial wood continue to be decommissioned each year.
Currently, the most common disposal method for this material is landfilling. Opening burring is not permitted and concentrates the metals in the ash. Landfilling is not an environmentally friendly means of disposal, and landfill tipping fees and regulations are continually becoming more expensive and cumbersome, respectively. Moreover, landfilling exposes parties in the production chain to potential legal liability risks.
The research team, led by Dr. Todd Shupe, professor with the LSU AgCenter’s Louisiana Forest Products Development Center, used a chemical process composed of combinations of two acids -- acetic, oxalic and phosphoric acids. The wood and acid were mixed in a microwave reactor similar to those commonly used in households. They found that the addition of acetic acid into phosphoric acid enhanced the chromium recovery rate of the mixed-acid solution. The minimal reaction conditions for extracting the maximum percentage of metals was 2.75% phosphoric acid, 0.5% acetic acid and 130°C. The total recovery rate approached 100% for arsenic, 96.7% for chromium and 98.6% for copper in a one-step process that ran for 10 to 15 minutes. The patent is titled “process for rapid microwave-enhanced detoxification of CCA-treated wood” and is U.S. Patent No. 8,043,399. The process is thought to be economically viable due to the short reaction time, minimal chemical input and low energy requirements.
The process is currently being tested at a pilot plant in cooperation with researchers with the Chinese Academy of Forestry in Beijing, China. The results from the scaled-up study will provide economic data and optimization of the process on a larger scale. The EPA has not indicated that CCA or any other approved wood preservative is unsafe if properly treated, installed and maintained. EPA has classified CCA as a restricted-use product for use only by certified pesticide applicators. CCA remains in use for various industrial applications such as poles and pilings.
According to Shupe, this process should assist Louisiana wood treaters and the utility industry because disposal has been widely regarded as the Achille’s heel of the industry. Nearly half of all southern pine timber is pressure treated, and it is important that the wood-treating industry remain viable for the long term so that landowners continue to receive maximum value for their timber, said Shupe. Also, studies have shown that pressure-treated wood uses less energy to produce than other non-wood alternatives.
The forest products industry in Louisiana and the United States is a multi-billion dollar industry; therefore, even improvements in processes and products can create huge benefits. The forest products industry is economically the second largest industry in Louisiana, adding over $4.9 billion per year directly to the economy and providing jobs for over 30,000 of Louisiana’s citizens, which makes it the third largest employer. There are currently 16 wood-preservative treating plants in Louisiana that directly employ approximately 600 people and produce gross sales of approximately $170 million. Forestry continues to be the most economically important agricultural commodity in Louisiana. According to the LSU AgCenter’s Ag Summary, forestry added $3.8 billion to the Louisiana economy in 2011.