Rapid Microwave-Assisted Acid Extraction of Metals from Chromated Copper Arsenate Treated Wood

Preservative-treated wood products are well known to significantly prolong service life, and thereby extend the forest resource and enhance its sustainability. Inevitably, large volumes of preservative-treated wood are decommissioned each year. It is estimated that about 3-4 to 12 million tons of spent preserved wood will be removed from service in the United States and Canada in the next 20 years (Kazi and Cooper, 2006). Disposal of the spent CCA-treated wood has become a major concern because of its residual toxic components, particularly arsenic and chromium. Conventional waste disposal options for spent preserved wood, such as burning and landfilling, are becoming more costly or even impractical because of increasingly strict regulatory requirements (Townsend et al., 2004). The burning of treated wood can be extremely dangerous and even more so when the wood has been treated with CCA. Studies have shown that burning of preservative–treated wood waste emits highly toxic smoke and fumes in the environment (Solo-Gabriele, 2002). In the case of landfills, studies have shown that CCA3 compounds can be gradually leached out (Townsend, 2005; Moghaddam, 2008). Moreover, there is concern regarding landfill capacity. Therefore, there is an urgent need for developing techniques for recycling CCA-treated out of service wood. Several chemical methods have been proposed to extract the metals from CCA-treated wood. Solvent extraction can dissolve the preservatives and partially remove them from the wood. The used of acid extraction to remove CCA components from wood has been extensively studied (Kartal and Clausen, 2001; Son et al., 2003; Clausen, 2003; Clausen, 2004; Gezer, 2006; Kakitani 2006; Kakitani 2007). One of the advantages for acid extraction is based on its potential ability of reversing the CCA fixation process, thereby converting CCA elements into their water-soluble form (Kartal and Clausen, 2001). However, among the disadvantages of this recycling method are the huge amount of chemical solvents used and the long duration of the process. The prevailing treatment times reported ranged from 16 h for sawdust (Clausen and Smith, 1998) to 24 h for chips (Kartal and Clausen, 2001), which are considered to be major factors hindering commercial development. Therefore, to develop an economically viable industrial process, the focus of our study was on treatment time and acid concentration. Thus, the time saving potential of microwave heating led us to its application with acid extraction. The specific objectives of this study were to: (1) develop a new CCA recovery system based on the application of microwave energy, and (2) optimize reaction time, temperature, and acid concentration for the process.