Improving the two-step remediation process for CCA-treated wood: Part I. Evaluating oxalic acid extraction
Introduction
Concern about disposing of CCA-treated wood in landfills has increased because of the risk of environmental contamination. Some alternative disposal methods for this material include incineration, reconfiguration and reuse, composting with decay fungi, and acid extraction and bioleaching of metals by bacteria (Clausen, 2003). These alternative methods could divert CCA-treated material from landfills by reducing the biomass, removing and recycling the metals, or simply increasing the useful service life by reusing the material in a secondary application. All these alternative methods have one thing in common—they are costly compared with disposal in landfills.
Research on remediation of CCA-treated wood has increased during the past decade as a result of concern about chromium and arsenic disposal in landfills. CCA-treated wood is banned for use in a number of European and Asian countries and will be phased out for residential use in the United States by 2004. There are copper-based replacements for CCA in residential uses, but these products are likely to raise concerns about leaching of copper into groundwater when wood treated with this generation of preservatives begins to come out of service. A two-step remediation process, involving a combination of oxalic acid extraction and bacterial culture with the metal-tolerant bacterium, Bacillus licheniformis, substantially reduces the amount of copper (78%), chromium (97%), and arsenic (93%) in CCA-treated wood (Clausen, 2000, Clausen & Smith, 1998). This remediation process has been shown to be equally effective on a number of copper-based preservatives (Crawford and Clausen, 1999) and meets the need to divert wood treated with either CCA or copper-based organics from our landfills.
Chemical leaching of copper, chromium, and arsenic with acid extraction has been studied (Kim & Kim, 1993, Stephan et al., 1993, Pasek, 1994, Clausen & Smith, 1998, Kazi & Cooper, 2002). Stephan et al., 1993, Stephan et al., 1996, Illman & Highley, 1996, Yang & Illman, 1999 evaluated microbial conversion of CCA-treated wood with species of Antrodia and Meruliporia, brown-rot fungi known for their copper tolerance and production of high levels of oxalic acid (Clausen et al., 2000). The theory is that the high production of oxalic acid increases the acidity of the substrate, thereby increasing the solubility of the metals. Warner and Solomon (1990) reported that 100% of the copper was leached from CCA-treated wood with NaOH/citric acid buffer at pH 3.5 after 40 days of continual leaching. Minimal exposure of wood fiber to acids is desirable if fiber recovery is desired, because prolonged exposure to strong acid damages the fiber integrity by hydrolyzing carbohydrates. Clausen (2000) minimized damage of wood fiber by oxalic acid in the remediation process by optimizing the oxalic acid concentration and exposure time of the wood substrate.
Remediated wood fiber has been reassembled into medium-density particleboard panels (Clausen et al., 2001). An evaluation of the properties of particleboard made from remediated fiber showed a 28% and 13% decrease in internal bond strength and modulus of rupture (MOR), respectively, and an increase of 8% in modulus of elasticity (MOE) compared with particleboard prepared from virgin fiber. Compared with the cost of manufacturing particleboard from virgin southern pine stock, manufacturing costs using remediated wood fiber is approximately six times as expensive, mostly due to the cost of oxalic acid and the nutrient culture medium. On the other hand, savings can be incurred through extraction by avoidance of landfill fees and by recovery and reuse of the metals. With increased restrictions internationally on landfilling CCA-treated wood, landfilling costs may become prohibitive, increasing the desirability of alternative management options.
Eliminating or reducing the cost of metal extraction with oxalic acid would decrease the overall processing cost for the two-step remediation of CCA-treated wood. There are several ways to accomplish this: (1) reuse oxalic acid for multiple extractions, (2) maximize the ratio CCA-treated wood to acid, or (3) use an alternative source of oxalic acid, such as Aspergillus niger, a mold fungus commonly associated with wood. This fungus is capable of producing high concentrations of oxalic acid in culture (Cameselle et al., 1998). Sayer and Gadd (1997) have shown that A. niger can solubilize inorganic metal compounds and transform them into insoluble metal oxalates. Furthermore, when Price et al. (2001) evaluated fungi for their ability to remove metals from wastewater, they demonstrated that A. niger internally absorbs copper as a means of detoxifying its environment.
Oxalic acid has many industrial applications, including being used as a pharmaceutical purifying agent, a precipitating agent in mineral processing, a bleaching agent for textiles and wood, and in wastewater treatment. The objective of this study was to evaluate the three alternative oxalic acid extraction approaches mentioned previously to improve the economic feasibility of the two-step remediation process for CCA-treated wood.
Section snippets
Treated wood
CCA-treated southern yellow pine lumber (6.4 kg/m3 retention) was used throughout this study. The treated lumber was hammer-milled and sorted to approximately 1- to 3-mm (6- to 16-mesh) particles.
Elemental analysis
Ovendried samples, ground to pass a US Standard 20-mesh (850-μm) screen, were digested and analyzed for copper, chromium, and arsenic content by inductively coupled plasma (ICP) emission spectrometry according to American Wood Preservers’ Association (AWPA) standard A-21-00 (AWPA, 2001).
Oxalic acid/CCA-treated wood ratio
One hundred
Oxalic acid loading and reuse
Combined results evaluating the ratio of CCA-treated wood to oxalic acid concentration and use of commercial oxalic acid for multiple extractions are shown in Fig. 1. Oxalic acid (0.8%) at a ratio of 100:1 (v/w) to treated wood particles was reused in three subsequent 24-h extractions, and each extraction resulted in the removal of copper, chromium, and arsenic with equal efficiency. During subsequent extractions, 65–68% copper, 48–51% chromium, and 68–71% arsenic were removed with 1:100 ratios
Conclusions
Oxalic acid extraction is an important first step in the two-step remediation process; neither acid extraction nor bacterial exposure alone is as effective as they are in combination. Using commercial oxalic acid to repeatedly extract CCA-treated particles appears to be the most cost-effective method for acid extraction. The CCA-treated wood to acid ratio was optimized at 1:100 (w/v). The ability of Aspergillus niger to produce oxalic acid and effectively remove significant amounts of copper
Acknowledgements
The author thanks Dan Foster, Chemist, for conducting ICP analyses.
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