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3.4. Cleaning Wood for Recycling After physical separation, in certain grades of recycled wood, the chemical contam- inants within the wood may remain a challenge, especially for treated timber contained within household waste (Category C) or within construction and demolition waste (Cate- gory B). The obviously treated materials (e.g., fencing panels) may be visually identified and removed prior to shredding to minimise the need for segregation later within the process and deflected into Category D. However, it would be desirable to find cleaning treatments, either chemical or biological, that can remove the contaminants and enable this feedstock to be returned into new products. In the context of particleboard, the feedstock must be sufficiently clean. Potential contaminants include adhesives, coatings, additives, and chemical treatments. Researchers have long considered methods for sampling and quantification of contaminant levels and their influence on board properties [43,44]. Hydrolysis is the most commonly proposed method for deactivation of urea formaldehyde adhesive bonds [5,45], as discussed later, but other adhesives may prove more challenging—for example pMDI, MUF, and PF resins are resistant to hydrolysis and deliberately selected to provide moisture resistance in certain products. Many additives, e.g., waxes and hydrophobising agents, are of low toxicity and, thus, of limited benefit to remove, but could alter the bond formation within next generations of product. Coatings are a complex group—ranging from paints and stains or varnishes through to solid laminates or polymers which can be identified and removed by physical methods. The larger challenge is associated with preservative treatments and fire-retardant treatments [8,46]. Helsen and Van den Bulck [47] considered the specific case of CCA-treated timbers, and identified chemical extraction, bioremediation, electrodialytic remediation, and ther- mal destruction as options. This particular grade of treatments posed multiple challenges, including the possibility for arsenic release, or hazards associated with chromium if in certain valent states, as well as the potential formation of dioxins and furans during combustion reactions. However, the quantity of CCA-treated timber within the waste stream is diminishing as the use of this treatment agent was restricted in Europe in 2006 [48]. Chemical cleaning treatments, solvent systems, biological treatments, and liquefaction have been tested to remove preservative treatment chemicals from the wood. In some cases, the technology has focused on the reclamation of the copper, chromium, and arsenic and conversion to a form suitable for re-use as a wood treatment chemical [49,50], although this is of lower interest in Europe where such treatments are not permitted. The use of organic acids has been demonstrated to be effective in removing copper-based treatments [46,51,52], especially citrate ions. Shiau et al. [53] demonstrated that citric acid extraction gave a steep chromium and arsenic removal when the pH was dropped to 3.5. Electrokinetic processes have been investigated at the bench scale; for example, Sarah- ney et al. [54] demonstrated the removal of 74% of chromium, 97% of the copper, and 88% of the arsenic from CCA-treated timber. The electrokinetic process was enhanced with an oxalic acid–EDTA solvent mixture. Bioremediation offers a sustainable approach to removing inorganic wood preserva- tives like heavy metals from treated wood, mitigating ecological risks and human haz- ards [55]. This process uses microorganisms to break down or transform contaminants into less harmful substances [56]. Xing et al. [55] reviewed three main direct bioremediation options: fungal bioremediation, bacterial bioremediation, and non-living bio-sorbents. It has been suggested that the decontaminated wood can be reused in products such as particleboard [57]; however, there can be strength and toughness loss, depending on the organism deployed and treatment conditions.
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