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Work with bacteria such as Lactobacillus and Streptococcus shows some potential [58] but may require several days to achieve metal removal. The laboratory study used milled wood powder so that it may generate material that is better suited to wood plastic com- posite production than particleboard. Milling was shown to be unnecessary for bacterial fermentation by Bacillus licheniformis when oxalic acid was used as a pre-treatment [59]. Enzymatic systems such as pectinolytic and cellulolytic from Bacillus and Pseudomonas spp. may assist in releasing copper, chromium, and arsenic from wood [59]. Some researchers have investigated the production of cleaned chips for future re- use [60,61]. The pilot scale work by Coudert et al. produced wood chips with a significantly reduced copper content from CCA, ACQ, MCQ, and copper-azoletreated woods [61]. Up to 97.5% of the arsenic, 87.9% of the chromium, and 96.1% of the copper present in CCA treated wood was removed by the three-step leaching process. The particles were sufficiently clean for compost production but only suitable for use in particleboards if mixed with clean wood chip at 10 to 50% of the chip feedstock (US regulations). It is clear that while approaches for chemical or physical cleaning upgrading methods for treated wood exist, none have yet become a reality due to economic constraints. The move from small-scale to industry-level applications has not yet taken place. Cleaning could benefit the particleboard manufacture from recycled timber, opening potential for lower-quality material to enter a material second life rather than combustion. It could also lead to new options and products if the cleaning process leads to a transformation of state (e.g., pyrolysis oils and nanocellulose). 4. Cascading Use of Wood and the Circular Economy Innovative thinking on the recovery and recycling of materials can lead to materials ‘cascading’ through many life cycles before the material is rendered only suitable for incineration and (hopefully) energy recovery [62,63]. The aim of the waste reduction measures, and the philosophy of cascading use of materials, is to reduce the quantity of waste reaching landfill. The initial diversion of timber away from landfill is largely complete in the UK, but the cascade can continue to be refined and developed. More recently, the concept of cascading has become a key part of the approach to optimising choices in the wood value chain to enhance carbon storage and to reduce GHG emissions—both necessary to mitigate climate change [12]. An ideal material cascade for waste wood may be to move from primary use in structural timber, furniture, pallets, etc., into a second use within particleboard; thereafter, the particleboard may be recycled once or more into new generations of particleboard, finally leading to a tertiary product or use for energy recovery (Figure 1). During this process, some losses will occur at each step, e.g., non-recoverable off-cuts of particleboard, painted material, and material too degraded for recycling. Most of these small portions will reach landfill or incineration, some may decompose in the environment, and a few pieces may also reach new artisan or alternative applications. For the timber cascade system it is possible to use the model to estimate residence time within the wood products pool, relating to carbon storage benefits (greenhouse gas removal and storage) within the wood products sector [11,64]. This has become a subject of intense interest and has renewed efforts to enhance the recycling and reuse of wood, to move wood through different product lives down the cascade, prolonging the time period for which the sequestered atmospheric carbon remains in solid form, removed from the atmosphere [64].
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