To extend the service life of softwood timber, such timbers are commonly treated with chromated copper arsenate (CCA) – a preservative that inhibits the natural decaying process of timber. However, due to the toxic nature of the preservative contained within the wood, its disposal after service-life has proven to be problematic. Not only the preservative entering the disposal sector, but also the limited opportunities for alternative use and growing quantities of this waste, are further complicating the problem. The linear product pathway of cradle-to-grave promotes the need for continued use of virgin CCA preservative, resulting in ever larger quantities entering the immediate environment. Currently, CCA-treated timber waste is disposed of in Class II landfills in New Zealand. In order to mitigate the environmental concerns and to achieve a circular economy, an alternative and more sustainable waste management strategy is required, shifting away from landfill disposal of CCA timber wastes.

The objectives of this thesis are to explore suitable processing technologies for CCA-treated timber wastes for recovery, reuse and recycling; furthermore, to develop a corresponding waste management strategy applicable to the New Zealand circumstance. From literature review, pyrolysis has been identified as the key recovery technology for a waste management strategy that enables the recovery of the treatment metal(oid)s in the CCA-treated timber waste. Thus, this thesis sets out to test the hypothesis that pyrolysis can provide a feasible solution, not only allowing for the decomposition of the woody biomass (including the recovery of its energy value) but also recovering the arsenic in a suitable form for future application.

Proof of this hypothesis will open up the opportunity for the development of a circular economy in regard to the recovery and reuse of the preservative chemicals, thereby reducing environmental harm by limiting the quantities of CCA-treated timber waste entering landfills. In addition, the preservative product route could be adapted to that of a cradle-to-cradle pathway, thereby reducing the requirement for virgin preservative use.

The emphasis in this thesis was placed on arsenic recovery. As arsenic is a highly toxic and volatile chemical, a thorough understanding of its release processes from wood structures during pyrolysis is essential for its effective and safe recovery. Experimental and thermodynamic modelling was undertaken to identify the optimal conditions for arsenic recovery into the pyrolysis bio-oil product. To this end, experimental and thermodynamic investigations were carried out, in regard to arsenic interactions, oxidation states and chemical reactions that occur during pyrolysis. Furthermore, pyrolysis product yields and metal(oid) concentrations were determined. The findings were visualised and integrated into a conceptual process design suitable for industry application.

Experiments were conducted in a batch pyrolysis system at operating temperatures from 280°C to 580°C with a residence time of 30 minutes. In the experiments, 25 g of freshly treated H4-CCA-treated Pinus radiata wood were pyrolysed in an inert nitrogen environment. Liquid (bio-oil), solid (char) and non-condensable gas yields were measured, and their compositions were analysed. For comparison purposes, untreated Pinus radiata wood was also pyrolysed as a control.

The experimental results indicate that batch pyrolysis of CCA-treated wood - in comparison to that of untreated wood - results in lower liquid yields but higher yields of char and non-condensable gas products. This is attributed to the presence of CCA compounds in the wood, which promote charring and secondary cracking reactions. The onset of arsenic release from the solid state (arsenic pentoxide or As(V), As2O5) was observed to occur at 300°C, with a maximum total recovery at 475°C. The arsenic release at temperatures within the pyrolysis regime is attributed to the breakdown of As2O5, which has remained in the wood in its unfixated form. The other form of arsenic compound contained within the wood, CrAsO4, is stable during pyrolysis. This form of arsenic is fixated through the chromium compound to the wood structures.

As stated above, the highest bio-oil yield is achieved at a pyrolysis temperature of 475°C. Under this condition, the resulting liquid yield was 29 wt% with an arsenic concentration of 606 ppm – equating to 6.7 wt% of the original arsenic content. The arsenic present in the bio-oil was found to be As(III) (As2O3), indicating that the reduction of As(V) to As(III) took place during the pyrolysis process at temperatures lower than those that would have occurred in an inert environment. The catalytic effect of lower a reaction temperature is attributed to the reducing environment, created during pyrolysis. At temperatures higher than 475°C the volatilisation of arsenic is promoted, driving it into the gaseous non-condensable pyrolysis product.

To clarify the pyrolysis reactions involved, when using CCA-treated wood as a feedstock, a pyrolysis model was proposed, based on studies reported in literature. Additionally, a thermodynamic equilibrium model was developed to investigate the decomposition process of arsenic compounds.

Experimental results and thermodynamic equilibrium modelling showed that chromium and copper are essentially retained in the char. Temperature was found to be the key parameter in determining the onset of arsenic release. In addition, comparison of this study’s findings and those reported in literature indicate that both heating rate and residence time play important roles in regard to the extent of arsenic release and its recovery in liquid or gaseous products.

In order to achieve the maximum bio-oil yield and arsenic recoveries within the pyrolysis liquid product, a fast heating rate and short residence time are deemed to be favourable. Consequently, continuous fast pyrolysis is the preferred technology for pyrolysis decomposition of the biomass. In this thesis, to meet the above parameters of maximum oil yield and arsenic recovery, results from continuous fast pyrolysis experiments, as reported in the literature review, were used. On the basis of these data, a simplified process model to simulate industrial performance was developed and, in addition, the feasibility for recovery of the treatment preservatives for further use in timber treatment was examined. The results were then applied to establish a conceptual waste management scheme and engineering design for a possible commercial plant. The proposed plant with a capacity of processing 1 tonne h-1 of dry CCA-treated timber wastes produces 211 kg of CCA-free char, alongside 237 kg of a CCA-free LOSP analogue liquid and 331 kg of CCA-free gas. A heavy oil fraction (151 kg) containing 0.61 kg of As(III) and 69 kg of CCA-contaminated char can also be recovered, together with 0.35 kg of As4O6, which needs to be removed from the gas product.

The applicability of the bio-oil as a wood preservative still requires experimental proof. However, literature shows a potential suitability in its crude form. Furthermore, a light organic solvent preservative (LOSP) analogue liquid can be recovered that has similar properties to LOSP and which could be augmented with LOSP for wood treatment.

This thesis provides an initial proof-of-concept for pyrolysis as a waste management technology for CCA-treated timber waste and validates that further scientific investigation into this process is warranted. Importantly, the outcome of the research and engineering appraisals undertaken in this thesis offers a novel end-of-life solution for CCA-treated timber waste that moves beyond current disposal methods towards a circular economy approach.