Characterisation and Optimisation of Pyroligneous Acid Properties for Agricultural Applications

Conventional agrochemicals were synthesised from non-sustainable sources and often left a high carbon footprint profile. Recognising the issues associated with conventional agrochemicals, this study proposed pyroligneous acid (PA) as a high-potential green agrochemical biomass-derived alternative. Inherently equipped with biodegradability and a low ecological impact, PA offered the industry high-performing active components of organic acids and phenolics for a wide range of applications. Furthermore, PA showed biocidal and plant regulatory effects - reviews had illustrated that the concentration of 10% v/v had biocidal effects, while 2% v/v PA promoted plant growth regulation. Not only was PA sustainable, it also generated a hundred-fold less greenhouse emissions than commercial agrochemicals like paraquat, and could effectively compete with industry fungicides like Polyoxin B, with better biodegradability and less ecotoxicity. PA yield was optimally obtained at a 350 to 450 oC temperature range; however, the upper limit of PA yield was attributed to the hemicellulose degradation of the biomass feedstock, especially wood-based materials. A novel rapid calculation of higher heating value and organic matter was based on the organic carbon content of PA, enabling a quick assessment of the chemical profile. It was found that wood-based PA, having up to 40 wt.% of organics, showed potential to serve as an insecticide, herbicide, and plant growth regulatory agent at a lower dilution factor. On the other hand, paper-based PA was evaluated to possess preferred characteristics to be used as a nematicide or fungicide and to open new horizons for pyrolysis studies on pulp materials. PA also showed promising potential for removing heavy metal minerals from contaminated soil, eliminating approximately 88.2% of Zn, 89.1% of Cd and 31.6% of Pb, compared to the original level. Contaminated soil treated with PA retained up to 50% of its original phosphorus content, which was about 2.5 times higher than that maintained by conventional washing agents, suggesting its application could enhance the reusability of post-treated soil for agriculture. 5 % v/v PA could leach thirteen-fold more phosphorus than water, and twice the amount of acetic acid, citric acid, or sulfuric acid during a 48-h leaching time. The pronounced P mobilisation effects of PA worked at 1000 g/kg soil; however, PA had shown biocidal properties at 10 g/kg soil, requiring the natural biodegrading of PA of 5 weeks to utilise. Ozonation was found to effectively create a cost-effective copper-PA chelate that had superior water solubility (99% and 80% solubility at pH = 6 and 8) and comparable metal stability with lignosulfonate chelate (7.5 % remaining copper in alkaline soil conditions). Overall, the work has delved into the characterisation and optimisation of PA to remediate soil minerals - heavy metal contamination and P immobilisation, and suggested a novel method to enhance the binding of PA with minerals to create chelated micronutrients. Navigating and extending the potential applications of PA in agriculture not only improved the adoption of this green liquid in various industries but also provided a great contribution to the circular economy, aligning with the global trend of green industry transformation.