Hydrothermal Humification of Extracted Spruce Greenery Biomass as a Tool for End-of-Waste Approach for Production of Artificial Humic Substances

Forestry sector produces a considerable amount of underutilised biowaste that can be valorised into high added value products. A prospective material is Norway spruce branches and needles, also known as spruce greenery. This study is a follow up from a previously explored green chemistry application of conifer greenery extraction, using green solvents. After extraction, a significant amount of spruce lignocellulose waste is produced, that can be thermochemically converted to artificial humic substances that mimic the properties of natural humic substances. The proposed technology is hydrothermal humification, which is considered to be climate neutral, as it produces no greenhouse gas emissions during the process. This study focuses on hydrothermal humification process application on extracted spruce greenery lignocellulose waste. Reaction conditions have been evaluated, to achieve the highest product yield, while maintaining least energy consumption possible, and the conditions were found to be 185 °C and 9 hours. Reaction byproducts have been identified, such as carbohydrates and polyphenol-like structures and their dynamics depending on reaction conditions. Contemporary methods have been applied to investigate the formation and structure of artificial humic acids. UV-Vis, FTIR and solid-state 13C NMR revealed that higher reaction temperatures increase the aromaticity and robustness of said humic acids, however, applying temperatures over 200 °C promotes the formation of potentially harmful free radicals, proven by EPR spectroscopy.