Unlocking the potential of undervalued forest residues: a comprehensive characterization of eucalypt stump in the biorefinery context

Abstract

Eucalyptus globulus stumps are a by-product from the coppice pulp plantation after three generations. In this study a stump was fractionated in three discs (60 cm between them), and their constituent tissues—heartwood, sapwood and bark—were subjected to further chemical characterization by summative analysis, evaluation of the phytochemical profile and antioxidants activities, plus GC/MS and analytical pyrolysis aiming at their valorization. Wood density was similar between tissues and disc level: values ranging from 0.652 to 0.705 g/cm³ (Disc 1) and 0.605 g/cm³ (Disc 5). Bark had high ash (3.5%), extractives (7.5%) and holocellulose (68.4%) but lower lignin contents (22.0%). Original heartwood contained 0.7% ash, 7.0% extractives, 27.1% lignin, and 67.3% holocellulose. Heartwood showed high extractives (12.1–15.8%), less lignin (23.9–24.5%), and high holocellulose (61.7–64.7%) compared to sapwood which contained 3.9–5.4% extractives, 26.9–27.3% lignin and 68.6–71.5% holocellulose. Water extracts had poor antioxidant activity in contrast to ethanol extracts with high activities in heartwood. All tissues presented GS lignin type with S/G ratios varying from 3.0 to 3.4 (heartwood), 3.2–3.4 (sapwood), bark (3.5) and 3.8 (original heartwood). In wood, fibers and vessels were highly lignified with SG and G-lignin respectively; while rays had low lignin with G-type. Light and fluorescence macroscopic observation of the tissues in Disc 1 revealed a lower proportion and larger vessels in sapwood and high emission fluorescence at 488nm. Overall, these results show that stumps are valuable raw material to be used under the biorefinery context.