Exploring the Potential of Residues from Industrial Tannins Extraction through Comprehensive Characterization

The industrial extraction of tannins from maritime pine (Pinus pinaster) bark generates multiple solid by-products at different stages, including the raw bark, partially extracted material, and final residue, whose valorization remains largely unexplored. This study presents a comprehensive chemical and structural characterization of these three materials using a multi-analytical approach, with the aim of assessing their potential for material recovery and sustainable reuse. Fourier-transform infrared spectroscopy (FTIR) revealed depletion of hydroxyl-rich phenolics and enrichment in aliphatic associated structures. Proximate analysis demonstrated increased volatile content and fixed carbon in the final residue, indicating improved suitability for thermochemical conversion. Despite successive extractions, the final residue retained significant levels of total phenolics (373.86 mg GAE/g), condensed tannins (116.8 mg CE/g), and strong antioxidant activity (up to 425 µg AAE/mg), confirming the presence of bioactive compounds. However, the Stiasny number remained below 40%, suggesting limited reactivity with formaldehyde. In terms of structural composition, the final residue was enriched in acid-insoluble lignin (≈ 80%) and showed a dramatic loss in cellulose and hemicellulose content (< 7%), highlighting extensive structural degradation. Solvent extractive tests (Soxhlet, TAPPI T 264) revealed unusually high solubility (up to 85%), linked to depolymerized fragments. Size exclusion chromatography confirmed a decrease in molecular weight, suggesting breakdown of macromolecules during processing. Overall, these residues exhibit promising characteristics for energy recovery and extraction of phenolic-rich fractions, supporting their integration into circular bioeconomy strategies.