Hot‑water extraction of phenolic‑rich bark fractions from Ireland and New Zealand: Process optimisation and evaluation as methylene blue adsorbents

Forestry residues from Ireland and New Zealand provide a variety of bark materials, each with unique species-specific tannin profiles influenced by their respective ecological conditions. This study investigated the hot‑water extraction of tannin‑containing fractions from bark samples of Quercus robur L. (Ireland), Sequoia sempervirens (D. Don) Endl., and Eucalyptus nitens (Deane & Maiden) Maiden (New Zealand), focusing on their potential as bio-based adsorbents for dye removal. Extraction parameters, including bark type, temperature, time, and solvent-to-bark ratio, were optimised using a Taguchi design, with extraction yield and total phenolic content (TPC) as key indicators. E. nitens exhibited the highest extraction yield (13.6%) and TPC (619.41 mg catechin equivalents [CE]/g extract), followed closely by Q. robur , which had an extraction yield of 12.7% and TPC of 501.83 mg CE /g extract. In contrast, S. sempervirens exhibited the lowest extract yield at 0.8% and the lowest TPC value at 66.22 mg CE /g extract. Gel permeation chromatography (GPC) showed that Q. robur contained more condensed tannins, while E. nitens and S. sempervirens consisted of lower molecular weight oligomers. Fourier transform infra-red spectroscopy (FTIR) and 13 C nuclear magnetic resonance (NMR) spectroscopy confirmed additional differences in the structures of polyphenols and carbohydrates. As a proof-of-concept application, selected tannin-rich extracts were crosslinked with formaldehyde to produce bio-based adsorbents for methylene blue removal. The resulting adsorbents demonstrated promising adsorption performance under controlled single-dye conditions. Overall, the study demonstrates a systematic approach for valorising forestry bark through optimised extraction and characterisation of tannin-rich fractions, while highlighting their potential as precursors for functional bio-based materials.