Thermal characterization of softwood lignin modification by termite Coptotermes formosanus (Shiraki)

Wood-feeding termites (WFT) pretreat wood cell walls to achieve efficient cellulose utilization by modifying the protecting lignin polymers. An analytical protocol consisting of three-stage-pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry (TG/DTG) was employed along with attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy to investigate softwood degradation by the lower termite Coptotermes formosanus. The results revealed that softwood-lignin moieties were altered during termite digestion. This was characterized by preferential ring demethoxylation, dehydroxylation, and methylation, side chain carboxylation and oxidation, methylation, and oxidation on the ring hydroxyl group, as well as depolymerization and ring destruction. The TG/DTG data demonstrated remarkable changes in the thermal behavior of the processed softwood and also in termite feces, showing decreased activation energy and pre-exponential factor at the second stage (330–400 °C) of lignin pyrolysis. Meanwhile, ATR-FTIR analysis suggested aromatic ring destruction, methoxyl and hydroxyl group modification, C–H deformations in –CH3 and –CH2 groups, splitting/modification of aliphatic side chains in lignin, and conformation changes of lignin. Such modifications induced changes in pyrolysis products and bond energies in lignin, and provided information on softwood digestion by termite.