The Changing Structure of Residual Lignin in the Unbleached Bagasse Pulp During Chlorine Dioxide Delignification

Herein, residual lignin was extracted from unbleached sugarcane bagasse pulp via the sequential enzymatic-weak acid hydrolysis method. Subsequently, chlorine dioxide delignification was applied to the extracted lignin. The structural changes of lignin during chlorine dioxide delignification were characterized by Fourier transform infrared spectroscopy (FT-IR), 13C-nuclear magnetic resonance (13C NMR), 1H-13C heteronuclear single quantum coherence (2D HSQC), and quantitative 31P-nuclear magnetic resonance (31P-NMR) spectroscopy. The degradation products of lignin dissolved in the simulated bleaching liquor, extracted, and quantified via gas chromatography-mass spectrometry (GC-MS). Lignin was significantly degraded during the delignification process, showing cleavage of the aromatic ring, removal of the methoxyl group, and cleavage of major interunit linkages. However, the carboxylic acid content increased and GC-MS analysis indicated that lignin was degraded into several organic substances dissolved in the liquor. However, only a few absorbable organic halides (AOX) were generated, confirming that chlorine dioxide bleaching is an ideal bleaching method.