Effect of hydrogen peroxide bleaching on anionic groups and structures of sulfonated chemo-mechanical pulp fibers

The properties of the chemo-mechanical pulp were determined by the position and quantity of anionic groups in the pulp fibers. In this study, the effects of anion group content and structure in sulfonated mulberry stem chemo-mechanical pulp (SCMP) were investigated during the hydrogen peroxide (H₂O₂) bleaching process. Surface morphology and anion group structure of the pulp fibers were analyzed by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GCMS). The results indicate that the sulfonic group content in bleached pulp decreases with the concentration of H₂O₂ to a minimum content of 71.53 μmol g⁻¹, which is reduced by 22.68% in comparison to the unbleached pulp content of 92.51 μmol g⁻¹. The content of carboxyl groups in pulp first increases and then decreases with the concentration of H₂O₂. The maximum carboxyl group content is 164.79 μmol g⁻¹, which is increased by 53.97% compared with unbleached pulp at 107.03 μmol g⁻¹. The total anion groups content of SCMP first increases and then decreases with the concentration of H₂O₂. FTIR and XPS analysis show that the content of anion groups on the surface of the pulp fibers increases slightly after H₂O₂ bleaching. GCMS analysis demonstrates that the content of uronic acid increases after H₂O₂ bleaching and the order of increase is 4-O-MeGlcA > GlcA > GalA. The contents of sulfonic acid group, total anion groups (TAGs) and surface anion groups (SAGs) of bleached pulp increase because of the hydroxyl group oxidation, terminal group reduction, methyl esterification of pectin and decomposition of polyxylose, resulting in the formation of new carboxyl groups and more uronic acid. This study reveals the mechanism of the effect of hydrogen peroxide bleaching on the anionic groups in the SCMP pulp, which is conducive to the development of bleaching technology for high yields of mulberry stem pulp.