Neutral Sulfite Treatment of Lignocellulosic Pulps

Birch and eucalyptus kraft pulps were treated with sulfite solutions in neutral conditions (pH 7) at 170 °C for 60 min and at 190 °C for 180 min. The pulps were analyzed for kappa number, viscosity, carbohydrate composition, and optical properties. Additionally, UV resonance Raman (UVRR) spectroscopy was applied to collect information on the contents of hexenuronic acid (HexA), lignin, and its phenolic hydroxyl groups. The sulfite treatments resulted in (i) depolymerization and partial dissolution of cellulose, (ii) partial dissolution of xylan and substantial decrease in its HexA content, and (iii) removal of the major part of lignin and increase in its phenolic hydroxyl group content. The extensive removal of HexA and depolymerization of cellulose by neutral sulfite were unexpected phenomena that have not been previously reported. According to their degree of polymerization (DP), the pulps were mainly in the form of microcrystalline cellulose after the more drastic treatments.