Investigating Lignin Re-Deposition during Pretreatment of Pine and Alternative Methods for Pulp Washing

The use of ionic liquids in pretreatment of waste biomass has recently become a major step in the pulping process of breaking down biomass into its major constituents: cellulose, hemicellulose, lignin and various other chemicals such as proteins and other forms of carbohydrates. Previous experimental results on pulp recovery and lignin extraction showed that ionic liquids effectively separate lignin from the pulp and its cellulose and hemicellulose components. However, the question of lignin re-deposition onto the pulp during the pretreatment process remains unanswered. Experiments were conducted to investigate the cause of lignin re-deposition, and whether it occurs during the pretreatment process or in the washing step. These experiments focused only on the pretreatment of sieved pine using 80% aqueous triethyl ammonium hydrogen sulfate (TEA:HSO4). The Klason test, in which absorbance of light by lignin at 280 nm occurs, was used to quantify the presence of lignin after it was recovered by ethanol and Soxhlet extracted for 20 hours. It was found that the re-deposition occurs when pulp was washed with ethanol, whose high volatility allow for the solvent to easily be evaporated after the washing procedure. Afterward, proceeding experiments utilized 65% TEA:HSO4 as a washing solvent for one step, followed by ethanol for three more steps. Klason tests were performed for both IL-washed and ethanol-washed pulp for comparison. The Klason test results for IL-washed pulp yielded average ASL (acid soluble lignin) value of 1.24% and an average AIL (acid insoluble lignin) value of 0.023%. For ethanol-washed pulp, an average ASL value of 1.47% and an AIL value of 0.025% were found.