The results of PE and DE after each pretreatment are summarized in Table 19.3.

For samples after organosolv extraction (Group 1), the PE and DE were 51.4 % and 76.5 %, respectively. The results indicated that organosolv extraction had a significant effect on delignification. The delignification efficiencies are comparable or slightly higher than those reported in previous studies [42, 50]. The higher PE and higher DE might be accounted for by the slightly higher temperature (by 20 °C) employed in this work. Pasquini et al. [51] found that both temperature and pressure can affect PE and DE: A higher temperature and pressure could lead to an increase in the PE and DE. In a study by Pasquini et al., the DE was on the order of 93.1 % for P. tarda wood chips when 16.0 MPa and 190 °C were employed. This process also leads to the formation of acetic acid or other carboxylic acids that can act as catalysts accelerating the rupture of lignin-carbohydrate polymers in the presence of water.

For Group 2 samples (with organosolv extraction followed by ultrasonic treat­ment), slightly improved PE (53.3 %) and PE (77.2 %) were obtained. The results suggest that the ultrasonic treatment did affect PE and DE, although the effect was not significant. This could be caused by the decreased particle size of pine sawdust after ultrasonic treatment. A decrease in particle size would lead to an increase in specific surface area and thus release of hemicellulose and lignin. Ultrasonic treat­ment has been widely used to enhance the extraction of hemicellulose in alkaline solutions by introducing violent cavitation. In this study, the ultrasonic treatment of pine sawdust accounted for only 3-5 wt% weight loss, due to the fact that distilled water rather than alkaline solution was used as liquid phase in the ultrasonic treat­ment. Extractives from hemicellulose and cellulose could not be dissolved easily in water. The comparable DE values with and without ultrasonic treatment (76.5 % vs. 77.2 %) suggest that ultrasonic treatment was not very effective for removing lignin.

For Group 3 samples (after organosolv extraction followed by NaOH treatment), the PE (57.7 %) and DE (81.5 %) were significantly higher, implying the high effi­ciency of NaOH treatment in the extraction of hemicellulose and lignin. When the organosolv extracted solid was added into NaOH solution, its color turned to dark brown, and the mixture was much finer and more viscous than it was before the treat­ment. This could be explained by the fact that the NaOH treatment of lignocellulosic materials can cause swelling which leads to an increase in internal surface area, a decrease in the degree of polymerization, a decrease in crystallinity, and disruption of the lignin structure [13, 32].

It is not surprising to observe that organosolv extraction followed by ultrasonic and NaOH treatment in sequence (Group 4) led to the highest PE (61.1 %) and DE (86.4 %), approximately 10 %, respectively higher than that for the Group 1 pretreatment, as shown in Table 19.3.