Impact of Sulfite (SPORL) Pretreatment on the Lignin Carbohydrate Complex Structure
The mode of action of sulfite pretreatment (SPORL) process involves the addition of sulfur dioxide during pretreatment to promote sulfonation of lignin molecules; the lignin molecules changed its character from being hydrophobic to hydrophilic as a result of this sulfonation, which in turn promotes enzymatic hydrolysis in the absence of nonproductive enzyme binding which is otherwise inherent because of the lignin hydrophobic character (Shuai et al. 2010; Cheng et al. 2011). It is important to understand the sulfite pulping chemistry of the sulfonation reaction which requires that the SO2 is impregnated into the biomass, which actually gets chemically attached within the lignin structure during SPORL pretreatment conditions. The resultant lignin therefore from the sulfite pretreatment (lignosulfonate) cannot be easily water washed to completely remove the sulfur and will require the use of an organophilic phase to remove the impurities. An organophilic phase such as ethanol reduces the surface tension and penetrates the structure to remove the impurities from lignosulfonate. Lignosulfonates resulting from the sulfite pretreatment could find their applications as pesticides, emulsifiers, dyeing for textile, etc. Like dilute acid pretreatment, during SPORL pretreatment, sul- fonation is enhanced at lower pH, i.e., acidic SPORL pretreatment reaction conditions increase the sulfonation and make it more difficult to decontaminate the lignosulfonate with water washing (Bu et al. 2012). According to Mendonca et al. (2004), during SPORL pretreatment, lignin sulfonation and degradation resulted in delignification and its distinctive composition varied based on the SPORL pretreatment temperature and pH conditions (Mendonca et al. 2004; Zhu et al. 2009). SPROL pretreatment under alkaline reaction conditions has also been reported to lead to an increase in sulfonation and lignin solubilization, whereas the cellulose structure was found to remain unchanged (Mikhail and Adriaan 2012). Alkaline SPORL conditions reported higher lignin solubilization and removal than acidic or neutral reaction conditions. Using ethanol in place of alkaline, SPORL pretreatment resulted in higher sugar yields. This was attributed to the fact that ethanol has a lower boiling point than water so during the SPORL pretreatment reaction pressure becomes higher at the reaction conditions and penetrates the structure, increasing the hydrophilicity of the lignin (Bu et al. 2012) and confirmed the higher degree of sulfonation with ethanol by using the FT-IR spectroscopic studies. The fact that the bands that were attributed to the absorption of sulfonic groups were stronger in the spectrum of the ethanol SPORL-pretreated sample than SPORL-pretreated samples without ethanol indicates that the degree of sulfonation of the ethanol sulfite- treated sample was higher. According to Eriksson et al., chemical and physical structure of lignin plays a critical role in enzyme interactions with lignin and the rate and extent of hydrolysis, and therefore addition of surfactants increases cellulose hydrolysis to D-glucose (Eriksson et al. 2002).