Structural Features of Dilute Acid Pretreated Acacia mangium and Impact of Sodium Sulfite Supplementation on Enzymatic Hydrolysis

Lignocellulosic biomasses (LCB) differ in their chemical composition and cell wall architecture from one another and within the same LCB due to varying geographical conditions. Thus, pretreatment parameters need optimization for recovery of sugars across different biomasses. In the current study, Acacia mangium has been investigated at bench scale for its potential as a feedstock for fermentable sugar production. Attempts were first made to target hydrolysis of hemicellulose using dilute acid (DA) as a catalyst. Pretreatment at temperature of 160 °C, residence time of 30 min and H₂SO₄ concentration of 2% (w/w) yielded highest overall saccharification efficiency (50%) corresponding to a glucan conversion in enzymatic hydrolysis (57.8%). Further enhancement in glucan conversion and reduction in the formation of inhibitors was brought about by using sodium sulfite (SS). It was observed that SS caused a significant increase in overall sugar recovery. Interestingly, the order in which SS and DA were added to the pretreatment medium was an important strategy to improve enzymatic saccharification. The condition (SS→DA) where sodium sulfite (SS) was added right in the beginning along with the biomass followed by dilute acid (DA) addition at the desired temperature was more effective in improving the glucan conversion yield (77.0%). Highest BET surface area of SS→DA residue (3.7 m²/g) among all the pretreated residues is one of the factors contributing to this high conversion yield. To get further insight into the basis for improved saccharification, cellulase adsorption studies were conducted. The results showed that the solid residue obtained by SS→DA despite having the lowest maximum adsorption capacity (σ_max) resulted into highest saccharification yield which was supported by its highest affinity constant (K_a = 0.25 mL/mg) for the enzyme. Among the pretreated residual solids, SS→DA residue showed lignin modification and cellulose peak alterations by FT-IR spectroscopy and increased surface area by BET measurement revealing implication in improved enzymatic saccharification and overall sugar recovery.