Enhancing enzymatic hydrolysis and biogas production from rice straw by pretreatment with organic acids

• Oxalic acid is the best chemical for pretreatment to enhance saccharification.
• Citric acid is the best chemical for pretreatment to enhance biogas production.
• Certain types of organic acid has advantages over inorganic acid for pretreatment.
• Mechanisms of organic acid pretreatment were proposed.
• RSM assisted to evaluate the influence of interactions of pretreatment parameters.

Lignocellulosic biomass is recalcitrant to enzymatic and microbial degradation due to its chemical compositions and physical structures that inhibit the hydrolysis reactions. To obtain a high sugar yield for biofuel production, pretreatment process is generally needed to improve efficiency of hydrolysis. Here, organic acids were used to pretreat rice straw. Using Response Surface Methodology (RSM), the effect of three pretreatment parameters, including acid concentration, treatment time, and reaction temperature, on pretreatment efficiency were evaluated and used to generate mathematic optimization model. The hydrolysis results indicated that oxalic acid pretreatment led to the highest enhancement of enzymatic saccharification up to 213.4 mg (starting from 500 mg of pretreated sample) when using 5.01% oxalic acid concentration at 135.91 °C for 30.86 min, which was 2.68 times higher than the untreated rice straw. The structural changes in the pretreated biomass were investigated using FT-IR analysis. The results suggested that lignin composition was significantly removed from the pretreated biomass, and the crystalline cellulose was also modified to amorphous form during pretreatment. Both untreated and pretreated rice straw were subjected to anaerobic digestion to evaluate the influence of organic acid pretreatment on biogas production. The highest biogas yield was 322.1 ml/g-rice straw when using citric acid pretreatment which was 7.40 times higher than untreated biomass. These results demonstrated that organic acids can be used for pretreatment of lignocellulosic materials to enhance hydrolysis and biogas production.