Comparative evaluation of oxalic acid reuse approaches in biomass pretreatment for enhanced sugar and ethanol production

The study presents a comparative evaluation of three reuse approaches for oxalic acid in the pretreatment of sugarcane leaves, Napier grass, and pandan pulp for bioethanol production. The optimal pretreatment conditions were established at 6% oxalic acid, 100 °C, and 105 min for Napier grass, yielding maximum sugar release. Pretreatment under this condition was repeated across three approaches: Approach A involved supplementing reused liquid with fresh oxalic acid to maintain 10% solid loading; Approach B added water to the reused liquid to maintain solid loading; Approach C adjusted acid content through titration and oxalic acid addition. Subsequent pretreatments under the same conditions across three approaches for the biomasses revealed significant improvements in cellulose content, while hemicellulose showed partial removal in early cycles of reuse. The highest sugar yield attained for sugarcane leaves, Napier grass, and pandan pulp was 84.81% (approach C), 93.84% (approach A), and 95.24% (approach C), respectively. Among the approaches, approach A demonstrated the highest ethanol yields of 0.37 g/g sugarcane leaves, 0.40 g/g Napier grass, and 0.43 g/g pandan pulp, correlating with lower acetic acid accumulation as an inhibitor. Although Approach C achieved elevated sugar yields in sugarcane leaves and pandan pulp, its greater oxalic acid consumption led to higher inhibitor formation and reduced ethanol productivity. Approach B, involving water addition during reuse, could maintain comparable ethanol yields as in approaches A and C. Overall, Approach B emerged as the most economical and feasible approach for bioethanol production across diverse biomasses with reduced chemical usage for 5.2–6.4 times.

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