Microwave-assisted acid and alkali pretreatment of Napier grass for enhanced biohydrogen production and integrated biorefinery potential

Abstract

Napier grass, a promising lignocellulosic energy crop, presents a complex composition that limits its bioconversion into fermentable products. To address this challenge, we applied microwave (MW) pretreatment assisted by acid and alkali, using varying chemical concentrations (0.5–1 % w/v) and pretreatment times (3–10 min). Acid-catalyzed MW pretreatment achieved a maximal hemicellulose removal of 69.8 %, while alkali-catalyzed MW pretreatment resulted in significant lignin removal of 65.5 %. Without chemical catalysis, the pretreated hydrolysate significantly increased hydrogen yield to 38.0 ± 2.9 mL H₂/g volatile solid (VS), five times greater than that obtained from untreated biomass. Hydrogen yield was further enhanced when the MW-pretreated solid underwent simultaneous saccharification and fermentation. The highest hydrogen yield of 89.2 ± 7.2 mL H₂/g VS was achieved from alkali-catalyzed MW pretreated solid (0.5 % w/v NaOH, 5 min), with a chemical oxygen demand (COD) solubilization of 62.6 %. Increasing the NaOH concentration to 1 % (w/v) slightly decreased hydrogen yield but significantly increased COD solubilization to 85.8 %. The high carbohydrate content facilitated rapid cellulase hydrolysis, producing and accumulating a high concentration of fermentable sugars. However, this accumulation subsequently led to a shift towards lactic acid formation. The improved hydrogen yield and increased COD solubilization, along with the shift towards lactic acid production, suggest the possibility of optimizing this process for simultaneous production of multiple valuable products in an integrated biorefinery approach, potentially enhancing the economic viability of biomass conversion.