Low-Power Ultrasound-Assisted Methanesulfonic Acid–Formic Acid Pretreatment Coupled with Simultaneous Saccharification and Fermentation for Bioethanol Production from Sugarcane Leaves

This study investigates an energy-efficient and environmentally sustainable approach to enhancing bioethanol production from sugarcane leaves through synergistic pretreatment using low-power ultrasound and an organic acid co-solvent system. To reduce the energy demand and environmental impact associated with conventional pretreatment methods, methanesulfonic acid (MSA) and formic acid (FA) were combined under low-power ultrasonic conditions. A Box–Behnken design (BBD) based on response surface methodology (RSM) was employed to optimize the FA concentration, ultrasonication time, and ultrasound power. The optimal conditions were identified as 29.17% FA, 18.70 min of ultrasonication, and an ultrasound power of 16.61 W, with the MSA concentration maintained at 5%. These conditions promoted the disruption of the lignocellulosic structure through acoustic cavitation and acid-assisted modification, thereby enhancing cellulose accessibility while minimizing energy consumption. Subsequent simultaneous saccharification and fermentation (SSF) using Saccharomyces cerevisiae at 32 °C with an enzyme loading of 50 FPU/g achieved an ethanol yield of 73.22% of the theoretical yield (on a cellulose basis), corresponding to an ethanol concentration of 12.602 mg/mL after 48 h. Extending the fermentation time to 72 h resulted in only a marginal increase in ethanol yield of 0.8%, suggesting that a 48 h incubation period represents a potentially techno-economically optimal operating window by reducing processing time and associated production costs. Compared with separate hydrolysis and fermentation (SHF), SSF achieved a comparable ethanol concentration while shortening the overall processing time and eliminating the need for a separate hydrolysis step at 50 °C. These findings demonstrate that low-power ultrasound-assisted MSA–FA co-solvent pretreatment is a promising strategy for converting sugarcane leaf residues into bioethanol under mild and potentially cost-effective operating conditions.