Ultrasonic-Assisted Alkaline Pretreatment for Enhanced Bioethanol Production from Water Hyacinth (Eichhornia crassipes) Biomass

Water hyacinth (Eichhornia crassipes) is an abundant lignocellulosic biomass with strong potential as a feedstock for bioethanol production, but its relatively high lignin content limits the accessibility of cellulose and inhibits the subsequent hydrolysis process. This study aims to analyze the effect of ultrasonic-assisted pretreatment duration and NaOH concentration on the lignocellulosic composition and the resulting bioethanol yield from water hyacinth biomass. Pretreatment was carried out at 60°C and 60 W power with time variations of 10, 15, 20, 25, and 30 minutes and NaOH concentrations of 1 M and 2 M. The pretreated samples were analyzed for lignin, cellulose, and hemicellulose content using the Chesson method, then hydrolyzed with 3% H2SO4, fermented for 120 hours using Saccharomyces cerevisiae, distilled, and characterized. The results showed that the optimum pretreatment condition was obtained at a treatment time of 25 minutes with 2 M NaOH, which reduced the lignin content from 12.1% to 5.4%, reduced the hemicellulose content to 10.8%, increased the cellulose content from 56.0% to 75.1%, and produced the highest glucose content of 8.2%. Under these optimum conditions, the resulting bioethanol had an ethanol content of 14%, a density of 0.980 g/mL, a pH of 6.5, and a distillate volume of 42 mL. Gas chromatography-mass spectrometry analysis further confirmed ethanol as the dominant compound in the distillate, supporting the validity of the refractometric measurement. Beyond 25 minutes of pretreatment, however, the lignin content, cellulose content, and bioethanol yield all showed a slight decline, indicating that excessively prolonged ultrasonic exposure can degrade part of the biomass structure and reduce process efficiency. Overall, these results indicate that ultrasonic pretreatment combined with 2 M NaOH effectively enhances delignification and improves the potential for bioethanol production from water hyacinth biomass, although further optimization is still required to achieve a fuel-grade ethanol content.