Effect of nitric acid on pretreatment and fermentation for enhancing ethanol production of rice straw

•HNO3 is a potent acid catalyst to release xylose from rice straw and allow for efficient enzymatic hydrolysis.•RSM was applied to optimize HNO3 pretreatment condition for maximum xylose yield.•Under the optimal condition, a maximum xylose yield of 86.5% in the pretreatment and an enzymatic digestibility of 83.0% were achieved.•Nitrate, the neutralized form of nitric acid, was found to be used as a nitrogen source in the fermentation process. In this study, nitric acid (HNO3) was evaluated as an acid catalyst for rice straw pretreatment, and, after neutralization, as a sole nitrogen source for subsequent fermentation. Response surface methodology was used to obtain optimal pretreatment condition with respect to HNO3 concentration (0.2–1.0%), temperature (120–160°C) and reaction time (1–20min). In a condition of 0.65% HNO3, 158.8°C and 5.86min, a maximum xylose yield of 86.5% and an enzymatic digestibility of 83.0% were achieved. The sugar solution that contained nitrate derived from the acid catalyst supported the enhancement of ethanol yield by Pichia stipitis from 10.92g/L to 14.50g/L. The results clearly reveal that nitric acid could be used not only as a pretreatment catalyst, but also as a nitrogen source in the fermentation process for bioethanol production. It is anticipated that the HNO3-based pretreatment can reduce financial burden on the cellulosic bioethanol industry by simplifying after-pretreatment-steps as well as providing a nitrogen source.