Improved hydrolysis yields and silica recovery by design of experiments applied to acid-alkali pretreatment in rice husks

Abstract In this work, a two-step pretreatment using acid and alkali was optimized for rice rusks (Oryza sativa) using a 25−1 fractional factorial design (FFD), followed by a central composite design (CCD) to further optimization of enzymatic saccharification. The effect of five variables was simultaneously evaluated: H2SO4 concentration (from 0–5.4 % w/w); NaOH concentration (0–6 % w/w); temperature (85–125 °C); time (20–100 min) and solid to liquid ratio (S/L = 5–12.5 % w/w). The best pretreatment conditions were: 1.8 % w/w of H2SO4 in the first step and 6 % w/w of NaOH at 85 °C for 100 min at a S/L = 12.5 % (w/w) in the alkaline step, which resulted in 58.7 mg of glucose/g substrate, an 8-fold increase compared to the sample in natura (7 mg/g). In rice husks, in contrast to the results commonly found in literature, NaOH extracts mainly silica instead of lignin, while H2SO4 has an important role in lignin removal. High purity silica (97 %) was isolated at high yields (70 %) from the alkaline liquor by a simple and scalable process, which could contribute to making ethanol production from this biomass economically viable.