A comparison of chemical pretreatment methods for improving saccharification of cotton stalks

The effectiveness of sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and ozone pretreatments for conversion of cotton stalks to ethanol was investigated. Ground cotton stalks at a solid loading of 10% (w/v) were pretreated with H2SO4, NaOH, and H2O2 at concentrations of 0.5%, 1%, and 2% (w/v). Treatment temperatures of 90 °C and 121 °C at 15 psi were investigated for residence times of 30, 60, and 90 min. Ozone pretreatment was performed at 4 °C with constant sparging of stalks in water. Solids from H2SO4, NaOH, and H2O2 pretreatments (at 2%, 60 min, 121 °C/15 psi) showed significant lignin degradation and/or high sugar availability and hence were hydrolyzed by Celluclast 1.5 L and Novozym 188 at 50 °C. Sulfuric acid pretreatment resulted in the highest xylan reduction (95.23% for 2% acid, 90 min, 121 °C/15 psi) but the lowest cellulose to glucose conversion during hydrolysis (23.85%). Sodium hydroxide pretreatment resulted in the highest level of delignification (65.63% for 2% NaOH, 90 min, 121 °C/15 psi) and cellulose conversion (60.8%). Hydrogen peroxide pretreatment resulted in significantly lower (p ⩽ 0.05) delignification (maximum of 29.51% for 2%, 30 min, 121 °C/15 psi) and cellulose conversion (49.8%) than sodium hydroxide pretreatment, but had a higher (p ⩽ 0.05) cellulose conversion than sulfuric acid pretreatment. Ozone did not cause any significant changes in lignin, xylan, or glucan contents over time. Quadratic models using time, temperature, and concentration as continuous variables were developed to predict xylan and lignin reduction, respectively for sulfuric acid and sodium hydroxide pretreatments. In addition, a modified severity parameter (log M0) was constructed and explained most of the variation in xylan or lignin reduction through simple linear regressions.