Pretreatment Using Ozonolysis And Ball Milling To Improve Enzymatic Saccharification Of Corn Straw

With the development of economy and society, the growing demand for energy is leading to the gradual depletion of conventional fossil fuels. Therefore, exploitation of nonpetroleum-based sources of energy is important. Bioethanol produced from lignocellulosic materials has the advantage of high efficiency, being environmentally friendly and renewable. Owing to the recalcitrant structures of lignocellulosic materials, pretreatment is an essential step. The aim of the pretreatment is to break down the lignin structure and disrupt the crystalline structure of cellulose for enhancing enzymes accessibility to the cellulose, thus improving the enzymatic saccharification.In this study, pretreatments using ozonolysis(OZ) and planetary ball milling(BM) were applied separately and in combination and their effect on enzymatic hydrolysis of corn straw was analyzed. The results are as follows.(1) Pretreatment of corn straw using BM individually was effective in reducing the particle size and crystallinity index(Cr I) and increasing the specific surface area of corn straw. The optimal milling time was 8 min, which resulted in glucan conversion of 75.0% and xylan conversion of 45.3%, increasing by 3.3- and 4.1-fold that of untreated corn straw, respectively. Sample treated with ball milling for 8 min(BM8) existed as quite smaller particles, as well as greatly destroyed and shorter fiber bundles compared to raw material, whereas the later existed as much larger sheets, as well as compact, smooth and well-organized fiber bundles.(2) Pretreatment of corn straw using OZ individually was effective in degrading partial lignin and a few xylan of corn straw. The optimal OZ time was 90 min, which resulted in a glucose yield of 152.9 mg/g-straw and a xylose yield of 81.9 mg/g-straw after emzymatic saccharification, increasing by 1.5- and 3.2-fold that of untreated corn straw, respectively. Prehydrolysis washing of corn straw sample OZ for 90 min(OZ90) before enzymatic hydrolysis increased the glucose yield to 332.3 mg/g-straw and xylose yield a little. As partial lignin removed, the morphology of OZ90 showed much rougher surfaces and looser fiber bundles than raw material, which was benefit for enzymatic saccharification.(3) OZ followed by BM-treatment(OZ-BM) was effective in not only degrading partial lignin but also reducing the particle size and cellulose Cr I and increasing the specific surface area of corn straw. The optimal sample was OZ for 90 min followed by BM for 8 min(OZ90-BM8), which resulted in a glucose yield of 407.8 mg/g-straw and a xylose of 101.9 mg/g-straw after emzymatic hydrolysis, increasing by 6.6- and 5.2-fold that of untreated corn straw, respectively. In comparison to that of BM8 sample, glucose yield of combined OZ90-BM8 increased by up to 54%, while xylose yield improved slightly. In comparison to that of OZ90 sample, glucose yield and xylose yield of combined OZ90-BM8 increased by 23% and 13%, respectively. The microscopy image of OZ90-BM8 was similar to that of BM8, which existed as quite smaller particles, as well as greatly destroyed and shorter fiber bundles compared to raw material.(4) BM followed by OZ-treatment(BM-OZ) improved the delignification efficiency of subsequent OZ pretreatment and therefore was beneficial for efficient enzymatic saccharification. The optimal sample was BM for 1 min followed by OZ for 90 min(BM1-OZ90), which resulted in a glucose yield of 373.2 mg/g-straw and a xylose yield of 82.5 mg/g-straw after emzymatic hydrolysis, increasing by 5.1- and 3.2-fold that of untreated corn straw, respectively. In comparison to that of BM1 sample, glucose yield and xylose yield of combined BM1-OZ90 increased by up to 4.3- and 2.7-fold. In comparison to that of washed OZ90 sample, glucose yield of combined BM1-OZ90 sample increased by 11%, while xylose yield decreased slightly. The morphology of BM1-OZ90 sample was similar to that of OZ90 sample. As partial lignin removed, it exhibited much rougher surfaces and looser fiber bundles than raw material, which was benefit for enzymatic saccharification.(5) When the amount of Celluclast 1.5L cellulase decreased from 15 to 1.5 FPU/g of corn straw, the sugar yields of BM8, OZ90, OZ90-BM8 and BM1-OZ90 maintained at nearly the same level, whereas those of untreated corn straw decreased significantly after emzymatic hydrolysis, indicating that the cellulase loading for enzymatic hydrolysis of pretreated samples decreased greatly. The enzymatic hydrolysates of BM8, OZ90, OZ90-BM8 and BM1-OZ90 could be used directly in the fermentation of ethanol by Saccharomyces cerevisiae. The conversion ratio of glucose to ethanol from all the four hydrolysates was above 90%.In this study, OZ and BM were combined to improve the enzymatic hydrolysis of corn straw. It was a promising pretreatment for lignocellulosic biomass, taking advantage of environmentally friendly, mild reaction condition and low enzyme loading.