Effects of ethylene glycol - ferric chloride pretreatment on the enzymatic hydrolysis of cotton stalks

Abstract: Cotton stalk rich in carbohydrates can widely be hydrolyzed and saccharified to produce biofuels and biochemicals. However, the dense and complex network structure makes it difficult to be hydrolyzed in normal cases. In this work, the cotton stalk was pretreated by ethylene glycol and ferric chloride, and subsequently enzymatic hydrolyzed to fermentable sugars for the production of value-added products. An orthogonal experiment was also carried out to optimize the pretreatment conditions, including the ferric chloride concentration, solid/liquid ratio, reaction temperature, and reaction time. Results showed that the influencing factors on the hemicellulose and lignin removal of the cotton stalk were ranked in order of the ferric chloride concentration > solid/liquid ratio > reaction temperature > reaction time. The optimal conditions for the removal of lignin and hemicellulose were achieved, where 0.1 mol/L ferric chloride, 90%(v/v) ethylene glycol, solid/liquid ratio of 1:15 (w/v), reaction temperature 160 °C, reaction time 20 min with 79.7% cellulose remaining, while 85.7% lignin and 88.9% hemicellulose removal, and cellulose content increased by 133.0%, hemicellulose and lignin content decreased by 67.2% and 58.0%, respectively. In addition, the structure and physical properties of the original and pretreated cotton stalk were characterized by N2-adsorption desorption (BET), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA). Experimental results indicated that the specific surface area and pore volume increased approximately by 420.8% and 450.0%, respectively, compared with the original one, particularly that the pore size rose to 1.7 nm. The increase of specific surface area, pore volume, and size greatly contributed to the enzyme accessibility to lignocelluloses. The crystallinity of pretreated cotton stalk also increased by 9.5%, compared with the original one. The removal of amorphous portion, hemicellulose, and lignin led only to the exposure of inner crystalline cellulose, thereby to fully access to cellulase for hydrolysis. The weight loss below 400 °C was 63% for the original stalk, and 89% for the pretreated residue in the TGA test, respectively, indicating that the removal of lignin effectively made cotton stalk less thermally stable after pretreatment. The enzymatic hydrolysis of pretreated stalks was conducted in a constant temperature shaker, where the cellulase loading of 8.3 FPU/g-dry substrate, substrate concentration of 5% (w/v), pH 4.8, temperature 50 °C, and the rotary speed of 150 rpm. It was found that the concentration of ferric chloride posed a significant effect on the enzymatic hydrolysis, compared with solid/liquid ratio, reaction temperature, and time. The glucose yield reached the highest (88.5%) after 48 h from the sample pretreated under the optimal conditions. Furthermore, the cellulose in the pretreated cotton stalk was completely converted to glucose at the hydrolysis time of 72 h, and cellulase loading of 8.3 FPU/g. The glucose yield increased by 7.6 times than before. Consequently, the ethylene glycol and ferric chloride effectively removed the lignin and hemicellulose, while remained cellulose to promote hydrolysis efficiency. Longer hydrolysis time and enzyme loading can also enhance the enzymatic hydrolysis for cotton stalk pretreatment.