Isolation and physicochemical characterization of different lignin streams generated during the second-generation ethanol production process

Four different lignin samples were isolated during the processing of rice straw for ethanol production: one after mild alkaline pretreatment (deacetylation) of rice straw, and other three after simultaneous saccharification and fermentation (fermentation residues), under different process conditions. Then, a complete chemical characterization in terms of main components (lignin, carbohydrates, among others) and elemental composition was performed for all the samples. Additionally, the structural and morphological characteristics, calorific value, thermal stability, solubility in solvents and antioxidant potential were also determined. For comparison, a standard lignin (Kraft lignin) was also assayed. The results revealed that the process used for lignin isolation has an important influence in the final properties of the sample. In general, the fermentation residues contained non-hydrolyzed polysaccharides in their composition and lower lignin content when compared to the lignin solubilized in the alkaline liquor. Among several organic solvents and mixtures, acetone 80% (v/v) was the most efficient for lignin solubilisation. Interesting properties and characteristics were observed for all the lignin samples isolated from rice straw processing, which would allow their application in different industrial areas. Based on the properties, potential applications were suggested, which could be of value for integration in biorefineries.