Extraction of high-purity lignin from the kraft pulping black liquor by enzyme purification process with alkaline-resistant xylanase and cellulase

In this study, the response surface methodology was first utilized to optimize the enzyme treatment conditions as reaction pH, temperature, time and enzyme dosage of 9.5, 45 °C, 94.5 min and 100 U/L. Under these parameters, the kraft pulping black liquor was treated with alkaline-resistant xylanase and cellulase, followed by acid precipitation to obtain enzyme-purified lignin (EPL). The yield, purity and physicochemical characteristics of EPL were contrasted with acid-purified lignin (APL) prepared at the same pH values. Results showed that the enzyme purification method generated lignin with lower molecular weight of 3532 g/mol, greater purity of 96.79 % and higher yield of 2.89 %. Compared with APL, EPL exhibited stronger UV absorption capacity. SEM images revealed that EPL had a rough and porous surface, whereas the surface of APL was relatively smooth. TGA analysis indicated the thermal stability of EPL (T_max = 333.5 °C) was superior to APL (T_max = 309.2 °C). Moreover, no significant differences were observed in the chemical functional groups and molecular structures of APL and EPL, suggesting that the addition of alkaline-resistant xylanase and cellulase didn't change the chemical structure of lignin. The favorable properties of EPL make it a promising application in the development of high-value composite materials and biodegradable plastics.