Influence of residual lignin and thermal drying on the ultrastructure of chemical hardwood pulp and its enzymatic hydrolysis properties

Abstract Recycled pulp can be used as a potential and sustainable resource for production of fermentable monosaccharides. In this work, the effects of residual lignin and thermal drying on ultrastructure, physical properties and further enzymatic digestibility of chemical pulps were investigated. The results showed that drying process (at 105 °C for 24 h) did not significantly change the functional groups but it resulted in increased cellulose crystallinity and fibril aggregation, pore shrinkage and decreased swelling ability of fibers. On the other hand, residual lignin (~ 3%) in unbleached pulp restrained partially the deterioration of those properties during drying, compared to bleached pulp (lignin content 0.3%). Compared with once-dried bleached pulp, the unbleached pulp after drying had a slightly higher proportion of amorphous region, higher average pore size and specific surface area, and lower hornification degree, which are favorable for enzymatic hydrolysis. Moreover, the fibrils may have wrapped the lignin when they aggregated, reducing the unfavorable adsorption of lignin to cellulase. Enzymolysis also showed that the once-dried unbleached pulp was much easier to digest, and yielded 66% glucose after 12-h hydrolysis, which was much higher than the yield (54%) of the once-dried bleached pulp and should have good prospects. Graphical abstract