Investigation of the chemical basis for inefficient lignin removal in softwood kraft pulp during oxygen delignification

In this study, an industrial softwood kraft pulp sample was subjected to an oxygen-alkali treatment over three consecutive times as part of a broader research effort to identify methods for selectivity improvements and lignin reactivity inefficiencies. Chemical properties of the pulp were determined that included active functional group analysis of the residual and effluent lignins. Further data for the analysis of the chemical reactions were obtained by collecting and identifying the low molecular weight compounds in the effluent liquors. We determined that the residual lignin cannot be removed beyond 75% of the original levels, although the first oxygen delignification reaction can remove nearly half of the original resident kraft residual lignin. The main factor limiting lignin removal during the oxygen delignification process is the penetration of oxygen and oxygen-active species into the lignin matrix along the microfibrils in the pulp. The matrix structure of carbohydrate and the lignin carbohydrate complex constitute the main barriers against oxidative reactions. We have evidence that the hemicelluloses in the pulp are part of the lignin-carbohydrate complexes, and the robust nature of the native cellulose crystal structure in the pulp fiber is partly responsible for the barrier to lignin removal during oxygen-alkali treatment. In fact, xylan-linked lignin is more resistant to oxidative reactions and tends to remain in the pulp, while galactan-linked lignin tends to dissolve during oxygen delignification. The reduction of reactive groups including hydroxyl and carboxyl groups during oxygen delignification is also responsible for the low level of subsequent delignification. We also support the finding that the p-hydroxylphenyl structures and 5,5'-biphenolic units in residual lignin are stable and accumulate during oxygen delignification.