On the mechanistic and kinetic role of hydrogen sulfide during kraft delignification

Journal article, 2026

A mechanistic understanding of kraft delignification remains incomplete, particularly regarding the role of hydrogen sulfide in the reaction kinetics. This work examines its influence from both mechanistic and kinetic perspectives. Laboratory-scale kraft cooks on birch wood meal were conducted in a flow-through reactor, providing two main advantages: (i) continuous sampling of the immediately cooled cooking liquor, enabling snapshots of dissolved components throughout the cook, and (ii) rapid adjustments of the liquor composition, enabling kinetic evaluation without a priori assignment to simplified rate constants. The sulfide dependency was found to cease partway through the process, persisting longer than expected from the reactions typically considered to control delignification, while simultaneously indicating that said reactions are unlikely to control the rate throughout the entire process. Furthermore, significant and previously unreported differences in the molecular weight distribution between kraft and soda lignin were observed. These differences were further investigated through transient experiments and 2D NMR analysis. The results suggest that HS- also enhances the delignification rate by the more extended demethylation occurring during kraft pulping, thereby enabling larger fractions of dissolved lignin to be removed compared with soda pulping.