Trade-offs between sulfidity and physicochemical properties of recovered <i>Eucalyptus</i> kraft lignin

Abstract Kraft lignin is a promising biopolymer for product diversification in the pulp and paper industry and integrated biorefinery concepts. This study evaluated the effect of white liquor sulfidity (0–40 %) on Eucalyptus spp. kraft lignin recovered by acid precipitation (CO 2 and HCl). The recovered lignins were characterized in terms of elemental composition, thermal behavior, and chemical structure. While sulfidity had a limited impact on the overall thermal stability and decomposition profiles, it strongly affected pulping performance, including total yield, H-factor, and cooking severity. Notably, the sulfur and sodium contents of the recovered lignin varied systematically with sulfidity, reflecting changes in the delignification pathway and black liquor chemistry. Higher sulfidity enhanced sulfide-assisted lignin fragmentation, leading to the incorporation of sulfur-containing functionalities (e.g., thiols and mercaptans) and a reduced dependence on sodium for lignin solubility. The outcomes provided insights into how sulfidity can influence lignin properties with implications for its subsequent use in advanced materials applications.