Review on Separation and Modification Technologies of Kraft Lignin and Their Potential Applications as Environmental Materials

Kraft lignin is a major biomass byproduct generated in large quantities from the kraft pulping process and has attracted attention as a renewable aromatic polymer with the potential to replace petroleum-based aromatic compounds. Since the kraft process accounts for the majority of global pulp production, large amounts of lignin are continuously generated, providing a stable supply of biomass-derived resources. Kraft lignin contains a complex aromatic structure and various functional groups; however, variations in molecular weight distribution and functional group composition caused by differences in feedstock species and pulping conditions lead to significant quality inconsistency. In addition, sulfur-containing structures and a high degree of condensation act as major technical barriers during separation, purification, and subsequent modification processes, thereby limiting its application in high-value materials. Various separation and purification technologies, including acid precipitation, membrane separation, and solvent-based fractionation, have been applied to obtain lignin fractions with properties suitable for specific applications. In particular, the abundant phenolic hydroxyl and carboxyl groups of kraft lignin have enabled its development as water-treatment adsorbents and porous carbon precursors for the removal of heavy metals and organic pollutants, demonstrating its potential as an environmental material. In applications such as adhesives, resins, and carbon materials, structural reproducibility is also an important requirement, emphasizing the importance of selective separation and structural control technologies. However, the high capital investment and production costs required for high-purity purification and modification processes remain major challenges for large-scale industrial utilization. Furthermore, quality standardization and the establishment of large-scale production systems are essential for practical applications. Future research should focus on the development of selective separation and fractionation technologies capable of precisely controlling molecular weight and functional group composition, along with the transition to continuous processing systems to enhance industrial applicability.