Toward the green disaggregation of kraft lignin and characterization of aggregation beheviour of disaggregated lignin

Abstract

Lignin is an aromatic biopolymer mainly present in lignocellulosic biomass. Its industrial application is severely constrained by the heterogeneous, complex structure and aggregation behaviours. This study investigated a mild chemical route to alter the aggregation behaviours of softwood kraft lignin using sodium bicarbonate/carbonate-assisted NaOH dissolution and a NaOH-only dissolution, both followed by precipitation at pH 5 and pH 3 using citric acid. The main objective of the research was to investigate how controlled alkaline dissolution followed by citric acid treatment could disrupt the lignin intermolecular aggregation without structural degradation of its main aromatic structure.

Raw and treated samples were characterized using ATR-FTIR, UV-Vis spectroscopy, DLS, WAXS, GPC, zeta potential, nitrogen adsorption analysis and TGA. ATR-FTIR analysis showed that the main aromatic structure of lignin was preserved after treatment. Changes in the O-H region and the introduction of a carbonyl/carboxyl peak near 1700 cm-1 indicated the non-covalent functionalization of the softwood kraft lignin. The UV-Vis and DLS results demonstrated that the treated lignin exhibited a reduction in particle size, a decrease in PDI and improved dispersion in an aqueous medium. WAXS analysis illustrated increased d-spacing in the treated samples, indicating a clear change in short-range molecular packing. Furthermore, GPC analysis showed a distinct shift in the molecular weight distribution toward lower molecular weight regions, indicating the disaggregation of the treated samples. TGA results illustrated improved thermal stability in the treated samples, which may be attributed to possible citric acid esterification with lignin hydroxyl groups during heating. The findings suggested that the controlled alkaline dissolution followed by citric acid precipitation is an effective strategy for the non-covalent functionalization of lignin, successfully disaggregating softwood kraft lignin without significant change in core molecular structure. However, nitrogen adsorption analysis showed a lower surface area of dried, treated samples compared to raw lignin. This may be due to re-aggregation and compact packing during open-air drying. Future work should focus on quantifying the degree of citric acid functionalization using advanced analytical techniques. Additionally, strategies should be developed to preserve these disaggregation features in the dry state for long periods to ensure sustainable industrial applications.