In Situ Characterization of Lignin Precipitation by Synchrotron Small-Angle X-Ray Scattering

Lignin is an underutilized industrial byproduct with strong potential for sustainable applications. In organosolv pulping, lignin is dissolved into the spent liquor and can be recovered by antisolvent precipitation to form micro- and nanoscale particles. However, the relationship between mixing conditions and structural properties of the lignin particles remains poorly understood, limiting the controlled production of high-value lignin products. By studying this precipitation process in situ using synchrotron X-rays, we obtain information about the nanoscale structure of the particles and their formation. In the small-angle X-ray scattering experiment, structural units consistent in scale with lignin subunits reported in previous literature were initially observed. Later in the process, the precipitated lignin consisted of larger structural units in the size scale of 60–120 nm, which aggregated to form yet larger structures. An upper time limit of 1 second is obtained for this process. The aggregation and the relevant size scale is verified by scanning electron microscopy. Understanding the hierarchical structure formation in lignin precipitation promotes exploitation of lignin biomass at industrial scale.