Fractional Recovery of Lignin from Black Liquor Using Acid-Assisted Flocculation with Lignin-Based Flocculants

Black liquor (BL), the alkaline effluent from straw pulping, is rich in lignin with diverse molecular weights but poses serious environmental challenges. Here, a stepwise acid-assisted flocculation (AAF) strategy was developed for fractional lignin recovery and water treatment, in which a quaternary-ammonium-modified lignin flocculant (L-CTA) was employed following the “using-waste-to-treat-waste” principle. Through sequential AAF, high- (>5000 Da), medium- (1000–5000 Da), and low-MW (800–1000 Da) lignin fractions were selectively recovered (recovery rates of 91.7, 88.0, and 89.5%, respectively) under mild conditions. Structural analyses revealed a molecular transition from β-O-4-rich, syringyl-dominated, and compact product of high-MW fraction, to more oxidized and guaiacyl-enriched low-MW lignin fractions with lower thermal stability. Mechanistic studies, by a combination of instrumental analyses (zeta potential, quartz crystal microbalance, and isothermal titration calorimetry) and theoretical computations (density functional theory and molecular dynamics), demonstrated that the AAF process followed enthalpy–entropy compensation: high-MW lignin separation relied on both an enthalpy reduction of electrostatic attraction and entropy increases of bound-water release during π–π stacking, whereas low-MW lignin favored an enthalpy-dominated process. This study provides a mechanistic-guided and environmental-benign route for simultaneous primary treatment of BL and fractional recovery of lignin, thereby contributing to sustainable development within the pulping industry.