Hydrothermal-process-based direct extraction of polydisperse lignin microspheres from black liquor and their physicochemical characterization

Lignin nano-/microstructures are widely employed for agricultural drug delivery and heavy metal removal from wastewater, and facile low-cost methods of their large-scale production are therefore highly sought after. Herein, uniform-morphology polydisperse lignin microspheres were directly extracted from black liquor by lowering its pH to <4 followed by hydrothermal treatment and featured several lignin-typical characteristics, e.g., functional groups, thermal stability, amorphousness, and monolignol units. It was assumed that lignin rearranged and assembled into microspheres of various size, shape, and uniformity depending on pH, temperature, and hydrothermal treatment time. Lignin microsphere extraction efficiency was estimated as 15.87–21.62 g L−1, and the average size of microspheres obtained under different conditions was calculated as ∼1 µm, while the C, H, O, and N contents equaled 48–62, 5–6, 30–36, and 0.2–1.5%, respectively. Thus, our method was deemed suitable for direct large-scale extraction of lignin microspheres from black liquor.