Green Transforming of Rice Straw-Based Pulping Black Liquors: Synergistic Valorization of Lignin and Cellulose Nanoparticles for UV-Shielding Applications

Lignin, the second most abundant natural polymer after cellulose, is a complex aromatic biopolymer whose structure and properties are strongly governed by the extraction process, including kraft, soda, organosolv, and sulfite pulping. It typically constitutes 25–35% of softwoods and 10–20% of non-wood biomass, playing a critical role in determining mechanical strength and chemical reactivity. Lignin is primarily composed of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, which form a highly conjugated aromatic network responsible for its strong ultraviolet (UV) absorption capability. This inherent UV-shielding property positions lignin as a promising, sustainable alternative to conventional synthetic UV filters, particularly in light of growing concerns regarding UV-induced environmental and biological damage. In addition to lignin, other biopolymers such as cellulose, chitosan, tannins, and flavonoids are increasingly being explored as eco-friendly functional materials. Notably, nanostructured lignin demonstrates enhanced UV-blocking efficiency, along with significant antioxidant and antimicrobial properties, attributed to its high surface area and abundant reactive functional groups. These multifunctional characteristics make nano-lignin a highly attractive component in advanced biopolymer composites for applications in UV-protective coatings, sustainable packaging, cosmetics, and biomedical materials. The valorization of lignin into high-performance nanomaterials not only enhances material functionality but also contributes to waste minimization and supports the advancement of sustainable technologies within a circular bioeconomy framework.