Lignin microparticles from coconut fiber: Chemically alkaline extraction, antibacterial, antioxidant, and UV barrier...

Coconut fiber is an abundant lignocellulosic waste with high lignin content, yet its valorization into functional materials remains limited. This study reports, for the first time, the isolation of lignin microparticles from coconut fiber (MPLs-CF) via alkaline hydrolysis and provides a comprehensive evaluation of their structural, thermal, and multifunctional biological properties in comparison with commercial lignin microparticles (MPLs-C). Morphological analysis revealed that MPLs-CF possess a porous architecture with a relatively homogeneous particle size distribution. Spectroscopic and elemental analyses confirmed a higher abundance of oxygen-containing functional groups in MPLs-CF, which may contribute to their chemical reactivity. X-ray diffraction patterns showed that the lignin microparticles exhibit predominantly amorphous to semi-amorphous structures. Thermogravimetric analysis demonstrated that MPLs-CF display a slightly lower primary degradation temperature (249 °C) than MPLs-C (268 °C), while both materials produced high carbon residues (∼25%), highlighting their suitability as sustainable carbon precursors. The in vitro multifunctional performance of MPLs-CF was systematically investigated at concentrations of 0-20% (w/w). Total phenolic content and antioxidant activity increased in a concentration-dependent manner, reaching maximum values at 15-20% (w/w). Antibacterial assays revealed enhanced inhibitory activity with increasing concentration, with the highest effectiveness against Staphylococcus aureus and Salmonella typhimurium, followed by Escherichia coli and Bacillus cereus. In addition, MPLs-CF dispersions exhibited excellent UV-shielding performance, effectively blocking UV-C and UV-B radiation while moderately attenuating UV-A, without compromising visible-light transparency. Overall, MPLs-CF exhibit favorable structural, thermal, antioxidant, antibacterial, and UV-barrier properties under the in vitro conditions tested. These findings suggest that MPLs-CF can be explored as an eco-friendly, biomass-based multifunctional material.