Valorisation of waste lignocellulosic biomass into UV-absorbing lignin: comparative extraction and characterization

Background Agro-industrial lignocellulosic waste streams are widely available and under-utilised sources of biomass which can be valorised for the production of high-value bio-derived materials. The non-edible aromatic biopolymer lignin holds great prospects as a renewable UV-protective agent for safer and sustainable applications such as coatings and agri-food packaging. Methods Lignin was isolated from four lignocellulosic substrates including wheat straw, cotton stalk, bamboo sawdust and eucalyptus sawdust by the Klason lignin process. In addition, FTIR was used to detect functional groups (guaiacyl, syringyl, carbonyl) of the isolated lignins; XRD for fundamental structure determination, TGA for thermal stability and UV–Vis spectroscopy by which the absorbance of UV radiation was analysed including UV-B spectrum area (290–320 nm), compared with TiO₂ and ZnO). Results Lignin yield Bamboo sawdust showed the highest lignin yield (24.11%) and was followed by eucalyptus (21.42%), wheat straw (17.31%), and cotton stalk (15.25%). FTIR demonstrated consistent lignin-associated aromatic and carbonyl functionalities in all samples, however XRD showed differential structural changes of the lignins. The TGA results presented a higher thermal stability for bamboo and eucalyptus lignins, with the other two lignins beginning to decompose at temperatures below 250°C. Moreover, the absorbance in UV range was analyzed by UV–Vis and it has been observed that bamboo lignin has strong absorption in UV region; particularly compared to the absorbance of TiO₂ and ZnO under tested conditions due to its higher absorbance toward UV-B at wavelengths ranging from 290 to 320 nm. Conclusion The results reveal a potential biomass and waste valorization route for functional lignin production from agro-industrial wastes, in which bamboo sawdust is identified as an exceptional feedstock mainly due to its high yield, thermal stability and excellent UV-blocking capability. This research paves a way towards the growth of safer and renewable UV absorbing materials and is also built upon socially solid and environmentally responsible practices helping humanity and the planet.