Effect of Lignin Content on Ultrasound-Induced Nanocellulose Formation in Biorefinery Lignin–Cellulose Mixtures

Lignin–cellulose mixtures (LCMs) generated as intermediates in wood biorefineries are commonly separated into lignin and cellulose. However, using ultrasound (US) to process these mixtures could create novel, valuable materials not possible with conventional methods. This study looked at how lignin affects the US modification of these mixtures. Crude and partially delignified LCMs were successfully prepared using aqueous solutions of EtOH, THF and dilute NaOH and then subjected to short, high-power US treatment. The resulting materials were characterised using FT-IR spectroscopy, particle size analysis, water retention value analysis, SEM and XRD. Sonication rapidly reduced the mean particle size, generating cellulose nanofibril-like structures in all samples according to SEM. The response depended strongly on lignin content, with samples containing lower amounts of lignin exhibiting substantially higher hydration capacity and stronger US responsiveness. At the molecular level, lignin removal exposes cellulose surfaces and enhances hydrophilic interface formation, increasing water uptake and suspension stability. Thus, results show that lignin limits accessible hydrophilic cellulose surface area rather than preventing fragmentation by sonication. US is therefore a chemical-lean strategy to tune the physicochemical properties of partly delignified LCMs and expand the product portfolio of integrated wood biorefineries towards novel advanced lignocellulosic materials.