Agro-industrial residues are an abundant and underutilized source of lignocellulosic biomass with
strong potential for the development of bio-based materials. In this study, onion peels—a major
by-product of the agri-food sector—were valorised for the production of cellulose nanofibers
(CNF) through a green pretreatment based on Natural Deep Eutectic Solvents (NADES), followed
by mechanical fibrillation (microfluidization). The NADES system, composed of betaine
(hydrogen bond acceptor, HBA) and tartaric acid (hydrogen bond donor, HBD) in a 1:2 molar ratio
(HBA:HBD), was applied at 120 °C for 6 hours. This treatment yielded a solid residue enriched in
cellulose (72%) and lignin (15%), suitable for further processing. Subsequently, the pretreated
biomass was the treated using a Microfluidics LM-20 unit equipped with a Z-type interaction
chamber (H210Z, 200 μm). The influence of homogenization pressure (up to 150 MPa) and the
number of passes was evaluated. The resulting CNFs exhibited a high aspect ratio as confirmed by
morphological analyses. Additionally, they demonstrated promising techno-functional properties,
including excellent water retention capacity and potential as emulsifying agents. This process
provides a sustainable and efficient alternative to conventional chemical pretreatments and aligns
with circular bioeconomy principles by converting low-value agricultural waste into high-value
nanomaterials. These findings support the implementation of NADES-based technologies in
integrated biorefinery strategies aimed at producing advanced materials from renewable sources.
