In alignment with circular economy principles and the sustainable valorisation of biomass, this
work addresses an intermediate stage in a novel green biocascade strategy aimed at converting
onion peel waste, an abundant agro-industrial by-product, into cellulose nanofibers. At this stage,
the process focuses on the selective removal of non-cellulosic structural components to enrich the
cellulose fraction within the lignocellulosic matrix. To achieve this objective, Natural Deep
Eutectic Solvents (NADES) were employed as environmentally benign alternatives to
conventional toxic solvents for the pretreatment step. A systematic screening was conducted to
evaluate various combinations of hydrogen bond acceptors (HBAs), such as choline chloride and
betaine, with different hydrogen bond donors (HBDs), primarily organic acids. Key operational
parameters, including temperature, NADES-to-onion peel ratio, HBA:HBD molar ratio, and
reaction time (up to 10 hours), were thoroughly investigated. Among the tested formulations,
choline chloride:lactic acid (ratio 1:2) and choline chloride:tartaric acid (ratio 1:2) mixtures
demonstrated the most promising performance, operating effectively at temperatures between 100
and 120 ºC. Under these conditions, and after 6 hours of processing time, an increase in cellulose
content exceeding 50% was achieved in the treated biomass, with cellulose and lignin identified
as the main remaining constituents in the solid fraction. This approach represents a sustainable and
efficient pathway for the valorisation of agri-food residues through green chemistry, supporting
the advancement of integrated biorefineries and fostering the transition towards circular
bioeconomy models.
