Natural deep eutectic solvents as green pretreatment agents in the biocascade processing of onion peel waste

Abstract

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.