Analytical assessment of a ZnCl₂–Urea deep eutectic solvent for fruit and vegetable waste biorefining: synergistic catalysis, fractionation efficiency, and bioethanol yield enhancement

The global surge in fruit and vegetable (F&V) waste represents a critical challenge, necessitating innovative valorization strategies aligned with the circular bioeconomy. This study developed a novel, low-cost Type IV Deep Eutectic Solvent (DES) composed of zinc chloride and urea (ZnCl₂:Urea, 1:4 molar ratio) for the pretreatment of a blended lignocellulosic biomass from banana peel, lemon peel, and cabbage. The synthesized DES was thoroughly characterized using FTIR, NMR, TGA, and physicochemical analyses, confirming its formation via an extensive hydrogen-bonding network and high thermal stability (decomposition onset: 195 °C). The DES pretreatment under mild conditions (80 °C, 5 h) resulted in significant biomass fractionation, achieving 71.9% delignification and a 44.9% reduction in cellulose crystallinity index. This multi-scale deconstruction led to a high enzymatic digestibility, yielding 85.2% glucose after 72 hours. Subsequent fermentation of the hydrolysate, which contained negligible fermentation inhibitors (furfural < 0.1 g/L, HMF < 0.05 g/L), yielded 42.8 g/L of bioethanol with 84% fermentation efficiency. Comparative analysis with conventional 1% H₂SO₄ pretreatment revealed that the DES process, while marginally lower in ethanol titer, offered superior environmental benefits, including minimal inhibitor formation, reduced corrosivity, and excellent solvent recyclability over five consecutive cycles. Collectively, the results demonstrate that ZnCl₂:Urea DES is a highly effective, sustainable, and economically viable pretreatment agent, paving the way for its application in advanced biorefineries for F&V waste management and biofuel production.