Natural deep eutectic solvent for cellulose isolation from almond shell powder: A sustainable approach for by-product valorization

Cellulose was isolated from almond shell through different pretreatments: a conventional 7.5 % NaOH method and a natural deep eutectic solvent method using choline chloride:urea (2:1), both treatments carried out at 80 °C for 4 h. A control without pretreatment was also included. All samples underwent subsequent bleachings with NaClO 2 (1.3 %, 2 h, 75 °C, twice) and with NaOH 10 %:H 2 O 2 6 %:H 2 O (1:1:100) (1.5 h, 60 °C). Pretreatment with ChCl:urea proved to be the most effective, removing up to 90.1 % of lignin and increasing cellulose content from 28.4 to 63.2 g/100 g dry matter. Fourier Transform Infrared Spectroscopy confirmed lignin removal through 1726 and 1125 cm⁻¹ bands attenuation. X-Ray analysis indicated cellulose enrichment up to a 68 % (p < 0.05) in crystallinity. Thermogravimetric analysis revealed improved thermal stability, with degradation onset and peak temperatures between 250 and 271 °C and 355–360 °C, respectively, due to reduced amorphous content. High whiteness (average 86.7 %) was observed after bleachings. SEM micrographs confirmed cellulose fibrillation accompanied with a decrease in the degree of polymerization. These results highlight the importance of pretreatment for effective delignification and cellulose isolation. ChCl:urea treatment emerged as an efficient option against conventional methods, supporting future strategies for agricultural waste valorization.