A soft processing technology for the extraction of cellulose from plant residues and agri-food wastes

In this study, cellulose was extracted from giant cane (GC), Posidonia oceanica seagrass (PO), coffee silverskin (CS), and brewer’s spent grain (BSG) as alternatives to conventional sources of cellulose. The extraction protocol involved three steps: i) hemicellulose and lignin removal through alkaline hydrolysis in a 5% (w/v) NaOH so- lution (solid-to-liquid ratio = 1:100 g/mL, T = 25 ◦C, t = 2 h, ω = 300 rpm), ii) removal of organic compounds and ashes through a 95% (v/v) ethanol solution (solid-to-liquid ratio = 1:25 g/mL, T = 25 ◦C, t = 0.5 h, ω = 500 rpm), and iii) double bleaching in a 1% (w/v) acidic (pH = 4) NaClO2 solution (solid-to-liquid ratio = 1:50 g/mL, T = 90 ◦C, t = 1.5 h, ω = 500 rpm). Yield, purity, crystallinity degree, and morphology of cellulose extracted through a soft-chemical cascade process were assessed by gravimetric, infrared (FT-IR), nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. Averaged cellulose extraction yields of 36.4, 38.6, 23.1, and 22.2% for GC, PO, CS, and BSG were obtained, respectively. All cellulose samples had high purity, though lower than the ultra-pure bacterial cellulose, which was due to the slight contamination from unremoved hemicellulose and lignin residues. Cellulose samples exhibited similar chemical features and the typical fibril-like morphology of microcrystalline cellulose (6–13 μm in width). The versatility of the proposed extraction procedure supports the sustainable conversion of low-cost organic biomasses to valuable products with manifold industrial applications (e.g., food packaging).