Studying the effect of raw material particle size on the properties of isolated nanocellulose is crucial for enhancing material performance, optimizing processing techniques, and promoting sustainability in various industrial applications. Therefore, this study aims to extract and characterize nanocellulose from spent coffee ground (SCG) with different particles size (A: 850-1000, B: 500-850, C: 350-500 μm). For this purpose, SCGs were purified using alkaline and bleaching agents to extract cellulose, followed by acid hydrolysis (sulfuric acid 64%, 45 °C, 1 h) combined with ultrasonication treatment to produce nanocellulose. The obtained nanocelluloses were characterized in terms of morphology, size distribution, sulfur content, ζ-potential, crystallinity, crystal size, thermal stability, and re-dispersibility in water. Atomic force microscopy confirmed the production of spherical nanocellulose (CNS) with diameter ranging from 65.21 nm in A-CNS to 49.31 and 48.06 nm in B and C-CNS, respectively. The ζ-potential of fresh dispersions decreased with particle size, from -44 mV for A-CNS to -41 mV and -40 mV for B and C-CNS, respectively. The sulfur content also decreased gradually from 214.30 (A-CNS) to 191.29 mmol/kg (C-CNS). All CNS were relatively thermally stable, with thermal analysis showing enhanced stability for A-CNS. The crystallinity index (64-69%) and the crystal size (2.21-2.39 nm) of CNS were not affected by the particle sizes. In summary, the CNS dimensions, ζ-potential of fresh dispersions, sulfur content and thermal stability of CNS varied significantly among samples, depending on the particle size of SCG. Therefore, it is strongly recommended that researchers choose the optimum particle size for producing nanocellulose for the desired applications.

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