Extraction of cellulose nanocrystals from millet (Eleusine coracana) husk waste: optimization using Box Behnken design in response surface methodology (RSM)

Abstract

Cellulose nanocrystals (CNCs) and their applications have attracted growing interest in both research and industry due to their appealing properties, such as excellent mechanical properties, high surface area, rich hydroxyl groups for modification, and natural properties that are 100% environmentally friendly. Cellulose nanocrystals were extracted from millet husk residue waste using a homogenized acid hydrolysis method. The effects of the process variables homogenization speed (A), acid concentration (B), and acid to cellulose ratio (C) on the yield and swelling capacity were investigated and optimized using the Box Behnken design (BBD) method in response surface methodology. The cellulose and nano-cellulose obtained were characterized using transform infrared microscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The numerical optimization analysis results showed that the maximum yield of CNCs and swelling capacity from cellulose was 93.12% and 2.81% obtained at homogenization speed, acid concentration, and acid to cellulose ratio of 7464.0 rpm, 63.40 wt%, and 18.83 wt%, respectively. ANOVA revealed that the most influential parameter in the model was homogenization speed for Yield and acid concentration for swelling capacity. The mathematical models to predict cellulose nanocrystals’ yield and swelling capacity were developed with R 2 of 98.9% and 97.9%, respectively. The TGA showed that the thermal stability of cellulose was higher than that of CNCs. FTIR results showed that functional groups of CNCs and cellulose were similar. SEM image of CNCs is porous and displayed narrow particle size with needle-like morphology compared to cellulose. The XRD pattern presented an increase in the intensity of CNCs. Graphical abstract

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