From Agricultural Residue to Nanomaterial: Extraction and Characterization of Cellulose Nanocrystals from Pigeon Pea Husk (Cajanus cajan)

Abstract

Cellulose nanocrystals (CNCs) were produced from pigeon pea husk (Cajanus cajan), an underutilized agro-industrial residue, through controlled sulfuric acid (H₂SO₄) hydrolysis. Hydrolysis with 64 wt.% H₂SO₄ at 45 °C for half an hour, followed by dialysis, probe sonication, and freeze-drying, yielded 61.25% CNCs relative to purified cellulose. Scanning and transmission electron microscopy (SEM & TEM) confirmed rod-shaped nanocrystals with a mean length of 335 ± 12.91 nm, width of 28.80 ± 2.67 nm, and aspect ratio of 11.63. X-ray diffraction (XRD) analysis showed a crystallinity index of 91.83%, indicating a highly crystalline structure. Dynamic light scattering (DLS) measurements showed a hydrodynamic diameter of 288.08 ± 9.07 nm and a polydispersity index (PDI) of 0.451, while the zeta potential of −43.21 ± 2.34 mV indicated favourable electrostatic characteristics for aqueous dispersion. Fourier-transform infrared (FTIR) spectroscopy confirmed the characteristic cellulose functional groups, while differential scanning calorimetry revealed a prominent thermal event near 300 °C. These findings highlight pigeon pea husk as a sustainable, renewable precursor for cellulose nanocrystal (CNC) production and demonstrate the potential of agricultural residues as valuable feedstocks for advanced nanocellulose materials. The physicochemical and structural characteristics of the extracted CNCs provide a basis for further research into their material performance and potential applications in nanocomposites, functional materials, and sustainable material development.