Effect of selective and aggressive alkaline delignification using Na₂SO₃ and NaOH on the structural characteristics of cellulose nanocrystals derived from Calotropis gigantea seed fiber

Cellulose nanocrystals (CNCs) are promising biomass-derived nanomaterials for biocomposite reinforcement applications due to their high crystallinity, large surface area, and excellent mechanical properties. This study investigated the effect of Na₂SO₃ and NaOH alkaline pretreatments on the structural characteristics of CNCs isolated from C. gigantea seed fibers. CNCs were produced through sequential delignification, bleaching, and sulfuric acid (H₂SO₄) hydrolysis processes. The obtained materials were characterized in terms of yield, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Particle Size Analysis (PSA). The results demonstrated that the type of alkaline treatment significantly influenced the physicochemical properties of the CNCs. Na₂SO₃ treatment produced a higher CNC yield (31.11 ± 1.04%) than NaOH (22.83 ± 1.60%), indicating a more selective delignification mechanism with greater cellulose preservation. In contrast, NaOH treatment generated CNCs with superior nanostructural characteristics, including a smaller particle size (0.43 µm), higher crystallinity index (85.13%), and lower crystallite size (2.27 nm), compared with Na₂SO₃-treated CNCs, which exhibited a particle size of 1.35 µm, crystallinity index of 73.25%, and crystallite size of 3.52 nm. FTIR analysis confirmed the progressive removal of lignin and hemicellulose through the reduction of characteristic lignocellulosic absorption bands. At the same time, XRD patterns revealed enhanced crystalline ordering of cellulose type I after chemical treatment. SEM observations showed a morphological transformation from hollow tubular fibers into aggregated rod-like nanocrystalline domains, particularly in the NaOH-treated samples. PSA results further indicated that NaOH promoted more intensive fibril fragmentation than Na₂SO₃. Overall, NaOH treatment was more effective in producing CNCs with a smaller particle size and higher crystallinity, whereas Na₂SO₃ was more efficient in preserving cellulose yield. The CNCs obtained from NaOH treatment demonstrated greater potential as reinforcement fillers for polymer-based biocomposite applications.