Effect of bleaching process on the allomorph, crystalline index, morphology, and thermal stability of cellulose nanocrystals derived from rice straw

Abstract

Rice straw (RS), a renewable byproduct of rice production, represents a promising source of cellulose nanocrystals (CNC). Bleaching is essential for obtaining high-purity cellulose and reducing RS recalcitrance by removing hemicellulose and lignin. This study compares two bleaching methods—alkaline hydrogen peroxide (AHP, Route 1) and hydrogen peroxide-acetic acid (HPAA, Route 2)—and evaluates their effects on the structure and properties of RS-derived cellulose and CNC following H₂SO₄ hydrolysis. Analytical techniques were employed to assess chemical composition, structure, crystallinity, morphology, and thermal properties. Thermogravimetric analysis using the K–K method provided effective measurement of hemicellulose and lignin removal. Route 1 did not fully remove lignin, resulting in CNC1 with a cellulose I structure, high thermal stability, and a rod-like shape. In contrast, HPAA bleaching efficiently removes lignin and introduces carbonyl groups on the cellulose surface, increasing fiber flexibility. This enhances sulfuric acid penetration and disrupts hydrogen-bond networks in both crystalline and disordered regions. H₂SO₄ molecules contribute to dissolution, regeneration, and acid hydrolysis. After H₂SO₄ hydrolysis, CNC2 converts to cellulose II and displays a spherical shape with lower crystallinity and thermal stability than CNC1, mainly due to surface sulfate groups. These results demonstrate that HPAA bleaching is both effective and environmentally preferable for whitening cellulose derived from agricultural waste.