Chemical and crystal evolution of the lignocellulosic components of coconut fibers during the alkaline and ultrasound treatments

Abstract

In this study, we demonstrate that a combined alkaline/ultrasonic treatment is highly effective in modifying the chemical composition, crystallinity, and morphology of coconut fibers (CCFs). The dual treatment primarily extracts surface hemicellulose while preserving the internal hemicellulose; notably, the lignin content remains largely unaffected. Furthermore, silicon microparticles and other inorganic components are almost completely eliminated during the dual treatment. SEM, TGA, and FTIR analyses reveal that alkaline treatment is more influential than ultrasonic treatment, though their combination yields a synergistic effect. After 15 min of dual treatment, the crystallinity index (CI) of the treated CCFs increased significantly to 74%, compared to 58% for the pristine fibers as shown in the XRD analysis. However, extended sonication decreased CI, a phenomenon attributed to localized erosion in the crystal regions induced by acoustic cavitation. Consequently, the water retention capacity of the fibers decreased from 68 to 48 wt%. Solid-state ¹³C NMR further corroborated the removal of hemicellulose and the partial extraction of lignin. This study demonstrates the effectiveness of dual alkaline/ultrasound treatment to control the properties of coconut fibers, which can be used as reinforcement in polymer matrices.