Isolation and characterization of cellulose nanocrystals from reticulated fibers covering the stem of milk thistle (Silybum marianum L.).

This study investigates the valorization of the reticulated fibrous sheath covering the stems of Silybum marianum (milk thistle) as a novel lignocellulosic source for the production of cellulose nanocrystals (CNCs). A modified Kruscher-Hoffer pretreatment, followed by alkaline extraction and multi-step bleaching, was applied to isolate purified cellulose with a yield of 49.0%. Subsequent sulfuric acid hydrolysis was performed to obtain two distinct products: D2 (single-step bleaching) and D3 (two-step bleaching). CNC production through the D2 process achieved a total yield of 26.46% based on the raw material, whereas the D3 process resulted in a yield of 21.56%. Structural characterization using FTIR, XRD, SEM, and TEM confirmed the effective removal of non-cellulosic components. XRD analysis revealed high crystallinity indices of 86% and 77% for D2 and D3, respectively. Thermal analysis indicated that the D2 sample (single-step bleaching) exhibited higher thermal stability than the D3 sample (two-step bleaching), with the main degradation peak occurring at 368.1 °C, compared with 299.5 °C for D3. This difference may be related to differences in the surface chemistry of the isolated CNCs. The isolated CNCs exhibited high colloidal stability, with a maximum zeta potential of -37.7 mV, which was further supported by long-term visual observations showing no sedimentation after 30 days. Morphological observations confirmed the formation of rod-like nanocrystals, indicating the successful isolation of a CNC-rich fraction. These results suggest that Silybum marianum stem fibers represent a promising, high-yield agricultural residue for the sustainable production of reinforcing agents for bio-based composite applications.