Effects of Acid and Alkali Pretreatments on the Degradation Patterns and Structural Properties of Lignocellulose in Energy Crop Arundo donax L.

Arundo donax L. is a significant energy crop and perennial grass, with its efficient conversion holding substantial implications for the utilization of agricultural biomass resources. However, the distinct effects of acid and alkali pretreatments on its lignocellulose degradation patterns and structural modifications remain inadequately characterized. This study utilized Arundo donax L. as raw material to compare the effects of dilute sulfuric acid and sodium hydroxide pretreatments on its component degradation and structural modifications. Single-factor experiments were conducted, and the mechanisms were investigated using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and scanning electron microscopy (SEM) analyses. The results indicated that dilute sulfuric acid pretreatment primarily degraded hemicellulose (up to 85.8%) with limited lignin removal (<13%), whereas sodium hydroxide pretreatment effectively removed lignin (66.8%). XRD analysis revealed that crystallinity after dilute acid treatment was significantly higher than that of untreated samples (p < 0.05). Sodium hydroxide treatment induced a concentration-dependent non-monotonic change in crystallinity: the crystallinity index (CrI) peaked at a 1% concentration, was significantly lower at 3% and 4%, and showed intermediate values at 2% and 5%. The apparent crystallite size remained at 3.0–3.3 nm, suggesting that both pretreatments primarily targeted amorphous regions. FTIR analysis confirmed that alkali treatment more thoroughly disrupted ester bonds and lignin. SEM images revealed that alkali-treated fiber bundles were more loosely packed with relatively smoother surfaces. In acid treatment, 100 °C was identified as the critical temperature for a significant increase in crystallinity, whereas in alkali treatment, temperature had no significant effect on crystallinity.