Multi-scale characterization and mechanistic comparison of acid–alkali pretreatment during cell wall deconstruction of Achnatherum splendens

Abstract

Achnatherum splendens was used as a high-lignin herbaceous feedstock to compare the effects of dilute sulfuric acid and sodium hydroxide pretreatments on component degradation and cell wall structure. Chemical composition analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used for multi-scale characterization. The results showed that dilute sulfuric acid pretreatment mainly promoted hemicellulose degradation, with a hemicellulose degradation rate of 79.2% under 1.5% H2SO4, while lignin dissolution remained limited at 20.5%. In contrast, NaOH pretreatment showed stronger lignin removal and fiber separation, with lignin dissolution increasing to 44.8% at 4% NaOH. Both pretreatments increased cellulose crystallinity from 47.0% to between 53.4 and 60.8%, whereas crystallite size showed no significant change, indicating that the crystalline cellulose domains were largely preserved. FTIR results showed spectral inconsistencies after acid treatment, including increased absorbance at 1510 cm−1 and limited reduction at 1730 cm−1, which may be associated with pseudo-lignin-related interference. SEM observations showed that acid treatment generated pores and cracks, whereas alkali treatment resulted in more obvious fiber bundle separation and surface cleaning. These results indicate that acid and alkali pretreatments have complementary effects on the deconstruction of Achnatherum splendens. The findings provide structural-level evidence for pretreatment selection and optimization of high-lignin energy grasses.