# Steam Explosion-Based Method for the Extraction of Cellulose and Lignin from Rice Straw Waste
## 3 Results and Discussion
### Characterisation of Cellulose Fibres
These findings highlight the importance of peak broadening and the amorphous background when evaluating the crystallinity of the samples. The higher crystallinity index observed in the steam-treated sample for 33 min without CO2, despite not being the highest, may be more reflective of the true crystalline content, as it is less influenced by the broadening effects seen in other samples. This finding is consistent with the untreated natural fibres and demonstrates that the cellulose extracted from rice straw exhibits a crystallinity index that matches that which was reported for untreated natural fibres [32; 69], suggesting that rice straw cellulose could be a suitable candidate for high-quality nanocrystalline materials. Figure 11 contains SEM images illustrating the morphology of the extracted cellulose fibres. The rice straw image depicts its structure before undergoing treatment, revealing the presence of a cell wall composed of hemicellulose and lignin, which serves to protect the cellulose. Throughout the analysis, significant differences have been noted between the treatments with and without ScCO2. The samples treated with ScCO2 exhibited an increased compactness in the cellulose fibres, which is indicative of a higher degree of amorphousness, in line with the previous discussion on the XRD. These fibres appeared denser and less defined, suggesting a profound alteration in the cellulose structure due to the presence of ScCO2. This effect is likely attributed to an enhanced breakdown of cellulose's crystalline structure, leading to the predominance of amorphous cellulose. Contrastingly, the samples treated without ScCO2, especially the 33 min treatment, presented better preserved fibrous structure of cellulose [32; 74; 75]. These fibres are more defined and less compact, indicating the retention of crystalline structures and consequently, less amorphousness. As confirmed in the FTIR and XRD tests, this treatment was more effective in removing lignin and hemicellulose without significantly altering the crystalline structure of cellulose. The SEM images also reveal varying degrees of defibrillation in the cellulose fibres. The treatments, particularly those involving ScCO2, facilitated a more complete defibrillation, breaking down the fibres into finer structures. In contrast, the treatments without ScCO2 led to the fibres retaining more of their original, less fragmented form. Furthermore, this analysis indicated the presence of possible remnants of cell wall materials or silica oxide crystals, characteristic of rice straw. These remnants were more evident in the samples treated less aggressively, preserving more of the original biomass structure. These findings provide valuable insights into the effects of steam explosion treatments with and without ScCO2 on the structure of the cellulose obtained from rice straw. Therefore, they underline the crucial role of the treatment conditions in determining the final characteristics of the cellulose fibres. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Steam Explosion Process** & **CI (\%)** & **CS (nm)** \\ \hline ScCO2 33 min & 66.47 & 12.18 \\ ScCO2 1 h & 49.90 & 12.90 \\ ScCO2 2 h & 64.32 & 4.37 \\ ScCO2 4 h & 37.03 & 1.65 \\
33 min & 46.20 & 11.30 \\
1 h & 53.57 & 14.14 \\
2 h & 36.31 & 13.34 \\
4 h & 52.72 & 15.51 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Crystallinity index (CI) and crystallite size (CS) obtained for the different steam explosion processes performed. Figure 11: _Cont._