# Steam Explosion-Based Method for the Extraction of Cellulose and Lignin from Rice Straw Waste
## 3 Results and Discussion
### Characterisation of Cellulose Fibres
These results are in line with the previously shown FTIR spectra, evidencing the structural preservation of cellulose in the biomass at the 33 min mark without ScCO\({}_{2}\) (Figure 7). This is evidenced by the stable ratio between the -OH stretching band at 3400 cm\({}^{-1}\) and the C-H stretching band around 2900 cm\({}^{-1}\) in the FTIR spectrum, which are associated with the hydroxyl groups of cellulose and its overall molecular structure. This aligns with the high cellulose content observed in the TGA. Conversely, the increasing intensity in the ratio between the -CH stretching vibration at 2900 cm\({}^{-1}\) and the C-O stretching band at 1054 cm\({}^{-1}\)--representing cellulose and hemicellulose bonds--in the CO\({}_{2}\)-treated samples over time aligns with the reduced cellulose yield from the TGA, suggesting cellulose destabilisation. Furthermore, X-ray diffraction (XRD) patterns, shown in Figures 9 and 10, revealed the presence of crystalline cellulose I\(\beta\) in the extracted samples, with peaks at 16.5\({}^{\circ}\), 22.6\({}^{\circ}\), and 32.5\({}^{\circ}\) 20 aligning with the (110), (200), and (004) planes, supporting the identification of this polymorph, which is consistent with the diffraction pattern of the reference cellulose (ICDD patent PDF-2 database, File no. 00-050-2241). Cellulose I\(\beta\) is the most dominant allomorph in higher plants and is frequently targeted for the production of high-quality nanocellulosic materials due to its crystalline nature [73]. The additional diffraction peaks observed throughout the 90\({}^{\circ}\) 20 range indicate the presence of other crystalline constituents such as silica, a known component of rice straw. The presence of broad peaks throughout the diffractograms is indicative of a substantial amorphous fraction, which must be considered when interpreting the crystallinity indices (CI) and crystallite sizes (CS) provided in Table 4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Steam Explosion** & **Hemicellulose (\%)** & **Cellulose (\%)** & **Lignin (\%)** & **Yield (\%)** \\ \hline ScCO\({}_{2}\) 33 min & 22.40 \(\pm\) 2.84 & 60.12 \(\pm\) 0.50 & 17.48 \(\pm\) 3.34 & 25.00 \(\pm\) 1.70 \\ ScCO\({}_{2}\) 1 h & 30.79 \(\pm\) 7.47 & 56.55 \(\pm\) 1.67 & 12.66 \(\pm\) 9.14 & 19.10 \(\pm\) 1.90 \\ ScCO\({}_{2}\) 2 h & 20.82 \(\pm\) 1.63 & 66.74 \(\pm\) 3.01 & 12.44 \(\pm\) 1.38 & 17.60 \(\pm\) 0.45 \\ ScCO\({}_{2}\) 4 h & 26.49 \(\pm\) 0.70 & 55.49 \(\pm\) 0.89 & 18.02 \(\pm\) 0.19 & 15.20 \(\pm\) 1.70 \\
33 min & 14.31 \(\pm\) 0.84 & 77.36 \(\pm\) 0.94 & 8.33 \(\pm\) 0.10 & 23.00 \(\pm\) 0.15 \\
1 h & 23.42 \(\pm\) 1.27 & 52.00 \(\pm\) 2.10 & 24.58 \(\pm\) 0.83 & 19.69 \(\pm\) 1.00 \\
2 h & 27.79 \(\pm\) 0.21 & 59.44 \(\pm\) 0.24 & 12.77 \(\pm\) 0.03 & 18.55 \(\pm\) 0.55 \\
4 h & 28.15 \(\pm\) 2.00 & 57.49 \(\pm\) 2.54 & 14.36 \(\pm\) 0.54 & 15.19 \(\pm\) 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Compositions and yields of the rice straw waste subjected to different treatment conditions. When comparing the steam explosion processes, the data evidence the influence of \(\mathrm{ScCO_{2}}\) on the crystalline and amorphous contents of cellulose. A longer exposure to steam in the presence of \(\mathrm{ScCO_{2}}\) is associated with a reduced crystallinity and crystallite size, as reflected by the calculated values. The steam-treated sample for 33 min without \(\mathrm{ScCO_{2}}\) exhibits a lower crystallinity index compared to the one with \(\mathrm{ScCO_{2}}\), which could be affected by the amorphous broad component on the crystalline peaks. In that sense, the presence of broader peaks along the \(\mathrm{ScCO_{2}}\) diffractograms suggests that the absence of \(\mathrm{ScCO_{2}}\) potentially favours the preservation of the crystalline structure of cellulose. Figure 10: XRD profiles of extracted fibres after different durations of the steam explosion stage using \(\mathrm{ScCO_{2}}\). Figure 9: XRD profiles of extracted fibres after different durations of the steam explosion stage.