# Steam Explosion-Based Method for the Extraction of Cellulose and Lignin from Rice Straw Waste
## 3 Results and Discussion
### Characterisation of Lignin  
Transitioning from the characterisation of cellulose to the exploration of lignin extraction, this study narrows its scope to samples that showed exceptional outcomes in the cellulose extraction. Specifically, samples subjected to 33 min and 1 h of steam explosion with and without ScCO\({}_{2}\) were selected for lignin isolation from the lignin- and hemicellulose-rich supernatant post-steam explosion. The rationale for this selection was underpinned by the FTIR, TGA, XRD, and SEM analyses, which justified the preferential choice of the 33 min steam explosion without ScCO\({}_{2}\), reducing the time of this stage. Utilising the supernatant from the steam explosion stage, which contains high concentrations of lignin and hemicellulose, the main goal was to isolate lignin for future purification steps. Table 5 indicates the percentage composition of hemicellulose, cellulose, and lignin in each sample, derived from the established ranges in the TGA derivatives and standardised TGA data. Figure 11: SEM images of the fibres extracted at different steam explosion conditions. (**a**) Untreated rice straw, (**b**) 33 min, (**c**) 1 h, (**d**) 2 h, (**e**) 4 h, (**f**) 33 min ScCO\({}_{2}\), (**g**) 1 h ScCO\({}_{2}\), (**h**) 2 h ScCO\({}_{2}\), and (**i**) 4 h ScCO\({}_{2}\). A comparative analysis of the TGA data revealed notable differences between the samples treated with and without ScCO\({}_{2}\). The samples subjected to steam explosion for 33 min without ScCO\({}_{2}\) demonstrated a more favourable lignin yield compared to their counterparts. Moreover, the absence of ScCO\({}_{2}\) in the steam explosion process appeared to enhance the selectivity of the lignin extraction, as evidenced by the lignin content. Notably, the lignin content in the samples without ScCO\({}_{2}\) was consistently higher across the stages, particularly in the optimised 33 min condition. Furthermore, the FTIR analysis performed (Figure 12) provides a molecular-level insight into the lignin structure of rice straw treated with steam explosion under various conditions. The FTIR spectra show the analysis of the lignin extracted from a biomass substrate through a steam explosion process at 33 min and 1 h, with and without the presence of ScCO\({}_{2}\). The four spectra show different peaks that can be correlated with lignin content. These include the aromatic skeletal vibrations around 1500 cm\({}^{-1}\), the -C-O stretching indicative of guaiacyl and syringyl lignin units, and the -CH stretching which suggests aliphatic hydrocarbons [49]. The prominence of -OH stretching near 3400 cm\({}^{-1}\) suggests a high concentration of phenolic and aliphatic OH groups, typically abundant in lignin [76]. When comparing the 33 min without ScCO\({}_{2}\) spectrum to the other treatments, the former displays sharper and more defined peaks, suggesting less degradation and a higher preservation of the aromatic ring structure of lignin. The addition of ScCO\({}_{2}\) in the 1 h treatment seems to affect the lignin structure, as evidenced by the slight shifts and changes in the peak intensities, particularly in the functional groups associated with the aromatic  
Figure 12: FTIR spectra performed for the supernatants obtained after carrying out the different steam explosion processes. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Steam Explosion** & **Hemicellulose (\%)** & **Cellulose (\%)** & **Lignin (\%)** & **Yield** \\
**Process** & & & & \\ \hline
33 min & 23.78 \(\pm\) 1.78 & 24.57 \(\pm\) 1.19 & 51.65 \(\pm\) 2.88 & 19.96 \(\pm\) 0.47 \\
1 h & 25.99 \(\pm\) 1.46 & 27.27 \(\pm\) 1.33 & 46.74 \(\pm\) 2.79 & 13.64 \(\pm\) 1.58 \\
33 min ScCO\({}_{2}\) & 26.96 \(\pm\) 0.49 & 31.17 \(\pm\) 0.92 & 41.87 \(\pm\) 0.43 & 14.12 \(\pm\) 1.53 \\
1 h ScCO\({}_{2}\) & 32.04 \(\pm\) 0.96 & 23.52 \(\pm\) 0.85 & 44.44 \(\pm\) 1.81 & 17.21 \(\pm\) 0.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Compositions and yields of the steam explosion supernatant at different treatment conditions.