## 2 Results and Discussion
### Investigating the Effect of Operational Parameters on Modified Fenton Reaction Efficiency
In Figure 2d, incremental time led to flax fibers with a more reduced average diameter, where the average diameter of the fibers after 15 h reached 5.2 \(\pm\) 0.3 \(\upmu\)m. It was shown that there was negligible improvement in the process efficiency by increasing the reaction time from 15 h to 24 h, according to the fiber diameter size profiles. Based on the obtained results in Figure 2, it is evident that the optimum condition was determined as 10 wt.% H2O2 and 2 wt.% FeSO4 for a 15 h reaction under neutral pH conditions. For the most favorable conditions, there was a significant reduction in fiber diameter distribution, including the average diameter of the FFs. Cellulose has a lower binding affinity to iron, as compared with other biomass fractions such as lignin, hemicellulose, and pectin that contain -COOH groups. Lignins and pectins can facilitate favorable binding with metal ions such as Fe (II), especially in the ionized state [28; 29]. As well, these components are more susceptible to Fenton oxidation, which results in the ability to produce micron-sized cellulose fibers [30]. Figure 3 represents the obtained diameter of the raw flax fiber (a; no treatment) and modified flax fiber (b; after Fenton AOR treatment). It was found that the modified Fenton AOR process removed non-cellulosic components such as lignin that comprise the fiber structure, resulting in the isolation of thinner fibers, as compared to alkali treatment (cf. Section 2.3). Also, Figure 4 shows that there was no significant change in the diameter of the samples upon treatment with alkali media (step 2, an additional treatment step) after the Fenton AOR process (step 1). Degumming of FFs with a modified Fenton oxidation AOR process affords isolation of cellulosic fibers, since oxidation treatment can effectively remove lignin, which serves as a binding agent for cellulosic composites in such lignocellulosic materials [31]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **ID Code** & **Experimental Conditions** \\ \hline Raw flax fibers & **Raw** & \\ Treated flax fiber with Fenton AOR & **F-T15** & Flax fiber with Fenton AOR at \\  & (Fenton AOR) & described conditions 1  \\ Treated flax fiber with Fenton AOR & **F-T15-Alk** & Flax fiber with Fenton AOR at \\  & (Fenton AOR & described conditions 1, followed + alkali treatment + Alkali) & by NaOH 5\% for 1 h \\ Treated flax fiber with alkali & **Alk** & NaOH 5\% for 1 h \\ treatment & (no Fenton AOR) & NaOH 5\% for 1 h \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental conditions and sample ID codes. Figure 4: Impact of various degumming techniques on the surface of flax fibers, with their respective standard deviations. The reader is referred to Table 2 for definitions of the sample ID codes used in this figure. Figure 3: Optical microscopy analysis of flax fiber samples. (**a**) Raw flax fiber with its respective diameter distribution, and (**b**) treated flax fiber with its respective fiber diameter after treatment by the Fenton AOR process at optimal conditions.