## 3 Results and Discussion
The Fe\({}^{2+}\) 2p spin-orbit doublet was resolved at binding energies of 711.2 eV (2p\({}_{3/2}\)) and 724.6 eV (2p\({}_{1/2}\)), and the Fe\({}^{3+}\) 2p spin-orbit doublet was resolved at binding energies of 713.4 eV (2p\({}_{3/2}\)) and 727.4 eV (2p\({}_{1/2}\)). Figure 8k presents the XPS spectra of Fe 2p for FS-FT (0.36 g, MO) after photodegradation. The Fe\({}^{2+}\) 2p spin-orbit doublet was resolved at binding energies of 711.0 eV (2p\({}_{3/2}\)) and 724.4 eV (2p\({}_{1/2}\)) and 724.4 eV (2p\({}_{1/2}\)), and the Fe\({}^{3+}\) 2p spin-orbit doublet was resolved at binding energies of 713.3 eV (2p\({}_{3/2}\)) and 727.3 eV (2p\({}_{1/2}\)). Figure 8k presents the XPS spectra of Fe 2p for FS-FT (0.36 g, MO-Na\({}_{2}\)SO\({}_{4}\)) after photodegradation. The Fe\({}^{2+}\) 2p spin-orbit doublet was resolved at binding energies of 711.0 eV (2p\({}_{3/2}\)) and 724.4 eV (2p\({}_{1/2}\)), and the Fe\({}^{3+}\) 2p spin-orbit doublet was resolved at binding energies of 713.3 eV (2p\({}_{3/2}\)) and 727.3 eV (2p\({}_{1/2}\)) [32; 33]. The Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio is 73:100 after iron doping. The reason for the occurrence of the Fe\({}^{2+}\) peak may be that Fe\({}^{3+}\) was reduced during the preparation process, or that Fe\({}_{3}\)O\({}_{4}\) in the core may be detected. In Figure 8k, the Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio changes to 63:100 for FS-FT (0.36 g, MO) after degradation in MO solution; it was found that the proportion of Fe\({}^{3+}\) decreased slightly, which could have led to a decline in photocatalytic performance [34; 35]. In Figure 8l, the Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio is 51:100 for FS-FT (0.36 g, MO-Na\({}_{2}\)SO\({}_{4}\)) after the degradation of MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution. The proportion of Fe\({}^{3+}\) further decreases, indicating that Fe\({}^{3+}\) was reduced to Fe\({}^{2+}\) during the reaction process, leading to the decrease in Fe\({}^{3+}\) and electron concentrations and in the photocatalytic activity [36; 37; 38]. Since the concentration of Na\({}_{2}\)SO\({}_{4}\) used in the printing and dyeing process is approximately 12.5 g/L, the selected concentration range for further studying the impact caused by Na\({}_{2}\)SO\({}_{4}\) is 10-17.5 g/L. Figure 9 shows an evaluation of the photodegradation rates of MO using FS-FT (0.36 g) in different MO-Na\({}_{2}\)SO\({}_{4}\)-blended solutions. The four sets of photocatalytic samples were placed in the dark chamber of the photocatalytic experimental setup for approximately 20 min to attain the adsorption-desorption equilibrium and subjected to irradiation with a HPML. The degradation rates of MO in different MO-Na\({}_{2}\)SO\({}_{4}\)-blended solutions were 99.21%, 97.16%, 90.12%, and 83.36% for CN\({}_{2}\)SO\({}_{4}\) = 10.0, 12.5, 15.0, and 17.5 g/L, respectively. These results indicate a decreasing trend in the degradation efficiency from increasing the concentration of Na\({}_{2}\)SO\({}_{4}\) within the range of Na\({}_{2}\)SO\({}_{4}\) concentration from 10 to 17.5 g/L. Firstly, protonation of the photocatalyst surface generates positively charged species, establishing a surface charge conducive to photocatalytic reactions, which makes the dye with negative charge compete with SO\({}_{4}\)\({}^{2-}\) in the process of adsorption [39]. This competition reduces the chance of contact between the photocatalyst and dye. Secondly, the results of some researchers showed that in the process of the degradation of dyes using photocatalysts based on titanium dioxide, photogenerated electrons played a major role [24]. Fe\({}^{3+}\) combines with photogenerated electrons that should have reacted with MO, which inhibits the reaction between photogenerated electrons and MO, and decreases the number of Fe\({}^{3+}\)-capturing electrons, thus reducing the degradation effect [25]. These findings are in agreement with those of XPS in Figure 8j-l. Thirdly, sulfate radicals (SO\({}_{4}\)\(\bullet\)) are generated via the interaction of photogenerated holes and SO\({}_{4}\)\({}^{2-}\).