## 3 Results and Discussion
The degradation efficiency of FS-FT (0 g) in five cycles was 90.25%, 89.18%, 77.31%, 88.66%, and 85.03%, respectively, and the average degradation rate was 86.08%. In Figure 5b, the degradation rates of MO using FS-FT (0.36 g) were 99%, 97.18%, 95.09%, 96.68%, and 95.21%, respectively, and the average degradation rate was 96.63%. The findings show that the moderate amount of metal doping can improve degradation performance. The cause of the gradual decrease in the degradation efficiency in the first three cycles is that during the degradation process, a water-insoluble intermediate product is adsorbed at the photocatalyst surface. This reduction in the catalyst-dye interfacial contact area limits surface light absorption, consequently lowering the photocatalytic efficiency [17]. Figure 6 illustrates the photodegradation of MO in MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution (C\({}_{\text{MO}}\) = 10 mg/L, C\({}_{\text{Na2 SO4}}\) = 12.5 g/L). First, 0.02 g of FS-FT with different levels of iron doping was placed into 10 mL of MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution in a dark room until it reached the absorption-desorption equilibrium. The adsorption rates of MO-Na\({}_{2}\)SO\({}_{4}\) using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g) were 12.25%, 11.11%, 11.86%, 12.23%, and 10.96%, respectively. Figure 6a illustrates the UV-VIS spectra of MO degradation in MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g). Figure 6b illustrates the absorption spectra of MO in the UV-VIS range using FS-FT (0.36 g) in MO-Na\({}_{2}\)SO\({}_{4}\) at various degradation time intervals. Figure 6c shows the degradation efficiency using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g) in MO-Na\({}_{2}\)SO\({}_{4}\). The photodegradation rates were 92.38%, 80.37%, 85.32%, 97.16%, and 77.54%, respectively. The findings demonstrate that the FS-FT (0.36 g) photocatalyst exhibits optimal degradation efficiency in the MO-Na\({}_{2}\)SO\({}_{4}\), but its photocatalytic efficiency is inhibited compared to its performance in degrading pure MO solution. When FS-FT is doped with an appropriate amount of Fe, the addition of Na\({}_{2}\)SO\({}_{4}\) suppresses the degradation of MO. The possible explanation for this phenomenon is that SO\({}_{4}\)\({}^{2-}\) competes for the surface-active sites of the photocatalyst, inducing competitive adsorption and thus reducing the photocatalytic performance [24; 25]. The relevant photocatalytic efficiency data of the produced materials for MO and MO-Na\({}_{2}\)SO\({}_{4}\) degradation are summarized in Table 1[26; 27; 28]. Comparative analysis reveals that the FS-FT (0.36 g) composite exhibits superior photodegradation efficiency. Furthermore, it demonstrates a recyclability rate of up to 90%, highlighting its potential for sustainable applications in photocatalytic systems. Figure 5: The degradation rates of MO in pure MO solution using (**a**) FS-FT (0 g); (**b**) FS-FT (0.36 g) for five cycles. Figure 6: (**a**) UV-VIS spectra of MO in MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution prior to and after photodegradation using FS-FT (0–0.45 g); (**b**) time evolution of UV-VIS spectra of MO degraded using FS-FT (0.36 g) in MO-Na\({}_{2}\)SO\({}_{4}\) during photocatalytic degradation process; and (**c**) degradation rates of MO degraded using FS-FT (0–0.45 g) in MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution. The recovered FS-FT (0 g, 0.36 g) was used to degrade MO in MO-Na\({}_{2}\)SO\({}_{4}\)-blended solution in the same environment (C\({}_{\text{MO}}\) = 10 mg/L, C\({}_{\text{Na2 SO4}}\) = 12.5 g/L). Figure 7a illustrates the photocatalytic performance of the recovered FS-FT (0 g). The degradation efficiency of MO using recovered FS-FT (0 g) for five cycles was 92.38%, 90.41%, 82.61%, 90.48%, and 85.48%, respectively, and the average degradation rate was 88.27%. In Figure 7b, the degradation efficiency of MO using recovered FS-FT (0.36 g) over five cycles was 97.16%, 96.71%, 93.50%, 96.07%, and 94.71%, respectively, and the overall degradation rate was 95.63%.