## 3 Results and Discussion
The insoluble intermediates hindered the contact between the photocatalyst and the dye and reduced the absorption of the light by the photocatalyst surface, which resulted in a decrease in photocatalytic activity. In order to improve the degradation effect of cycle test, at the end of the third cycle of the experiment, the photocatalyst was self-cleaning. The self-cleaning process was as follows: the recovered photocatalyst was placed in deionized water and illuminated with a high-pressure mercury lamp for about 1 h, then washed by deionized water and dried. The UV-VIS absorption spectra of MO degraded by FS-NT (0 and 2-5 mL) in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution are shown in Figure 7a. The results suggest that the photocatalytic degradation effect of FS-NT (3 mL) was the best. Figure 7b shows UV-VIS absorption spectra of MO degraded by a random batch of FS-NT (3 mL) in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution at different degradation times. Figure 7c shows the average degradation rate of MO degraded by three different batches of FS-NT (0 and 2-5 mL) in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution, and  
Figure 5: UV-VIS spectra of MO solution after the photocatalytic degradation reaction by **(a)** FS-NT (0 and 2-5 mL). **(b)** Time-dependent UV-VIS spectra of MO degraded by a random batch of FS-NT (3 mL) in MO during the photocatalytic degradation process. **(c)** The average degradation rate of MO degraded by three different batches of FS-NT (0 and 2-5 mL). Figure 6: The degradation rate of MO by **(a)** FS-NT (0 mL) and **(b)** FS-NT (3 mL) for five cycles. the experimental results of the three different batches were averaged and standard errors were added. In a dark environment, the removal rates of MO-Na\({}_{2}\)SO\({}_{4}\) by FS-NT (0 and 2-5 mL) were 10.51%, 10.74%, 11.42%, 8.83%, and 8.53%, respectively. The results showed that the average degradation rate of MO using three different batches of FS-NT (3 mL) in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution reached 97.79% \(\pm\) 2.09% within 32 min under irradiation conditions. The results indicate that the FS-NT (3 mL) photocatalyst exhibited better degradation performance in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution. It was found that when FS-NT has an appropriate amount of N, Na\({}_{2}\)SO\({}_{4}\) can promote photocatalytic degradation. The possible reason is that Na\({}_{2}\)SO\({}_{4}\) can improve the separation efficiency of photogenerated electron-hole pairs and promote the formation of free radicals with high activity [39]. The pH value of the MO solution was 4.0, and the pH value of the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution was 6.22. MO is an anionic molecule that exhibits higher adsorption efficiency on the photocatalyst at lower pH values, which is very beneficial to photocatalytic degradation. After the addition of Na\({}_{2}\)SO\({}_{4}\), the increase in pH was not conducive to adsorption, but SO\({}_{4}\)[2] reacted with h\({}^{+}\) to form SO\({}_{4}\)-, which enhanced the photocatalytic activity of the photocatalyst and improved the photocatalytic degradation effect [39; 40]. In order to study the effect of photocatalyst recycling in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution, the photocatalytic degradation of MO degraded by recycled FS-NT (0 and 3 mL) was studied. The degradation rate of MO using a batch of recycled FS-NT (0 mL) in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution is shown in Figure 8a, and the average degradation rate of MO for the five-cycle experiment was 91.38%. Figure 8b shows the degradation rate of MO by a batch of FS-NT (3 mL) recovered in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution, and the average degradation rate was 98.78%. It was found that in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution, the recovered FS-NT (3 mL) showed better photocatalytic degradation than recovered FS-NT (0 mL). The photocatalytic degradation effect of the recycled FS-NT (3 mL) and the recycled FS-NT (0 mL) in the MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution was better than that in the pure MO solution, respectively.