Controllable Preparation of TiO2/SiO2@Blast Furnace Slag Fiber Composites Based on Solid Waste Carriers and Study on Mechanism of Photocatalytic Degradation of Urban Sewage

Photocatalytic composite materials (TiO2/SiO2/BFSF) were first fabricated using the sol–gel method of loading SiO2 and TiO2 on blast furnace slag fibers (BFSFs) in sequence and using them as a new carrier. Then, TG-DTA, XRD, BET, SEM-EDS, and UV-Vis absorption spectra, as well as spectrophotometric measurements, were employed to analyze the physicochemical properties of TiO2. The influence of SiO2 coating, the number of impregnations in TiO2 sol, the calcination temperature, and the number of repeated usages on the activity of TiO2/SiO2/BFSF was researched by analyzing the degradation of methylene blue (MB) aqueous solution. The results show that SiO2 could increase the load of TiO2, impede the growth of TiO2 grains, and inhibit the recombination of electron–hole pairs, ultimately enhancing the photocatalytic activity of samples. The activity of TiO2/SiO2/BFSF first quickly increased and then slowly decreased with an increase in the loading times of TiO2 sol and calcination temperature. After three impregnations in TiO2 sol and calcining at 450 °C for 2.5 h, a uniform and compact anatase TiO2 thin film was deposited on the surface of TiO2/SiO2/BFSF, showing the strongest activity. When this sample was used to degrade MB aqueous solution for 180 min under ultraviolet light irradiation, the degradation proportion reached a maximum of 96%. After four reuses, the degradation ratio could still reach 67%. In addition, three potential photocatalytic mechanisms were proposed. Finally, the high-value-added application of blast furnace slag for preparing photocatalytic composite materials was achieved, successfully turning solid waste into “treasure”.