## 4 Modified Steel slag as Catalysts
### Photocatalysis  
[PERSON] et al. [24] modified the steel slag with hydrochloric acid, dimethylformamide solvent, p-phthalic acid, and methylene blue (MB) to prepare MIL-53(Fe)/SiO\({}_{2}\) photocatalyst; the modification process is shown in Figure 8. Using photodegradation experiment of MB to evaluate its photocatalytic efficiency, the results showed that the prepared MIL-53(Fe)/SiO\({}_{2}\) photocatalyst had more photoexcited electrons (e\({}^{-}\)) and holes (H\({}^{+}\)) under UV irradiation, and these electron holes can generate oxygen-free radicals and hydroxyl-free radicals, and then degrade MB. Moreover, the electron hole pair generated by the steel slag modified catalyst has a long life. Studies have shown that the long life of the electron hole pair [56] also contributes to the improvement of degradation efficiency. Moreover, the modified steel slag had large photocatalytic active sites on the photoluminescent surface. Under ultraviolet light, the maximum photodegradation of methylene blue reached 66.3%, which was higher than other similar photocatalysts. [PERSON] et al. [57] used steel slag as a carrier and industrial titanium liquid as a raw material, and used a sol-gel method followed by calcination at 500 \({}^{\circ}\)C to prepare a TiO\({}_{2}\) photocatalyst supported on steel slag, and studied its catalytic degradation performance for phenol. The results showed that the photocatalytic performance of the catalyst was better under the irradiation of ultraviolet light, and with the increased dosage of catalyst, the number of photogenerated electrons and photogenerated hole pairs generated by ultraviolet light will also increase, thus improving the photocatalytic reaction efficiency. The degradation of phenol was up to 78.3% when 10 g/L of catalyst was added into 100 mL of phenol (40 mg/L). When the number of loading times is three, the photocatalytic degradation rate of phenol can reach 86.7%, which indicates that the proper increase inthe loading times can help to improve the light catalytic properties of the modified steel slag. After the steel slag was treated with aqueous solution containing alkaline, [PERSON] et al. [29] synthesized a novel nickel and calcium cementitious materials-based catalyst with a two-step reaction of polymerization and ion exchange, then the catalyst for the photocatalytic degradation of MB dye. The modification process is shown in Figure 9. Results showed that under ultraviolet light, the degradation rate of MB can reach 94.39% by using nickel and calcium cementitious material-based catalysts, which was much higher than that without catalysts. In the photocatalytic oxidative degradation process, Ni\({}^{2+}\) and iron oxide  
Figure 8: Preparation of MIL-53(Fe)/SiO\({}_{2}\) photocatalyst for photodegradation of MB. Figure 7: KPEs catalyst production for catalytic degradation of NOR. played an important role in the catalytic process. Among them, Ni\({}^{2+}\) contributed to the generation of photo-generated electrons, while iron oxides contributed to the generation of the catalytic potential of electrons, both of which increased the photocatalytic efficiency. [PERSON] et al. [58] used alkali modification to convert steel slag into steel slag-derived calcium silicate hydrate (CSH), used CSH to adsorb heavy metals, and then carried out photocatalytic degradation experiments on organic pollutants. Studies showed that this kind of CSH modified by steel slag showed good adsorption performance for heavy metals and good catalytic performance for the degradation of organic pollutants. Under low-power visible light conditions, the MB removal rate with this modified photocatalyst can reach 63% in 8 h. Using steel slag-derived CSH to adsorb heavy metals and photocatalysts in wastewater not only met the requirements of high performance, but also met the requirements of low cost.