## 4 Modified Steel slag as Catalysts
### Organic Matter Degradation  
Studies have shown that the presence of the element Fe in steel slag can produce Fenton or Fenton-like reactions in the degradation process of organic matter, thus improving the degradation of organic matter [42]; moreover, the presence of iron made the steel slag particles present magnetic properties [43], and thus it had a good recycling performance. [PERSON] et al. [44] studied the modified slag as a Fenton-like catalyst to treat alachlor wastewater. Due to the dissolution of calcium minerals in the steel slag, the pH of the solution rapidly increased to about 10, and the Fenton-like reactions had low reaction efficiency under high pH conditions [45]. To eliminate this effect, they modified the steel converter slag (SCS) with salicylic acid-methanol (SAM); the specific process of modification is shown in Figure 3. The results show that iron oxides in steel slag can be dissociated into Fe2+ after SAM treatment. Fe2+ and H2O2 react to generate hydroxyl radical (-OH), and then oxidize and degrade organic pollutants. Secondly, after steel slag is treated with acid, a large number of calcium minerals can be removed to prevent the steel slag from dissolving alkaline substances in the catalytic process, which increases the pH and leads to the precipitation of Fe2+. SAM solution could selectively remove calcium silicate minerals on the surface of steel slag, and according to the degradation experiment data, it can be concluded the catalytic ability of the modified steel slag in the Fenton-like reaction is higher than that of the unmodified steel slag. Further research showed that the prepared SAM modified steel slag catalyst had high recyclability and can be reused four times without significant loss of activity. The steel slag modified by the SAM Fenton-like reaction system had the characteristics of low cost and high catalytic performance, and may be an important technology for large-scale practical application. Figure 1: Nickel-modified steel slag catalysts to catalyze pine sawdust native biomass pyrolysis tar. Figure 2: FAU-SL catalyst modification for pyrolysis of peel fiber. [PERSON] et al. prepared a new type of catalyst through acid modification treatment of steel slag, and studied its degradation rate of sulfamethazine (SMZ) in a Fenton-like system [26]. In this study, a new type of Fe/Co composite catalyst was synthesized through the modification of SCS by SAM and cobalt nitrate Co(NO\({}_{3}\))\({}_{2}\). The modification flowchart is shown in Figure 4. Given the above discussion, it is known that through acid modification, steel slag's catalytic efficiency has been greatly improved. After the addition of Co\({}^{2+}\), Co\({}^{2+}\) can also carry out Fenton-like reactions under neutral conditions, which can further break through the constraining conditions of pH. The results showed that with a Co-SAM-SCS/H\({}_{2}\)O\({}_{2}\) Fenton-like system with a wide pH range, the SMZ degradation was very effective. In the initial pH value of 7, the SMZ degradation rate in the Co-SAM-SCS/H\({}_{2}\)O\({}_{2}\) system was 16.21 times that of the unmodified SCS/H\({}_{2}\)O\({}_{2}\) system in the SMZ degradation rate. The high surface area and synergy of Co-SAM-SCS from Co and Fe introduced in the sample contribute to good stability and reusability of catalyst. Additionally, Co-SAM-SCS had low iron and cobalt leaching. All of this showed that this modified method was effective and reasonable. [PERSON] et al. [46] prepared a new catalyst with magnetically separable steel slag (MSS) as a carrier on the premise that there were magnetic substances in steel slag that can remove para-nitrophenol (p-Np) in wastewater. MSS was prepared by the grinding, separation, impregnation, and calcination of steel slag. After five magnetic separations of MSS, a Cu loaded catalyst Cu@MSS was prepared by wet impregnation method.