## 4 Modified Steel slag as Catalysts
### Electrocatalysis  
Electrochemical oxidation is an important advanced oxidation technology (AOT) that had been successfully used in many fields [48], but conventional AOTs were not very efficient. Therefore, it was necessary to find more suitable electrode materials and new technical means. Steel slag contains a variety of metal oxides and non-metal oxides, such as CaO, Fe\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\)[49]. Considering the composition and characteristics of steel slag, it was feasible and cost-effective to prepare particle electrodes from steel slag. Combined with the three-dimensional electrode oxidation reactor, some studies in recent years found that this was feasible and had good application prospects. The reaction mechanism of electrocatalysis is as follows: when the electrode is energized, H\({}_{2}\)O\({}_{2}\) will be generated, and the active component in the catalyst reacts with H\({}_{2}\)O\({}_{2}\) to produce \(\cdot\)OH, which then degrades the pollutants [50]. [PERSON] et al. [43] ground the clean steel slag into powder, mixed it with clay and a pore-forming agent, and then rolled the mixture into particles with a diameter of 3-5 mm. The particles were heated in a tube furnace at 1050 \({}^{\circ}\)C to obtain a particle electrode, as shown in Figure 5, which was used in the research of advanced treatment of oil field wastewater. Studies showed that the electrochemical reactor with magnetic steel slag particles as the particle electrode can remove more than 85% of the total organic carbon within 2 h, which was considered to be effective, and had high reusability and better performance in practical applications. [PERSON] et al. [51] prepared a steel slag particle electrode after heating mixed particles of steel slag in a tube furnace at 1050 \({}^{\circ}\)C for 30 min, and studied the use of modified steel slag particle electrodes for electrocatalytic degradation of rhodamine B (RhB). In the catalytic process, Fe\({}^{2+}\) played a major role in the steel slag electrodes. Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\) oxidize to produce the active substance \(\cdot\)OH, and then degrade RhB. In the process of degradation, air can be passed to ensure the content of H\({}_{2}\)O\({}_{2}\). These new particle electrodes showed excellent catalytic activity in the degradation of RHB compared with ceramic particles of other electrode materials. Within 60 min, the degradation rate of RHB reached an astonishing 82.4% under ventilation and 65.45% without air. The electrode particles exhibited a very satisfactory cycle performance. The materials commonly used to prepare particle electrodes are activated carbon particles, ceramic filter materials, and zeolite [52]. The specific surface area and pore structure of zeolite enabled it to absorb electrolytes and electrochemical reaction species [53]. However, the conductivity of zeolite is poor, leading to high energy consumption, which hindered the application of electrochemical systems [54]. Zeolite had a certain degree of adhesion [55]; therefore, a mixture of steel slag and zeolite can prepare a particle electrode with excellent performance, good electrical conductivity, and low cost. [PERSON] et al. [32] used a particle electrode made of modified steel slag in a three-dimensional  
Figure 5: Steel slag particle electrode catalyst modification process. electrocatalytic system for RhB degradation. They mixed steel slag powder, zeolite powder, and Sn/Mn of the active ingredient in a certain proportion into a spherical shape, and sintered it in a muffle furnace at 1190 \({}^{\circ}\)C to obtain Sn/Mn loaded zeolite particles steel slag electrodes. The modification flowchart is shown in Figure 6. With the addition of ZnO and MnO\({}_{2}\), the catalytic activity of the particle electrode significantly improved, and stronger and more active substances such as -OH, -O\({}^{2-}\), -O\({}^{-}\) and -HO\({}^{2-}\) could be produced.