## 4 Modified Steel slag as Catalysts
### Carbon Capture and Storage  
With the massive global burning of fossil fuels, the emission of CO\({}_{2}\) has an increasingly serious impact on global warming. At present, the global artificial emission of carbon dioxide is more than 3.2 billion tons, while the total utilization of carbon dioxide is less than 200 million tons per year. Compared with the emissions, the utilization rate is far less than the emissions [77], and CO\({}_{2}\) capture and storage technology (CCSU) is considered to be a very promising approach to carbon dioxide emission reduction [78]. Therefore, it is very necessary to strengthen the research on CCSU. It is estimated that 1.4 tons of CO\({}_{2}\) will be emitted for each ton of steel produced in steel plants [79]. Steel slag and CO\({}_{2}\), as wastes of the steel industry, can be treated by means of \"using waste to treat waste\", thus achieving economic synergies.  
Ca/Mg-rich phase is used to capture and fix carbon dioxide through mineral carbonation, and carbon dioxide combines with Ca/Mg to eventually generate carbonate products [80]. Studies have reported that 70-85% of Ca/Mg is extracted from steel slag or magnesium silicate rock [81]. Therefore, it is very necessary to study the leaching of Ca/Mg phase in steel slag. [PERSON] et al. [82] investigated the leaching rate of chromium element and Ca/Mg in stainless steel slag, and they first added MnO to the slag, and then carried out acid leaching treatment. The results show that the content of MnO and acid treatment have great effects on the fixation of chromium and the leaching of Ca/Mg from stainless steel slag. When the content of MnO is about 5%, the pH of acid leaching solution is 2, the leaching time is 120 min, and the temperature is 20 \({}^{\circ}\)C, the leaching rates of Ca and Mg are about 65% and 55%, respectively. [PERSON] [81] took BOF and EAF as research objects and successfully extracted from them calcium and magnesium, respectively.  
In addition to extracting carbon sequestration substances from steel slag for CO\({}_{2}\) capture and storage, steel slag can also be directly used for CO\({}_{2}\) capture and storage after removing impurities by simple modification. [PERSON] et al. [79] synthesized lithium silicate-based CO\({}_{2}\) adsorbents using iron and steel metallurgical waste residue as raw material. Using steel slag as a silicon source, lithium silicate group was synthesized by adding (10-30 wt% K\({}_{2}\)CO\({}_{3}\)). The eutectic phase is formed between K\({}_{2}\)CO\({}_{3}\) and Li\({}_{2}\)CO\({}_{3}\), which promotes the diffusion of CO\({}_{2}\) to the adsorption material, thus improving the capture efficiency of CO\({}_{2}\). Under the condition of 600 \({}^{\circ}\)C, the CO\({}_{2}\) capture rate of steel slag adsorption material containing 20 wt% K\({}_{2}\)CO\({}_{3}\) reached the maximum. After 10 cycles, the capture efficiency of CO\({}_{2}\) only decreases by 10%, indicating that the lithium silicate base has good stability. [PERSON] et al. [83] enhanced CO\({}_{2}\) absorption capacity by adding steel slag to seawater. The results show that MgO, CaO, and Fe\({}_{2}\)O\({}_{3}\) in the steel slag can enhance the CO\({}_{2}\) collecting capacity of seawater. Moreover, MgO and CaO have a strong synergistic effect on CO\({}_{2}\) capture by seawater. Fe\({}_{2}\)O\({}_{3}\) has more significant CO\({}_{2}\) capture ability than other steel slag components. Under the optimal conditions, the addition of steel slag can ensure that the CO\({}_{2}\) capture rate of seawater can reach 90%. This is feasible both in terms of efficiency and cost, and will provide some coastal steel mills with a more rational method of handling steel slag while reducing CO\({}_{2}\) content.