## 3 Results and Discussion
### Effects of Elements in Steel Slag Leaching Solution on Concentration of CO2 (aq)
Since all the experiments in this research were carried out under the condition of near neutral pH, though MgO could react with acid, reactivity of MgO leaching from slag was not high. As a result, from the perspective of the nature of MgO, MgO would not show high reactivity in the medium. Thus, this research studied the behavior of Ca\({}^{2+}\) specifically. Figure 5 shows the relations between contents of different forms of carbonate calculated by the Henderson-Hasselbalch equation and pH value. In this research, under an experimental condition of pH = 7.3, among different the forms of carbonate, content of HCO\({}_{3}\)\({}^{-}\) was the highest. As a result, Ca\({}^{2+}\) in leaching solution might react with HCO\({}_{3}\)\({}^{-}\), as is shown in Formula (10). According to Formulas (2) and (3), bicarbonate compound might help the solution absorb CO\({}_{2}\) from air. \[\text{Ca}^{2+}+2\text{HCO}_{3}\text{${}^{-}$}\rightarrow\text{Ca(HCO}_{3})_{2} \text{(aq)} \tag{10}\]  
Ca(HCO\({}_{3}\))\({}_{2}\) could completely ionize, thus in the form of HCO\({}_{3}\)\({}^{-}\), Ca(HCO\({}_{3}\))\({}_{2}\) might disturb the chemical equilibrium among different forms of carbonate. The concentration ratio of different forms of carbonate in solution must be constant, an increase in the concentration of HCO\({}_{3}\)\({}^{-}\) makes Formula (3) proceed to the left, causing the increase of CO\({}_{2}\) (aq), while the increased CO\({}_{2}\) (aq) might be consumed by microalgae as raw materials for photosynthesis. Figure 4: Concentrations of aqueous CO\({}_{2}\) in control, 10 vol% converter slag leaching solution and 10 vol% EAF slag leaching solution. In order to test the relations between contents of Ca\({}^{2+}\) and Mg\({}^{2+}\) in leaching solution of slag and concentration of CO\({}_{2}\) (aq), 50, 100, 200, and 400 mg/L Ca(OH)\({}_{2}\) and MgO were added into F/2 medium, respectively. Then concentration of CO\({}_{2}\) (aq) in compound was tested. As is shown in Figure 6, the concentration of Ca(OH)\({}_{2}\) was related to the concentration of CO\({}_{2}\) (aq). The concentration of CO\({}_{2}\) (aq) increased with the increase in concentration of Ca(OH)\({}_{2}\). However, since its solubility was relatively low, the concentration of MgO in the test concentration range showed no great effects on the concentration of CO\({}_{2}\) (aq) in the compound. With regard to effect of Ca\({}^{2+}\) on concentration of CO\({}_{2}\) (aq), a part of Ca\({}^{2+}\) in solution might remain in the medium, so it could not be completely absorbed by _Nannochloropsis_ sp. In order to test effect of Ca\({}^{2+}\) on concentration of CO\({}_{2}\) (aq), the concentrations of Ca\({}^{2+}\) (Figure 2) in F/2 medium after the addition of converter slag and EAF slag leaching solutions before and after culturing _Nannochloropsis_ sp. were tested, and concentrations of Mg\({}^{2+}\) under the same condition were listed to be used for reference. Figure 5: Concentrations of aqueous CO\({}_{2}\), HCO\({}_{3}\)\({}^{-}\), and CO\({}_{3}\)\({}^{2-}\) for each pH (mol%). Figure 6: Relation between the amount of total carbonic acid and Ca(OH)\({}_{2}\) content in solution. In this research, it was found that Ca\({}^{2+}\) was a factor for promoting the increase of concentration of CO2 (aq) in steel slag leaching solution. The experiment proved that adding Ca(OH)2 could increase the concentration of CO2 (aq) (Figure 6). In the F/2 medium with the addition of converter slag and EAF slag leaching solutions before and after culturing _Nannochloropsis_ sp., enough amount of Ca\({}^{2+}\) and Mg\({}^{2+}\) existed to react with carbonate (Table 7). Apart from Ca(OH)2 and MgO, some solutes in the steel slag leaching solution could also react in the F/2 medium to form complex ions and other molecular forms. These hypothetical molecular morphologies composed by second-group elements might be related with the increase of concentration of CO2 (aq).