## 3 Results and Discussion
### Effects of Elements in Steel Slag Leaching Solution on Concentration of CO2 (aq)  
Until now, it is still unclear why the accumulation of chlorophyll a in the medium containing steel slag leaching solution is higher than in that without steel slag leaching solution. To a great extent, accumulation of chlorophyll a of terrestrial plant, microalgae, and other photosynthetic organisms relies on photosynthetic efficiency. Photosynthesis can be mainly divided into two metabolic systems, that is, light-dependent reaction (attaining light energy from sunshine) and Calvin cycle (immobilizing CO2 to synthesize glucose). An increase of CO2 (aq) concentration can increase the photosynthetic rate, inhibit the respiration rate, promote the accumulation of chlorophyll a, and increase biomass [30]. The increase of CO2 (aq) promoted microalgae to absorb CO2 through photosynthesis [31]; however, the concentration of CO2 (gas) in air determined the concentration of CO2 (aq) soluble in water. Thus, the concentration of CO2 (gas) was the key factor driving the rate of photosynthesis [32] not only for terrestrial plants using CO2 (gas) directly but also for aquatic plankton using dissolved CO2 (aq) in water. This research studies the concentration of CO2 in relation to the Calvin cycle. The mixture configured by NaHCO3 and HCl was used as reference standard of CO2 (aq). Emergence of CO2 (aq) depended on the concentration ratio of these two materials. According to Formula (7), the calculation was performed. \[\text{NaHCO}_{3}+\text{HCl}\rightarrow\text{NaCl}+\text{CO}_{2}\left(\text{aq }\right)+\text{H}_{2}\text{O} \tag{7}\]  
CO2 (aq) was measured by a PXS-CO2 type carbon dioxide concentration meter, and the concentration of CO2 (aq) under all the experimental conditions was tested (Figure 4). The pH value of the medium containing steel slag leaching solution was the same as that of the control group (pH = 7.3). According to the Henderson-Hasselbalch equation, under 10 vol% converter slag and EAF slag leaching solutions condition, the concentration of CO2 (aq) in the medium containing 10 vol% of converter slag leaching solution was 3.8 mg/L, while it was 3.5 mg/L in the medium containing 10 vol% of EAF slag leaching solution, both higher than that of the control group (3.0 mg/L). Thus, adding steel slag leaching solution can improve the water environment of microalgae. Compared with the experiment that only used F/2 medium, photosynthesis of microalgae could be further promoted. Additionally, compared with the control group, an increased concentration of CO2 (aq) by adding steel slag leaching solution could further promote the accumulation of chlorophyll a of _Nannochloropsis_ sp. (Figures 2 and 3). Compared with EAF slag leaching solution, converter slag leaching solution could better promote the accumulation of chlorophyll a of _Nannochloropsis_ sp., which could also be explained by the different concentrations of CO2 (aq) between converter slag and EAF slag leaching solutions. Relations between Concentrations of Ca\({}^{2+}\) and CO\({}_{2}\) (aq) in Steel Slag Leading Solution  
The hardness of water is usually expressed by the contents of Ca\({}^{2+}\) and Mg\({}^{2+}\). Ca\({}^{2+}\) and Mg\({}^{2+}\) are alkaline-earth metals and easily react with carbonate. Meanwhile, contents of these elements in steel slag leaching solution are higher than that of other elements (Figure 2). Leaching process of CaO in slag can be assumed as the expressions in Formulas (8) and (9). \[\text{CaO(solid)}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2}\text{( solid)} \tag{8}\]  
\[\text{Ca(OH)}_{2}\text{(solid)}\rightarrow\text{Ca}^{2+}+2\text{OH}^{-} \tag{9}\]  
Although reactions of MgO and CaO could be assumed to be similar, the solubility of hydrated magnesium in water was much lower than that of hydrate calcium [33]. It was assumed that very low content of MgO (whose solubility in water is slightly higher than that of hydrated magnesium) could dissolve in water.