## 4 Conclusions  
This research carried out a leaching test of converter slag and EAF slag leaching solutions. Effects and mechanisms of steel slag leaching solution on _Nannochloropsis_ sp. were explored. Conclusion of this research can be summed up in the following points.  
1. Adding converter slag and EAF slag leaching solutions into the F/2 medium influenced the accumulation of chlorophyll a of _Nannochloropsis_ sp. When the addition of these two kinds of leaching solutions was less than 50 vol%, the accumulation of chlorophyll a of _Nannochloropsis_ sp. was promoted. While when addition was at 70 vol%, it was inhibited. When the addition of converter slag leaching solution was 10 vol%, accumulation of chlorophyll a reached the highest (4.2 mg/L), 1.45 times as much as that of control group. When 10 vol% of EAF slag leaching solution was added, accumulation of chlorophyll a was 3.2 mg/L, 1.10 times as much as that of the control group. Under the same adding ratio, promoting and inhibitory effects of converter slag leaching solution on accumulation of chlorophyll a were higher than that of EAF slag leaching solution.
2. The reason to the increase of chlorophyll a of _Nannochloropsis_ sp. was that Ca\({}^{2+}\) in converter slag and EAF slag leaching solutions increased the content of CO2 (aq), then the rate of photosynthesis and the growth rate of algae were improved.  
Results of this research show that the use of slag not only promotes the growth of aquatic photosynthetic organisms in aquatic environment but it also promotes the absorption of CO2 through photosynthesis. Additionally, the increased CO2 (aq) makes secondary contributions to enhance yields of fish and shellfish, because the increased CO2 (aq) promotes the growth of both microalgae in aquatic ecosystem and that of fish and shellfish feeding on the phytoplankton.  
Meanwhile, there are such elements as Fe, P, Ca, and Mg in the slag, which can be used as nutrients to promote the growth and reproduction of microalgae and to improve the accumulation of chlorophyll a and metabolites of microalgae. Thus, the use of slag is beneficial to decrease the cost of the culturing of microalgae.  
Conceptualization, Y.W.; writing--original draft preparation, T.L.; writing--review and editing, Q.Y., S.C., D.G., X.W., F.W., Y.W., J.L. and Y.Z.; funding acquisition, Y.W. and Y.Z. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Concentration of Ca\({}^{2+}\) (mg/L)**} & \multicolumn{2}{c}{**Concentration of Mg\({}^{2+}\) (mg/L)**} \\ \cline{2-5}  & **Before Culturing** & **After Culturing** & **Before Culturing** & **After Culturing** \\ \hline Converter slag Leaching Solution & 371.14 & 271.71 & 1134.56 & 483.10 \\ EAF slag Leaching Solution & 367.73 & 254.11 & 1134.07 & 471.38 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Concentrations of Ca\({}^{2+}\) and Mg\({}^{2+}\) in F/2 medium containing steel slag leaching solution before and after culturing _Nannochloropsis_ sp.  
**Funding:** The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (Nos.: 52004095 and 51704119) and the Natural Science Foundation of Hebei Province (E2017209243) and the Department of Education of Hebei Province (BJ2019038).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors give thanks to the anonymous reviewers and all the editors in the process of manuscript revision.  
**Conflicts of Interest:** The authors declare no conflict of interest.