# Density Functional Theory Study on the Adsorption of Co(II) in Aqueous Solution by Graphene Oxide
## 6 Conclusions  
By establishing the GO simulation configuration, the stability of the structure, the electrostatic potential, the orbital energy level and the binding energy between the structure and Co\({}^{2+}\) under different environmental conditions were calculated, the adsorption mechanism and adsorption conditions of graphene oxide were analyzed, and the calculated results were verified by static adsorption experiments. Thus, the influences of the carboxyl group, the hydroxyl group, the epoxy group, the graphene sheet and the external environment such as pH, temperature and co-existing ions on the adsorption effect were determined, which provided theoretical guidance for the modification direction of graphene oxide and the selection of adsorption conditions. Through calculation and experiment, the following conclusions can be drawn:  
1. With the increase in the number of six-membered rings on the graphene sheet, the material structure becomes more and more stable. The oxygen-containing functional groups on graphene oxide not only have certain electronegativity but can also combine with Co\({}^{2+}\) to form stable complexes. However, the adsorption capacity of different functional groups is also different under different conditions.
2. pH has a great influence on the existence form of graphene oxide, and the adsorption effect is the best under weak acidic conditions. At this time, the carboxyl group and hydroxyl group on the base level are negatively charged due to proton loss and bind to Co\({}^{2+}\) through electrostatic adsorption, while the epoxy group and Co\({}^{2+}\) have strong stabilization energy and more easily bind to Co\({}^{2+}\) through coordination.
3. The influence of temperature change on the adsorption capacity of different functional groups on GO is uncertain. Although it will increase the contact probability between adsorbent and adsorbent to a certain extent, it will also affect the stability of the adsorbed materials. Through static adsorption experiments, it was found that the adsorption capacity of Co\({}^{2+}\) by GO reaches its maximum value at 50 \({}^{\circ}\)C.
4. For the wastewater containing Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) and other interfering ions, although the carboxyl group on GO can effectively adsorb co-existing ions, it is difficult for the hydroxyl group and epoxy group on the base level to combine with it to form a stable structure, indicating that GO still has certain selective adsorption properties for Co\({}^{2+}\), and the static adsorption experiment also verified this calculation result. Since interfering ions can destroy the stability of the binding of epoxy groups to graphene sheets, increasing the number of hydroxyl groups on GO may be an effective way to improve the selective adsorption performance of materials.  
Figure 12: Effect of co-existing ions on the adsorption of Co\({}^{2+}\) by GO.  
**Author Contributions:** Conceptualization, P.B.; methodology, X.W.; software, J.M.; validation, Y.X.; writing--original draft preparation, P.B.; writing--review and editing, X.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.