# Density Functional Theory Study on the Adsorption of Co(II) in Aqueous Solution by Graphene Oxide
## 4 Study on the Mechanism of GO Adsorption Process
### Influence of External Environment on Adsorption
#### 4.2.3 Interfering Ions  
Radioactive wastewater produced by nuclear facilities is often mixed with a certain amount of equipment, valves, pipeline drainage and leakage; in addition, surface drainage and domestic water, which contain Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\), may affect the adsorption capacity of GO on Co\({}^{2+}\). In order to analyze the influence of co-existing ions on adsorption, Gaussian software was used to calculate the adsorption process when GO and the above ions were placed in the same space under suitable pH and temperature conditions.  
The following results were found via the Gaussian calculations:  
(1) Co-existing ions can bind to the hydroxyl oxygen atom on GO(COO)\({}^{-}\). The energy of Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\) in the stable state is \(-\)0.159212401 Hartree, \(-\)28.09549138 Hartree, \(-\)36.4406424 Hartree and \(-\)0.628838916 Hartree. The binding energy between GO(COO)\({}^{-}\) and co-existing ions can be obtained as shown in Table 3:  
It can be seen from Table 3 that the adsorption capacity of GO containing only carboxyl groups is equivalent to that of all interfering ions. Compared with Co\({}^{2+}\), the relationship between binding energy is E[GO(COO)\({}^{-}\)...Co\({}^{2+}\)] > E[GO(COO)\({}^{-}\)...Na\({}^{+}\)] > E[GO(COO)\({}^{-}\)...K\({}^{+}\)] > E[Go (COO)\({}^{-}\)...Mg\({}^{2+}\)] > E[GO(COO)\({}^{-}\)...Ca\({}^{2+}\)]; it can be seen that the carboxyl group on GO has a certain selectivity for Co\({}^{2+}\).  
(2) As shown in Figure 6, GO(O)\({}_{-}\)s forms a stable configuration with each interfering ion. After Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\) combine with the O atom on the hydroxyl group, the C-O bond length gradually extends and finally breaks, forming graphene sheets and H-C-X (X represents interfering ion).  
(3) When GO(O) is placed in the same space with Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\), the metal ion is bound to the O atom on the epoxy group and adsorbed on the GO surface. However, through calculation, it was found that the adsorption energy of GO(O) and interfering ions is negative, indicating that the adsorption process is energy release and adsorption does  
Figure 6: Stable configuration of GO(O)-s and each interfering ion: (**a**) the stable configuration of GO(O)\({}^{-}\)s and Na\({}^{+}\); (**b**) the stable configuration of GO(O)\({}^{-}\)s and K\({}^{+}\); (**c**) the stable configuration of GO(O)\({}^{-}\)s and Ca\({}^{2+}\); (**d**) the stable configuration of GO(O)\({}^{-}\)s and Mg\({}^{2+}\).  
\begin{table}
\begin{tabular}{c c} \hline
**The Type of [GO(COO)...Different Ions]** & **Binding Energy/Hartree** \\ \hline GO(COO)\(\ldots\)Na & 0.3798 \\ GO(COO)\(\ldots\)K & 0.3736 \\
[GO(COO)...Ca]\({}^{+}\) & 0.3308 \\
[GO(COO)...Mg]\({}^{+}\) & 0.3670 \\ \hline \end{tabular}
\end{table}
Table 3: Adsorption energy of GO(COO)\({}^{-}\) with different ions.  
not occur easily. The adsorption energy of GO(O) combined with different ions is shown in Table 4.  
It can be seen from the above calculation results that during the adsorption of Co\({}^{2+}\) by GO, different functional groups and different adsorption conditions have different influences on the adsorption effect: appropriately increasing the pH of the solution within a certain range is conducive to the adsorption of metal ions by carboxyl groups and hydroxyl groups on the base plane. The effect of temperature change on adsorption efficiency should be considered comprehensively. The interaction between co-existing ions and the hydroxyl group on the GO surface is weak, but they can be strongly adsorbed by the carboxyl group on the GO surface and destroy the epoxy group on the surface of the material. In order to improve the selectivity of GO to Co\({}^{2+}\), GO can be modified by increasing the number of hydroxyl groups.