# Efficient Removal Of U(VI) Ions from Aqueous Solutions by Tannic Acid/Graphene Oxide Composites
## 3 Results
### Adsorption Isotherms  
The adsorption isotherms of U(VI) ions on TA/GO are displayed in Figure 7, and the adsorption capacity was calculated from these isotherms. The Langmuir and Freundlich models were used to fit the experimental data to better understand the adsorption mechanism of U(VI) ion on TA/GO [39; 40]. The Langmuir isotherm model was used to describe the monolayer adsorption, and its fitting equation can be expressed as [39]:  
\[\mathrm{C_{s}}\ =\ \frac{\mathrm{bC_{s\ max}C_{e}}}{1+\mathrm{bC_{e}}} \tag{3}\]  
The above equation can be further transformed into the following linear form:  
\[\frac{\mathrm{C_{e}}}{\mathrm{C_{s}}}\ =\ \frac{1}{\mathrm{bC_{s\ max}}}+\frac{ \mathrm{C_{e}}}{\mathrm{C_{s\ max}}} \tag{4}\]  
where \(\mathrm{C_{s}}\) (mol\(\cdot\)g\({}^{-1}\)) and \(\mathrm{C_{s\ max}}\) (mol\(\cdot\)g\({}^{-1}\)) are the amount of uranium ions adsorbed by the adsorbent and the maximum amount of uranium ions adsorbed per unit weight of the adsorbent, respectively,  
Figure 6: Effects of ionic strength on U(VI) ions adsorption at m/V = 0.6 g/L and pH = 3.6 \(\pm\) 0.05.  
and b (L mg\({}^{-1}\)) is the Langmuir adsorption coefficient representing the enthalpy of adsorption. The Freundlich isotherm model is an empirical equation based on the adsorption behavior on a heterogeneous surface, and it can be expressed as [40]:  
\[\text{C}_{\text{s}}\ =\ \text{K}_{\text{F}}\text{C}_{\text{e}}^{\text{n}} \tag{5}\]  
The above equation can be further transformed into the following linear form:  
\[\text{logC}_{\text{s}}\ =\ \text{logK}_{\text{F}}+\frac{1}{\text{n}}\text{logC}_{ \text{e}} \tag{6}\]  
where K\({}_{\text{F}}\) (mol\({}^{1\text{-n}}\cdot\)L\({}^{\text{n}}\)/g) and n are the adsorption strength at a specific temperature and the Freundlich constant of the adsorption capacity when the equilibrium concentration of metal ions is equal to 1, respectively. The linear fitting results for the Langmuir models of U(VI) ions on TA/GO composites are presented in Figure 7, and the relevant parameters are listed in Table 3. It is noticeable from Table 3 that the correlation coefficient (R\({}^{2}\)) of the adsorption isotherm fitted by the Langmuir model was larger than that fitted by the Freundlich model, which indicates that the Langmuir model could better fit the isotherm; hence, the adsorption of uranium ions by TA/GO occurred in a single-layer coverage mode [4]. According to the fitting curve of the Langmuir model, the maximum adsorption capacity (C\({}_{\text{s, max}}\)) of TA/GO was obtained as 87.8 mg/g at pH = 3.6 \(\pm\) 0.03 and T = 298 K. The adsorption capacities of uranium ions on other adsorbents are shown in Table 4. It can be seen from the table that the adsorption capacity of TA/GO is 266.56 mg/g when the experimental conditions are pH = 5.0 and T = 298 K. Compared with other adsorbents, the adsorption capacity of TA/GO is the largest. Oxygen-containing functional groups on the TA/GO surface reduced the aggregation of TA/GO particles through van der Waals forces, resulting in a good dispersion performance [27]. In addition, in comparison to GO (BET specific surface area = 131.9678 m\({}^{2}\)/g), TA/GO has a larger specific surface area (150.9678 m\({}^{2}\)/g). The above results indicate that TA/GO is a suitable material to remove uranium ions from aqueous solutions.  
Figure 7: Sorption isotherms of U(VI) ions on TA/GO at pH = 3.6 \(\pm\) 0.03, T = 298 K, m/V = 0.6 g/L, I = 0.001 M NaCl.