## 3 Results and Discussion
### Effect of the Chemical Structure of Phenols on Adsorption  
It was also noted that the five phenols might be divided into two groups. The first group included PE, ME and IPE, which have only one benzene ring in their molecules. The second group included BPA and PPE, which have two benzene rings in their molecules. For the first group of phenols including PE, ME and IPE, the \(q_{\rm e}\) value of IPE was greater than that of ME and that of PE (Figure 4). In other words, on the two tested adsorbents, the \(q_{\rm e}\) values of the first group of phenols decreased in the order of IPE > ME > PE. For the second group of phenols, the \(q_{\rm e}\) values decreased in the order of PPE > BPA on the two tested adsorbents. Moreover, the \(q_{\rm e}\) values of the first group of phenols were generally less than that of the second group of phenols. To account for the observations, we may consider the following aspects. (1) The increased number of benzene rings in the molecules of the phenols was favorable to the \(\pi\)-\(\pi\) interaction between the phenolic molecules and the RGO surface and hence to the adsorption of the phenolic compound. Therefore, the \(q_{\rm e}\) values of the second group of phenols were generally more than that of the first group of phenols. (2) The increased steric hindrance of the substitution groups on the benzene ring was unfavorable to the \(\pi\)-\(\pi\) interaction between the phenolic \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Adsorbents**} & \multirow{2}{*}{**Phenolics**} & \multicolumn{3}{c}{\(\Delta G^{0}\) (kJ/mol)} & \multicolumn{3}{c}{\(\Delta S^{0}\) (J/mol)} & \(\Delta H^{0}\) (kJ/mol) \\ \cline{3-6}  & & **298 K** & **313 K** & **333 K** & & \\ \hline \multirow{4}{*}{RGO} & PE & \(-1.37\) & \(-0.88\) & \(-0.39\) & \(-27.33\) & \(-9.48\) \\  & ME & \(-3.42\) & \(-3.28\) & \(-2.55\) & \(-24.32\) & \(-10.74\) \\  & IPE & \(-4.99\) & \(-4.73\) & \(-4.20\) & \(-21.77\) & \(-11.50\) \\  & BPA & \(-9.33\) & \(-9.00\) & \(-8.87\) & \(-12.21\) & \(-12.92\) \\  & PPE & \(-10.57\) & \(-10.04\) & \(-10.31\) & \(-6.76\) & \(-12.44\) \\ \hline \multirow{4}{*}{GO} & PE & \(-0.51\) & \(-0.34\) & \(-0.12\) & \(-10.92\) & \(-3.76\) \\  & ME & \(-1.63\) & \(-1.47\) & \(-1.11\) & \(-14.38\) & \(-5.93\) \\ \cline{1-1}  & IPE & \(-2.43\) & \(-1.91\) & \(-1.51\) & \(-25.68\) & \(-10.03\) \\ \cline{1-1}  & BPA & \(-3.39\) & \(-3.01\) & \(-2.82\) & \(-15.91\) & \(-8.09\) \\ \cline{1-1}  & PPE & \(-4.37\) & \(-3.876\) & \(-3.583\) & \(-21.786\) & \(-10.8\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermodynamic parameters of phenolics adsorbed on GO and RGO. molecules and the RGO surface. Among the second group, there was a connector (-C(CH\({}_{3}\))\({}_{2}\)-) between the two benzene rings in BPA, but no connector in PPE. The steric hindrance of the connector can account for the poorer adsorption of BPA than PPE. Among the first group of phenols, the group at the para-position on the basic phenol structure was -H, -CH\({}_{3}\), and -CH(CH\({}_{3}\))\({}_{2}\) for PE, ME and IPE, respectively. The steric hindrance of the substitution groups increased in the order of -H < -CH\({}_{3}\) < -CH(CH\({}_{3}\))\({}_{2}\), which suggested that the adsorption capacity of the phenolic compound on RGO should increase in the order of IPE < ME < PE. This order was reversed to the observed order of IPE > ME > PE in \(q_{\rm e}\) values. Consequently, the steric hindrance of the substitution groups on the benzene ring was not the major parameter influencing the \(\pi\)-\(\pi\) interaction between the first-group phenolic molecules and the RGO surface in the tested cases. (3) An electron donor will increase the electron density of the benzene ring, strengthening the \(\pi\)-\(\pi\) interaction. The order of the electron donating ability of substituent group was -H < -CH\({}_{3}\) < -CH(CH\({}_{3}\))\({}_{2}\), which was consistent with the observed order of IPE > ME > PE in \(q_{\rm e}\) values.