## 3 Results and Discussion
### Adsorption Isotherms and Thermodynamics  
The adsorption isotherms of PE, ME, IPE, BPA and PPE on GO and RGO samples were measured (Figure 6), which were well estimated with the Freundlich adsorption model:  
\[\ln q_{\mathrm{e}}=(1/\mathrm{n})\ln c_{\mathrm{e}}+\ln Q_{\mathrm{F}} \tag{2}\]  
where \(q_{\mathrm{e}}\) (mmol g\({}^{-1}\)) is the equilibrium adsorption capacity of the adsorbent, \(c_{\mathrm{e}}\) (mmol L\({}^{-1}\)) is the equilibrium concentration of adsorbate, and \(Q_{\mathrm{F}}\) and n are the model constants on behalf of the adsorption capacity and the adsorption intensity, respectively. \(K_{\mathrm{m}}\) (\(K_{\mathrm{m}}\) = \(Q_{\mathrm{F}}\)\({}^{\mathrm{n}}\)) is the median binding affinity. The related parameters fitted from the Freundlich model are listed in Table 2. It was found that for all the tested phenols, the adsorption capacity on RGO was more than that on GO. Therefore, the reduction of GO was of benefit to the adsorption of phenols. For closer comparison, a significant difference regarding the adsorption capacities of phenols with different chemical structures and solubility properties from each other was observed and increased in the order of PE < ME < IPE < BPA < PPE, which will be discussed later. The adsorption isotherms were also fitted with the Langmuir model [37]. The result showed that at the test concentration range, the Langmuir adsorption model fit the adsorption of phenols no better than the Freundlich model. The order of the adsorption capacities of phenols was similar to the Freundlich model (Table S3). The effect of temperature on the adsorption of phenols on GO and RGO were investigated, including 298, 313 and 333 K, from which we could obtain \(Q_{\mathrm{F}}\) and \(K_{\mathrm{m}}\) (Figure 7 and Table S4). The results showed that for all the tested phenols, the adsorption capacities were higher at relatively low temperatures, suggesting that a low temperature was good for adsorption. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Phenols** & **Adsorbents** & \(Q_{\mathrm{e}}\) (mmol\({}^{-1}\)\({}^{\mathrm{in}}\)\(\mathrm{L}\)\({}^{\mathrm{in}}\) g\({}^{-1}\)) & **1/n** & \(K_{\mathrm{m}}\) (mmol\({}^{-1}\)\(\mathrm{L}\) g\({}^{-1}\)) & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{Phenol (PE)} & GO & 0.645 & 0.745 & 0.555 & 0.997 \\  & RGO & 0.882 & 0.758 & 0.847 & 0.992 \\ \hline \multirow{2}{*}{4-methylphenol (ME)} & GO & 0.939 & 0.751 & 0.92 & 0.986 \\  & RGO & 1.695 & 0.703 & 2.118 & 0.993 \\ \hline \multirow{2}{*}{4-isopropylphenol (IPE)} & GO & 1.264 & 0.723 & 1.382 & 0.995 \\  & RGO & 2.393 & 0.599 & 4.292 & 0.981 \\ \hline \multirow{2}{*}{Bisphenol A (BPA)} & GO & 1.124 & 0.566 & 1.229 & 0.992 \\  & RGO & 3.675 & 0.321 & 57.667 & 0.973 \\ \hline \multirow{2}{*}{2-phenylphenol (PPE)} & GO & 2.194 & 0.667 & 3.248 & 0.99 \\  & RGO & 3.819 & 0.272 & 137.89 & 0.968 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters for adsorption of phenols on GO and RGO obtained by data fitting with the Freundlich model at 298 K.  
Figure 6: Adsorption isotherms of phenols on GO and RGO samples at 298K. (**a**) GO and (**b**) RGO. Figure 7: Adsorption isotherms of phenolics on GO (**a**,**c**,**e**,**g**,**i**) and RGO (**b**,**d**,**f**,**h**,**j**) at different temperatures. The thermodynamic parameters \(\Delta G^{0}\), \(\Delta H^{0}\) and \(\Delta S^{0}\) calculated from Equations (S1) and (S2) at three different temperatures are listed in Table 3. The negative values of \(\Delta G^{0}\) at three tested temperatures for all the five phenols revealed that the adsorption was a spontaneous process. Furthermore, a more negative \(\Delta G^{0}\) at lower temperatures meant a greater driving force to facilitate the adsorption. If performed carefully, it could be found that the values of \(\Delta G^{0}\) were generally low, falling in between \(-0.12\) and \(-10.31\) kJ mol\({}^{-1}\). This was enough to show that the adsorption was a physisorption process with a major \(\pi\)-\(\pi\) interaction.