## 2 Experimental Section
### Adsorption of ILs onto Ion-Exchange Resins  
The ion-exchange resin (Amberlite IR 120 Na) was used to remove ILs from the solution by batch adsorption. During the adsorption process, the adsorbate ions transfer from the bulk liquid phase to a porous adsorbent in three consecutive steps: (i) mass transfer of the adsorbate ions through the boundary layer film of liquid to the external surface of adsorbent (film diffusion); (ii) diffusion of the adsorbate ions within the liquid-filled pores of adsorbent (intraparticle diffusion); (iii) adsorption of the adsorbate ions through ion exchange on the internal or external surface of the adsorbent [29]. The ion exchange process is illustrated in Figure 1. The matrix of the ion-exchange resin is the styrene divinylbenzene copolymer, where styrene is the monomer and divinylbenzene is the crosslinking agent. The functional group of the resin is sulfonic acid. When ion-exchange resin is introduced to ILs solution, H\({}^{+}\) of the sulfonic acid has been exchanged with the cationic part (represented by A\({}^{+}\)) of ILs, thus allowing the adsorption of IL cations onto the ion-exchange resin. Simultaneously, the H\({}^{+}\) ions \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Matrix** & **Type** & \begin{tabular}{c} **Functional** \\ **Group** \\ \end{tabular} & \begin{tabular}{c} **Total** \\ **Exchange** \\ **Capacity** \\ \end{tabular} & \begin{tabular}{c} **Ionic** \\ **Form as** \\ **Shipped** \\ \end{tabular} & \begin{tabular}{c} **Particle** \\ **Size** \\ \end{tabular} & \begin{tabular}{c} **Moisture** \\ **Content** \\ \end{tabular} & \begin{tabular}{c} **Shipping** \\ **Weight** \\ \end{tabular} & \begin{tabular}{c} **Physical** \\ **Form** \\ \end{tabular} \\ \hline \begin{tabular}{c} Styrene \\ divinylbenzene \\ copolymer \\ \end{tabular} & Gel & \begin{tabular}{c} Sulfonic \\ acid \\ \end{tabular} & \begin{tabular}{c} \(\geq\)2.0 \\ eq/L \\ (Na\({}^{+}\) \\ form) \\ \end{tabular} & Na\({}^{+}\) & \
\begin{tabular}{c} 0.6\(\sim\)0.8 \\ mm \\ \end{tabular} & 45\(\sim\)50\% & 840 g/L &
\begin{tabular}{c} Amber \\ spherical \\ beads \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of ion-exchange resin (Amberlite IR 120 Na). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ionic Liquids** & **1-Butyl-3-Methylimidazolium** & **1-Ethyl-3-Methylimidazolium** \\ \hline \begin{tabular}{c} Molecular structure \\ \end{tabular} & \begin{tabular}{c} \\ **Tetrafluorate** \\ \end{tabular} & \begin{tabular}{c} \\ **Tetrafluorate** \\ \end{tabular} \\ \hline \begin{tabular}{c} Acronym \\ Chemical formula \\ Molecular weight (g/mol) \\ \end{tabular} & \
\begin{tabular}{c} [Bmim][BF\({}_{4}\)] \\ C\({}_{8}\)H\({}_{15}\)BF\({}_{4}\)N\({}_{2}\) \\ 226.02 \\ 153.8 (298.1 K) \\ 1.171 (298.1 K) [28] \\ \end{tabular} &
\begin{tabular}{c} [Emim][BF\({}_{4}\)] \\ C\({}_{6}\)H\({}_{11}\)BF\({}_{4}\)N\({}_{2}\) \\ 197.97 \\ 66.5 (298.1 K) \\ 1.248 (298.1 K) [6] \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical and chemical properties of ionic liquids used in this study. are released from the resin and transferred into the bulk aqueous solution. The adsorption process can be expressed by Equation (1):  
\[\text{Resin-H}+\text{A}^{+}\rightarrow\text{Resin-A}+\text{H}^{+} \tag{1}\]  
Before the adsorption, the ion-exchange resin was pretreated by being immersed in NaCl aqueous solution (15 wt%) for 12 h. This treatment avoids the breaking of resin caused by water uptake. After the salt pretreatment, the resin was further soaked in diluted HCl aqueous solution (5 wt%) for 4 h to transform the ionic form from Na\({}^{+}\) to H\({}^{+}\). Subsequently, the resin was washed by DI water until the pH of the water was about 7. The pretreated ion-exchange resins were added into glass conical flasks (250 mL) containing ILs solution (150 mL).