## 3 Results and Discussion
### Adsorption Isotherms  
The adsorption isotherms can provide the adsorbate molecules distribution between the solid and liquid phases when the adsorption process reaches the equilibrium state. Herein, the Langmuir, Freundlich, and Sips models were used to analyze the equilibrium data obtained from ILs adsorption. The parameters, along with a correlation coefficient (R\({}^{2}\)) of three isotherms, are given in Table 4. The fitting results are presented in Figure 5. The three adsorption models are given as Equations (5)-(7), respectively:  
\[q_{e}=\ \frac{q_{m}K_{L}C_{e}}{1+K_{L}C_{e}} \tag{5}\]  
\[q_{e}=K_{F}C_{e}^{1/n} \tag{6}\]  
\[q_{e}=\ \frac{q_{m}K_{S}C_{e}^{1/p}}{1+K_{S}C_{e}^{1/p}} \tag{7}\]  
where \(q_{e}\) is the equilibrium adsorption capacity, mg/g; \(C_{e}\) denotes the equilibrium concentration of ILs in solution, mg/L; \(q_{m}\) stands for the maximum adsorption capacity, mg/g; \(K_{L}\) is the Langmuir constant related to the affinity between the adsorbent and adsorbate, L/mg; \(K_{F}\) refers to the Freundlich constants correlated with the adsorption capacity for a unit equilibrium concentration, and \(1/n\) is the  
Figure 4: Effect of (**a**) resin dose; (**b**) temperature; (**c**) adsorption time on the adsorption ratio of ionic liquids ([Bmim][BF\({}_{4}\)] and [Emim][BF\({}_{4}\)]) onto ion-exchange resin (Amberlite IR 120 Na). heterogeneity factor; \(K_{S}\) represents the Sips constant relating to the median binding affinity; \(1/p\) stands for the heterogeneity factor varied from 0 to 1. The Langmuir equation assumes homogeneous and monolayer adsorption, with no lateral interaction and steric hindrance between the adsorbed molecules [35]. The essential features and feasibility of the Langmuir model can be expressed by a dimensionless constant commonly known as separation factor [29], which is used to predict if an adsorption system is \"favorable\" or \"unfavorable\" (Equation (8)):  
\[R_{L}=\ \frac{1}{1+K_{L}C_{0}} \tag{8}\]  
where \(R_{L}\) represents the separation factor; \(C_{0}\) refers to the lowest initial concentration of ILs, mg/L. The value of \(R_{L}\) indicates the shape of isotherm to be either unfavorable (\(R_{L}>1\)) or linear (\(R_{L}=1\)) or favorable (\(0<R_{L}<1\)) or irreversible (\(R_{L}=0\)). From Table 4, [Bmim][BF\({}_{4}\)] showed a higher \(K_{L}\) value than that of [Emim][BF\({}_{4}\)], which was in concert with the higher maximum adsorption capacity obtained from [Bmim][BF\({}_{4}\)]. This suggested the higher affinity between the [Bmim][BF\({}_{4}\)] and the resins compared with that of [Emim][BF\({}_{4}\)]. Values of \(R_{L}\) were in range of 0 and 1 for two ILs, indicating that the ion-exchange resin is favorable for adsorption of two ILs under the conditions studied. The Freundlich model can be applied to multilayer adsorption of adsorbate onto heterogeneous surface. It assumes that: (i) the heat of adsorption decreases with the increase of surface coverage of adsorbent; (ii) the binding sites on the surface have different adsorption energies [36]. As presented in Table 4, the \(K_{F}\) for [Bmim][BF\({}_{4}\)] was higher than that for [Emim][BF\({}_{4}\)]. This is in line with the results obtained from the Langmuir model, suggesting that the uptake is higher for IL with longer alkyl chain length of the imidazolium cation. Similar behavior was observed in the adsorption of ILs onto other adsorbents, such as activated carbons and soils [37; 38]. The enhanced adsorption capacity for IL with longer alkyl chains is mainly attributed to higher van der Waals and polar interactions between the IL and the resins [25].