## 3 Results and Discussion
### Separation of RB8 Based on Ion Exchange Using ILs  
Our previous works [32; 44; 45] reported that the addition of appropriate amounts of quaternary ammonium compounds (R4 NX, ILs) to the aqueous solutions of acidic halogenated compounds (substituted with -COO\({}^{-}\) or -SO3\({}^{-}\) groups), including dyes or drugs, results in ion exchange and the formation of respective insoluble ion pairs (see Equation (1) above and Scheme 1 below) that can be separated from aqueous solutions by simple filtration. The formation of the mentioned ion pairs has been earlier proven by NMR analysis; see refs. [32; 44; 45].  
Therefore, the subsequent experiments were focused on the separation of RB8 dye using two ILs--BAC [52] and A336 [53] (for chemical drawings, see Figure 2 above). We established that 2 g L\({}^{-1}\) of BAC (50% aq. solution) enables the decolorization of the RB8 solutions up to 99% and the elimination of COD up to 88%; see Figure 5A. Furthermore, we also observed rapid precipitation of the tested dye using BAC within 30 min of action; see Figure 5A. On the other hand, A336 provided a comparable efficiency (DE = 99%) of the RB8 dye after 30 min using 6 g L\({}^{-1}\). Furthermore, the COD removal efficiency reached only 39% using the mentioned dosage of A336; see Figure 5B. The hydrophobic A336 is extremely viscous, and thus its precise application is troublesome [53]. The low solubility in water, along with high viscosity, can cause problems with the application of A336 in practice.  
Figure 4: Comparison of the sorption rate of RB8 (1 g L\({}^{-1}\) model aq. solution) on PAC (40 g L\({}^{-1}\)) and MTM (40 g L\({}^{-1}\)) and alternative sorbents BC and BT (40 g L\({}^{-1}\)).  
The better separation of RB8 using BAC can be caused by the better solubility of BAC in water and thus a better contact of BAC and RB8 dissolved in water. Based on these findings, BAC combines both sufficient aqueous solubility and the removal efficiency of acid halogenated pollutants such as RB8 dye; for more information, see our previous studies [32; 44; 45].  
The better solubility of BAC in water than A336 can be supported with the octan-1-ol/water partition coefficient \(\mathrm{P_{ow}}\) determined for the ion pair of RB8 with BAC with A336; see Figure S6 in the Supplementary Materials. The octan-1-ol/water partition coefficient for (BAC)\({}_{4}\)RB8 is lower than the octan-1-ol/water partition coefficient for (A336)\({}_{4}\)RB8; see Figure S6 in the Supplementary Materials.  
The formation of ion pairs can also be supported by the content of BAC or A336 after RB8 dye separation using these ILs. According to the cationic surfactant tests, the content of BAC or A336 after the formation of respective ion pairs within the separation of the RB8 decreased almost by 80% or by 69%, respectively. It is in good agreement with the separation efficiency of the RB8 dye using BAC and A336 (see Figure 5) and the octan-1-ol/water partition coefficient for the relevant ion pairs (see Figure S6 in the Supplementary Materials).  
In addition, the mentioned cationic surfactant tests proved that the content of R\({}_{4}\)NX after the separation of RB8 dye using BAC (in optimized dosages) does not reach the limits of surfactant contents in sewage waters nor the levels of no-observed-adverse-effect concentration (NOAEC) for aquatic organisms [54], and it is under concentrations that can cause acute or chronic toxicity problems for aquatic organisms [55]. The removal of the cationic surfactant also corresponds with the great removal of COD (see Figure 5) after the mentioned treatment (in the second case, the remaining BAC in treated water would increase the content of COD). Therefore, most of the BAC applied for the separation of dyes does not remain in the aqueous solution, and these treated aqueous streams can be discharged into the biological WWTP.