## 3 Results and Discussion
### Leaching  
Results for IL leaching into the aqueous phase during extraction are summarized in Table 3.  
In general, we were able to reproduce the low leaching values reported in previous extraction experiments [28]. Values ranged between a partial dissolution of 0.1%-0.8%, implying a high potential for reusability of the three ILs in liquid-liquid extraction setups. In contrast to the multi-elemental setup of past works [28; 34], the single metal experiments in the present study enabled an examination of metal-dependent changes in leaching. This provides insights into the prevalent extraction mechanisms. Overall, differences in leaching due to varying sample matrices or the respective target metal, however, were small. The most pronounced effects were recorded for [N\({}_{\text{1888}}\)][HNA], which we previously reported to be more susceptible to sample composition [28]. This difference could reflect a higher probability of a cation exchange extraction mechanism for [N\({}_{\text{1888}}\)][HNA] in the pure water feed  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Leaching \(\pm\) SD (mg L\({}^{-1}\))**} \\ \hline
**Sample** & **[P\({}_{\text{66614}}\)][HNA]** & **[P\({}_{\text{1888}}\)][HNA]** & **[N\({}_{\text{1888}}\)][HNA]** & **[N\({}_{\text{1888}}\)][HNA]** \\ \hline
**Cu** & & & & & \\ Pure water pH 3.5 & 76.7 \(\pm\) 1.1 & (0.38) & 24.7 \(\pm\) 2.0 & (0.12) & 139.3 \(\pm\) 23.8 & (0.70) \\ Pure water pH 8.0 & 75.9 \(\pm\) 0.9 & (0.38) & 25.7 \(\pm\) 2.3 & (0.13) & 136.8 \(\pm\) 30.7 & (0.68) \\ Mineral water & 62.9 \(\pm\) 1.1 & (0.31) & 27.6 \(\pm\) 3.8 & (0.14) & 78.6 \(\pm\) 6.0 & (0.39) \\ \hline
**Ag** & & & & & \\ Pure water pH 3.5 & 75.0 \(\pm\) 2.4 & (0.38) & 25.3 \(\pm\) 0.4 & (0.13) & 168.1 \(\pm\) 5.7 & (0.83) \\ Pure water pH 8.0 & 80.3 \(\pm\) 0.7 & (0.40) & 23.2 \(\pm\) 0.9 & (0.12) & 132.4 \(\pm\) 9.8 & (0.66) \\ Mineral water & 74.4 \(\pm\) 1.6 & (0.38) & 24.8 \(\pm\) 7.9 & (0.12) & 99.8 \(\pm\) 9.3 & (0.50) \\ \hline
**Cd** & & & & & \\ Pure water pH 3.5 & 83.7 \(\pm\) 2.5 & (0.42) & 25.6 \(\pm\) 1.5 & (0.13) & 97.4 \(\pm\) 6.1 & (0.48) \\ Pure water pH 8.0 & 78.6 \(\pm\) 2.5 & (0.39) & 25.4 \(\pm\) 1.2 & (0.24) & 80.4 \(\pm\) 10.4 & (0.39) \\ Mineral water & 84.3 \(\pm\) 0.5 & (0.42) & 43.5 \(\pm\) 3.2 & (0.33) & 81.8 \(\pm\) 6.0 & (0.35) \\ \hline
**Hg** & & & & & \\ Pure water pH 3.5 & 80.2 \(\pm\) 1.6 & (0.40) & 25.6 \(\pm\) 2.0 & (0.13) & 137.1 \(\pm\) 15.0 & (0.68) \\ Pure water pH 8.0 & 78.4 \(\pm\) 0.6 & (0.39) & 24.8 \(\pm\) 0.4 & (0.12) & 87.5 \(\pm\) 2.3 & (0.44) \\ Mineral water & 65.4 \(\pm\) 0.8 & (0.33) & 27.6 \(\pm\) 2.4 & (0.14) & 80.6 \(\pm\) 6.0 & (0.40) \\ \hline
**Pb** & & & & & \\ Pure water pH 3.5 & 73.2 \(\pm\) 8.0 & (0.36) & 25.4 \(\pm\) 2.4 & (0.13) & 76.7 \(\pm\) 2.7 & (0.38) \\ Pure water pH 8.0 & 74.6 \(\pm\) 1.7 & (0.37) & 28.5 \(\pm\) 1.5 & (0.14) & 107.1 \(\pm\) 12.5 & (0.53) \\ Mineral water & 63.6 \(\pm\) 1.2 & (0.32) & 27.3 \(\pm\) 0.2 & (0.14) & 110.8 \(\pm\) 4.8 & (0.55) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Leaching of the respective ionic liquid into the aqueous feed solution during extraction for an extraction time of 2 h; percentage of dissolution given in brackets, n = 3.  
solution at pH 3.5, enabled through the weak ionic strength and acidic conditions in the sample [17; 44]. This effect is best visible for the extraction of Ag, where leaching decreased from 0.83% to 0.50% for the mineral water solution. [P\({}_{66614}\)][HNA] and [P\({}_{1888}\)][HNA] displayed almost equal leaching data for each metal and feed solution, respectively, suggesting that the mode of extraction remained constant under all conditions. Based on the consistent data and the highly hydrophobic character of the two ILs, we assume that all target metals were mainly extracted via a neutral extraction mechanism. This is further supported by constant leaching, regardless of whether or not the respective target metal could be extracted.