## 2 Materials and Methods
### Extraction Experiments  
In order to evaluate the potential of the synthesized ILs for metal extraction, 100 mg of the respective IL were weighed into glass vials and 5 mL of the respective feed solution were added. The two ILs that were solid at room temperature, [P18 ss][HNA] and [N18 ss][HNA, were melted, weighed in molten state, and the feed solution was added after solidification. The ILs remained solid during extraction, resulting in a solid-liquid extraction in these cases. Firstly, a time-dependent extraction using a feed solution with 1 mg L\({}^{-1}\) mercury adjusted to an initial pH of 3.5 was performed. For this, samples were kept at room temperature and shaken with 300 rpm for 1, 2, 3, 4, 5 and 6 h, respectively. Afterwards, the aqueous phase was recovered by pipetting and centrifuged at 5000 rpm before being measured for the respective metal and DOC content. Experiments were conducted in triplicates. To consider a loss of metal due to possible oxidation, precipitation and adhesion effects in the glass vials, reference samples containing the respective metal concentrations without IL were treated equally to the samples with ILs; the metal concentration was determined before and after the respective extraction time. Secondly, extraction experiments were carried out for an extraction time of 2 h with spiked pure water feed solutions adjusted to pH 3.5 and pH 8.0, as well as with bottled natural mineral water whose major composition was characterized before use (Table 1). These feed solutions were spiked with 20 ug L\({}^{-1}\) of the respective metal.  
The extraction efficacy was calculated as the percentage of metal removed from the feed solution after the extraction experiment compared to reference samples, using Equation (1)  
\[\textbf{Extraction efficacy}\ (\%)=\frac{\textbf{C}_{\textbf{Ref}}-\textbf{C}_{ \textbf{t}}}{\textbf{C}_{\textbf{Ref}}}\textbf{+}\textbf{100} \tag{1}\]  
C\({}_{\textbf{Ref}}\) represents the metal concentration in the reference samples after the respective extraction time and C\({}_{\textbf{t}}\) the metal concentration in the sample after extraction.  
To evaluate the degree of leaching, the dissolved organic carbon concentration in samples after extraction was measured and leaching expressed in mg L\({}^{-1}\) as well as relative loss of IL (Equations (2) and (3))  
\[\textbf{Leaching}\left(\textbf{mg L}^{-1}\right)=\frac{\textbf{DOC}}{\textbf{ C}_{\textbf{IL}}} \tag{2}\]  
\[\textbf{Leaching}\left(\%\right)=\frac{\textbf{DOC}\textbf{*}\textbf{V}_{ \textbf{S}}}{\textbf{m}_{\textbf{IL}}}\textbf{*}\textbf{100*}\textbf{C}_{ \textbf{IL}} \tag{3}\]  
DOC (mg L\({}^{-1}\)) was the measured concentration of dissolved organic carbon in the sample after extraction, vs. (L) the feed solution volume and m\({}_{\textbf{IL}}\) (mg) the mass of IL used. C\({}_{\textbf{IL}}\) represents the carbon content of the respective IL and was used to calculate the leaching of the entire liquid based on the carbon leaching, whereby the cation and anion of the IL were assumed to leach equally. The calculated value therefore represents the percent loss of IL during extraction and serves as an estimation of the reusability in liquid-liquid/solid-liquid extraction setups.