## 3 Results and Discussion
### Extraction Experiments
#### 3.1.2 Extraction of Cu, Ag, Cd, Hg and Pb from 20 ug L\({}^{-1}\) Feed Solutions  
The results on the extraction capability are summarized in Table 2, given as residual concentration remaining in feed solutions after extraction and the respective relative efficacy in brackets. The highest  
Figure 1: Time-dependent Hg extraction (filled symbols) from a pure water feed solution at pH 3.5 containing 1 mg L \({}^{-1}\) Hg and leaching of the ILs into the aqueous phase (open symbols); n = 3, error bars = \(\pm\) standard deviation (SD). extraction efficacies were achieved for Hg and Ag using [P\({}_{66614}\)][HNA], which is additionally depicted in Figure 2.  
Remarkably, Hg could be extracted to final concentrations under the limit of detection of 0.1 \(\mu\)g L\({}^{-1}\) from all three feed solutions using [P\({}_{66614}\)][HNA]. Changing the feed solution concentration from 1 to 20 \(\mu\)g L\({}^{-1}\) decreased the metal content after extraction from 50 to less than 0.1 \(\mu\)g L\({}^{-1}\), displaying a successful removal even of trace levels of Hg. In both cases, the residual concentration would conform to the Water Framework Directive 2000/60/EC of the European Union concerning limit values and quality objectives for industrial mercury discharges, with a threshold value of 50 \(\mu\)g L\({}^{-1}\)[36]. Furthermore, concentrations after the trace level extraction experiment are below the Austrian drinking water act threshold of 1.0 \(\mu\)g L\({}^{-1}\)[37] and potentially comply with the threshold value of 0.07 \(\mu\)g L\({}^{-1}\) defined in the directive on environmental quality standards for surface waters in the European Union [38]. Regarding toxicity to freshwater algae, the achievable extraction also conforms with predicted no observed effect concentrations (NOEC) for Hg towards several freshwater algae, including _R. subcapitata_, of 0.6-3.2 \(\mu\)g L\({}^{-1}\), reported by [PERSON] et al. (2013)[39]. [P\({}_{66614}\)][HNA] was equally suitable for extracting Ag, achieving \(>\) 99% extraction from the three feed solutions. The remaining concentrations below the limit of detection of 0.03 \(\mu\)g L\({}^{-1}\) were significantly lower than the EC\({}_{50}\) values found in the literature: the lowest EC\({}_{50}\) for aquatic biota have been reported for fish, with LC\({}_{50}\) (refers to the concentration of a toxicant in the respective medium, that causes the death of 50% of a tested population after a specified time) between 2 and 30 \(\mu\)g L\({}^{-1}\)[40]; EC\({}_{50}\) for certain freshwater algae are as low as 10 \(\mu\)g L\({}^{-1}\)[41], whereas 125 \(\mu\)g L\({}^{-1}\) were reported for _R. subcapitata_[42]. Cu was extracted \(>\) 90% from the mineral water feed solution for all three investigated ILs. The positive effect that we previously reported [28] of a mineral water composition on Cu extraction at mg L\({}^{-1}\) levels remained valid in the present extraction setup. The low residual concentration after  
extraction, approx. 1 \(\mu\)g L\({}^{-1}\), is below the lowest reported NOEC of 4.2 \(\mu\)g L\({}^{-1}\) for _R. subcapitata_ in natural waters [43], underlining the high capability of the three ILs to extract Cu from freshwater matrices. Regarding Pb, [N\({}_{\text{1888}}\)][HNA] achieved an extraction of 93.0\(\%\)\(\pm\) 0.6%, but only from pure water at pH 3.5. For mineral water, the extraction efficacies were comparably low for all three ILs, with values ranging from 28% to 45%, equivalent to residual concentrations of 4.5-6.0 \(\mu\)g L\({}^{-1}\). Cd extraction likewise was decreased compared to studies using 1 mg L\({}^{-1}\) feed solutions [28], revealing comparably high residual concentrations between 6.5 and 18.5 \(\mu\)g L\({}^{-1}\). The highest efficacy was achieved from pure water at pH 3.5 using [P\({}_{66614}\)][HNA]: 64.4% \(\pm\) 2.1%. Both Pb and Cd displayed a dependence on initial metal concentration, as described by [PERSON] et al.