The task-specific ionic liquid di-[trioctyl-(8-phenyloctyl)-phosphonium] pamoate, [TOPP]2[PAM], was designed as a highly hydrophobic substance for “greener” metal extraction. During metal extraction, it exhibits considerably reduced leaching into the aqueous phase compared to similar phosphonium-based ionic liquids. In order to assess the ecotoxicological potential of this novel compound, we investigated its algal toxicity towards the three freshwater green algae species Acutodesmus obliquus, Chlorella vulgaris and Raphidocelis subcapitata. In addition, algal toxicity was compared to commercially available ionic liquids Aliquat® 336, Cyphos® IL 101 and the task-specific ionic liquid trihexyltetradecylphosphonium 3-hydroxy-2-naphthoate, [P66614][HNA], over a prolonged period (7 days). In order to reach sufficient IL concentrations in the testing media, [TOPP]2[PAM] was leached either with or without support of ultrasonication. When leaching was supported by ultrasonication, the half-maximal inhibitory concentration (IC50) for [TOPP]2[PAM] ranged from 0.05 mg L−1 for C. vulgaris, 0.23 mg L−1 for A. obliquus to 0.45 mg L−1 for R. subcapitata. This classifies the substance as a short-term (acute) aquatic hazard in the category ‘Acute 1’. Yet, during leaching without ultrasonication, these concentrations were reached only for C. vulgaris, yielding an IC20 for A. obliquus and an IC10 for R. subcapitata instead. This demonstrates a clear advantage over Aliquat® 336 and Cyphos® IL 101, where IC50 values were easily reached. The results show a significantly reduced growth inhibition compared to commercial compounds when applying [TOPP]2[PAM] according to suggested metal extraction procedures. Further comparisons to [P66614][HNA] demonstrated the advantage of using ILs with functional anions to reduce toxic effects on freshwater green algae. Evaluating the algal vitality with pulse-amplitude modulated fluorescence also demonstrated the lower impact of ILs with functional, hydrophobic anions.
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