Abstract
We investigated the applicability of three task-specific ionic liquids (ILs) as heavy metal extracting agents by contrasting extraction capabilities with algal toxicity. The compounds tested were trihexyltetradecylphosphonium-, methyltrioctylphosphonium- and methyltrioctylammonium 3-hydroxy-2-naphthoates. Experiments were performed to assess if these ILs can provide environmentally safe residual concentrations of the target metals after extraction. Both pure water and natural mineral water samples were spiked with 20 µg L−1 of Cu, Ag, Cd, Hg and Pb, respectively. Quantitative extraction (> 99%) of Hg and Ag was achieved. Cu and Hg were below the respective no-observed-effect-concentrations (NOECs) after extraction and Ag below 0.03 µg L−1. Acute toxicity assays were conducted using two freshwater green algae Raphidocelis subcapitata and Tetradesmus obliquus. Growth inhibition and maximum photochemical quantum yield of photosystem II after 72 h were assessed. ILs were less toxic than similar compounds, but still must be classified as acute toxicants for algae. An inhibiting effect on both growth and chlorophyll fluorescence was observed. The leaching of the ILs into the samples remains a limitation regarding their environmental-friendly applicability. Nonetheless, the extremely efficient removal of Cu, Ag and Hg under environmentally relevant conditions calls for further research, which should focus on the immobilization of the ILs.
Highlights
Ionic liquids (ILs) are generally defined as low-melting organic salts, often described as liquid below 100 ◦ C, and represent a relatively new class of chemical compounds [1]
Successful uses of ILs were reported in the fields of solvent extraction and separation [3,4], organic synthesis [5], electrochemistry [6], among many others
To further assess their applicability with respect to toxicological concerns, the toxicological potential of the Ils towards the freshwater green algae Raphidocelis subcapitata and Tetradesmus obliquus was investigated in acute toxicity assays. These tests included growth inhibition assays and were enhanced by monitoring the photosynthetic performance via chlorophyll fluorescence. We propose this non-invasive and fast method as a first ecotoxicity screening of novel task-specific ionic liquids (TSILs) used as extracting agents in aqueous phases
Summary
Ionic liquids (ILs) are generally defined as low-melting organic salts, often described as liquid below 100 ◦ C, and represent a relatively new class of chemical compounds [1] They exhibit a variety of useful physico-chemical properties, including very low volatility, high thermal, chemical and electrochemical stability, a wide liquid range and varying lipophilicity. Successful uses of ILs were reported in the fields of solvent extraction and separation [3,4], organic synthesis [5], electrochemistry [6], among many others. Their versatile properties enable numerous applications ranging from carbon dioxide (CO2)
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