Environmental Implications of Ionic Liquid and Deep Eutectic Solvent in Geothermal Application: Comparing Traditional and New Approach Methods

The significant surge of ionic liquids (ILs) research over the past decade has led to the formation of various novel ionic liquid compounds and their diverse applications. Enhanced geothermal systems (EGS) for geothermal power generation are an emerging IL application as a heat extraction fluid. The once widely held belief in the environmentally friendly characteristics of ionic liquids, mainly due to their insignificant vapor pressure, is now being scrutinized. It has become apparent that while ILs do not readily evaporate into the atmosphere, they are not guaranteed to remain entirely isolated from the environment. Recent attention has been directed toward toxicological studies, including ecotoxicity impacts, with the long-accepted assumption of ILs having low toxicity being invalid. This paper aims to shed light on the toxicity of hexylepyradinium bromide (HPyBr) IL and a deep eutectic solvent (DES) comprising choline chloride with magnesium chloride hexahydrate (ChCl:MgCl₂·6H₂O) to five test species, an algal species (<i>Raphidocelis subcapitata</i>), the water flea (<i>Ceriodaphnia dubia and Daphina magna)</i>, the fathead minnow (<i>Pimephales promelas</i>), and the earthworm (<i>Eisenia fetida</i>), to measure acute and chronic toxicity. Additionally, new approach methods (NAMs) are presented using the fathead minnow embryo and the rainbow trout (<i>Oncorhynchus mykiss</i>) gill cell line and the RTgill-W1 assay to compare sensitivity across species. Overall, ChCl:MgCl₂·6H₂O displayed lower toxicity, while HPyBr demonstrated higher toxicity, highlighting the need for caution in handling it to prevent harm to aquatic ecosystems. Comparative analysis underscored the potential threat of ChCl:MgCl₂·6H₂O to aquatic life, highlighting the cumulative effects of the environmental components.