Hydrophobic (deep) eutectic solvents (HDESs) as extractants for removal of pollutants from water and wastewater – A review

Deep eutectic solvents (DESs) are a new generation of solvents that attracted increasing attention in diverse applications. In last years, growing number of studies on hydrophobic deep eutectic solvents (HDESs) as an alternative extractants for diverse chemicals from aqueous environments have been reported. This article provides an overview on the usage of HDESs in liquid–liquid extraction (LLE) of different pollutants from water and wastewater, where purified water tends to be further used or released into the environment. Discussed applications were developed for several emerging organic pollutants, such as pharmaceuticals (including antibiotics – ciprofloxacin, sulfamethoxazole, trimethoprim and vasoprotectant – calcium dobesilate), pesticides (neonicotinoids – imidacloprid, thiamethoxam, nitenpyram, acetamiprid), phenolic compounds, metal ions, among other. The particular attention was given to discuss chemical stability of HDESs after contact with water. On this basis, a matrix of possible water stable DESs was proposed. Furthermore, simple protocols to control HDESs solubility were suggested. Finally, the suggestions and guidelines for future research were provided, along with focus on most important physicochemical properties of HDESs playing a key role in presented application. A perspective on their future was discussed suggesting that such HDESs-based LLE should be mainly used for pre-treatment of wastewater with high pollution load, followed by adsorption or biological treatment process for removal of HDES traces. This review also highlights a serious environmental issue related to application of HDESs for sample preparation (microextraction) in analytical chemistry. Waste aqueous samples can contain hazardous – HDES related – substances and their utilization should be done with proper care on this aspect