# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Inorganic Compounds in Environmental Samples
Besides, regardless of the procedure used, most cases involve the use of external assistance including UA [27; 30; 33; 37; 38; 39; 41], VA [31], AA [34; 35], magnetic assistance [29; 40], or current intensity [28], among others, as it was mentioned for the extraction of organic compounds. An interesting application of additional assistance on the extraction of inorganic compounds was the work of [PERSON] et al. [28]. In this case, the authors performed an electrodyalitic system based on DESs for the removal and separation of tungsten and arsenic from mining residues. With this aim, three extraction systems were compared: (i) a conventional SLE, in which 2.2 g of sample was extracted with 20 mL of DES for 10 days; (ii) an SLE of 11.1 g of mining residues in 98.5 mL of water mixed with 1.5 mL of DES and applying current intensities of 50 and 100 mA for 4 days; and (iii) an electrodyalitic system with two compartments: the cathode in which 39 g of samples were mixed with 345 mL of water and 5 mL of DES and the anode, in which 250 mL of 0.01 M NaNO\({}_{3}\) were placed. Both parts were divided by an anion exchange membrane, and current intensities of 50 and 100 mA were applied for 4 days. In the initial study, four DESs were tested composed of ChCl and malonic, oxalic, and lactic acids at molar ratios of 1:2, 1:1, and 1:2, respectively, and urea:propionic acid at a molar ratio of 1:2, showing that the best extraction yields were obtained with the first two DESs for As (16.2%) and W (8.8%), respectively. ChCl:malonic acid presents lower viscosity which improved the mass transfer of As. For its part, W extraction is enhanced by high concentrations of chloride ions provided by ChCl and oxalate ions through the availability of tungsten complexes in solution. Thus, these DESs were selected for further experiments. When this experiment was carried out applying a current intensity of 100 mA, better results were obtained using ChCl:oxalic acid DESs (recovery of 27.6% for As and 15.6% for W). The use of an electrical field may contribute to accelerate the dissolution of tungsten, which could explain this improvement. In the case of the electrodialytic removal, the absence of DESs during the extraction performance caused the W recovery to fall lower than 1%. In DES-assisted extraction, the removal of 35% for As and 22% for W was obtained using ChCl:oxalic acid and a current intensity of 100 mA. However, the best electromigration conditions for As and W were obtained using ChCl:malonic acid (100 and 50 mA, respectively) with migration percentages of 82.3 and 76.5%. This work constitutes a promising strategy for the remediation of As and W from mining residues and the subsequent exploitation of treated residues in the construction sector, although the material is not free of contaminants and further studies should be carried out in this way. A commonly used strategy for the extraction of metal ions from aqueous samples or solutions is to use a complexing agent that allows the efficient extraction of analytes [30; 31; 33; 34; 38; 39; 41]. In this case, the formation of a hydrophobic complex between metal ions and the complex ligand takes place, favouring the extraction of the metal-ligand complex into the extraction phase and improving the selectivity of the procedure. An interesting approach is the one developed by [PERSON] [27], in which taking advantage of the versatile properties of DESs, an HDES constituted of the IL THTTDPC (HBA) and thiosalicylic acid (HBD) was synthesised at a molar ratio of 1:2 and used as both extraction and complexing agent. Thanks to the presence of thiol groups, the HDES was capable of executing this dual role, obtaining good recovery values (99-102%) as well as low LODs (0.05-0.13 \(\upmu\)g/L) for divalent cations of lead, cadmium, cobalt, and nickel.