# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Inorganic Compounds in Environmental Samples
One of the current concerns of government agencies, the scientific community, and popular society is the lack of resources prognosticated for the next century. For this reason, great efforts are being made aimed at the recovery of valuable resources and the reuse of wastes and secondary resources. With this aim, [PERSON] et al. [32] presented a broad study of the potential of some biodegradable DESs for the extraction of metal salts and oxides, as well as phosphates from incienerated sewage sludge ash, as the main secondary resource of sewage sludge. In this regard, seven DESs based on ChCl and d-sorbitol, tartaric or citric acid, and urea, or DMU with d-fructose, mannose, and citric acid, as HBA and HBDs, respectively, were evaluated. As can be seen in Figure 2, the solubility in terms of mass fraction (wt%) of twelve metal oxides [Al(III), Ca(II), Fe(III), Cu(II), Zn(II), Mo(III), Ru(IV), Sn(IV), Ce(IV), Pt(IV), Pb(II/IV)], seven metal chlorides [Fe(III), Cu(II), Zn(II), La(III), Ce(III), Eu(III), Pb(II)], three metal acetates [Cu(II), Zn(II), Pb(II)], two metal phosphates [Al(III), Ca(II)], CaSO\({}_{4}\), and CaCO\({}_{3}\) in the tested DESs showed that metal oxides were better dissolved in eutectic mixtures containing carboxylic acids while those formed by urea or derivatives are preferable by metal chlorides or phosphates. These facts can be partially explained because of the pH values of the eutectic mixtures. DESs containing organic acids present pH values in the range of 0-2, very favourable for metal oxides dissolution, and metal chlorides dissolve in neutral or slightly basic media such as the main values of urea or DMU (pH of 6.9-7.7). However, although phosphates also dissolve in neutral or slightly basic DESs media, this does not appear to conform to this pH trend. For their part, acetates tend to dissolve in DESs composed of ChCl. Thus, based on these data, the selective extraction of the studied compounds is possible, however, as indicated by the authors, it should be determined experimentally. As incinerated sewage sludge ash contained both phosphate-based compounds and trace components such as metal oxides, which possessed very similar solubility properties in conventional solvents, the authors proposed the evaluation of neutral to slightly basic DESs containing urea or derivatives in order to selectively separate both kinds of compounds. Thus, ChCl:urea (1:2), DMU:D-tructose (3:1), and DMU:mannose (5:1) were tested, while ChCl:d-sorbitol (5:1) was selected as the reference medium because of its slightly acidic pH and does not possess a urea derivative. As expected, the best extraction yields of around 45 and 47% were found for DMU with fructose and mannose, respectively, after 24 h of extraction. However, when the accumulation of phosphates was studied, it showed that the main accumulation took place in the first few hours (31% in 2 h for DMU:mannose), while the accumulation was very little in the following hours (7% in 24 h). Additionally, the authors studied the content of toxic heavy metals in both the original matrix and in the extract following the requirements of the German Ordinance for the Recovery of Sewage Sludge. Results showed that the amounts of toxic metals, which are relatively low in the raw material, not only do not decrease, but some of them also accumulate, such as cadmium and nickel. In conclusion, the work developed by [PERSON] et al. served as a preliminary study for phosphorus resources obtention, however, further studies should be carried out in order to improve the extraction performance and removal of undesirable components such as toxic metals. In general terms, for the evaluation of inorganic species in environmental samples, there is not much variability in the use of DESs and they are mainly formed by ChCl as HBA and/or alcohols, organic acids, or urea as HBDs. Several metal ions were evaluated, although other inorganic compounds such as ammonia and metal oxides and salts were also determined.