(460g) First Principles of Metal Ion Extraction from Non-Buffered Water with Hydrophobic Deep Eutectic Solvents

Some metals, such as Cd, Pb, and As, are highly toxic at low concentrations.^[1,2] A technique often applied for the removal of metal ions from water is liquid-liquid extraction, due to the many advantages it offers over other methods.^[3] These advantages include easy to operate equipment, continuous removal of the contaminants and small amounts of extractant needed.^[4]

A disadvantage of the commonly used techniques is the need for conventional solvents, which are often toxic, volatile, and flammable. Recently, ionic liquids (ILs) were introduced for the removal of metal ions from water.^[5] Due to the electrostatic forces between the cation and the anion, they have a low volatility and are not flammable.^[6,7] However, their production is costly and therefore less interesting for industry.

Alternative solvents are deep eutectic solvents (DESs), first introduced in 2003. DESs are composed of two or more solid components interacting via hydrogen bonding and Van der Waals interactions.^[8] These interactions transform the two solids into a liquid phase.

A hydrophobic variant of these innovative solvents was presented in 2015 by a number of us.^[9] The first hydrophobic DESs consist of decanoic acid and quaternary ammonium salts, which were subsequently used for the extraction of volatile fatty acids from water.^[9] In 2015 also a second paper regarding hydrophobic DESs was presented in the literature. In this manuscript biomolecules were removed from an aquatic environment.^[10]

Here, metal ion extraction from a non-buffered aquatic environment is shown for the first time.^[11] The results show that transition metals can be removed from water with distribution coefficients (D) higher than 0.99. When the initial concentration of the model metal ion Co²⁺ is increased from 1 g/L to 4 g/L, a decrease in D to approximately 0.83 is observed. When the solvent to feed ratio is varied, as low as 1:10, a decrease of D to approximately 0.80 is measured. It was found that the prevailing mechanism is ion exchange between the positively charged Co²⁺ and lidocaine. Furthermore, it is shown that Co²⁺ can be removed within 5 s. Finally, it is shown that it is possible to recover the DES.

References

[1] T. Martin and D. Holdich, Water Research, 1986, 20, 1137–1147.

[2] P. Madoni and M. G. Romeo, Environ. Pollut., 2006, 141, 1–7.

[3] J. Rydberg, Solvent extraction principles and practice, revised and expanded, CRC Press, 2004.

[4] S. Wellens, B. Thijs, and K. Binnemans, Green Chemistry, 2012, 14, 1657–1665.

[5] D. Parmentier, S. J. Metz, and M. C. Kroon, Green Chemistry, 2013, 15, 205–209.

[6] J. P. Hallett and T. Welton, Chemical reviews, 2011, 111, 3508–3576.

[7] P. Wasserscheid and T. Welton, Ionic liquids in synthesis, Wiley Online Library, 2008, vol. 1.

[8] M. Francisco, A. van den Bruinhorst, and M. C. Kroon, Angew. Chem. Int. Ed., 2013, 52, 3074–85.

[9] D. J. G. P. van Osch, L. F. Zubeir, A. van den Bruinhorst, M. A. A. Rocha, and M. C. Kroon, Green Chemistry, 2015, 17, 4518–4521.

[10] B. D. Ribeiro, C. Florindo, L. C. Iff, M. A. Coelho, and I. M. Marrucho, ACS Sustainable Chemistry & Engineering, 2015, 3, 2469–2477.

[11] D. J. G. P. van Osch, D. Parmentier, C. H. J. T. Dietz, A. van den Bruinhorst, R. Tuinier, and M. C. Kroon, Chemical Communications, 2016, 52, 11987–11990.