Boron extraction from aqueous medium using novel hydrophobic deep eutectic solvents

In this work, four novel hydrophobic deep eutectic solvents (DES) were prepared from a combination of menthol (Men) or thymol (Thy) as hydrogen bond acceptors (HBA) with 1-decanol (Dec) or 2-methyl-2,4-pentanediol (MPD) as hydrogen bond donors (HBD), for the extraction of boron from aqueous solutions. The DESs were characterized in terms of their physical (density, viscosity and water content), and thermal (melting point and decomposition temperature) properties. The DES' critical properties were estimated using a group contribution method based on the modified Lydersen-Joback-Reid and Lee-Kesler mixing rules. Later, the solvents were evaluated for their capability to extract boron from different aqueous solutions and the extraction and stripping efficiencies were used as performance evaluation criteria. The hydrophobic DES made with MPD showed high extraction efficiency of boron (in the boric acid form), up to 90.1% and 83.2% for Thy:MPD and Men:MPD, respectively. Stripping of boron from the DES phase was done using 0.1M NaOH as stripping solution. Moreover, FTIR spectra were recorded before and after extraction of boron to better understand the extraction mechanism. The extraction was due to the complexation between MPD in the DES and boric acid. The hydrophobic DESs offer an environmentally benign alternative as extractants, and allow for the use of water miscible diols, like MPD, in the extraction of boron from aqueous environments such as industrial wastewater or boron-rich brines.