Design and Application of Hydrophobic 2,2,4-Trimethyl-1,3-Pentandiol Deep Eutectic Solvents for Boron Extraction

Abstract

Recently, new genres of ionic liquids and deep eutectic solvents (DES) have emerged as eco-efficient "designer solvents". Herein, we report the liquification of the water soluble diol 2,2,4-trimehtyl-1,3-pentanediol (TMPD) into a novel hydrophobic DES with thymol or menthol. The intrinsic supercooling properties of the constituents yielded eutectic mixtures with a wide liquid window and superior capabilities for boron extraction. The design framework of the TMPD-based DESs considered multiple criteria in which the trade-offs between the solvents' properties, its boron extraction efficiency and leachability to the aqueous phase were thoroughly mapped. Formation of stable TMPD-boric acid complexes provided exceptional selectivity towards boron extraction from a synthetic brine solution. Thymol:TMPD and Menthol:TMPD at a molar ratio of 2:1 were selected as they exhibited superior extraction efficiencies for boron ('98%) without the drawbacks of lower DES stability or higher DES leachability into the aqueous phase. A remarkably stable performance of the TMPD-based DESs was verified with the sustained boron extraction efficiency over a wide range of pH (2-8), temperature (10-60 °C), aqueous feed boron concertation (100-6000 ppm) (Fig. 1) and most notably, the high A/O mass ratio. The DESs were highly selective for boron in the presence of various co-existing ions, and stable over multiple extraction-stripping cycles. This method of integrating TMPD as the active extracting agent within a DES mixture allows for a higher recovery and better extraction efficiency of boron compared to other traditional LLE systems in which TMPD is diluted with hazardous and volatile organic solvents. The sustainability of these novel solvents allow for its integration into membrane-based separation technologies such as Supported Liquid Membranes.

Keywords Boric acid; Economic and social effects; Efficiency; Eutectics; Hydrophobicity; Ionic liquids; Mixtures; Molar ratio; Phenols; Solvent extraction; Solvents