Creation of Deep Eutectic Solvents Based on Alkyl Salicylate Derivatives and Selective Extraction of In(III) and Ga(III) from Acidic Media

To recover In(III) and Ga(III) from waste electronics and solar panels, we examined 3,5-di-tert-butylsalicylic acid (DBSA) as a selective extractant for these metals. DBSA selectively extracted In(III) and Ga(III) from Zn(II) and Cd(II), although their mutual separation was not achieved. Therefore, a deep eutectic solvent (DES) that is liquid at room temperature was prepared by mixing solid DBSA with solid tri-n-octylphosphine oxide (TOPO). TOPO alone did not extract any metals. The DES exhibited higher selectivity for In(III) and Ga(III) over Zn(II) and Cd(II) at lower pH values due to its synergistic effect with TOPO. To clarify the extracted species of In(III), the extraction equilibria of In(III) with DBSA alone and the DES were quantitatively investigated. Quantitative back extraction of In(III) was achieved using dilute acidic solutions. In addition, the DES without organic solvents succeeded in the mutual separation of In(III) and Ga(III) from Zn(II) and Cd(II).