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With environmental problems and a shortage of resources, it is urgent to recover gold from electronic waste (e-waste). Meanwhile, it is necessary to explore the relationships between the structures and extraction performances of ionic liquids (ILs) to further improve the extraction capacities of ILs for gold. Herein, three pyrrolidinium-based ILs with various N-substituents were synthesized for solvent-free extraction of Au(III). The three pyrrolidinium-based ILs exhibited ultrahigh extraction capacities for Au(III) with the order of [Pyr-EA][NTf2] (524.6 mg·g–1) > [Pyr-Bu][NTf2] (457.3 mg·g–1) > [Pyr-C2OC2][NTf2] (403.5 mg·g–1). The effects of N-substituents on extraction performances were quantitatively investigated by electrostatic potential (ESP) and quantitative NMR. Among them, the ester group (−COOR) is the most beneficial to the extraction process, which is associated with an increased ESP value and boosted hydrophobicity. Notably, [Pyr-EA][NTf2] displayed outstanding selectivity in multimetal solutions; especially, it could selectively and efficiently extract Au(III) from the actual CPU. Furthermore, [Pyr-EA][NTf2] possesses significant reuseability and can maintain superior extraction efficiency of 84.3% after five cycles. These excellent performances together with low cost ($0.44·g–1 [Pyr-EA][NTf2]) and considerable profit ($29.72·g–1 [Pyr-EA][NTf2]) confirmed that [Pyr-EA][NTf2] is a promising prospect for sustainable recovery of gold from e-waste.

Two ionic liquid (IL) thermomorphic systems were designed. The thermoregulated property exhibited by the IL systems was utilized for the recovery of Au­(III) from acidic medium. The cloud point order of the two ILs was proposed and evidenced via a combination of experimental measurements and quantum chemical calculations. The impacts of salt ions and acid on the thermomorphic phase behavior of the IL–H2O binary mixture system were investigated. After comprehensive comparison, a systematic study of Au­(III) extraction by the [Suc]­[Tf2N]2 (succinyl bis­(trifluoromethylsulfonyl)­imide) system was carried out because of its outstanding hydrophobicity and extraction ability. Then, a comparison between traditional liquid–liquid extraction (TLLE) and homogeneous liquid–liquid extraction (HLLE) was made during the process of optimization of various extraction parameters. In addition, the anion-exchange mechanism in the extraction process was revealed. The thermoregulated [Suc]­[Tf2N]2 system presented an excellent selectivity for Au­(III) in a solution of coexisting multimetallic ions. Finally, Au­(III) was almost completely stripped from the loaded IL phase by oxalic acid. The extraction efficiency of Au­(III) using regenerated IL still maintains a surprising stability after five turns of cycling experiments. Hence, the dicationic IL [Suc]­[Tf2N]2 was regarded as a potential extractant for the recovery of gold.

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