Aqueous biphasic systems (ABSs) that are based on deep eutectic solvents (DESs) are environmentally benign systems to use for metal ion separation. In this work, a series of DESs was synthesized for the first time with PEG 400 as hydrogen bond donors and tetrabutylphonium bromide (P₄Br), tetrabutylammonium bromide (N₄Br), or tetrabutylammonium chloride (N₄Cl) as hydrogen bond acceptors, and then they were combined with citrate (Na₃C₆H₅O₇), which is eco-friendly, to construct an ABS for use in the separation of Au(I) from an aurocyanide solution. Phase diagrams of DESs + Na₃C₆H₅O₇ + H₂O systems were constructed using the experimentally measured data. Multiple factors that affect the efficiency of the gold extraction were studied; these factors were the species of salt or DES and their content, the equilibrium pH, the oscillation time, and the initial gold concentration. Gold(I) is preferentially retained in the DES-rich phase, and the P₄Br:PEG 1:2 + Na₃C₆H₅O₇ + H₂O system has a high extraction efficiency of 100.0% under optimized conditions. FT-IR, NMR, and TEM characterizations and DFT calculations show that the migration of Au(I) from the salt-rich to the DES-rich phase follows an ion exchange mechanism. Specifically, Au(CN)₂^- replaces Br^- in the original P₄Br and generates a stable ion pair with the quaternary phosphonium salt cation, P⁺, and this replacement is driven by electrostatic attractions. A new strong hydrogen bond network simultaneously forms between the anionic Au(CN)₂^- and the -OH group in the PEG 400 component. Finally, the gold of Au(I)-loaded P₄Br:PEG 1:2 can be successfully reduced by sodium borohydride with an efficiency of 100.0%. The strategy to extract gold(I) from alkaline cyanide solutions using an ABS based on DESs as proposed in this work provides a potential platform for developing green technology for recovering gold.

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