Selective Gold Recovery via Thermally Generated Water Microdroplets at Oil-Water Interfaces

Efficient and environmentally benign recovery of precious metals from complex aqueous systems remains a central challenge. Here, we demonstrate that thermally generated water microdroplets at oil-water interfaces function as highly reactive interfacial microenvironments that enable selective gold reduction and macroscopic particle growth without external reductants, catalysts, or sorbents. Mild heating of oil-water mixtures containing Au ions produces abundant interfacial water microdroplets (5-50 μm) that continuously generate redox-active species, including solvated electrons, hydrogen atoms, and hydroxyl radicals, thereby driving reduction reactions that do not occur in bulk water. Selectivity is governed by a dual criterion: (i) thermodynamic accessibility above the solvent reduction limit imposed by hydrogen evolution and (ii) chemical stability of the reduced phase under strongly acidic conditions. Au satisfies both requirements, and extension experiments with Pt and Ag validate the broader applicability of this principle, whereas competing base metals are excluded either by redox constraints or by instability of their reduced forms. Dynamic microdroplet coalescence and rupture further couple redox chemistry with particle growth, promoting nanoparticle aggregation into micrometer- to millimeter-scale Au particles that are readily separated. The oil phase is recyclable over multiple cycles, enabling high-yield gold recovery with minimal inputs and waste generation. These findings, validated across capillary, glass-vial, and upgraded reactor systems, establish interfacial microdroplet chemistry as a scalable framework for selective precious-metal recovery.