Here we show that diamide L acts as a highly selective reagent for the precipitation of gold from a variety of aqueous acidic solutions and, by tuning the HCl concentration, allows other valuable and critical metals to be separated by precipitation. A resulting selective precipitation could both eradicate the use of potentially hazardous organic solvents in SX separations and simplify a hydrometallurgical separation process. 1a) would readily assemble into a proton-chelated structure suitable for metalate recognition along with a potential to form insoluble, infinitely extended structures.

The observation of proton-linked amides by us and others in gold separations suggested that the simple diamide (L) (Fig.

In some cases, mixed-metal third phases formed, along with a tin-containing precipitate 35. Protonation of the monoamides formed intermolecular proton-chelated receptor cations in the organic phase that dynamically assembled with AuCl 4 − into supramolecular clusters. We reported that simple monoamides showed high selectivity in the separation of gold by solvent extraction (SX) from acidic solutions of metals representative of e-waste 34, 35. Extended supramolecular network structures were also formed upon selective precipitation of HAuCl 4 from acidic solutions comprising Au, Ni, Cu, Zn, alkali-, and alkaline-earth metals by the biomolecule niacin, a pyridine carboxylic acid 33. Supramolecular interactions between acyclic durene diamides and HAuCl 4 resulted in gold precipitation as extended networks, although in these cases no selectivity for gold over other metals was explored 32. This rationale has also been extended to the separation by precipitation of platinum from palladium and rhodium in which the cucurbituril displays a preference for the hexachloroplatinate(IV) dianion 31. Molecular recognition processes involving cyclodextrins 26, 27 and cucurbiturils ( n = 5–8) 28, 29, 30 use the curvature and donor-group decoration of these capsular guest molecules to host selectively both alkali-metal cations and AuCl 4 − within superstructures that ultimately precipitate gold from aqueous acidic solutions. Pre-formed porous network materials such as metal-organic frameworks (MOFs), polymers, and electroactive materials are selective for gold adsorption, and often exploit the accessible reduction potential of gold to deposit the metal within the porous cavities 20, 21, 22, 23, 24, 25. Selective and reusable precipitation and adsorption methods are becoming increasingly popular for gold and other metal separations as they provide significant advantages over traditional, single-use precipitants 15, 16, 17, 18, 19.

Recycling these latter materials would also provide impetus to global circular economy visions 11, 12, but their complexity requires highly selective and environmentally benign recycling technologies 13, 14. Alternatives to the current industrial practice of cyanidation 5, 6 and informal mercury alloying 7, 8 that are based upon selective extraction, precipitation, or adsorption of gold from leach solutions are being actively pursued 9, with the recycling of electronic waste (e-waste) the focus of much attention as it presents a significantly higher concentration of gold than its ores 10. The extraction of gold from its ores and its recycling from waste materials represent significant technological and environmental challenges 1, 2, 3, 4. This discovery could be exploited in metal refining and recycling processes due to its tuneable selectivity under different leaching conditions, the avoidance of organic solvents inherent to biphasic extraction, and the straightforward recycling of the precipitant. At 6 M HCl, complete collection of gold, iron, tin, and platinum occurs, demonstrating that adaptable selective metal precipitation is controlled by just one variable.

The diamide is highly selective, with its addition to 29 metals in 2 M HCl resulting in 70% gold uptake and minimal removal of other metals. Gold is released from the precipitate on contact with water, enabling ligand recycling. The X-ray crystal structure of the precipitate displays an infinite chain of diamide cations interleaved with tetrachloridoaurate. Here we show that a simple tertiary diamide precipitates gold selectively from aqueous acidic solutions, including from aqua regia solutions of electronic waste. Precipitation processes are increasingly popular and are reliant on designing and understanding chemical recognition to achieve selectivity. The efficient separation of metals from ores and secondary sources such as electronic waste is necessary to realising circularity in metal supply.

Precipitate gold