Metallic materials a few atomic layers thick can very quickly extract gold from water and solutions. The researchers report an efficiency of 99.99% in 15 seconds for dissolved gold ions. They thus recovered 0.15 g of gold from 30 old computer processors.
The recycling of electronic waste often relies on a preliminary dissolution of the metals. This step produces a liquid containing many different ions. The main obstacle is then to capture gold with high selectivity, without simultaneously recovering copper, nickel, or other much more abundant elements.
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The team studied several two-dimensional transition-metal dichalcogenides. These materials combine a metal with sulfur and can form very thin sheets. The tested variants notably included molybdenum disulfide, tungsten disulfide, tantalum disulfide, and titanium disulfide.
These sheets captured Au3+ ions very quickly. The measured efficiency reached 99.99% after 0.25 minutes.
Titanium disulfide exhibited the highest capacity. One gram of material could theoretically bind up to 9,093 mg of gold under the experimental conditions. This value is roughly an order of magnitude higher than that of many reference adsorbents compared by the authors.
The mechanism combines adsorption and chemical reaction. Gold ions attach to the electron-rich surface, then are reduced to metallic gold.
To test a situation closer to recycling, the researchers produced 5 g of titanium disulfide nanosheets. They assembled them into membranes, then treated a solution obtained from 30 used processors. The process yielded 0.15 g of 23.8-carat gold.
This demonstration has been carried out only at the laboratory scale. The manufacturing cost, membrane lifetime, and their regeneration will need to be evaluated in an industrial chain. The authors have filed a patent application and now plan to study larger-scale production as well as continuous treatment of recycling solutions.