Gold extracted from 20 computer motherboards
Swiss scientists have invented a way to extract gold from e-waste from cheese production waste.
Gold has always been at the center of social development due to its precious and inert metal properties. At present, gold has become an important raw material for electronics, catalysis, aerospace, medical biology and other industries due to its excellent conductivity, ductility, reflectivity, biocompatibility and stability.
E-waste caused by discarded electronic devices is currently the fastest-growing solid waste in society, with about 61.3 million tons of global e-waste in 2023, including copper, cobalt, lithium, gold and other high-value metals. However, current methods of recovering gold from e-waste often require the use of highly toxic chemicals and energy-intensive, as well as toxic waste. The development of a safe, environmentally friendly and efficient method for extracting gold is the pursuit of many chemists.
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Specific aerogel adsorption of gold
Raffaele Mezzenga's team at ETH Zurich, Switzerland, has successfully recovered gold from e-waste using protein aerogels made of protein starch nanofibers (AF) as a new adsorbent. This approach is not only efficient and cost-effective, but more importantly it is sustainable.
They first dissolve the motherboard of a computer or mobile phone in a suitable solvent (such as aqua regia) to make a gold-containing solution of e-waste. This AF aerogel selectively adsorbs gold from e-waste solutions with a gold adsorption capacity of 166.7 mg per gram (166.7 mg/g) and an adsorbed gold purity of 21-22 carats (mass percentage 90.8 wt.%). These results indicate that AF aerogel is a highly efficient gold adsorbent with directional properties, and has significant gold adsorption capacity and selectivity.
Refining process
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How to get aerogel
To create this aerogel, Mohammad Peydayesh, a member of Professor Mezzenga's team, chose whey, the main by-product of the dairy industry, and denatured the whey proteins under acidic conditions and at high temperatures, allowing them to aggregate into protein nanofibers in the gel. The scientists then dried the gel and used these protein fibrils to make a sponge-like AF aerogel.
This aerogel is very light
With a density of 33.18 g/L and a porosity of 97%, this AF aerogel is very lightweight and has excellent mechanical and water stability. After treatment with AF aerogel, the gold concentration in the mixture decreased from 950 ppm to 130 ppm, indicating that the AF aerogel adsorbed 3320 μg of gold.
In addition, with the exception of gold, the concentration of other ions changed only slightly. The removal efficiency of aerogel was 93.3%, and the adsorption capacity was 166.7mg/g. In contrast, the adsorption rate and adsorption capacity are lower for Cu, Ni, Pb, Zn, Cr, Fe and Mn plasma. These results were obtained from the passive adsorption of extremely high concentrations of heavy metal solutions, demonstrating the superior selectivity of AF aerogels for gold ions compared to other metal ions.
Adsorption rate % for gold and other metals (left) and adsorption capacity mg/g (right)
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Why is the ability to attract gold so strong?
If the concentration of gold in the solution is low (10 ppm), gold nanoparticles will form on the surface of the AF aerogel, and these particles will be 32 nm in size. When the gold concentration is increased to 1000 ppm, the AF aerogel not only adsorbs the gold particles, but also reduces them to a single crystal form, which forms a triangular or hexagonal crystal sheet under the microscope, which always has a 120° angular symmetry in shape, and the thickness of all the wafers is unified at 55 nm. Researchers believe this is due to the complex interactions between amino acids and gold ions that allow gold to grow along its planes.
Gold ions grow into hexagonal wafers
Even with a low concentration of 1.44 ppm gold in solution, much lower than other metals such as copper (2711ppm) and iron (5033ppm), the gold content of AF aerogel treatment was reduced to 0.48 ppm, which adsorbed 66.8% of gold, while copper (the second largest adsorbed metal) had a removal efficiency of only 15.6%. The AF aerogel after adsorption of gold ions was heat treated, and the gold nuggets obtained mainly contained 90.8wt.% gold, equivalent to 21-22 carats, and the remaining metals were 10.9wt.% copper and 0.018wt.% nickel. The high gold selectivity of AF aerogel makes it possible to recycle gold-containing e-waste.
Since the raw materials come from by-products of the food industry, the cost of AF aerogel is not high, and all raw materials and process costs are only one-fiftieth of the value of the recovered gold, so this new technology is commercially viable.